Deformation history of the Ballino-Garda line in the Southern Alps, Italy
Deformation history of the Ballino-Garda line in the Southern Alps, Italy
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Author: Co-workers: Supervisors: Nynke Hoornveld Judith van Hagen Dr. E. Willingshofer Lieke de Jong Dr. D. Sokoutis Master Research Project; Solid Earth, code 450200, 27 ects 05-01-2009
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 1 Content 1 Summary/Abstract 3 Introduction 5 Geological outline 7 Tectonic evolution of the Alps 7 Evolution of the Southern Alps 9 Tectonics of the Southern Alps 11 Statigraphy and lithology of the Southern Alps 14 Trento Plateau 14 Lombardian Basin 15 Adamello intrusions 16 Field research 19 Introduction 19 Methods 20 Introduction 20 Profiles 20 Fold axes calculation 20 Paleo stress analyses 23 Results 27 Introduction 27 Garda Area 28 Pinzolo Area 34 The Garda & Pinzolo Areas 39 Discussion 41 Garda Area 41 Pinzolo Area 42 Reactivation along the Ballino-Garda line 42 Conclusion 43 Analogue modelling 45 Introduction 45 Methods 47 Model materials 47 Scaling 48 Model construction 48 Deformation 50 Limitations 50 Results 50 Pre-existing structure models 50 Rheology difference models I 50 Rheology difference models II 54 Rigid indenter models 57 Discussion 59 Comparison with a natural example: Ballino-Garda line, Southern Alps 60 Conclusion 62 Acknowledgements 63 References 63 Appendix I 66
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 3 Summary/Abstract This research focuses on the deformation history of the Ballino-Garda line area in the Southern Alps of Italy. The Ballino-Garda line is a Norian-Liassic normal fault, which separates the Lombardian basin from the Trento Plateau. This area has been influenced by multiple deformation phases since the Paleogene (Alpine orogeny). It is thought that the Ballino-Garda line got inverted in the Neogene, as a thrust with a sinistral strike-slip component.
The field observations gathered from 2 areas near the Ballino-Garda line and the southern segment of the Giudicarie fault, were processed in order to deduce sigma 1 directions from structural data like fold axes, fault slip data and styllolites. This provided 4 shortening directions, in chronological order: 1W-E, 4NNE-SSW, 2NW-SE and 3NNW-SSE. These directions were used in analogue modelling to test the sensitivity of an already existing fault to reactivation upon applying different shortening directions with respect to the fault. The experimental work focused on parameters like the (a) inclination of the pre-existing fault, (b) the fault orientation with respect to the shortening direction, (c) the rheology of the fault, and (d) the rheology difference between the weak Lombardian basin (mainly flysch deposits) and the strong Trento Plateau (mainly limestones).
It became clear that the smallest angle, between the orientation of the shortening direction and the orientation of the fault zone, favors reactivation. Also a low fault inclination, a weak fault zone and a big rheology difference enhance reactivation. Analogue models suggest that the Ballino- Garda line was a weak zone, which could have been reactivated by all 4 shortening directions.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 5 Introduction One of the most striking features in the Alps is the Periadriadriatic Fault (PAF) and its sharp inflection in the central region of the Alps. This fault that links both sections of the PAF; the Tonale fault to the West and the Pustertal fault to the East is the Giudicarie fault. The origin and kinematics of the Guidicarie fault have long been, and still are, one of the most fascinating problems in Alpine geology.
Since the first half of the last century researchers have suggested several theories for its nature (Castellarin et al., 2005 and references therein): - an oblique shear zone resulting from Tertiary N-S Alpine compression, with the accompanying necessary abundant sinistral strike-slip movement along Giudicarie North fault.
- a Tertiary arc resulting from the push of the Dolomites to the N on the Austroalpine units. - a Neogene inverted remnant of Norian/Liassic extensional tectonics and continental rifting (the formation of the Mesozoic Alpine Thetys ocean). Recent research has indicated that the latter theory is the most probable (Prosser, 2000; Castellarin et al., 2005). The aim of this research project is to gain a deeper understanding of the effects of these kinds of inherited structures, and the resulting sedimentary geometries, on the subsequent deformational evolution of a region. In what way do these structural and mechanical heterogeneities in the crust control the location and style of later deformation? In order to attempt to answer these questions a detailed field study has been carried out in the region of the Giudicarie South fault zone.
The field study investigates the relative timing and the direction of movements along several important structures in this region. This is done by investigating structural data in 2 regions; along the Ballino-Garda line around the village Arco/Riva del Garda and the Giudicarie line around the village Pinzolo (fig. 1). Also, a series of analogue models have been conducted, aiming at specifically investigating the role of pre-existing structures and variations on the style and localization of deformation within different stress directions. This structural field study and analogue modelling report is the result of a mandatory research project of three master students studying at the Vrije Universiteit Amsterdam; Lieke de Jong, Nynke Hoornveld and Judith van Hagen.
Figure 1: Location map of the field-areas of this research, with some of the major tectonic units and faults. In blue the Lombardian Basin in the West, in yellow the Trento (also called Venetian) plateau in the East, and in orange the Adamello batholith.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 7 Geological outline Tectonic evolution of the Alps The present-day Alps and their constituting rock-units are the result of several subsequent large scale processes like the opening and closure of different Oceans and continental collisions.
The oldest rift phase started in the Early Permian. This caused the initial break up of the super continent Pangea, that eventually resulted in the formation of a passive margin and the Neotethys Ocean. The westernmost branch of the Neotethys is known as the Meliata Ocean (fig. 2A). Subsequently, the area of the Alps was affected by two orogenic phases; one in the Cretaceous and one in the Paleogene.
The closure of the Meliata Ocean (fig. 2B) initiated by the Cretaceous rifting resulted in the merge of two opposing continents, North and South Apulia, to form an important micro- continent often mentioned in literature: the Apulian plate. Remnants of this plate now form the Austroalpine and the Southalpine units in the Alps (fig. 3 and 4). The Piedmont-Liguria Ocean and the Valais Ocean; together called the Alpine Tethys are kinematically linked to the opening of the Atlantic Ocean. The onset of the opening of these Oceans occurred during the cretaceous rifting. The Piedmont-Liguria Ocean opened in the Late Triassic.
In the Early Cretaceous, the Valais Ocean opened, resulting in the formation of a small landmass in between the two Oceans: the Briançonnais terrane. Rocks from the Briançonnais terrane and the European continental margin are still found as slices in the Penninic units (fig. 3 and 4). The Western Alps still carries remnants of the northern margin of the Alpine Tethys, called Helvetic nappes (figs. 2,3 and 4). The Paleogene orogeny started after the closure of the Alpine Tethys (fig. 2C).
(Schmid et al, 2004) Figure 2A B C: Paleogeographic maps of A: Late Triassic, B: Late Jurassic and C: Late Cretaceous. In the maps the evolution of the Meliata, Piedmont- Liguria, Valais and the Neotethys Ocean is showed. W: Vienna, G: Geneva. (after Schmid et al, 2004)
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 8 From the end of the Cretaceous / beginning of the Paleogene onwards, the Apulian plate started to move towards Europe. During the Eocene, the Piedmont-Liguria Ocean was entirely subducted and continental collision of the European and Apulian plates followed.
Subduction direction of the Piedmont-Ligurian Ocean was oblique, this resulted in asymmetric collision of the two continents. Units from the Briançonnais terrane and the European margin were subducted towards the south, pushed under the Apulian plate, experiencing deformation and metamorphosis up to eclogite facies. The Dinarides (Apulian plate units south of the Periadriatic lineament) being in the upper plate position, escaped intense Alpine deformation and indented the European upper plate inducing backthrusting along the Periadriatic fault (PAF) system (fig 3). (Schmid et al, 2004) In the Eastern Alps, backthrusting along the PAF is less important; here exhumation of deformed rocks is mainly controlled by orogen- parallel extension and erosion (Fügenschuh et al, 1997).
There is still a discussion on the plate configuration (upper/lower plate positions) along strike of the Alps as well as in time. In the Western Alps a uniform southward dip of the European plate is commonly accepted. In the Eastern Alps there is, as of jet, no agreement on the dip direction. Figure 3: geological map of the Alps and their constituting rock-units after Bigi et al, 1990 and simplified by Castallarin et al, 2006. The black bar gives the location of profile NFP-20 EAST.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 9 Evolution of the Southern Alps The Southalpine Unit (Apulia) is located south of the Periadriadic lineament/fault (or Insubric line) and north of the Po plain (Castallarin et al, 2006), see figs.
3, 5 and 7. The PAF is basically the backthrust of the Alpine orogen. Its eastern part is also a major dextral fault along which the Eastern Alps moved eastward relative to the Southern Alps. This movement plays a strong role in the extrusion of the Eastern Alps (Schmid et al, 2004), see fig.4.
The Southalpine unit has been subdued to two rifting phases and one compressional phase. The early Permian break-up of Pangea affected the Southern Alps by rifting from the SE. The Norian/Liassic rifting produced the opening of the Piedmont- Ligurian Ocean, this created a W-E oriented horst and graben structure (fig. 5), followed by a N-S directed compression (Alpine orogeny). (Castellarin et al, 2006) The Norian-Liassic rifting produced very slow extension rates, as a consequence only a small thermal anomaly was generated and most of the subsidence took place already during rifting (Bertotti et al, 1997).
Strong subsidence affects the Lombardian Basin. The extension is basically accommodated along few major normal faults, creating synsedimentary W-E oriented horst and grabens; Friuli platform, Belluno trough, Trento Plateau and Lombardian Basin (fig. 5). This resulted in extensional N-S striking (listric or domino style) normal faults, the Ballino-Garda line separates the Trento (also called: Venetian) Plateau from the Lombardian Basin. See figure 7 for the locations of the main tectonic units.
The convergence history of the Southern Alps (Alpine orogeny) includes the Late Cretacious pre-collisional, the Eocene collisional and the late Oligocene-pliocene post-collisional (Neo- Alpine compression). Melting due to the rise of the isotherms at this post-collisional thermal relaxation produced magmas and large emplacement of intrusive bodies all along the PAF: Bergell, Adamello and Riesenferner plutons of Rupelian age. In the Figure 4: cross cut NFP-20 EAST, after Schmid et al, 2004
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 10 Southern Alps this is the Adamello batholith.
Apulia, in the upper plate position, escaped the Alpine metamorphism. Therefore the Southern Alps consist of a south-vergent fold and thrust belt of backthrusts. (Castellarin et al, 2006) The overall compression direction in the Paleogene (alpine orogeny) is N-S, but can be divided into 5 sub stages (see fig. 6). The Giudicarie fault system, still shows the remnants of these deformation phases. (Castellarin and Cantelli, 2000 and Castellarin et al, 2006). These multiple deformation phases have created the structural system and/or reactivated older structures, depending on their orientation.
The location of the structural belts can be seen in fig. 7. (Castellarin et al. 2006).
Tectonics of the Southern Alps The PAF is a fault system that forms the northern boundary of the Southern Alps (fig. 7). The system includes several major faults; the Insubric/Tonale fault, the NE trending Giudicarie North fault and the Pustertal fault (fig. 7). Several kilometres to the N and W of the Giudicarie faults we find several other faults that are also considered part of the Periadriatic fault system; Peio-, Rumo-, Passeier-, Jaufen- and the Defereggen-Antholz-Vals (DAV) fault. (Müller et al, 2001), see fig. 7 for the exact positions of these faults. Because of their central position, knowledge about the age and kinematics of the Periadriatic- and Giudicarie fault systems are essential to understanding the evolution of the whole area.
The Passeier Fault Initiated during the Insubric- Helvetic phase (fig.6) by the Giudicarie North fault to accommodate sinistral strike slip movements (Castellarin et al., 2006). Pseudotachylyte dating by Müller et al. (2001) gave ages around 17Ma. The NE trending Rumo Fault was active in the Cretaceous (60-57 Ma ago, pseudotachylyte dating by Müller et al., 2001). Movements were WNW along a NW dipping normal fault. Figure 5: Location of the horst and graben system after the Norian Liassic rifting, modified after Winterer and Bosellini, 1981.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 11 The Peio fault was active around 45 Ma ago, according to Müller et al.
(2001) (age from pseudotachylyte dating). Later on (~35Ma ago) it was again reactivated. The fault shows sinistral transtensive with north side up kinematics related to a shallowly E plunging stretching lineation. The Giudicarie fault system, in its present orientation, is a restraining bend in the PAF and interacts strongly with the faults mentioned above. Establishing if the Giudicarie fault system is an inherited structural feature (pre-Oligocene), or a late, collision derived structure is crucial for estimating the maximum possible amount of accumulated dextral shearing along the PAF. This in turn imposes different constraints on the proposed orogen-scale tectonic models (Viola et al., 2001).
Some researchers (a.o. Martin et al., 1998; Prosser, 2000; Viola et al., 2001; Castellarin et al., 2006) believe there is enough evidence to suggest that the Giudicarie North inflection was in fact already present in the Late Cretaceous. This would imply that dextral displacement along the PAF in the Neogene Figure 6: five main Alpine orogeny deformation phases with the associated structural systems and stress fields. Made by Judith van Hagen, after Castallarin et al, 2006 and references therein.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 12 Figure 7: A detailed description of all faults in the region relevant for this research, in order to give a more detailed overview of the structures in the Southern Alps and their kinematics. Modified after Muller et al, 2001 (top figure) and Castallarin and Picotti 1990 (left figure).
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 13 (alpine orogeny) could not have been more than a few tens of kilometers. This is in accordance with recent studies that have estimated the amount of Neogene dextral displacement along the PAF at no more than 40 kilometres (Castellarin et al., 2006).
This also implies that if the PAF was originally a straight, continuous line, this could only have been the case before the Late Cretaceous. It is thought that (Martin et al., 1998; Prosser, 2000; Viola et al., 2001; Castellarin et al., 2006) the Giudicarie North line nucleated along the normal faults created during the Norian-Liassic passive margin formation as the transition between the Lombardian basin and the Trento platform and acted as a transfer zone (Castellarin, 1972). In this setting it should have accommodated about 50 kilometres of sinistral strike slip movement in total since its formation (estimated from the current location of the present regional inflection of the Alps along the Giudicarie fault) (Castellarin et al., 2006).
Sinistral strike slip movements in this region are partitioned mainly onto the Passeier fault (Fig 7). The amount of slip along the Giudicarie North fault is estimated around 21km (calculated from the depth of formation of the mylonites and the depth of tonalite emplacement). This corresponds to 11km of vertical offset on a N 145, 40o dipping slip vector along the Giudicarie North fault plane (Prosser, 2000). During the Oligocene-Neogene (Insubric- Helvetic phase, fig.6) the N-Giudicarie fault was inverted into a thrust with contemporaneous dextral displacement along the PAF. The main tectonic contact is a NW dipping fault that presently displays reverse movement and now juxtaposes Austroalpine basement rocks on the W against Southalpine sedimentary cover rocks on the E (Prosser, 2000).
It consists, besides the foliated tonalities and basement- and limestone derived mylonites, of a brittle fault zone, mostly developed in South Alpine cover rocks. The southern section of the Giudicarie fault system separates the Adamello pluton in the west, from the Southalpine sedimentary cover in the east. It is formed by several steep west dipping faults and it is closely associated with a large transverse fault and thrust zone to the east that is called the Giudicarie belt, which is in its turn, strongly linked with the Val Trompia system to the south (fig. 7). Unlike the Giudicarie North fault, the Giudicarie South fault is of Miocene age and was formed during the Valsugana deformation phase (fig.
6). (Castellarin et al., 2006). The N trending Ballino-Garda Line is a remnant of the Norian-Liassic rifting phase. This major N-S normal fault formed the principal divide between Lombardian basin on the W and the Trento platform on the E (fig. 5). Field data shows that the offset affected the sedimentation in this region. The faults were reactivated and show synsedimentary wedges up to the Maastrichtian.
According to Prosser (2000) the Trento-Cles Fault is a N trending branch-off of the Giudicarie North fault, that accommodated a large part of the sinistral strike slip movements during the Insubric-Helvetic deformation phase (fig. 6). It is however, a much older structure that was reactivated because of its favourable orientation with respect to the Oligocene-Miocene stress regime. Stratigraphic data indicate that it is a Late Triassic normal fault that originated due to the cretaceous rifting phase that eventually led to the formation of the Alpine Tethys. The Val Trompia Fault (fig. 7) is suspected to be of Permian age (Cassinis, 1983).
Reactivated in the Norian-Liassic rifting phase; opening of the Piedmont-Ligurian Ocean; together with the Vies-Trat-, Lenzumo-, Drosso del Vento lines. Its kinematics are closely linked with the Giudicarie belt towards the north. (Castallarin and Picotti, 1990 and Castellarin et al., 2006).
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 14 Stratigraphy and lithology of the Southern Alps The Southern Alps are considered to be a part of preserved continental margin of the Apulian microplate. The three main elements in the region are the Lombardian Basin, the Trento plateau and the Adamello intrusion. The Lombardian basin and the Trento plateau are related to the Piedmont-Ligurian Ocean. As mentioned before the Ballino line in the Guidicarie zone separates this basin and plateau. This section concentrates mainly on the stratigraphy present in the research area, which is situated around the transition of the Trento platform to the Lombardian basin.
The descriptions are based on the carta geologica d’Italia, no 59 Tione di Trento and 80 Riva del Garda, except when mentioned otherwise. Trento Plateau At the end of the Triassic (Norian-Liassic rifting) a carbonate platform of Bahamian type formed at an area of slow subsidence rates (50 m/my, according to Winterer and Bosellini, 1981). The Trento plateau builds up until the end of the Liassic, when the platform drowned (fig. 8). The sequence consists of the peritidal and suptidal limestones of the ‘Gruppo di calcari grigi’ (gray calcareous rocks) followed by the calcare oolitico di S. Vigilio (OSV) and the top is formed by the Rosso Ammonitico Veronese formation.
Gruppo de calcar grigi: the Monte di Zugna formation (FMZ) is formed by peritidal and subtidal sediments. The peritidal unit is build up by limestones, the subtidal sediments consist of biocalcarenite with brown micrite layers. These different rocks are intercalated and spread out over the research area. The top member consists of calcmicrite. The FMZ is deposited during the Rhaethian to Sinemurian and is about 900 m thick. Concordant on the FMZ formation follows the Calcare oolitico di Loppio formation (LOP) which is deposited during the Sinemurian. This max. 200 m thick formation consists of mainly calcareous grainstone with ooids.
The carbonate platform builds up further with the formation Rotzo (RTZ) during the Sinemurian to Pliensbachian. The deposition of the RTZ represents a transgressive period. The formation consists of wackestone and packstones (calcmicrite) with mollusks and foraminifera. The Tovel member is an intercalation not present everywhere. This member contains more bioclasts. The total thickness has a maximum of 300 m. The top of the Calcari grigi group is formed by the Calcare Oolitico di Massone (OOM), a grainstone formation. It is deposited during the upper Pliensbachian and is about 250 m thick.
At the end of the Lias (Toarcian) the OSV was deposited. The OSV has a discordant position on the Gruppo di Calcare Grigi. The formation consists mainly of grainstones with ooids and intercalations of micrite. After/during the deposition of this formation the carbonate platform started to drown.
The top of the Trento plateau is formed by the Rosso Ammonitico Veronese (ARV). Because of the drowning of the platform these sediments have a pelagic character, but the distribution of the formation is still limited to the plateau area. Within the calcmicrite, the ARV contains pelagic bivalves and red nodules with occasionally ammonoids (fig. 8). This formation reaches a maximum of 20 m, measured on top of the tilted blocks created by the normal faults of the Ballino-Garda Line. The ARV is deposited during the Bajocian (sup.) until the end of the Jurassic, Tithonian.
Lombardian Basin During the Jurassic deep water sediments are deposited in the area west of the Ballino-Garda line, in the Lombardian basin.
The basin is filled mainly with the cherty limestones of the Tofino formation (TOF) and the turbidites of the Val d’Oro formation (FVO). The top is formed by the radiolarites of the Selcifero Lombardo formation (SLO).
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 15 figure 8: overview of all stratigraphic units. The TOF formation is divided in four members, with the main difference being the fossil content. In all of the members intercalations of turbidites occur. These turbidites originate from the Trento platform; debris of the platform (loosened also by the normal faults of the Ballino-Garda line) was transported into the basin. These turbidites belong to the FVO formation. The TOF members consist of: calcareous dolomicrite, marly limestone with breccia levels, levels of radiolarite, chert in nodules and sponges, needles, crinoids and brachiopods.
The TOF and FVO are deposited during Hettangian to Sinemurian.
Simultaneously the turbidites of the FVO are deposited, carrying the platform material. The formation consists of dark calcmicrite with radiolarian, sponge needles, chert noduli and layers of breccia and turbeditic oolith, turbeditic calcarenite and calcsilt. Also breccia with marine fragment associated with slumping is present. The formation has a maximum thickness of 450 m, but varies locally because of the slumping and flow character (fig. 8). As can be seen in figure 8 the Selcifero Lombardo formation (SLO) forms the top of the Lombardian basinal sequence. This is a formation formed by radiolarite mainly, intercalations of limestone with some chert are present.
The SLO is between 0-80 m thick and is deposited during the Bajocian until Tithonian. Adamello intrusions The Adamello batholith is the largest (670 km2) and oldest (Eocene to Oligocene) of the plutons exposed along the Periadriatic fault (Pennacchioni et al. 2006).
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 16 The intrusion is located at the junction between the Giudicarie and the Tonale lines of the PAF (fig. 9). The Adamello batholith consists of 4 distinct, dominantly tonalitic-granodioritic intrusions: 1. Re di Castello - Corno Alto 2. Western Adamello 3. Avio –Central Peaks 4. Presanella These above mentioned intrusive units are distributed from south (Re di Castello) to north (Presanella) as can be seen in figure 9. Geochronological data shows a decrease in age from 42-38 Ma for the Re di Castello intrusion to 32-30 for the Presanella section (Pennacchioni et al.
In this research, three of these four intrusions are found. In the area west of Pinzolo, the Corno Alto (1) pluton is present. This is a medium to coarse grained granodiorite- trondheimite with poriric plagioclase crystals. North of the Corno Alto, in the E-W corridor of the Genova valley, the Val d' Avio (3) pluton is found. This intrusion is leucoquartzdiorite with biotite and rare amphibolite crystals. Nuclei of mafic crystals are often found. A clear tectonic foliation is present in this intrusion. The youngest pluton is the tonalite of the Presanella Figure 9: Location of the Adamello batholith with the different intrusive units indicated.
(Pennacchioni et al. 2006)
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 17 intrusion (4). This contains medium to coarse grained biotites and amphibolites. Nuclei of fine grained mafic crystals are also present in this unit. As mentioned above the sequential emplacement history of the Adamello batholith is indicated by the progressive younging of crystallization ages from south to north. This trend is confirmed by crosscutting relationships between the plutons. The mechanisms of final magma emplacement in the Adamello batholith differ from one batch of magma to another.
None of these mechanisms can be directly attributed to the activity of the PAF.
Structural and microstructural investigations of the northern Adamello plutons show that emplacement was syntectonic with respect to the PAF. Interestingly, the southern part of the batholith, which has no spatial relationship to any segment of the PAF, was intruded at 42 Ma, i.e., prior to the activity of the PAF. In contrast, the northern and northeastern parts of the batholith, which are adjacent to the PAF, yield intrusion ages (34 to 28 Ma) matching the inferred activity of the PAF. The magmas of the Adamello Batholith probably ascended independently of the PAF. Unlike the Bergell, the lithologies of the southern Adamello cannot be continuously traced northward into the PAF.
It is therefore unlikely that magmas ascended along the PAF and subsequently migrated southward, along a gently inclined path at a depth of ~7 km, over a distance of ~50 km.
(Rosenberg, 2004 and references therein)
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 18
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 19 Field research Introduction Data for this research has been gathered in the field by structural mapping. Focussing on two areas (red rectangles in Fig. 10), where the Ballino-Garda line and Giudicarie South fault can be found in outcrop. The southern area is located between Lago di Ledro and the town Riva del Garda.
In this area the Ballino-Garda line lies in the Garda Lake and can only be investigated by indirect data. The accessibility of this area is moderately good. The second area is about 50 km to the NW of Arco/Riva del Garda, in the Brenta Massif surrounding the towns Pinzolo and Tione. Both are mountainous areas with large height differences. In the Riva del Garda area the elevation is 400-1900m. The terrain is mainly bare rock and therefore many outcrops are present, the low areas are covered with forests and meadows. The area around Pinzolo in the Brenta Massif has an elevation of 600-2100m.
The number of outcrops is very good especially higher up the mountains, but these areas are not very accessible: bad quality roads, few footpaths, steep hillsides and glaciers. In the field, the focus was on finding evidence of brittle deformation: faults, fault planes, fault gouge, and kinematic indicators: slickensides, mineral steps, joint systems and stylolites. Also bedding planes were measured to derive fold axes.
Planes and lines were measured with normal compasses (no declination). The notation used for planar structures is the azimuth (from 0 to 360 clockwise from the N) and dip angle. For lines this is the azimuth and plunge. Data was arranged according to the numbered outcrops were they were gathered; numbers have prefix G for the Garda area and P for the Pinzolo area, also individual measurements have the prefix of the first name of the student in question. These locations have been registered with GPS handhelds in the field. The used coordinate system is the UTM projection (WGS 84, this part of Italy is zone 32).
The maps used in the field are: carta geologica d’Italia; no 59 Tione di Trento and 80 Riva del Garda, 1:50.000. And the topographic maps; Kompass- Figure 10: location of the field areas and the main tectonic units present.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 20 karten; 690 Alto Garda e Ledro and 688 Gruppo di brenta, 1:25.000. The structural data was processed and calculated by two computer programs. Fold axes were calculated by: StereoWinFull version 1.2.0, written by Rick Allmendinger, Cornell University New York: http://www.geo.cornell.edu/geology/faculty/RW A/programs.html, and the paleo stress tensors were obtained by using WinTENSOR, a software developed by Dr. Damien Delvaux of the Royal Museum for Central Africa, Tervuren, Belgium. The version used: 1.4.15: http://users.skygrid.be/damien.delvaux/Tensor/ tensor-index.html Methods Introduction The structural data has been divided in two field areas.
The southern area will be called Garda area and the northern area will be called Pinzolo area.
Profiles Profiles were taken from A to B (see figure 11 and 12 for the exact locations) and were deduced from the geological map carta geologica d’Italia, no 59 Tione di Trento and 80 Riva del Garda, and from the gathered information in the field along the gps points as has been indicated. Fold axes calculation Fold axes were used for indicators of shortening directions, they were measured directly or calculated from bedding planes. This data was grouped in locations per area. The locations were chosen for their outcrop spacing, position nearby faults or similarities in statigraphy. For Garda the locations are: Ledro 1, Ledro 2, Pergamo, Campi and Garda 1.
For Pinzolo the locations are: Stenico, Cacciatore, Mughi and Cassinei. See figure 11 and 12 for the emplacement of these locations. Some locations showed multiple oriented fold axes, we assume them to be related to multiple deformation phases.
Garda locations: Campi (outcrop: G21) was grouped as a location for its being the only outcrop with information within a kilometer. Pergamo (outcrops: G01-06) was grouped as a location for the spacing between the outcrops is very small and all outcrops are close to the Ballino-Garda lineament. Ledro 1 (outcrops: G09-13/20+L01-08) and Ledro 2 (outcrops: GJ01-03+G07/08) were divided in 2 locations for the reason that the bedding plane information exceeded the level of organisation in the computer program. Garda 1 (outcrops: N02-05+J05-08+G14-17) was grouped as a location for all outcrops are near the Ballino-Garda lineament, and as Garda 2 gave no information on bedding planes, the Garda area got divided in Garda 1 and 2.
Pinzolo locations: Casinei (outcrops: P01+P02) was grouped as a location for its being the only outcrops near the Ballino-Garda lineament reachable within several kilometers.
Mughi (outcrops: P08-15) was grouped as a location for the spacing between the outcrops is very small and all measurements represent the same mountain. Cacciatore (outcrops: P04/06/21-24/37/47/48) was grouped as a location for all outcrops are near the Ballino-Garda lineament or near one of its important divisions. Stenico (outcrops: P05+P25+P31-35) was grouped as a location for all outcrops represent the same mountain, near the same fault in one road cut.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 21 Figure 12: Main faults of the Pinzolo area.
Line A-B shows the location of the profile line, ellipses represent the locations of grouped data. Dots represent numbered outcrops. Figure 11: Main faults of the Garda area. Line A-B shows the location of the profile line, ellipses represent the locations of grouped data. Dots represent numbered outcrops.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 22 The computer program StereoWinFull was used to calculate the fold axes from the measured bedding planes. The output is an equal area grid plot with bedding planes as great circles and a cylindrical best fit as points. The fold axis is calculated by the action: “cylindrical best fit”. This function calculates the best fit plane to a distribution of lines and plots, the plane and the pole to the plane. This routine defines the density function of the Bingham distribution: an antipodally symmetric distribution on a sphere, it is a two-parameter distribution.
The density describes a wide range of distributions on the sphere and displays the eigenvalues and eigenvectors. The great circle is drawn through the two eigenvectors corresponding to the two largest eigenvalues, with the fold axis given by the vector corresponding to the smallest eigenvalue (number 3 in fig. 13). The Bingham' s distribution on the sphere is applied to orientation data from cylindrical folds. Data from cylindrical folds typically form two clusters, one cluster for each fold limb. The bimodal distribution is obtained by fitting a unimodal distribution to each cluster. One parameter of the distribution gives the fold axis, another parameter is directly related to the curvature of the fold limb (Kelker and Langenberg, 1976).
The existence of only cylindrical folds can of course be debated. Figure 13: results plot after adding the bedding planes followed by the action: “cylindrical best fit”. The shortening directions obtained from the fold axes have been grouped in 8 directions from now on called subsets. 8 subsets were made (see fig. 14). Each shortening direction found, belongs to 1 of the following subsets: W-E, WNW-ESE, NW-SE, NE-SW, ENE-WSW, NNW-SSE, N-S, NNE-SSW .
3 Figure 14: the 8 subsets Result of the density function of the bingham distribution
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 23 Paleo stress analyses Kinematic indicaters as slickensides, (conjugate) faults, joints and stylolites were gathered in the field. This data was grouped in locations. These locations were chosen for their outcrop spacing, same fault nearby or similarities in statigraphy. For Garda the locations are: Ledro 1+2, Garda 1, Garda 2 and Pergamo. For Pinzolo the locations are: Stenico, Cacciatore, Bleggio, Tione, Giudicarie and Cassinei.
See figure 11 and 12 for these locations. A computer program was used to calculate the paleostress tensor with these measured data.
Garda locations: The Garda paleostress locations are the same as for the fold axes calculation, extra locations are added below: Ledro 1 + 2 (outcrops: G07-13/20+L01- 08+GJ01-03) were grouped in 1 location for all outcrops are near Lake Ledro and around the same fault pattern (roughly W-E oriented faults with dextral movement). The used computer program could handle all this information at once. Garda 1 (outcrops: N02-05+J05-08+G14-17) was grouped as a location because all outcrops are near the Ballino-Garda lineament, as Garda 1 and 2 provided a bulk of kinematic information, separation by hand (which the computer program requires) was not realizable and the locations were divided.
Garda 2 (outcrops: N07-13+L09-11+G19) + (G18) was grouped as a location for all outcrops are near the Ballino-Garda lineament, and the amount of information manageable. G18 deserves special attention for its being a large fault zone with sufficient data to be regarded on its own. Pinzolo locations: The Pinzolo paleostress locations are the same as for the fold axes calculation, extra locations are added below: Giudicarie (outcrops: P03+P18-20+P44-45) was grouped as a location because all data is provided from the Adamello batholith near the Giudicarie South fault.
Bleggio (outcrops: P07+P17) was grouped as a location for its being the only Permian rock outcrops with information.
Tione (outcrops: P42+P46+P49) was grouped as a location for its being the only Neogene outcrops and also the only outcrops with information within a kilometre. Input data The computer program win_tensor is a sophisticated program which handles the following input: 1 - Fault plane with slip line (slickenside) 2 - Two conjugated shear planes 3 - Shear plane with tension fracture 4 - Plane alone (Fracture, Bedding, Foliation) 5 - Focal mechanism: Movement and auxilliary planes & type 6 - Focal mechanism: Movement plane, slip line & slip sense 7 - Focal mechanism: P and T kinematic axes 8 - Line alone (Fold or Boudinage axis, Stylolite peak..) Our study areas provided mainly data of type: 1, 2, 4 (fracture) and 8 (stylolite peaks and folds), where a fault represents a plane with measurable slip and a fracture/joint represents a plane without indication of movement.
In case of 1: A slickenside needs an orientation and a direction, given in: (N) = Normal (I) = Inverse or Reverse (D) = Dextral (S) = Sinistral (X) = Unknown The direction of the slickensides also needs a confidence level, given in: (C) = Certain (P) = Probable (S) = Supposed (X) = Unknown The fault with slickensides can be given a weight factor (1-9), this indicates the importance of the fault. The weight factor was kept at a standard of 2,0 because this kind of
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 24 classification was hard to establish in the field. An activation type (0-3) can be added for relative timing of the fault. The activation type was kept at 1: neoformed, for relative timing indicators were scarce in the field. The striae (slickensides) intensity can be addressed with a classification (0-4), in our research the intensity was kept at a standard of 0 because this kind of classification was hard to establish in the field.
Activation type (0) = Non activated (1) = Neoformed (2) = Reactivated (3) = Unknown Striae intensity: (0) = No striae (1) = Weakly marked striae (2) = Well marked striae (3) = Profound striae (4) = Corrugation Subset management For rocks that have been affected by multiple deformation phases, the raw data set consists of several subsets (or systems) of fault data.
A subset is declined as a group of faults and fractures that moved during or has been generated by a distinct tectonic event, which can be ascribed to a particular stress tensor/shortening direction. (Delveaux and Sperner, 2003) After this subset management it occurs that some data is incompatible with all subsets, possibly due to: measurement errors, incorrect data input, the presence of reactivated inherited faults, fault interaction, non-uniform stress field and non-coaxial deformation with internal block rotation (Dupin et al, 1993; Pollard et al, 1993; Angelier, 1994; Nieto-Samaniego and Alaniz-Alvarez, 1996; Twiss and Unruh, 1998; Maerten, 2000; Robert and Ganas, 2000).
A certain percentage of misfitting data is normal: 10-15%, but they have to be eliminated from the data set for better accuracy of the calculated results. A stress tensor is also still based on misfits and calculates the best fit for the entire data set (Delveaux and Sperner, 2003).
This subset data management is managed by hand. In the field measured kinematic indicators were already assigned to a certain shortening direction, in order to make the subset management easier. The 10-15% misfit range was exceeded with 37% at Stenico area. Further subset management needs to be done by the right dihedron and optimization technique. Please note that subset numbering does not indicate a time span, or relative timing. Right dihedron The right dihedron method is typically designed for building initial data subsets from the raw data set, and for making a first estimation of the four parameters of the reduced stress tensor.
The method has been designed by Angelier and Mechler (1977) and has been improved by Delveaux and Sperner (2003). The principle is that the sphere is divided in a grid, each grid segment gets a value: extensional= 100% compressive= 0%, all summed up and divided by the number of faults analyzed. In order to obtain compressive or extensional information, where slip information is essential. Possible orientations for 1 and 3 are defined by the orientations in the average counting grid (values 0-100%), see figure 15 for a visualization of this counting grid. 1 and 3 are defined separately and are not always perpendicular.
The improved version introduces the stress ratio R: R = ( 2- 3/ 1- 3). This is possible by introducing 2 (a counting value) by using the empiric relation: R~(100- 2)/100 R gives information about the stress ellipsoid. The counting deviation (CD) compares each datum counting with the average counting grid, depending on the weight of the datum. A low CD value attributes in the positive to the tensor and
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 25 a high CD influences the tensor in the negative. The CD values should not exceed 40%, by separating these high values from the low is the next step in making subsets. (Delveaux and Sperner, 2003) Figure 15: counting grid of the right dihedron method. Where S1 = sigma 1, 1, S2 = sigma 2, 2, S3 = sigma 3, 3, R the calculated stress ratio, QRw with quality D and QRt with quality E, n/nt 7/7, the CD does not exceed 40%, the big blue dots (=100%) represent extension and the small blue dots (0%) represents compression.
Rotation optimization The subsets made by the right dihedron method serve as a starting point for the rotation optimization technique. This technique is based on the testing of a great number of different stress tensors, with the aim of minimizing a misfit function. The right dihedron allows restriction of the search area during the inversion, so that the whole grid does not need to be searched. The minimization function used in this research is F5, which is very efficient for mixed data. As kinematic indicators type 1, 2, 4 and 8 were introduced in the tensor calculations. The rotation optimization function consists of a 4D grid search involving successive rotations of the tensor around the three principal stress axes and equivalent testing of stress ratio R.
For each stress axis the rotation angle is determined for which the misfit function has its minimum value. From 4 runs this minimum value can be determined from the polynomial regression curves (see fig. 16). After choosing the lowest F5 value the tensor is rotated accordingly (see fig. 17). The fault slip data are considered compatible with a stress tensor as soon as the deviation angle is less than 30° . In this stage subset management continues, after the optimization action certain data will exceed the required 30°and need to be separated from the tensor.
(Delveaux and Sperner, 2003) Figure 16: The axes stability of the three principal stresses after the F5 minimization function, the line represents the rotation angle per principle stress at the lowest F5 value, which is here 11,17.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 26 Figure 17: Rotation optimization with function F5 and value 11,17. Where S1 = sigma 1, 1, S2 = sigma 2, 2, S3 = sigma 3, 3 R the calculated stress ratio, QRw with quality D and QRt with quality D, n/nt 14/80, does not exceed 30° .
Data quality The quality of the results obtained by stress inversion is dependant on a number of factors such as: number of data per subset, type of data, and the experience of the user. The program consists of a quality ranking: A (best) to E (worst) n/nt = the ratio of slip data used in the inversion relative to the total number measured. Note that n/nt changes after each technique. QRt = a tensor quality rank, calculated after 7 steps of pre-quality ranking including, unit vectors representing the poles of fault planes and slip directions and n/nt. Note that QRt can change after rotation optimization (fig.
15 and 17).
QRw = initial WSM quality rank designed by Delveaux et al (1995b and 1997b) F5 value as described above is also a quality rank, the lower the better the quality. R’ gives the stress regime where: 0-1 is extensional, 1-2 is strike-slip and 2-3 is compressional (see fig. 18) (Delveaux and Sperner, 2003) Figure 18: axe stability of R’, the lower the y-axis value the more stable the R’, the x-axis indicates the stress regime; here 1,5 means pure strike-slip. Note that all figures used represent Stenico location subset 3.
Eventually these tensors were also addressed to 1 of the 8 estimated shortening directions as mentioned before (fig.
14). Data grouping: The grouping of the data is an arbitrary process. For example: Cacciatore is a big area with multiple faults (fig. 12 and 26). One subset is represented by only P04 and P38 which are near a big N-S striking sinistral strike-slip fault while P21/22/23/24 are near a big W-E striking sinistral strike-slip fault. P37 and P47+48 are near the Ballino-Garda line. This grouping makes subset comparing rather difficult, especially locations as Tione and Giudicari (fig. 12 and 26); because the information comes from rocks with immense age difference. Another implication is that the spacing between outcrops is too big to represent the results within the location.
The research area is small and the data gathered is very limited, this might imply that the tensors are local instead of applicable to the large scale stress field.
The subset management is mainly done by hand and is therefore arbitrary; data that does not fit the subsets become a misfit and get thrown away. The consequence of such a routine is that the 10%-15% misfit boundary gets exceeded.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 27 In the rotation optimisation technique, the program designs a stress regime (compression/strike slip) for the paleostress direction. In this research the shortening direction was most important and subsets have been classified by 1 direction.
It is obvious that the stress regimes do not fit the shortening direction/subset classification. The stress regimes could also have been classified in order to obtain information about different deformation phases instead of just looking at the 1 direction.
Despite the objections, 4 repeated distinct shortening directions have been found in both research areas, from now on called: subset: 1,2,3 and 4 (see fig. 19). W-E = subset 1 NW-SE = subset 2 NNW-SSE = subset 3 NNE-SSW = subset 4 Figure 19: the 4 subsets found in the field after data processing, correspond to 4 shortening directions. Results Introduction The research areas consist mainly of carbonates, dolomites and flysch deposits. The Lombardian Basin is build up with mainly “soft” rocks as flysch, clay, cherty limestones and turbidites. The Trento Plateau consists of “harder” rocks such as peritidal and suptidal limestones, as has been indicated in the chapter “geological outline”.
The geological maps and profiles (fig. 21 and 26) in these research areas show 10 statigraphical units (fig. 20), they are classified with respect to the depositing time. See figure 8 and the chapter “geological outline” for the details of the formations formed. The classification consists of the following units: • Pre-perm: Consists of basement rock containing micaschists with paragneis. • Perm: Mainly intrusion deposits, consisting of a coarse grained granodiorite and leucogranodiorite. The intrusions are followed by sedimentation in the form of rhyodacite lavas, terrigeneous deposits and conglomerates.
• Early Triassic: Contains calcareous breccie and conglomerates, locally dolomitized. • Middle Triassic: The DPR and ZUU formation. • Late Triassic: Contains the COR, RTZ, LOP, FMZ and OOM formations. At the Trento Platform these formations consist of peritidal and subtidal sediments. In the Lombardian Basin these formations contain cherty limestones and turbidites. • Jurassic: Contains the FVO, TOF and SLO formations, which contain cherty limestones, turbidites and radiolarites in the Lombardian Basin. At the Trento Platform these formations form pelagic calcmicrite.
• Cretaceous: Contains the MAI, SAA and VAG formations.
• Paleogene: The FPP and PTA formations. • Eocene-Oligocene: The Adamello intrusion. • Pliocene-Holocene: glacial deposits; debris flows and conglomerates.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 28 Figure 20: Statigraphic overview, classification as used in the profiles and the geological maps. This chapter will be further subdivided in: Garda area Pinzolo area Garda & Pinzolo areas. Garda Area The geological map and profile of the Garda area (fig. 21) show the main tectonic features: normal faults, thrusts, dextral and sinistral strike slip faults and folds. The normal faults and thrusts are mainly N-S oriented, which implies that the lengthening and shortening directions are W-E orientated.
The strike-slip movements also seem to correspond with a W- E compression direction. This direction does not correspond to any recent movement as has been described by Castellarin and Cantelli, (2000) and Castellarin et al, (2006). When looking at the profile please note that the area is very complicated and bedding planes were hard to measure, therefore the profiles were hard to deduce and gathered information was projected inclined on the profile line. Because of the peritidal character certain places miss statigraphic units.
The geological map shows that most deformation in this research area took place in the brittle domain, expressed in the formation of: fault-, joint-, and fracture systems with kinematic indicators. Minor ductile deformation is found, such as folds and pressure solution in the form of stylolites. This field area mainly consists of the basin formations at the west side of the Ballino-Garda line. The area consists of a remarkable big amount of breccias. Most of them are found around the Ballino-Garda line. During normal faulting the platform material got crushed and deposited in the basin as brecciated stones.
G08 and G10 show syntectonic brecciated Jurassic deposits. They have wedge shaped geometries and they are cemented with Jurassic deposits. All breciated outcrops are found around major faults: G08, G10, GN01, GN10 and GN13. The figures 22 A + B are pictures of syntectonic breccie deposits. Other outcrops show fault breccie, which are excellent examples of large scale brittle deformation. Next to the large amounts of fault breccie and brecciated deposits, the area also contains fault gouge. Especially outcrop G01, G10, G12 and G15 show several meters of fault gouge next to large fault planes.
Fault gouge can be used as a decollement when the fault gets inverted. The main structural features on this geological map do not show large scale folds, while the profile clearly states the presence of tilted bedding. In the field multiple small scale folds are present as can be seen in fig. 23. The measured fold axis in Fig 23 is: 257/18. It is a secondary fold in marls and clay, with a SE vergence. The measured fold axis gives a shortening direction of: NNW-SSE, represented by subset 3. This direction is not supported in the Pergamo location by other data. The big anticline that is part of these secondary folds was not found in the field.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 29 Figure 21: Geological map and profile of the Garda area.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 30 Figure 22: A + B belong to outcrop G10 part of the Ledro 1 location. A represent syntectonic brecciated carbonates. B represents syntectonic megabreccie (large scale brecciated rocks). Figure 23: parasitic folds in marls (scaglia rossa formation) with a SE vergence. The black bars represent the axial planes.
This picture represents outcrop G05 in Pergamo. Outcrops G15, G16, G17, J07 and GN02 contain pressure solution in the form of stylolites. These outcrops are late Triassic limestones. The orientation of the stylolite peaks, which are associated with the 1 direction, have been measured. Fig. 24A shows an outcrop with a 006/37 stylolite peak orientation. This direction would lie in the range of the NNW/SSE direction, represented by subset 4. Other measurements and data processing do not support this shortening direction in the Garda 1 location. The measured stylolite peaks in Fig 24B are oriented: 303/63.
This direction would lie in the range of the NW/SE shortening direction; subset 2. This shortening direction is supported by the fold axis calculation and the tensor calculation in the Garda 1 location.
Figure 24: stylolites; the black bar indicates the stylolite peaks; the 1 direction. A represents outcrop G15. B represents outcrop J07, both outcrops belong to the Garda 1 location.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 31 Campi In the Campi location 3 bedding planes were measured, which gave a fold axis calculation of 035/01, this fits in the shortening direction of subset 2. Pergamo In the Pergamo location 9 bedding planes were measured, the fold axis calculation gave an orientation of 132/09; this fits in the shortening direction of subset 4.
The tensor analyse gave 2 shortening directions. 6 fault planes with slickensides gave a sigma 1 of 204/02, which was put in the shortening direction of subset 4. 9 fault planes with slickensides gave a sigma 1 of 282/26, which was put in the shortening direction of subset 1. In Pergamo subset 4 was found twice and subset 1 was found once. Ledro 1 In the Ledro 1 location 24 bedding planes were measured which gave a fold axis of 012/34; this fits in the shortening direction of subset 1. Ledro 2 In the Ledro 2 location 9 bedding planes were measured which gave a fold axis calculation of 188/00, this fits in the shortening direction of subset 1.
Ledro 1 + 2 The tensor analyse gave 4 shortening directions. 12 kinematic indicators gave a sigma 1 of 119/18, which was put in the shortening direction of subset 2. 7 fault planes with slickensides gave a sigma 1 of 338/06, which was put in the shortening direction of subset 3. 5 other fault planes with slickensides gave a sigma 1 of 344/19, which was put in the shortening direction of subset 3. 5 kinematic indicators gave a sigma 1 of 201/24, which was put in the shortening direction of subset 4. In the Ledro 1 +2 location, subset 1 was found twice, subset 2 was found once, subset 3 was found twice and subset 4 was found once.
Garda 1 In the Garda 1 location 20 bedding planes were measured, the fold axis calculation gave an orientation of 038/05, this fits in the shortening direction of subset 2. The tensor analyse gave 1 shortening direction. 20 fault planes with slickensides and 4 stylolite peaks, gave a sigma 1 of 311/19, which was put in the shortening direction of subset 2. In the Garda 1 location, subset 2 was found twice. Garda 2 In the Garda 2 location, the tensor analyse gave 4 shortening directions. 7 fault planes with slickensides gave a sigma 1 of 300/30, which was put in the shortening direction of subset 2.
6 other fault planes with slickensides gave a sigma 1 of 301/20, which was put in the shortening direction of subset 2. 17 kinematic indicators gave a sigma 1 of 332/29, which was put in the shortening direction of subset 3. 11 other kinematic indicators gave a sigma 1 of 219/07, which was put in the shortening direction of subset 4. In the Garda 2 location, subset 2 was found twice, subset 3 was found once and subset 4 was found once. Fold axes calculation As described above, the calculated fold axes were addressed to a (shortening direction) subset. Table 1 gives the locations, the number of bedding planes per location, the calculated fold axis (or axes) for each location and the related subset.
The fold axes calculation produced results plots (see fig. 13). These plots have been put in the geological map (see fig. 25) each colour represents a different subset. As you can see in fig. 25; subset 1 is only represented near Lake Ledro. Subset 2 is represented near the Ballino-Garda line. Subset 3 is not represented by folds in the Garda area. Subset 4 is represented near the Ballino-Garda line. The Ballino-Garda line is represented by subset 2 and 4. Table 1 shows that the main deformation phases seem to be subset 2 and 1. All measurements in subset 2 have flat fold axes; the shortening direction is horizontal.
The other subsets show a difference in dip.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 32 Table 1: results of the fold axes calculation of the Garda area. Paleo stress analyses As described above, the calculated stress tensors were addressed to a (shortening direction) subset. Table 2 gives the locations for the Garda area, the calculated stress tensor(s) for each location, the related subset and the data quality. The stress tensor calculation plots have been put in the geological map; fig. 25, each colour represents a different subset.
The paleostress analyses show that all subsets represent strike-slip and compression as stress regime.
Subset 1 is represented near the Ballino- Garda line and subset 2, 3 and 4 are represented near the Ballino-Garda line and near Lake Ledro. Table 2 shows that the main deformation phase seems to be subset 2. Subset 1 has only been found once. Subset 2 and 4 are mainly found in the Ballino-Garda line area. Subset 3 is mainly found in the Lake Ledro area. Although the geological map shows structures that would fit in a W-E shortening direction, the fold axes and paleo stress calculations do not support this assumption. Subset 1 is the least abundant in the Garda area. Subset 2 and 4 are the most abundant subsets.
Garda Location Planes Calculated fold axis Shortening direction Subset Pergamo 9 132/09 NNE-SSW 4 Ledro 1 24 012/34 W-E 1 Ledro 2 9 188/00 W-E 1 Campi 3 035/01 NW-SE 2 Garda 1 20 038/05 NW-SE 2 Location Sigma 1 Stress regime Shortening direction nt n n/nt R R' F5 value Pergamo Subset 1 282/26 Pure Strike-Slip W-E 24 9 0,38 0,55 1,45 6,53 Subset 4 204/02 Oblique Compressive NNE-SSW 24 6 0,25 0,14 1,86 4,88 Garda 1 Subset 2 311/19 Compressional Strike-Slip NW-SE 42 24 0,57 0,12 1,88 8,5 Garda 2 Subset 2 300/30 Strike-Slip Compressional NW-SE 7 7 1 0,06 2,06 6,96 Subset 2 301/20 Pure Compressional NW-SE 11 6 0,55 0,33 2,33 5,07 Subset 3 332/29 Radial Compressional NNW-SSE 39 17 0,44 0,8 2,8 6,56 Subset 4 219/07 Pure Compressional NNE-SSW 39 11 0,28 0,57 2,57 14,11 Ledro 1+2 Subset 2 119/18 Compressional Strike-Slip NW-SE 25 12 0,48 0,21 1,79 10,48 Subset 3 338/06 Pure Compressional NNW-SSE 25 7 0,28 0,75 2,75 8,27 Subset 3 344/19 Pure Compressional NNW-SSE 7 5 0,71 0,34 2,34 3,54 Subset 4 201/24 Oblique Compressive NNE-SSW 25 5 0,2 0,84 1,16 6,32 Table 2: Results of the paleo stress tensor calculations for the Garda area.
nt = total number of slip data used. n = number of slip data used for the specific subset. n/nt = the ratio of slip data used. R = ( 2- 3/ 1- 3). Shape of the stress ellipsoid. R’ = the stress regime, F5 = minimization function used for the rotation optimisation. See the methods section for further explanation.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 33 Figure 25: The geological map of the Garda area, showing all subsets found in the field, including both the fold axis calculation and the paleo stress analyses.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 34 Pinzolo Area The geological map and profile of the Pinzolo area (fig. 26) show the main tectonic features: normal faults, thrusts, sinistral strike slip faults and folds. The profile shows a little more detail than the geological map.
The profile shows a syncline with a W-E shortening direction. This direction corresponds with the shortening direction of the thrust more to the west. This shortening direction also corresponds with the orientation of the Giudicarie south fault and the Ballino-Garda line. The geological map of the Pinzolo area reveals another deformation phase must have taken place: two thrusts and two folds have an orientation that fits in a NNE- SSW shortening direction. The strike-slip movements fit in the latter shortening direction, but it might very well be another shortening phase. Compared to the Garda area, the Pinzolo area consists of more folds and lesser strike-slip faults.
In the Garda area the Ballino- Garda line represents an obvious thrust, in the Pinzolo area this is unclear. The Pinzolo area seems more complicated than the Garda area. Please note that the area is very complicated and bedding planes are hard to measure, therefore the profiles were hard to deduce and gathered information was projected inclined on the profile line. Because of the peritidal character certain places miss statigraphic units and intrusions influence the stress field. The geological map (fig. 26) shows that most deformation in this research area took place in the brittle domain, expressed in the formation of: fault-, joint-, and fracture systems with kinematic indicators.
Also ductile deformation is found in the form of large scale and small scale folds and pressure solution is found in the expression of stylolites.
As mentioned before the outcrops were hard to access, but all outcrops visited, showed faults, joints or fractures. Outcrops P19, P27, P35, P42, P46 and P49 need special attention; they provided very useful information. P19 is part of de Adamello Batholith. The outcrop showed very clear slickensides in this relatively young tonalite. P42 and P46 are the only outcrops in the Pliocene deposits; they also provided fault planes with slickensides. These outcrops help in the relative timing of the shortening directions. Fig 27A shows a huge fault plane with red smoothed slickensides. This important fault plane has an orientation of: 072/87, unfortunately the direction of movement is not noticeable because of the smoothed slickensides.
Excellent slickensides can be found in fig. 27B; this normal fault (145/46) has been classified with a certain (C) for direction of movement. Therefore this fault is more important than others. The normal fault fits in a NNE-SSW shortening direction (subset 4), tensor calculations support this direction in the Tione location. The rocks in fig. 27C are of Permian age and consist of granites and tonalities. These rocks have also undergone brittle deformation. This mafic enclave gives a WNW-ESE displacement on a 256/75 plane. Folds are more frequent in the Pinzolo area than in the Garda area.
Outcrops P03, P05, P15 and P36 showed very clear folds. Fig. 28 shows a fold with bedding orientations: 310/25 and 135/38, this provided a fold axis of 200/08. This fold axis fits in the shortening direction NW-SE, represented by subset 2, tensor calculations support this direction in the Giudicarie location. P15 and all outcrops in the Mughi location provided tilted bedding and folds. See fig. 30 and table 3 for the results of the calculations. Pressure solution is not abundant in the Pinzolo area, but the few found provided much information. Outcrops P37 and P47, showed very clear stylolite peaks.
The stylolite plane in fig. 29 is parallel to the measured bedding; 116/15. The orientation of the peaks were too difficult to measure, therefore the pole of the plane was calculated to derive the stylolite peak orientation: 296/75. The orientation of the stylolite peaks were used in the tensor calculations. The outcome of these calculations will be further discussed in this chapter.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 35 Figure 26: Geological map and profile of the Pinzolo area.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 36 Figure 27: Examples of important kinematic indicators found in the field. A represents a big fault at outcrop P35 in Stenico. B represents slickensides at outcrop P49 in Tione. C represents a Permian granite at outcrop P27 in Bleggio. Figure 28: Fold found in the Adamello Batholith. The black bar represents the axial plane.
This picture represents outcrop P03 in the Giudicarie location. Figure 29: Stylolites; the black bar indicates the orientation of the stylolite peaks; the 1 direction. This picture represents P47 in Cacciatore. Casinei In the Casinei location 16 bedding planes were measured, the fold axis calculation gave an orientation of 044/04, this fits in the shortening direction of subset 2. The tensor analyse gave 1 shortening direction. 5 fault planes with slickensides gave a sigma 1 of 025/02, which was put in the shortening direction of subset 4.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 37 Mughi In the Mughi location 2 fold axes were calculated. 6 bedding planes were measured which gave an orientation of 230/08, this fits in the shortening direction of subset 2. 10 other bedding planes gave a fold axis orientation of 074/19, this fits in the shortening direction of subset 3. Bleggio In the Bleggio location, 1 shortening direction was found with the tensor analyse. 9 fault planes with slickensides gave a sigma 1 of 113/04, which was put in the shortening direction of subset 2.
Tione In the Tione location, also 1 shortening direction was found with the tensor analyse.
7 fault planes with slickensides gave a sigma 1 of 199/18, which was put in the shortening direction of subset 4. Giudicarie In the Giudicarie location, 2 shortening directions were found with the tensor analyse. 11 fault planes with slickensides gave a sigma 1 of 139/17, which was put in the shortening direction of subset 2. 33 kinematic indicators gave a sigma 1 of 014/06, which was put in the shortening direction of subset 4. Cacciatore In the Cacciatore location 2 fold axes were calculated. 19 bedding planes were measured which gave an orientation of 082/09, this fits in the shortening direction of subset 3.
8 other bedding planes gave a fold axis orientation of 345/00, this fits in the shortening direction of subset 1. The tensor analyse gave 3 shortening directions. 8 fault planes with slickensides gave a sigma 1 of 099/34, which was put in the shortening direction of subset 1. 8 other fault planes with slickensides gave a sigma 1 of 157/23, which was put in the shortening direction of subset 3. 10 fault planes with slickensides and 2 stylolite peaks, gave a sigma 1 of 207/29, which was put in the shortening direction of subset 4. In the Cacciatore location subset 1 was found twice, subset 3 was also found twice and subset 4 was found once.
Stenico In the Stenico location 11 bedding planes were measured; the fold axis calculation gave an orientation of 121/36, this fits in the shortening direction of subset 4. The tensor analyse gave 3 shortening directions. 20 fault planes with slickensides gave a sigma 1 of 073/18, which was put in the shortening direction of subset 1. 10 other fault planes with slickensides gave a sigma 1 of 145/29, which was put in the shortening direction of subset 3. 14 kinematic indicators gave a sigma 1 of 199/14, which was put in the shortening direction of subset 4. In the Cacciatore location subset 1 was found once, subset 3 was also found once and subset 4 was found twice.
Fold axes calculation As described above, the calculated fold axes were addressed to a (shortening direction) subset. Table 3 gives the locations, the number of bedding planes per location, the calculated fold axis (or axes) for each location and the related subset. The fold axes calculation produced results plots (see fig. 13). These plots have been put in the geological map (see fig. 30) each colour represents a different subset. Subset 1 is represented near the Ballino-Garda line and subset 2 and 3 are represented in the centre and near the Ballino-Garda line. Subset 4 is represented in the south of the Pinzolo area near a big sinistral strike-slip fault.
The Ballino- Garda line is represented by subset 1, 2 and 3. Table 3 shows that the main deformation phases seem to be subset 2 and 3. All measurements in subset 2 have flat fold axes; the shortening direction is horizontal. The other subsets show difference in dips.
Paleo stress analyses As described above, the calculated stress tensors were addressed to a (shortening direction) subset. Table 4 gives the locations for the Pinzolo area, the calculated stress tensor(s) for each location, the related subset and the data quality. The stress tensor calculation plots
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 38 have been put in the geological map; fig. 30, each colour represents a different subset. Table 3: results of the fold axes calculation of the Pinzolo area. The paleo stress analyses show that all subsets represent strike-slip and compression as stress regime, except for Cacciatore subset 4 (R’ is 0,38).
Although the R’ = 0-1 should represent extension, the program classifies it as oblique compression. Subset 1 is represented near the Ballino-Garda line and in the south of the Pinzolo area at a big sinistral strike-slip fault. Subset 2 is represented in the centre and near the Giudicarie fault. Subset 3 is represented near the Ballino-Garda line and in the south of the Pinzolo area near a big sinistral strike-slip fault.
Pinzolo Location Planes Calculated fold axis Shortening direction Subset Cacciatore 19 082/09 NNW-SSE 3 Cacciatore 8 345/00 W-E 1 Mughi 6 230/08 NW-SE 2 Mughi 10 074/19 NNW-SSE 3 Cassinei 16 044/04 NW-SE 2 Stenico 11 121/36 NNE-SSW 4 Location Sigma 1 Stress regime Shortening direction nt n n/nt R R' F5 value Casinei Subset 4 025/02 Pure Strike-slip NNE-SSW 6 5 0,83 0,5 1,5 13,72 Bleggio Subset 2 113/04 Compressional Strike-slip NW-SE 18 9 0,5 0,15 1,85 10,87 Giudicarie Subset 2 139/17 Extensional Strike-slip NW-SE 69 11 0,16 0,78 1,22 15,71 Subset 4 014/06 Extensional Strike-slip NNE-SSW 69 33 0,48 0,76 1,24 12,98 Cacciatore Subset 1 099/34 Strike-slip Compressional W-E 38 8 0,21 0.21 2,21 13.86 Subset 3 157/23 Compressional Strike-slip NNW-SSE 38 8 0,21 0,28 1,72 6.39 Subset 4 207/29 Oblique Compressive NNE-SSW 36 12 0,33 0,16 0,38 7,19 Stenico Subset 1 073/18 Pure Strike-slip W-E 80 20 0,25 0,5 1,4 10,51 Subset 3 145/29 Pure Strike-slip NNW-SSE 80 10 0,12 0,47 1,5 8,17 Subset 4 199/14 Pure Strike-slip NNE-SSW 80 14 0,18 0,49 1,5 11,17 Tione Subset 4 199/18 Compressional Strike-slip NNE-SSW 14 7 0,5 0,1 1,85 9,14 Table 4: Results of the stress tensor calculations for the Pinzolo area.
nt = total number of slip data used. n = number of slip data used for the specific subset. n/nt = the ratio of slip data used. R = ( 2- 3/ 1- 3). Shape of the stress ellipsoid. R’ = the stress regime, F5 = minimization function used for the rotation optimisation. See the methods section for further explanation.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 39 Subset 4 is represented in the whole area. The Ballino-Garda line is mostly represented by subset 1, and less abundantly also by subset 3 and 4. Table 4 shows that the main deformation phase seems to be subset 4 The geological map shows structures that would fit in a W-E and a NNE-SSW shortening direction, the fold axes and paleo stress calculations do support this assumption. Subset 4 is the most abundant subset. Subset 1 is however the least abundant in the Pinzolo area. This does not mean that the deformation phase is not important.
The Garda & Pinzolo Areas Table 5 summarizes the amount of shortening directions in the field per subset per area. The subsets found by fold axis calculations have been added to the subsets found by the paleo stress analyses. The two research areas have been compared to each other: In the Garda area, subset 1 is found in the whole area, but mainly in the Ledro 1+2 location. Subset 2 is found in the whole area, but mainly around the Ballino-Garda line. Subset 3 is found in the whole area, but mainly in the Ledro 1+2 location. Subset 4 is found in the whole area, but mainly around the Ballino- Garda line.
In the Garda area, the Ballino- Garda line is mainly represented by subset 2 and 4.
In the Pinzolo area, subset 1 is found near the Ballino-Garda line. Subset 2 is found in the whole area. Subset 3 is found mainly in the area of the Ballino-Garda line. Subset 4 is found in the area of Ballino-Garda line and around the area of the Giudicarie South fault. In the Pinzolo area, the Ballino Garda line is represented by all subsets equally. The Giudicarie fault is representen by subset 2 and 4 In both areas the main faults and folds suggest a W-E shortening direction. This direction has been found by fold axis calculations and paleo stress analyses. All 4 shortening directions found by data analyses are represented in both study areas.
Table 5 shows that both areas provide the same amount of shortening directions (indicated by the number of subsets). Subset 4 is most abundant in the Pinzolo area and subset 2 is the most abundant in the Garda area. Subset 2 and 4 are most abundant in both research areas. In the Garda area subset 2 and 4 are found near the Ballino-Garda line. In the pinzolo area subset 2 and 4 are found in the whole area.
subsets Pinzolo Garda total Amount of Folds 1 1 2 3 2 2 2 4 3 2 - 2 4 1 1 2 Amount of Tensors 1 2 1 3 2 2 4 6 3 2 3 5 4 5 3 8 Folds + Tensors 1 3 3 6 2 4 6 10 3 4 3 7 4 6 4 10 Total 17 16 33 Table 5: Amount of tensors and fold axes per subset per area.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 40 Figure 30: The geological map of the Pinzolo area, showing all subsets found in the field, including both the fold axis calculation and the paleo stress analyses.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 41 Discussion Castellarin & Cantelli (2000) and Castellarin et al (2006) predicted 5 deformation phases (see fig.
6 and 31) during and after alpine collision in these research areas. In this research only 4 deformation phases/subsets have been found. Figure 31 compares these subsets to the phases found by Castallarin et al (2006), they are found to agree relatively well. On the other hand Picotti et al (1995) suggested a NNE- SSW shortening phase followed by a NNW- SSE shortening phase and then a WNW-ESE shortening phase, from the cretaceous onward. This corresponds with later predictions by castallarin et al (2006), with the exception of the WNW-ESE shortening phase; deformation phase 3 corresponds with NNE-SSW and deformation phase 2 corresponds with NNW- SSE.
According to Castallarin et al (2006) subset 2 is the youngest phase (Adriatic phase). The second youngest and the oldest phase could be subset 3 (Valsugana or pre-Adamello phase), followed by subset 4 (Insubric-Helvetic phase). This study disagrees with this timing of Castellarin & Cantelli (2000) and Castellarin et al (2006): subset 1 is not represented by these theories, and deformation phase 4 is not supported by our field data. This timing issue will be further discussed in this chapter. In the Garda area as well as in the Pinzolo area the latest formed faults fit in a W-E shortening direction.
Picotti et al (1995) stated that the latest deformation phase was a WNW- ESE shortening direction. This study disregarded WNW-ESE and WSW-ENE directions and as you can see in fig. 19, 25 and 30 these directions were all grouped in a W-E shortening direction. Subset 1; the W-E shortening direction, might very well be the youngest deformation phase.
Figure 31: The deformation phases predicted by Castallarin et al, 2006 compared to the subsets found in the field. Garda Area In the Garda area, the Ledro 1+2 location shows that the shortening direction of subset 1 could have been responsible for the latest formed faults in that area: the prominent dextral strike- slip faults. This would suggest that subset 1 is the latest deformation phase. Ledro 2 and Pergamo contain outcrops of Pliocene age. Here subset 1 and 4 are present. The youngest rocks can only have been influenced by the
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 42 deformation phases younger than the rocks itself, therefore subset 4 must have been younger than Castallarin et al (2006) state.
Pinzolo Area As discussed in the previous chapter, the geological map (fig. 26) of the Pinzolo area reveals 2 recent shortening directions: W-E and NNE-SSW. A closer look at the geological map shows that the strike-slip faults and the WNW-ESE orientated fold axes are cut off by the N-S striking faults. This implies that the W- E (subset 1) shortening direction is younger than the NNE-SSW (subset 4) shortening direction.
In the Stenico location, subset 4 shows a stress regime compatible with the prominent sinistral strike-slip fault in that location. This fault is still present and should have been formed by one of the latest deformation phases. This would either mean that subset 4 found in the field is the youngest phase and that the youngest phases stated by Castallarin et al (2006) or subset 2 and 3, are not present in that location or did not succeed in overwriting the previous formed structures. The outcrops in the Adamello Batholith: the Giudicarie location, consists of young rocks (Eocene Oligocene age).
When Castallarin et al (2006) is right the shortening phase in this location should be subset 2 or 3 (Miocene- Pliocene age), the results show that subset 2 and 4 are found in the Giudicarie location. As described above subset 4 could be younger than theory states. The Tione location, where the youngest rocks (Pliocene age) are situated, can only have been influenced by the youngest deformation phase: subset 2. However this is not the case, the only subset found in the location is subset 4. This is the fourth argument for believing that subset 4 is younger than subset 2. Therefore this study would like to present a new theory visualized by fig.
32. Figure 32: New proposed deformation phases for the Ballino-Garda line (Southern Alps), modified after Castallarin et al (2006).
Reactivation along the Ballino-Garda line This study shows that the Ballino-Garda line experienced multiple deformation phases during the Cenozoic. The geological map of the Garda area (fig. 21) shows that the Ballino-Garda line now represents a thrust instead of a normal fault, therefore reactivation must have taken place. The fold axes calculations and the paleo stress analysis shows that subset 2 (Adriatic phase) and subset 3 (Valsugana phase) influenced the Ballino-Garda line. Castellarin et al (1993) and Picotti et al (1995) also indicate reactivation of the Ballino-Garda line during the Valsugana and Adriatic phases.
Furthermore, it is expected that newly formed faults are refracted along pre- existing weakness zones, as well as in the Garda area and in the Pinzolo area faults and folds are cut off by the N-S striking Ballino-Garda line and the Giudicarie fault. This suggests that
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 43 the Ballino-Garda line and the Giudicarie fault are pre-existing weak zones. Reactivation of pre-existing structures is easier when the zone is weak. The combined presence of fault systems with different strikes might have helped considerably with reactivation of the Ballino-Garda line, because total deformation can be easily partitioned into different movement vectors on differently striking faults. As is the case for the Val-Trompia/Giudicarie belt.
The relative timing indicators found in the field are not abundant enough to support or disregard these theories.
Therefore an analogue modelling study has been carried out to further constrain the influence of the parameters supposed to control reactivation in this research area. This study is important for understanding the evolution of the Southern Alps with regard to the Ballino-Garda line. To be more secure of the deformation history of the Ballino-Garda line, further research is needed. Conclusion After this research, it can be concluded that the research areas have been affected by at least 4 deformation phases. These deformation phases are based on fold axis calculations done by StereoWinFull and paleo stress analyses done by WinTENSOR, using bedding planes and kinematic indicators.
The shortening directions are: W-E = subset 1 NW-SE = subset 2 NNW-SSE = subset 3 NNE-SSW = subset 4 The youngest deformation phase is subset 1 (W-E), followed by subset 4 (NNE-SSW), then subset 2 (NW-SE) and the oldest deformation phase is subset 3 (NNW-SSE).
Subset 2 and 4 are the most abundant; they left the most traces in the rocks. The Ballino-Garda line has been influenced by all stress directions found in the field. To test the most probable reactivation direction, analogue modelling will be carried out. The hypothesis that: “the cretaceous normal fault; Ballino-Garda line, has been reactivated by one of the stress directions found in the field” is further tested in the next chapter: “Analogue modelling”.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 44
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 45 Analogue modelling Introduction A pre-existing fault remains mechanically weaker than the surroundings because the cohesive strength and the friction coefficient of these surfaces are lower than those of intact rocks (Anderson, 1951).
The role and feasibility of pre-existing structures in analogue models is still not fully established; dynamic and kinematic processes associated with structural reactivation are still in debate. Analogue models made of sand simulate the frictional/brittle behaviour of the sedimentary cover. In order to obtain reactivation, experiments have been carried out containing; ductile layers (Richard and Krantz, 1991, Richard et al, 1995), rubber sheets (Naylor et al, 1986) and basal plates (Schreurs and Coletta, 1998; Di Bucci et al, 2007). Following experiments simulate reactivation of a Neogene inverted remnant of Norian/Liassic extensional tectonics and continental rifting.
The purpose of these experiments is to understand the geometry and timing of the Ballino-Garda line, part of the Giudicarie fault system. This system has been subjected to several deformation phases since the Paleozoic. The structural system and reactivated older structures depend on the orientation of the current stress field. The Giudicarie fault has been modelled before by Viola et al (2004), by modelling the Giudicarie South fault as a reverse fault in a transpressive regime. They used a basal plate to obtain strike-slip and a metallic wire as a pre-existing fault. They illustrated a metallic wire can be used to govern reactivation.
Sassi et al (1993) used the same method of creating a pre-existing fault zone by pulling a wire through a sand model, which were reactivated as thrusts during compression. X-ray investigation done on their models proves that indeed the disturbed zones of less compacted grains left by the thread has a similar x-ray signature to a real fault zone “naturally” produced in the models. Other ways of making a pre-existing structure are the creation of a weak zone or induce a rheology difference. The area of interest has a distinct rheology contrast. To test our hypothesis as discussed in the previous chapter, we used different types of sand, rigid ramps and wires in order to obtain the pre-existing normal fault that divides the Trento Plateau from the Lombardian Basin.
To reactivate this structure; a machine with a pushing wall was used. Analogue models were employed for a pre-existing structure (1) and a rheology difference (2), with and without a detachment layer. The angles of strike-slip were changed according to the field observations and results of the fold axes calculation (StereoWinFull) and paleo stress analyses (WinTENSOR). An overview of the experiments and boundary conditions is presented in Table 6. These experiments are part of a research project on the evolution of the Alps.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 46 models L x W x H (cm) Materials Pre-existing structure (PS) (° ) (° ) Backstop Duration of the experiment Shortening (%) PS 01 PS 02 45 x 45 x 4.5 Q Rope disturbance 25 90 45 cm (full width backside) 2.00 hours 21 PS 03 45 x 45 x 4.5 Q FS Iron wire disturbance 25 90 45 cm (full width backside) +10 cm ‘frontstop’ 2.35 hours 29 RD 01 RD 02 45 x 45 x 4.5 Q FS Material boundary 25 90 45 cm (full width backside) 2.05 hours 2.00 hours 19 12 RD 03 RD 04 60 x 60 x 4.5 Q FS Material boundary 27 90 partial (45 cm) at FS side 3.45 hours 3.30 hours 35 29 RD 05 60 x 60 x 3 Q basal GB below FS Material boundary 27 90 partial (45 cm) at FS side 3.30 hours 29 RD 06 60 x 60 x 3 Q FS GB 27 27 partial (45 cm) at FS side 2.00 hours 17 RD 07 60 x 60 x 3 Q FS Material boundary 27 27 partial (45 cm) at FS side 2.00 hours 17 RD 08 60 x 60 x 3 Q and FS GB 27 90 partial (45 cm) at FS side 2.00 hours 17 WB 01-1 WB 01-2 WB 01-3 60 x 60 x 3 Q IR Material boundary 30 60 partial (45 cm) at WR side 1.15 hours 1.15 hours 1.15 hours 10 10 10 WB 02 60 x 60 x 3 Q IR Material boundary 45 60 partial (45 cm) at WR side 1.15 hours 10 WB 03 60 x 60 x 3 Q IR Material boundary 75 60 partial (45 cm) at WR side 1.15 hours 10 WB 04 60 x 60 x 3 Q IR GB 30 60 partial (45 cm) at WR side 1.15 hours 10 Table 6.
Overview of all model runs. Abbreviations: L= length, W= width, H= height, Q= quartz sand, FS= Feldspar sand, GB= glass beads, RI= Rigid Indenter, = angle between long side of model and fault plane, = angle of fault plane with horizontal.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 47 Methods Model materials For the experiments in this research quartz sand (Q), feldspar sand (FS), glass beads (GB), and wooden ramps (RI) were used, of which the mechanical properties are summarised in table 7. The sands represent upper-crustal sedimentary cover rocks because of their brittle behaviour, according to Mohr Coulomb and Byerlee laws (see Appendix 1). The dolomite rocks of the Trento Plateau ( =45° ) are represented by Q sand and the weaker Lombardian Basin limestones ( =27) by FS sand ( measurements according to Handin, 1969).
The GB are the strongest material (table 7), however because only thin layers (mm scale) are used these are the weakest layers. The GB have a very high sphericity and are almost perfectly round (van Mechelen, 2004), these properties make the GB very suitable to use as detachment layers. Therefore, in the experiments the GB are used to model weak pre-existing fault zones (detachments) and as part of the weak cover rock of the Lombardian basin (RD05). Wooden ramps are used as analogues for rigid indenters, representing the relatively strong rocks of the Trento plateau. Strength profiles of the models have been constructed (see fig.
33). The strength of the model materials is calculated with the following equations: The vertical normal stress is given by: = g h eq. 1 This equation can be rewritten to give the deviatoric stress: 1- 3. In a compressive regime, or during shortening, = 1 gives: 1- 3 = 2 g h eq. 2 (Brun, 1999) Brittle material Angle of internal friction ( ) Cohesion (C) Pa Density ( ) kg/m3 Thickness (h) m Strength 1 - 3 (Pa) Quartz sand 42° 48 1560 0,045 1376 Feldspar sand 37° 43 1280 0,045 1129 Glass beads 31° 0 1800 0,002 70 Table 7: brittle material properties, after van Mechelen, 2004. Calculations with the model materials and dimensions in order to obtain their strength according to equation 2.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 48 Figure 33: Strength profiles or failure curves for all 3 brittle materials. Values derived from Table 7 and calculated according to equation 2. The strength of the brittle materials is a linear function therefore, for each thickness the strength can be obtained. 3 cm feldspar: 753 Pa, 3 cm Quartz: 917 Pa. 4mm glass beads: 140 PA. Scaling Scaling relations between the natural prototype and the model are obtained by keeping the average strength of the brittle layers correctly scaled with respect to the gravity forces and the densities.
It is most important for modelling purposes that the dynamics (stresses, rheologies, densities) are comparable to the natural situation. This is governed by the equation of dynamics: ij/ ij + (g – ( 2 ij/ t2)) = 0 eq. 3 where ij are the components of stress, ij are the components of deformation (strain), ij the space coordinates, density, g gravity acceleration and t time (Brun, 1999). The following ratios also need to be taken into account: * g* L* eq. 4 * = g* (t*) eq. 5 Where L is the length of model or prototype area. The exponent * refers to the ratios between natural prototype and model.
As gravity acceleration does not change, inertial processes can be neglected (Hubert, 1937), therefore only equation 4 is of relevance. The ratio for the gravity in natural versus the experimental environment is 1. The densities of the model materials varies between 1300--1800 kg/m3. Natural densities for crustal rocks are in a range of 2300--3000 kg/m3. This is in the same order of magnitude; the ratio can be considered to be almost 1. Now, equation 4 can be simplified to: * ~ L* eq. 6 The ratio of stresses becomes nearly equal to the ratio’s of lengths (Brun, 1999). In order to put the experiments in a proper natural environment, we need a length scale ratio.
Brittle models simulate natural dynamic behaviour at any scale, therefore the length scale can simply be chosen at a practical value. Here, a 10-6 ratio is used (1 cm in the model represents 1 km in nature). Only the GB layers and pre-existing faults in the models are not to scale and too thick in comparison to their natural analogues. The cohesion (C) of crustal rocks is 50 MPa, knowing that the length scale ratio is 10-6 , the cohesion of the materials used in the models is negligible.
Model Construction Fig 34 shows the model set-ups and fig. 35 shows the deformation machine with a built up experiment. The size of the study area is 30 km by 40 km. To avoid any boundary effects models PS01-03 and RD01-02 were scaled to 45 km (45 x 45 cm). In experiments RD03-08 and RI01-04 the area is increased to 60 km (60 x 60 cm), to minimize boundary effects. Sediment pile thickness in the study area is approximately 4 to
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 49 5 km. The models PS01-03 and RD01-04 were scaled at a depth of 4,5 km (4,5 cm) and 3 km (3 cm) for models RD05-08, RI01-04.
Sands are sieved into the models with 300 micron sieves. Coloured layers are introduced as markers to observe deformational structures. A thin coloured layer is added to the top of the model to represent the original topography. After model deformation, a layer of white (post- kinematic) sand is added to protect the new topography. The model is wetted to increase cohesion and cut in serial cross sections perpendicular to the shortening direction and perpendicular to the created pre-existing structure to visualize the deformation/strike-slip component. In PS01-03 (Figure 34 A) vertical pre-existing faults are created by making a disturbed zone.
To create this zone, a thread was built into the model and then pulled upwards through the sand. In PS 01-02 a 5 mm thick sisal rope and in PS03 a 2 mm thick iron wire was used. The GB layer at the contact of the two sands in RD06, RD08 and RI04 (fig. 34 C, D) is 2 mm thick, the GB layer at the base in RD05 (fig. 34 C) is 4 mm thick, this represents 400 meters in nature. The 200 m thick fault zones modelled by the GB, iron wire and sisal, are out of proportion on the basin scale model. During the experiments it was practically not possible to make the GB layer thinner than 2 mm, although out of scale, this fault zone provided interesting results.
The angle of the fault dip ( ) was varied to represent the geometry (dip) of different types of pre-existing faults (normal or strike slip). In nature normal faults have angles varying from 40° to 80° . The angle of 27° in RD06 and RD08 is too low to represent normal faults, however it is a representation of a thrust fault. In the RI experiments the ; stress field angle, is changed according to the paleo stress regimes found in the field research.
Figure 34: Schematic sketches of the model set-up.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 50 Figure 35: The deformation apparatus with an experiment. Deformation Shortening was obtained by a moving wall (back stop), driven by a motor at a constant speed of 5 cm/h. This velocity needs no scaling since brittle deformation is time independent. At any velocity, brittle materials will generate similar structures. The model was pushed over a plastic sheet attached at the base to minimize basal friction.
Every 5 minutes top view images were produced, in RI01 and RI04 top views are made every minute in order to observe the thrust sequence and detect lateral movement. The amount of shortening (see Table 6) was not the same for all the models. The partial front stop of 10 cm width in PS03 is used to localize deformation to one side of the model and on the disturbed zone. The partial back stop of RD03-08 covers 45 cm of the model on the FS side in order to deform only the weak FS sand.
Limitations In nature erosion usually has a large influence on the location and amount of deformation. Through removal of overburden by erosion a fault can stay active for a longer period and deformation is not transferred to elsewhere. In the models this factor was not taken into account. The models focussed on intra-crustal rift basins/large scale crustal basins with sedimentary infill. The rigid indenter used in the RI experiments could not deform internally, like might be the case for stronger strata in nature. The position of 1 is confined to one side of the model, the other boundary is free to move.
In nature pressure is always present at both sides. On a mechanical point of view, the study of reactivation is very interesting. However such cylindrical boundary conditions are of course unlikely to occur in nature.
Results Pre-existing structure models As a first basic test, three experiments were performed to investigate the possibility of reactivation of pre-existing structures in simple mono-material quartz models; models PS01-03. In these models the pre-existing structure was formed by pulling different types of thread from the base upwards through the sand (after model construction). This produced a discontinuity, or pre-existing structure, in the sand layers. Model PS01 failed due to practical reasons. Models PS02 and PS03 produced thrusts at approximately 10 and 15 cm in front of the backstop.
The location and/or movement of these thrusts are not affected by the presence of the pre-existing structure.
Overall we can conclude that models of dry quartz sand with a roughly vertical pre-existing structure (disturbed zone) do not use this feature as a focus of fault movement or new fault creation. Rheology difference models I Subsequently, a series of models were carried out to provide insight into the influence of rheology variations on the style and location of deformation. Five models were constructed that consisted of a quartz and a feldspar sand domain (RD01-05), of which one of these models (RD05) had an additional basal glass bead layer below the feldspar sand to increase the rheology difference between the two domains.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 51 RD01 This model is the first experiment with the quartz-feldspar rheology, where the material transition boundary is roughly vertical. Information is only from the top-view photos because no cuts were made of this model. Initially (at 2% shortening) a first thrust is formed over the full width of the model, unaffected by the material boundary. Subsequent thrusts develop only in the feldspar domain; these curve/refract in the quartz domain and stop against the earlier formed thrust (see Figure 36 A and B).
In this way, at the end of the experiment at 19% shortening, two more thrusts are formed in the feldspar domain and only one in the quartz area.
RD02 This model setup is similar to RD01, a re-run was made to test the stability of the formed structures and to make cuts. Although this model was shortened twice as much, the end top view picture shows that only one thrust was formed. As is seen in Figure 36 E. This can be explained by the fact that much deformation was governed up by backthrusts. The fore-thrust (as in model RD01) is clearly affected by refraction along the material transition; the thrust in the FS-side is formed at a distance of 21 cm, where in the Q layer it is positioned at 17 cm in front the backstop. Curvatures on both sides of the model are due to boundary effects.
Figure 36: Model RD01 and RD02 A: Top view model RD01 at 19% shortening. B: Interpreted top view RD01. C: Top view model RD02 at 12% shortening. D: Interpreted top view RD02. E and F: Cross sections A and B respectively, both of model RD02, with interpreted fault, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 52 RD03 and RD04 These models are similar to RD01-02 but larger (60x60x4,5cm) to decrease the influence of the model boundaries.
Also the backstop size was reduced in order to produce only shortening in the feldspar domain. No cuts were made of model RD03. Model RD03 produced three thrusts (based on surface view alone, at 35% shortening) which all show a clear refraction on the material transition, especially the last (foremost) thrust is very strongly refracted.
Model RD04 also produced 4 fore-thrusts (after 29% short.), but from the cuts (Figure 37 D) we can discern that also several backthrusts govern deformation. The orientation/refraction of the thrusts is comparable to model RD03. Although difficult to discern without surface markers, there is no indication of lateral or thrust movements along the material contact. This is supported by the observations of cut B (Figure 37 E) were the vertical material contact is simply cut by a thrust and displaying active movement itself. Figure 37: RD04, top views and cross sections A: Top view at 10% shortening, and placing of presented cross sections.
B: Top view at 10% shortening.
C: Top view at 29% shortening, with roman numbers representing the thrust sequence. D and E: Cross section A and B respectively, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 53 RD05 This model was similarly constructed as RD03- 04, only the model thickness was reduced to 3cm and a thin basal glass bead layer was introduced underneath the feldspar sand. As can be seen in Figure 38 D, model RD05 produced a narrow, high wedge in front of the backstop.
Four fore-thrusts and one backthrust can be seen from surface view at the end of the experiment (Figure 38 C, at 29% shortening). There is no clear indication for thrust refraction. The cuts (Figure 38 D) reveal that thrusting occurred along one major fore- and a major backthrust, transporting the older thrusts upwards. Figure 38 E shows that the material contact was used for movement. Thrust 1 also cuts through part of the contact and transported the upper section of the quartz block towards the right.
In conclusion we can state that; in these models (back or fore) thrusting is the dominant mechanism of accommodating shortening. There are no clear indications of considerable amounts of lateral movements. Thrust refraction occurs along the material contact between quartz and feldspar sand, this is seen in all models (irrespective of the backstop size) except RD05. The vertical material contact, representing a pre- existing weak zone, is not reactivated but cut by thrusts in any of the models, comparable to the PS models, where reactivation does not take place either. The exception is again model RD05, in which a fault partially follows this contact but also cuts through a section.
Figure 38: RD05, top views and cross sections A: Top view at 10% shortening, and placing of presented cross sections. B: Top view at 10% shortening.
C: Top view at 29% shortening, with roman numbers representing the thrust sequence. D and E: Cross section A and B respectively, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 54 Rheology difference models II These series of models were carried out to provide insight into the influence of the contact between two rheologies, in order to study the style and location of deformation. Three models were constructed that consisted of Q and FS similar as the models mentioned above.
The contact was changed into a lower ; 27° , a GB layer to represent fault gouge and a combination of these two features. RD06 The dimensions of this model are similar to RD05, the rheology contact is a of 27° and covered with a GB layer. As is seen in the top views of Figure 39 A, B and C shortening is accommodated in the feldspar layer by 7 in sequence thrusts and 1 back thrust. The first thrust appears at 7 cm for the backstop, then a spacing of 2 to 3 cm is seen. The last thrust forms at 7 cm. These thrusts are curved towards the Q layer in the direction of movement, which might indicate strike-slip movement.
The white markers on top of the model do not support this Figure 39: RD06, top views and cross sections A: Top view at 10% shortening, and placing of presented cross sections. B: Top view at 10% shortening.
C: Top view at 17% shortening, with roman numbers representing the thrust sequence. D and E: Cross section B and A respectively, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 55 interpretation, these only show thrusting over the Q layer. Cross sections perpendicular to the shortening direction show minor thrusts in the Q close to the backstop. Cuts A and B in Figure 39 D and E show no deformation in the Q area.
Deformation at the contact area is poorly recorded in cross section A (Fig. 39 D), although thrusting in this area is clearly present from the top views. This is due to the practical problems of creating horizontal layers close to the boundary. Cut B (Fig 39 E) shows the shortening was accommodated along the contact with a minor distortion in the Q layer, interpreted as thrust (zone) 5. The movement along the contact is a clear indication that the created surface is reactivated and used for displacement.
RD07 The set up of this model is similar to RD06, only the GB layer at the contact is not included. The top views of Figure 40 A, B and C show shortening accommodated in the feldspar layer by 3 in sequence thrusts (spacing: 7, 2,5 and 3,5 cm) and 1 back thrust. Thrust 3 and 4 curve towards the Q layer in the direction of the movement, but straighten out as the experiment continues. In Figure 40 E cross section B movement along the Q-FS contact is interpreted as thrust 3. Close to this thrust more thrusting is present, which makes a deformation zone of the material contact area. This movement is a clear indication that the created surface was used for displacement.
This reactivation is only partly because the Q layer is also affected by thrust 6. Figure 40: RD07, top views and cross sections A: Top view at 10% shortening, and placing of presented cross sections. B: Top view at 10% shortening.
C: Top view at 17% shortening, with roman numbers representing the thrust sequence. D and E: Cross section A and B respectively, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 56 RD08 The set up of this model is similar to RD06; only the is 90° . Top views in Figure 41 A, B and C show 3 in sequence thrusts (spacing: 6, 2 and 8 cm) and 1 back thrust in the feldspar layer. Apart from the area close to the backstop, the Q layer is not affected by deformation.
The cross sections in Figure 41 E show thrusts 6,7 and 8 cut through the contact and transport parts of the Q layer towards the right. The thrusts in this experiment are straight, no curvature into the material boundary is noticeable. There is no indication for movement along the contact.
All three RD models show in sequence thrusting accompanied by backthrusts, however most of the deformation is accommodated by the fore- thrusts. Activation of the contact between the Q and FS sand occurs in this set of models, key feature for this is a low (RD06, RD07). In combination with weak material at the contact the activation is even enhanced: 3 cm reverse displacement for RD06 against 2 cm for RD07, based on the cross sections. Indications for activation in strike slip modus is more difficult to establish; only the surface views of RD06 give evidence for this.
Figure 41: RD08, top views and cross sections A: Top view at 10% shortening, and placing of presented cross sections.
B: Top view at 10% shortening. C: Top view at 17% shortening, with roman numbers representing the thrust sequence. D and E: Cross section A and B respectively, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 57 Rigid indenter models As explained in the methods section the wooden ramp is introduced to even enhance the rheology difference between the representation of the Trento plateau and the Lombardian basin. Another tested parameter is the angle ( ) between the shortening direction and material boundary. RI01 The set up of this experiment is different from the RD models; the wooden ramp is introduced with a of 60° and the angle is slightly changed to 30°for practical building purposes. This experiment results in 3 in sequence thrusts and two graben-like structures close to the ramp (Figure 42 C).
The zone of deformation is 3 cm wide and the height is increased with 0.8 cm. On Figure 42 B can be seen that apart from structures taking up deformation perpendicular to the ramp (thrusting), oblique structures with lateral movement also formed. Three of these faults developed after 2% shortening and small scale structures along the ramp formed after 5% shortening. These structures are identified as accommodators of strike slip movement, because of the orientation angle in respect to the shortening direction. However considering the scale and the fact these are not seen in the cross sections this is not ensured.
RI02 Deformation in this experiment with a of 45° , is taken up by thrusts, fore- and backwards directed (Figure 43 A). This in-sequence thrusting creates a deformation zone of 2.5 cm wide and 1 cm higher than the initial 3 cm. However after the formation of the first thrust, small scale (shallow) structures developed on top of the wedge (Figure 43 B). These are again the strike slip movements mentioned in experiment RI01 as well.
Figure 42: RI01, top view and cross section A: Top view at 10% shortening, and placing of presented cross section. B: Top view at 10% shortening, with roman numbers representing the thrust sequence. C: Cross section A, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement. Figure 43: RI02, top view and cross section A: Top view at 10% shortening, and placing of presented cross section. B: Top view at 10% shortening, with roman numbers representing the thrust sequence.
C: Cross section A, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 58 RI03 The deformation in RI03 with an of 75° is completely dominated by in sequence thrusting accompanied by two back thrusts (Fig 44 C). The deformation zone is 3.7 cm wide, the height of the sand is increased with 1.2 cm. RI04 This experiment resembles RI01, however glass beads are included on the contact between the wooden ramp and the quartz sand. The result is a deformation wedge of 2.8 cm wide and an increase of the sand thickness of 0.85 cm (Figure 45 C). Figure 45 B indicates again the ramp-parallel small scale structures found in experiment WB01 as well.
Figure 45 C shows the glass beads have moved up the ramp.
The influence of the angle of the material boundary to the compression direction ( ) is modelled in this set of experiments; the dimensions of the deformed zone (wedge) are increased with increasing . The nature of the structures changes as well, experiment RI03 is completely dominated by thrusting, where in experiment RI01 strike slip movement also occurred. RI02, with an intermediate results in a small amount of strike slip movement together with thrusting. The height of the deformed wedge after 10% shortening is related to the amount of deformation accommodated by thrust structures. All models are shortened 10%, the increased wedge height of the models at the end of the experiment varies from 0.8 to 1.2 cm.
Lower values are created by experiments RI01 and RI04, with both an angle of 30° .
Figure 44: RI03, top view and cross section A: Top view at 10% shortening, and placing of presented cross section. B: Top view at 10% shortening, with roman numbers representing the thrust sequence. C: Cross section A, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement. Figure 45: RI04, top view and cross section A: Top view at 10% shortening, and placing of presented cross section. B: Top view at 10% shortening, with roman numbers representing the thrust sequence.
C: Cross section A, with interpreted faults, numbers represent the faulting sequence and arrows indicate fault movement.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 59 Discussion Theory predicts that the most important factors controlling reactivation of pre-existing structures are; the orientation of the fault plane in relation to the new stress field , the dip angle of the structure and the frictional properties of the plane (Sassi et al, 1993). Here, the experimental results on these parameters are discussed. Frictional properties As presented in previous chapters, the PS models in this research (PS01-03) did not reactivate the pre-existing structures. This is remarkable because these types of discontinuities were successfully created and reactivated in previous research, e.g.
by Sassi et al (1993). Considering the fact that the pre- existing structure in the PS experiments was created in exactly the same way (Sassi et al used nylon wire to cut through dry quartz sand at different dip angles and locations), it can be assumed that the frictional properties of this contact are comparable. Therefore, the failure to activate structures in the models presented can be ascribed to differences in and/or of the contact and not to the frictional properties of the structure. Another aspect which should be considered is the width of the backstop, in the PS-models this was similar to the model width.
In the RD experiments the frictional properties of the contact in the models are different compared to the PS models; here the contact is the transition between quartz and feldspar domains. There are only minor changes in the structural development of the experiments. Thrusts still initiate independent of the pre- existing structure, only refraction at the contact occurs (the distance of the surface exposure of the thrust to the backstop is greater in the feldspar domain). This can be explained by the different strengths of the quartz and feldspar domain.
Weaker strata and inclined detachment zones were modelled by inserting thin glass beads layers because of their low frictional properties.
There is still an ongoing discussion about the natural counterpart that GB actually represent. Some researchers used GB to produce similar structures as in the presented models; a horizontal detachment in a sedimentary sequence (Maillot and Koyi, 2006), as a detachment in an accretionary wedge (Kukowski et al, 2002). In these studies movement along the GB layer occurred (i.e. it was used as a detachment level) when the internal friction was lower than the basal friction. In this research when the frictional component on the contact is strongly reduced by the implementation of a GB layer, larger amounts of deformation are transferred to the contact (both lateral and reverse movement).
This can be seen by the lower wedge in RD06 when compared to RD07, indicating a smaller amount of deformation partitioned to the contact in model RD06. RI04 is set up in the same manner as RI01, with the addition of a GB layer on the contact ramp. This did not induce significantly different results in the amount or style of deformation. In other research (Di Bucci et al, 2007) GB are used to represent a ca. 1000 m thick weak layer at the base of a platform succession. In this experiment, the GB failed to act as a detachment layer. This might be due to the experimental set-up and the deformation regime; strike-slip was induced by a basal plate, with the GB layer horizontally above these basal plates.
In RD05 the use of GB at the base provided partial movement along the contact (see Figure 38 E). According to Bonini et al (2000) weaker sediments can also be modelled by ductile materials like silicon putty. In this research ductile layers were not included, as the study area remains in the brittle domain. Depending on the model set-up; GB can act as weaker layers and detachment zones.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 60 In the experiments with the rigid indenter (RI), friction is higher and deformation is forced on the contact. These kinds of blocks are commonly used in analogue modelling as rigid indenters. The advantage of rigid blocks is the localization of deformation they induce. In this research, activation of the pre-existing structure is hard to establish, therefore forcing the deformation at the contact provides useful information about reverse movement on the normal fault plane. Other researchers used rigid indenters to represent the Adriatic plate (Bonini et al, 1999), a thrust footwall or pre- existing basement fault (Bonini et al, 2000), cool crust of higher strength (Persson and Sokoutis, 2002) or a thrust ramp (Maillot and Koyi, 2006).
Fault strike orientation For the PS and RD experiments one value is used (25° --27° ), this is based on the paleostress regimes found in the field study and will be discussed in the comparison with natural example section. A small angle is theoretically the most likely angle to reactivate a pre-existing structure in strike slip mode, especially when is steep. The influence of the angle is illustrated by the RI experiments, where a 30° , 45° and 75° angle is modelled. Low angles result in reverse movement combined with lateral movement, where RI03 with = 75°shows only thrusting. Because the more parallel to the strike is to the compression direction, the larger the percentage of the total deformation that is partitioned into lateral movement.
Fault dip angle Most experiments have been performed with a of 90° (PS01-03, RD01-05 and RD08) for practical purposes. A (nearly) vertical structure is not expected to initiate thrust movement since the normal angle for this is around 30° .
However, it is remarkable that there is also no proof of lateral movement along the pre- existing vertical fault planes in these models. There are two possible reasons for this: either this lateral movement was present but could not be detected or there was in fact no lateral reactivation. RD06-07 prove the importance of . Here is lowered to around 30° (see Table 6) and thrusting occurs at the contact. This is expected from theory, because this is the natural angle of thrusting in these settings and it is therefore used as activation zone of movement. In the RI-experiments the ramp is prepared at an angle of 60° , this is a good representation of a normal fault setting.
Because of the infinite strength of the rigid indenter deformation is localized at the contact zone. However, other research by (Bonini et al, 1999) has proven that this is not necessarily always the case. In their models, with a of 75° deformation takes place further away from the indenter and not at the contact zone. This is ascribed to the steeper that induces an ' effective'indenter by creating a sand pile in front of the rigid indenter that has an angle that fits the shear strength of the deforming sand wedge. A comparison with the RD experiments cannot be made, since no sand models were constructed with a of 60° .
Comparison with a natural example: Ballino- Garda line, Southern Alps The field study conducted prior to the analogue modelling is used to constrain the set up of the experiments. In this area reverse and lateral reactivation is observed along roughly N-S striking structures (e.g. Giudicarie and Ballino- Garda line). Fault dips of the Ballino-Garda line are assumed to be around 60° ; the formation angle of normal faults during extension. In specific compressive settings (where 1 is at a small angle to the fault strike), faults with such steep angles can be reactivated as a strike-slip zone with a reverse component.
This assumption is supported by the experiments and the field research. In the paleo stress analysis, two deformation events were found with 1 at a small angle to the N-S trending faults; with a NNW-SSE and NNE-SSW 1. These phases could have been responsible for left lateral strike
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 61 slip movement along the Giudicarie South and the Ballino-Garda line. These movement phases probably induced weakening of the fault area. Several mechanisms for this are suggested: brecciation at the fault zone, formation of fault gouge, clay smearing at the contact, processes induced by or the presence of fluids. Subsequent deformation phases more orthogonal to the strike of the fault zone like the E-W shortening phase found in the field research might have been able to produce the observed reverse movement along the resulting weakened fault zone.
The reactivational reverse movements in the experiments are significantly increased when the friction of the contact is lowered (RD06), as presented in the result section. This could be taken as a possible cause to explain the reactivation of the Ballino-Garda and Giudicarie as reverse faults.
Apart from the fault zone properties, the strength contrast between the sediments of the Trento (Venetian) platform and the Lombardian basin should also be considered. This has been modelled by rigid indenters and different sands. As mentioned before the feldspar and quartz sand are a good representation of the two different regions. Figure 46: Comparison of the study area and the model results. The coloured structures indicate which faults in natures have also been observed in the analoque models. BZ = Bolzano, TN = Trento, VR = Verona, BS = Brescia, VT = Val Trompia fault and thrust belt, TC = Trento-Cles fault.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 62 Although the extremely high strength contrast created by the rigid indenters is unlikely to occur in nature, it is a good experimental method to exaggerate the strong stable character of the Trento (Venetian) plateau in order to better observe the deformation in the models. This is also why a partial backstop deforming only in the feldspar sand was used. In conclusion, although the scaling of materials might not be entirely correct, it was necessary to model the deformation taken up in the Lombardian Basin.
See figure 46 for the analogue comparison with the natural example. A final controlling factor might be a significant influence of the presence of a basal block below the fault contact. This is not investigated in this research, however there has been a lot of attention to this subject in previous research. It is suggested that basal steps or blocks can act as places to localize stress and thereby force reactivation from the base (Viola et al, 2004).
Conclusions From these experiments can be concluded that: • The pre-existing structure is (re)activated when: 1 is oblique to the fault strike; the lower the angle of the PS trend with the 1 , the more strike-slip movement. 1 • The pre-existing structure is (re)activated when: the dip of the pre- existing structure is 30° in the RD models, when is 60°in the RI models. • The bigger the strength difference, the greater the amount of reactivation. • The lower the friction at the contact, the greater the amount of reactivation. The Ballino-Garda line as an inherited normal fault has been reactivated as a left lateral fault with a reverse component.
After the analogue modelling study the following conclusions can be made: • The strength difference between the two domains separated by the Ballino-Garda line (the Lombardian Basin and the Trento Plateau) is an important positive factor in reactivation.
• The models suggest the presence of a weak zone at the Ballino-Garda line. • The experimental results suggest that the reconstructed deformation phases with a 1 direction close to the N-S Ballino- Garda line trend are responsible for left lateral and reverse movement. And have possibly created the low friction on the Ballino-Garda contact. • The low friction of the contact makes it possible for E-W compression to reactivate a normal fault as a reverse fault.
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 63 Acknowledgements I thank Ernst Willingshofer for the supervision in the field and Dimitrios Sokoutis for the supervision of the analogue modelling.
A special thanks for Judith van Hagen and Lieke de Jong for their co-operation. I thank the Vrije Universiteit Amsterdam, for funding and use of the ISES Tectonic Laboratory of the Institute of Earth and Life Sciences. The technical staff and Arno Versteeg, thank you for the assistance.
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Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 66 Appendix I The Mohr-Coulomb-law for failure. Laboratory experiments have been carried out for calculating failure envelopes of natural crustal rocks. These all have the following characteristics: - Failure envelopes form a straight line (except when the confining pressure is extremely high or low). - Failure envelopes have a positive slope of approximately 30°(with variation between 20°and 45°depending on the rock type).
- Failure envelopes have a low positive value at their interception with the vertical axis of the Mohr-diagram ( s – vertical axis).
This behavior has been explained by the Mohr-Coulomb-law for failure. This law describes the critical value for shear stress ( s) that must be exceeded to break the rock. The equation of the Mohr-Coulomb-law for failure is: s = 0 + tan ( n) (1) s critical shear stress for failure angle of internal friction 0 cohesive strength tan coefficient of internal friction n normal stress The strength of rocks depends on its cohesive strength ( 0) and the internal frictional resistance to faulting. The stress must be large enough to overcome the cohesive strength and the internal friction in order to give displacement along a potential fault plane.
The cohesive strength is a property of the rock itself. The cohesive strength for a given rock varies little under different deformational regimes. The higher the density of a rock, the higher the cohesion will be. For example, the cohesive strength of basalts and granites is larger than that of sandstones and marls. The internal frictional resistance to faulting is not a rock property. This varies as a function of the coefficient of internal friction (tan , this is a rock property that varies depending on rock type) as well as the value of the normal stress ( n) that is acting on the potential fault plane.
The graph of Figure A1 shows a failure envelope of a limestone. A shear stress of 222000 psi is needed to break the rock with a confining pressure of 18500 psi. In terms of the Mohr-Coulomb- law this is the critical shear stress, s, which is needed to break the rock. Part of this value is the cohesive strength; 0 the interception of the failure envelope with the y-axis. The other part of the critical shear stress is the stress that is needed to overcome the internal frictional resistance. This component is the f. The value of f can be described as the normal stress, which is acting on the fault plane, and the angle of internal friction, which is the slope of the failure envelope.
tan = f / n (2) f = tan ( n) (3) s = 0 + tan ( n) (4) The stress angle upon which rocks break is strongly related to the angle of internal friction. For most natural basement and sedimentary rocks this is between 25°and 45° . Depending on rock
Deformation history of the Ballino-Garda line in the Southern Alps, Italy Nynke Hoornveld 2009 67 composition, values around 30° are most frequently encountered (van Mechelen, 2004 and references therein). For example, the angle of internal friction of natural limestone is around 27° , and dolomite ~45°(according to Handin, 1969). The angle of internal friction defines the angle between the fracture plane and the largest principal stress 1. = 90° - 2 (5) = (90 / 2 (6) Fig. A1. Illustration of the Mohr-Coulomb-law for failure (Mohr-diagram). Failure envelope for a limestone rock.