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 BallinoGarda 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 microcontinent 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, PiedmontLiguria, 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 orogenparallel 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 PiedmontLigurian 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 (NeoAlpine 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-, Jaufenand 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 Periadriaticand Giudicarie fault systems are essential to understanding the evolution of the whole area.
The Passeier Fault Initiated during the InsubricHelvetic 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 (InsubricHelvetic 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 basementand 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 granodioritetrondheimite 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; KompassFigure 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.