Jurassic to Neogene Quantitative Crustal Thickness Estimates in Southern Tibet - 10-13 Oct. GSA Connects 2021
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10–13 Oct. GSA Connects 2021
VOL. 31, NO. 6 | JUNE 2021
Jurassic to Neogene
Quantitative Crustal Thickness
Estimates in Southern Tibet
Structural and Thermal Evolution of
the Himalayan Thrust Belt
in Midwestern Nepal
By P.G. DeCelles, B. Carrapa, T.P. Ojha,
G.E. Gehrels, and D. Collins
Spanning eight kilometers of topographic relief, the
Himalayan fold-thrust belt in Nepal has accommo-
dated more than 700 km of Cenozoic convergence
between the Indian subcontinent and Asia. Rapid
aper 547
Special P tectonic shortening and erosion in a monsoonal
E v o l u t i o n of climate have exhumed greenschist to upper amphi-
bolite facies rocks along with unmetamorphosed
a n d T h e rm al N e p al rocks, including a 5–6-km-thick Cenozoic foreland
Stru c t u ra l i d we s t e rn basin sequence. This Special Paper presents
s tB e l t i n M new geochronology, multisystem thermochro-
a y a n T h ru nology, structural geology, and geological
e Himal
mapping of an approximately 37,000 km2 region
th in midwestern and western Nepal. This
Therm
work informs enduring Himalayan debates,
a l E vo
including how and where to map the Main
luti
Central thrust, the geometry of the seismi-
on of
cally active basal Himalayan detachment,
the H
processes of tectonic shortening in the
ima
context of postcollisional India-Asia con-
layan
vergence, and long-term geodynamics of
Thrust
the orogenic wedge.
Belt in
Midw
SPE547, 77 p. + insert
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JUNE 2021 | VOLUME 31, NUMBER 6
SCIENCE
4 Jurassic to Neogene Quantitative Crustal
Thickness Estimates in Southern Tibet
Kurt E. Sundell et al.
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Jurassic to Neogene Quantitative Crustal
Thickness Estimates in Southern Tibet
Kurt E. Sundell*, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, sundell@arizona.edu; Andrew K. Laskowski,
Dept. of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA; Paul A. Kapp, Dept. of Geosciences, University of
Arizona, Tucson, Arizona 85721, USA; Mihai N. Ducea, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, and
Faculty of Geology and Geophysics, University of Bucharest, 010041, Bucharest, Romania; James B. Chapman, Dept. of Geology and
Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
ABSTRACT 2009), and up to ~85 km (Wittlinger et al., proto-plateau (Kapp et al., 2007; Lai et al.,
Recent empirical calibrations of Sr/Y and 2004; Xu et al., 2015). The Tibetan Plateau 2019). Alternatively, Late Cretaceous to
La/Yb from intermediate igneous rocks as formed from the sequential accretion of con- Paleogene shortening may have been punc-
proxies of crustal thickness yield discrepan- tinental fragments and island arc terranes tuated by a 90–70 Ma phase of extension
cies when applied to high ratios from thick beginning during the Paleozoic and culmi- that led to the rifting of a southern portion
crust. We recalibrated Sr/Y and La/Yb as nated with the Cenozoic collision between of the Gangdese arc and opening of a back-
proxies of crustal thickness and applied India and Asia (Argand, 1922; Yin and arc ocean basin (Kapp and DeCelles, 2019).
them to the Gangdese Mountains in south- Harrison, 2000; Kapp and DeCelles, 2019). These represent two competing end-member
ern Tibet. Crustal thickness at 180–170 Ma The India-Asia collision is largely thought to hypotheses for the Mesozoic tectonic evo-
decreased from 36 to 30 km, consistent with have commenced between 60 and 50 Ma lution of southern Tibet that are testable
Jurassic backarc extension and ophiolite (e.g., Rowley, 1996; Hu et al., 2016); how- by answering the question: Was the crust
formation along the southern Asian margin ever, some raise the possibility for later col- in southern Tibet thickening or thinning
during Neo-Tethys slab rollback. Available lisional onset (e.g., Aitchison et al., 2007; van between 90 and 70 Ma?
data preclude detailed estimates between Hinsbergen et al., 2012). Despite ongoing Contrasting hypotheses about the Cenozoic
170 and 100 Ma and tentatively suggest ~north-south convergence, the northern tectonic evolution of southern Tibet are test-
~55 km thick crust at ca. 135 Ma. Crustal Himalaya and Tibetan Plateau interior are able by quantifying changes in crustal thick-
thinning between 90 and 65 Ma is consis- undergoing east-west extension, expressed ness through time. In particular, the Paleocene
tent with a phase of Neo-Tethys slab roll- as an array of approximately north-trending tectonic evolution before, during, and after
back that rifted a portion of the southern rifts that extend from the axis of the high the collision between India and Asia was
Gangdese arc (the Xigaze arc) from the Himalayas to the Bangong Suture Zone dependent on initial crustal thickness, and in
southern Asian margin. Following the con- (Molnar and Tapponnier, 1978; Taylor and part controlled the development of the mod-
tinental collision between India and Asia, Yin, 2009) (Fig. 1). ern Himalayan-Tibetan Plateau. Building on
crustal thickness increased by ~40 km at The Mesozoic tectonic evolution of the the hypothesis tests for the Late Cretaceous,
~1.3 mm/a between 60 and 30 Ma to near southern Asian margin placed critical ini- if the crust of the southern Asian margin was
modern crustal thickness, before the onset tial conditions for the Cenozoic evolution of thickened before or during the Paleocene,
of Miocene east-west extension. Sustained the Tibetan Plateau. However, much of the then this explains why the southern Lhasa
thick crust in the Neogene suggests the Mesozoic geologic history remains poorly Terrane was able to attain high elevations
onset and later acceleration of extension in understood, in part due to structural, mag- only a few million years after the onset of
southern Tibet together with ductile lower matic, and erosional modification during continental collisional orogenesis (Ding et
crustal flow works to balance the ongoing the Cenozoic. There is disagreement even al., 2014; Ingalls et al., 2018). However, if the
mass addition of under-thrusting Indian crust on first-order aspects of the Mesozoic geol- Paleocene crust was thin, then we can ask
and maintain isostatic equilibrium. ogy in the region. For example, temporal the question: When did the crust attain mod-
changes in Mesozoic crustal thickness are ern or near modern thickness? Answering
INTRODUCTION largely unknown, and the paleoelevation of this question is a critical test of alternative
The Tibetan Plateau is the largest (~1,500 the region is debated. Most tectonic models tectonic models that suggest rapid surface
× 3,500 km), high-elevation (mean of ~5,000 invoke major shortening and crustal thick- uplift from relatively low elevation (and pre-
m) topographic feature on Earth and hosts ening due to shallow subduction during the sumably thin crust) during the Miocene (e.g.,
the thickest crust of any modern orogen, Late Cretaceous (e.g., Wen et al., 2008; Guo Harrison et al., 1992; Molnar et al., 1993) or
with estimates in southern Tibet of ~70 km et al., 2013), possibly pre-conditioning the Pliocene (Dewey et al., 1988) as the product
(Owens and Zandt, 1997; Nábělek et al., southern Asian margin as an Andean-style of mantle lithosphere removal (England and
GSA Today, v. 31, https://doi.org/10.1130/GSATG461A.1. CC-BY-NC.
*Now at Dept. of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA
4 GSA Today | June 2021
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( Sample locality Active or recently active fault Triangle for thrust HW Ball and tick for normal HW Arrows for strike-slip Gangdese arc rocks
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Figure 1. Digital elevation model of southern Tibet with major tectonic features. Active structures from HimaTibetMap (Styron et al., 2010). The basemap is
from MapBox Terrain Hillshade. Lake locations are from Yan et al. (2019). Data points include only the filtered data (supplemental material [see text footnote
1]). HW—hanging wall.
Houseman, 1988). Finally, what happened whereas Sr and La have a higher affinity for METHODS
after the crust was thickened to extreme lev- plagioclase (Fig. 2A). Thus, high Sr/Y and Sr/Y and La/Yb (the latter normalized
els, as we have in the modern? Did the pla- La/Yb can be used to infer a higher abun- to the chondritic reservoir) were empiri-
teau begin to undergo orogenic collapse dance of garnet and amphibole and a lower cally calibrated using a modified approach
(Dewey, 1988) resulting in a net reduction in abundance of plagioclase, and may be used reported in Profeta et al. (2015). Calibrations
crustal thickness and surface elevation that as a proxy for assessing the depth of parent are based on simple linear regression of
continues to present day (e.g., Ge et al., melt bodies during crustal differentiation in ln(Sr/Y)–km and ln(La/Yb)–km; and
2015), as evidenced by the Miocene onset of the lower crust (Heaman et al., 1990). These multiple linear regression of ln(Sr/Y)–
east-west extension (e.g., Harrison et al., ratios have been calibrated to modern crustal ln(La/Yb)–km (Figs. 2B–2D). We also
1995; Kapp et al., 2005; Sanchez et al., 2013; thickness and paired with geochronological tested simple linear regression of ln(Sr/Y) ×
Styron et al., 2013, 2015; Sundell et al., 2013; data to provide quantitative estimates of ln(La/Yb)–km (see GSA Supplemental
Wolff et al., 2019)? Or did Tibet remain at crustal thickness and paleoelevation through Material1). Regression coefficients and resid-
steady-state elevation during Miocene-to- time (e.g., Chapman et al., 2015; Profeta et uals (known minus modeled thickness) are
modern extension (Currie et al., 2005) with al., 2015; Hu et al., 2017, 2020; Farner and reported at 95% confidence (±2s).
upper crustal thinning and ductile lower Lee, 2017). The revised proxies were applied to
crustal flow (e.g., Royden et al., 1997) work- We build on recent efforts to empirically geochemical data compiled in the Tibetan
ing to balance continued crustal thickening calibrate trace-element ratios of igneous Magmatism Database (Chapman and Kapp,
at depth driven by the northward under- rocks to crustal thickness and apply these 2017). Geochemical data used here comes
thrusting of India (DeCelles et al., 2002; revised calibrations to the eastern Gangdese from rocks collected in an area between 29
Kapp and Guynn, 2004; Styron et al., 2015)? mountains in southern Tibet (Fig. 1). This and 31°N and 89 and 92°E. Data were filtered
Igneous rock geochemistry has long been region has been the focus of several studies following methods reported in Profeta et al.
used to estimate qualitative changes in past attempting to reconstruct the crustal thick- (2015) where samples outside compositions of
crustal (e.g., Heaman et al., 1990) and litho- ness using trace-element proxies (e.g., Zhu 55%–68% SiO2, 0%–4% MgO, and 0.05–
spheric (e.g., Ellam, 1992) thickness. Trace- et al., 2017) as well as radiogenic isotopic 0.2% Rb/Sr are excluded to avoid mantle-gen-
element abundances of igneous rocks have systems such as Nd and Hf (Zhu et al., erated mafic rocks, high-silica felsic rocks,
proven particularly useful for tracking 2017; Alexander et al., 2019; DePaolo et al., and rocks formed from melting of metasedi-
changes in crustal thickness (Kay and 2019), and highlight discrepancies in dif- mentary rocks. Filtering reduced the number
Mpodozis, 2002; Paterson and Ducea, ferent geochemical proxies of crustal thick- of samples considered from 815 to 190 (sup-
2015). Trace-element ratios provide infor- ness. As such, we first focus on developing plemental material; see footnote 1).
mation on the presence or absence of miner- a new approach to estimate crustal thick- We calculated temporal changes in crustal
als such as garnet, plagioclase, and amphi- ness from Sr/Y and La/Yb, both for indi- thickness based on multiple linear regres-
bole because their formation is pressure vidual ratios, and in paired Sr/Y–La/Yb sion of ln(Sr/Y)–ln(La/Yb)–km (Fig. 2B).
dependent, and each has an affinity for spe- calibration. We then apply these recali- Each estimate of crustal thickness is
cific trace elements (e.g., Hildreth and brated proxies to data from the Gangdese assigned uncertainty of ±5 m.y. and ±10 km;
Moorbath, 1988). For example, Y and Yb mountains to test hypotheses explaining the former is set arbitrarily because many
are preferentially incorporated into amphi- the Mesozoic and Cenozoic tectonic evolu- samples in the database do not have reported
bole and garnet in magmatic melt residues, tion of southern Tibet. uncertainty, and the latter is based on
1
Supplemental Material. Filtered and unfiltered geochronology-geochemistry results are from the Tibetan Magmatism Database (Chapman and Kapp, 2017); the full data
set between 89–92 °W and 29–31 °N was downloaded 20 July 2020. All data are available online at jaychapman.org/tibet-magmatism-database.html. MATLAB code is
available at github.com/kurtsundell/CrustalThickness and incorporates the filtered data to reproduce all results presented in this work. Go to https://doi.org/10.1130/
GSAT.S.14271662 to access the supplemental material; contact editing@geosociety.org with any questions.
www.geosociety.org/gsatoday 5
A High Lithostatic Pressure (>1 GPa)
B z = (-10.6 ± 16.9) + (10.3 ± 9.5)x + (8.8 ± 8.2)y
Residuals: ± 8.1 km (2s)
R2 = 0.892
Thick Crust
Garnet Sr and La
not incorporated
Crustal Thickness (km)
Continental
Y 70
Crust 60
50
Melt 40
Amphibole 30
Yb 20 4.5
10 4
0 3.5
Plagioclase not stable 3
3.5 3 2.5 Y)
2.5 2 2 Sr/
1.5 1 0.5 1
1.5 ln(
ln(La/Yb) 0
C D
70 70
y = (19.6 ± 4.3)x + (-24.0 ± 12.3) y = (17.0 ± 3.7)x + (6.9 ± 5.8)
60 60
R2 = 0.810
Crustal Thickness (km)
Crustal Thickness (km)
R2 = 0.811
50 50
40 40
30 30
20 20
10 10
Residuals: ± 10.8 km (2s) Residuals: ± 10.8 km (2s)
0 0
1 1.5 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 3.5
ln(Sr/Y) ln(La/Yb)
Figure 2. (A) Schematic partitioning diagram for Y and Yb into minerals stable at high lithostatic pressures >1 GPa such as garnet and amphibole. (B–D)
Empirical calibrations using known crustal thicknesses from data compiled in Profeta et al. (2015) based on (B) multiple linear regression of ln(Sr/Y) (x-axis),
ln(La/Yb) (y-axis), and crustal thickness (z-axis); (C) simple linear regression of ln(Sr/Y) and crustal thickness; and (D) simple linear regression of ln(La/Yb)
and crustal thickness. Equations in parts B–D include 95% confidence intervals for each coefficient. Coefficient uncertainties should not be propagated
when applying these equations to calculate crustal thickness; rather, the 2s (95% confidence interval) residuals (modeled fits subtracted from known
crustal thicknesses) are more representative of the calibration uncertainty.
residuals calculated during proxy calibra- and Application of these equations yields mean
tion (Fig. 2). Temporal trends were calcu- absolute differences between crustal thick-
lated using two different methods. The first Crustal Thickness = (17.0 ± 3.7) × ln(La/Yb) nesses calculated with individual Sr/Y and
method employs Gaussian kernel regression + (6.9 ± 5.8), (2) La/Yb of ~6 km. Paired Sr/Y–La/Yb cali-
(Horová et al., 2012), a non-parametric bration yields absolute differences of ~3 km
technique commonly used to find nonlinear whereas multiple linear regression of compared to Sr/Y and La/Yb. Discrepancies
trends in noisy bivariate data; we used a ln(Sr/Y)–ln(La/Yb)–km calibration yields in crustal thickness estimates between Sr/Y
5 m.y. kernel width, an arbitrary parameter and La/Yb using the original calibrations in
selected based on sensitivity testing for Crustal Thickness = (−10.6 ± 16.9) Profeta et al. (2015) are highly variable, with
over- and under-smoothing. The second + (10.3 ± 9.5) × ln(Sr/Y) an average of ~21 km, and are largely the
method involves calculating linear rates + (8.8 ± 8.2) × ln(La/Yb). (3) result of extreme crustal thickness estimates
between temporal segments bracketed by (>100 km) resulting from linear transforma-
clusters of data that show significant Crustal thickness corresponds to the depth tion of high (>70) Sr/Y ratios (supplemental
changes in crustal thickness: 200–150 Ma, of the Moho in km, and coefficients are ±2s material [see footnote 1]); such discrepancies
100–65 Ma, and 65–30 Ma. Trends are (Figs. 2B–2D and supplemental material [see are likely due to a lack of crustal thickness
reported as the mean ±2s calculated from footnote 1]). Although we report uncertain- estimates from orogens with rocks that are
bootstrap resampling 190 selections from ties for the individual coefficients, propagat- young enough (i.e., Pleistocene or younger)
the data with replacement 10,000 times. ing these uncertainties results in wildly vari- to include in the empirical calibration.
able (and often unrealistic) crustal thickness For geologic interpretation, we use results
RESULTS estimates, largely due to the highly variable from multiple linear regression of Sr/Y–La/
Proxy calibration using simple linear slope. Hence, we ascribe uncertainties based Yb–km to calculate temporal changes in
regression of ln(Sr/Y)–km and ln(La/Yb)– on the 2s range of residuals (Figs. 2B–2D). crustal thickness (Figs. 3B–3D). Results
km yields Residuals are ~11 km based on simple linear show a decrease in crustal thickness from 36
regression of Sr/Y–km and La/Yb–km, and to 30 km between 180 and 170 Ma. Available
Crustal Thickness = (19.6 ± 4.3) × ln(Sr/Y) ~8 km based on multiple linear regression of data between 170 and 100 Ma include a sin-
+ (−24.0 ± 12.3), (1) Sr/Y–La/Yb–km. gle estimate of ~55 km at ca. 135 Ma. Crustal
6 GSA Today | June 2021
A Filtered Trace Element Ratios B Crustal Thickness Estimates
200 60 90
early Cenozoic Sr/Y La/Yb Paired
80 thickening
Crustal Thickness (km)
extension acceleration
Sr/Y
slab rollback
50
slab rollback
150 La/Yb 70
slab rollback?
40
60
La/Yb
Sr/Y
100 30 50
20 40
extension
onset of
50 30
10 limited data
20
0 0 10
0 25 50 75 100 125 150 175 200 0 25 50 75 100 125 150 175 200
Age (Ma) Age (Ma)
C Gaussian Kernel Regression Model D Linear Segment Rates
90 90
Paired calibration 1.3 ± 0.1 mm/a Paired calibration
80 80
Crustal Thickness (km)
Crustal Thickness (km)
−0.8 ± 0.05 mm/a
70 70
60 crustal 60 crustal
thinning thinning
50 50
crustal
40 crustal 40 thickening
thickening
30 30
20 crustal 20 crustal −0.7 ± 0.1 mm/a
thinning limited data thinning
10 10
0 25 50 75 100 125 150 175 200 0 25 50 75 100 125 150 175 200
Age (Ma) Age (Ma)
Figure 3. Results of new Sr/Y and La/Yb proxy calibration applied to data from the Tibetan Magmatism Database (Chapman and Kapp, 2017) located in the
eastern Gangdese Mountains in southern Tibet. (A) Filtered Sr/Y and La/Yb data extracted from 29 to 31°N and 89 to 92°W. (B) Values from part A converted
to crustal thickness using Equations 1, 2, and 3 (see text). (C–D) Temporal trends based on multiple linear regression of Sr/Y–La/Yb–km; trends are calcu-
lated from 10,000 bootstrap resamples, with replacement. (C) Gaussian kernel regression model to determine a continuous thickening history. (D) Linear
regression to determine linear rates for critical time intervals.
thickness decreased to 30–50 km by ca. -type orogen that existed until the Early (Kapp et al., 2007; Volkmer et al., 2007; Lai et
60 Ma, then increased to 60–70 km by ca. Cretaceous (Zhang et al., 2012), punctuated al., 2019), following shallow marine carbonate
40 Ma (Fig. 3). The two different methods for by backarc extension between 183 and 174 deposition during the Aptian–Albian (126–
calculating temporal trends in crustal thick- Ma (Wei et al., 2017). The latter is consis- 100 Ma) across much of the Lhasa terrane
ness (Gaussian kernel regression and linear tent with our results of minor crustal thinning (Leeder et al., 1988; Leier et al., 2007). Late
regression) produced similar results (Figs. from ~36 to ~30 km between 180 and 170 Cretaceous crustal thinning to ~40 km (closer
3C–3D). The Gaussian kernel regression Ma (Figs. 3B–3D) and supports models to the average thickness of continental crust)
model produces a smooth record of crustal invoking a period of Neo-Tethys slab roll- supports models that invoke Late Cretaceous
thickness change that decreases from ~35 to back (i.e., trench retreat), southward rifting extension and Neo-Tethys slab rollback that
~30 km between 180 and 165 Ma, decreases of the Zedong arc from the Gangdese arc, led to the development of an intracontinental
from ~54 to ~40 km between 90 and 75 Ma, and a phase of supra-subduction zone ophi- backarc basin in southern Tibet and south-
increases from ~40 to ~70 km between 60 olite generation along the southern margin of ward rifting of a southern portion of the
and 40 Ma, and remains steady-state from 40 Asia (Fig. 4A) (Kapp and DeCelles, 2019). Gangdese arc (referred to as the Xigaze arc)
Ma to present; the large uncertainty window Rocks with ages between 170 and 100 Ma from the southern Asian continental margin
between 160 and 130 Ma is due to the boot- are limited to a single data point at ca. (Kapp and DeCelles, 2019) (Fig. 4B). If a
strap resampling occasionally missing the 135 Ma and yield estimates of ~55 km backarc ocean basin indeed opened between
single data point at ca. 135 Ma (Fig. 3C). thick crust (Fig. 3B–3D). This is consistent Asia and the rifted Xigaze arc during this
Linear rates of crustal thickness change indi-with geologic mapping and geochronologi- time, it would have profound implications for
cate thinning at ~0.7 mm/a between 180 and cal data that suggest that major north-south Neo-Tethyan paleogeographic reconstruc-
170 Ma, thinning at ~0.8 mm/a between 90 crustal shortening took place in the Early tions and the history of suturing between
and 65 Ma, and thickening at ~1.3 mm/a Cretaceous along east-west–striking thrust India and Asia; this remains to be tested by
between 60 and 30 Ma (Fig. 3D). faults in southern Tibet (Murphy et al., 1997). future field-based studies.
The strongest crustal thinning trend in Paleogene crustal thickness estimates indi-
DISCUSSION our results occurs between 90 and 70 Ma at a cate monotonic crustal thickening at rates of
Early to Middle Jurassic crustal thickness rate of ~0.8 mm/a (Fig. 3D). Crustal thinning ~1.3 mm/a to >60 km following the collision
in southern Tibet was controlled by the takes place after major crustal shortening between India and Asia. This is in contrast to
northward subduction of Neo-Tethys oceanic (and thickening) documented in the southern models explaining the development of mod-
lithosphere (Guo et al., 2013) in an Andean Lhasa terrane prior to and up until ca. 90 Ma ern high elevation resulting from the removal
www.geosociety.org/gsatoday 7
A 174 − 156 Ma Extension
assimilation, to which pressure-based (not
Zedong Arc temperature-based) proxies such as Sr/Y and
Backarc basin
sea
Gangdese magmatic lull La/Yb from rocks filtered following Profeta
level Lha et al. (2015) are less sensitive.
sa Crustal thickness of 65–70 km between 44
Ter
rane and 10 Ma based on trace-element geochem-
Ne istry is similar to modern crustal thickness of
o-T
eth
ys
Yeba ~70 km estimated from geophysical methods
s volcanism
lab (Owens and Zandt, 1997; Nábělek et al., 2009)
accelerated asthenosphere
slab rollback upwelling and are 10–20 km less than upper estimates of
80–85 km (Wittlinger et al., 2004; Xu et al.,
2015). Upper-crustal shortening persisted in
B 90 − 70 Ma Xigaze backarc ocean basin southern Tibet until mid-Miocene time, but
Xigaze-Spong Arc
Gangdese Mountains
coeval rapid erosion (Copeland et al., 1995)
sea am
phi Lha may have maintained a uniform crustal thick-
level bol sa
i
(90 te exh ocean basin Ter ness. Our results are inconsistent with models
−80 um r ane
Ma ation
) that invoke net crustal thinning via orogenic
collapse (Dewey, 1988) beginning in the
Ne
o-T Miocene and continuing to present day (Ge et
eth
ysS
lab
asthenosphere al., 2015). Rather, our results are consistent
upwelling granulites
with interpretations of thick crust in southern
slab rollback (90−81 Ma)
Tibet by middle Eocene time (Aikman et al.,
Figure 4. Tectonic interpretation after Kapp and DeCelles (2019). (A) Middle–Late Jurassic accelerated 2008; Pullen et al., 2011), which continued to
slab rollback during formation of the Zedong Arc drives the opening of an extensional backarc basin. thicken at depth due to the ongoing mass addi-
This is consistent with the generation of the late-stage, juvenile (asthenosphere derived) Yeba volca-
nics (Liu et al., 2018). (B) Late Cretaceous slab rollback results in the opening of a backarc ocean basin.
tion of underthrusting India (DeCelles et al.,
2002) before, during, and after the Miocene
onset of extension in southern Tibet (e.g.,
of mantle lithosphere during the Miocene or Hammersley and DePaolo, 2006) assuming a Harrison et al., 1995; Kapp et al., 2005;
Pliocene (Dewey et al., 1988; England and depleted asthenospheric melt source with no Sanchez et al., 2013). We favor a model in
Houseman, 1988; Harrison et al., 1992; contribution from the mantle lithosphere; which continued crustal thickening at depth is
Molnar et al., 1993). The timing of crustal crustal thickness is then calculated based on balanced by upper crustal thinning (Kapp and
thickening in the late Paleogene temporally an assumed geothermal gradient on the prem- Guynn, 2004; DeCelles et al., 2007; Styron et
corresponds to the termination of arc magma- ise that a deeper, hotter Moho would result in
al., 2015), with excess mass potentially evacu-
tism in southern Tibet at 40–38 Ma and may more crustal assimilation than a shallower,
ated by ductile lower crustal flow (Royden et
indicate that the melt-fertile upper-mantle cooler Moho. In addition to using La/Yb to
al. 1997). In this view, late Miocene–Pliocene
wedge was displaced to the north by shallow- estimate Cenozoic crustal thickness, DePaolo
acceleration of rifting in southern Tibet
ing subduction of Indian continental litho- et al. (2019) use the flux-temperature model
(Styron et al., 2013; Sundell et al., 2013; Wolff
sphere (Laskowski et al., 2017). Crustal thick- to suggest that crustal thickening in southern
et al., 2019) is a consequence of the position of
ening during the Paleogene may be attributed Tibet was nonuniform based on Nd isotopes.
the leading northern tip of India (Styron et al.,
to progressive shortening and southward Specifically, they estimate crustal thickness
2015), because this region experiences local-
propagation (with respect to India) of the of 25–35 km south of 29.8° N until 45 Ma,
ized thickening at depth, which in turn
Tibetan-Himalayan orogenic wedge as Indian followed by major crustal thickening to
increases the rate of upper crustal extension in
crust was accreted in response to continuing 55–60 km by the early to middle Miocene.
order to maintain isostatic equilibrium.
convergence. We interpret that thickening Critically, they suggest that north of 29.9° N
depended mainly on the flux of crust into the the crust was at near modern thickness before
ACKNOWLEDGMENTS
orogenic wedge, as convergence between 45 Ma and that there was a crustal disconti- We thank Chris Hawkesworth, Allen Glazner,
India and Asia slowed by more than 40% nuity between these two domains, which two anonymous reviewers, and editor Peter Copeland
between 20 and 10 Ma (Molnar and Stock, Alexander et al. (2019) later interpret along for their detailed critique of this work. We also thank
2009), subsequent to peak crustal thickening orogenic strike to the east based on Hf isoto- Michael Taylor and Richard Styron for informal
rates between 60 and 30 Ma. pic data. In contrast, our results show that reviews; Sarah George and Gilby Jepson for
insightful discussions on proxies for crustal thick-
Estimates of crustal thickness based on crustal thickening was already well under
ness; and Caden Howlett and Aislin Reynolds for
Sr/Y and La/Yb differ both in time and space way by 45 Ma, potentially near modern discussions of Tibetan tectonics during the 2019 field
compared to estimates using radiogenic iso- crustal thickness, and with no dependence season as this research was formulated. KES was
topes. Determining crustal thickness from on latitude (Figs. 3B–3D and supplemental partially supported by the National Science Founda-
Nd or Hf relies on an extension of the flux- material [see footnote 1]). Radiogenic iso- tion (EAR-1649254) at the Arizona LaserChron
Center. M.N.D. acknowledges support from National
temperature model of DePaolo et al. (1992), topes such as Nd and Hf are not directly con-
Science Foundation grant EAR 1725002 and the
which calculates the ambient crustal temper- trolled by crustal thickness and concomitant Romanian Executive Agency for Higher Education,
ature and assimilation required to produce pressure changes. Rather, variability in Hf Research, Development and Innovation Funding proj-
measured isotopic compositions (e.g., and Nd is likely due to complex crustal ect PN-III-P4-ID-PCCF-2016-0014.
8 GSA Today | June 2021
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Zhang, Q., 2011, Metamorphic rocks in central between India and Asia: Proceedings of the Na- .06.001.
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crustal structure: Geological Society of America of America, v. 109, no. 20, p. 7659–7664, https:// Mo, X.X., 2017, Raising the Gangdese moun-
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12 GSA Today | June 2021Commitment to Care
The Geological Society of America considers the safety and IN PARTNERSHIP WITH THE OCC, WE WILL BE
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