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A 174 − 156 Ma assimilation, to which pressure-based (not
Zedong Arc Extension temperature-based) proxies such as Sr/Y and
Backarc basin La/Yb from rocks filtered following Profeta
sea Gangdese magmatic lull
level et al. (2015) are less sensitive.
Crustal thickness of 65–70 km between 44
and 10 Ma based on trace-element geochem-
Lhasa Terrane
istry is similar to modern crustal thickness of
Yeba ~70 km estimated from geophysical methods
volcanism (Owens and Zandt, 1997; Nábělek et al., 2009)
Neo-Tethys slab
accelerated asthenosphere and are 10–20 km less than upper estimates of
slab rollback upwelling
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 coeval rapid erosion (Copeland et al., 1995)
Gangdese Mountains
sea may have maintained a uniform crustal thick-
level amphibolite exhumation ness. Our results are inconsistent with models
ocean basin
that invoke net crustal thinning via orogenic
Lhasa Terrane
(90−80 Ma)
collapse (Dewey, 1988) beginning in the
Miocene and continuing to present day (Ge et
asthenosphere al., 2015). Rather, our results are consistent
upwelling granulites with interpretations of thick crust in southern
Neo-Tethys Slab
slab rollback (90−81 Ma) Tibet by middle Eocene time (Aikman et al.,
2008; Pullen et al., 2011), which continued to
Figure 4. Tectonic interpretation after Kapp and DeCelles (2019). (A) Middle–Late Jurassic accelerated
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- tion of underthrusting India (DeCelles et al.,
nics (Liu et al., 2018). (B) Late Cretaceous slab rollback results in the opening of a backarc ocean basin.
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,
Tibetan-Himalayan orogenic wedge as Indian followed by major crustal thickening to ized thickening at depth, which in turn
crust was accreted in response to continuing 55–60 km by the early to middle Miocene. increases the rate of upper crustal extension in
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
orogenic wedge, as convergence between 45 Ma and that there was a crustal disconti- ACKNOWLEDGMENTS
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
Estimates of crustal thickness based on crustal thickening was already well under insightful discussions on proxies for crustal thick-
Sr/Y and La/Yb differ both in time and space way by 45 Ma, potentially near modern ness; and Caden Howlett and Aislin Reynolds for
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
temperature model of DePaolo et al. (1992), topes such as Nd and Hf are not directly con- Center. M.N.D. acknowledges support from National
which calculates the ambient crustal temper- trolled by crustal thickness and concomitant Science Foundation grant EAR 1725002 and the
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