Page 4 - i1052-5173-31-6

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