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
Recent empirical calibrations of Sr/Y and La/Yb from intermediate igneous rocks as proxies of
crustal thickness yield discrepancies when applied to high ratios from thick crust. We
recalibrated Sr/Y and La/Yb as proxies of crustal thickness and applied them to the Gangdese
Mountains in southern Tibet. Crustal thickness at 180–170 Ma decreased from 36 to 30 km,
consistent with Jurassic backarc extension and ophiolite formation along the southern Asian
margin during Neo-Tethys slab rollback. Available data preclude detailed estimates between 170
and 100 Ma and tentatively suggest ~55 km thick crust at ca. 135 Ma. Crustal thinning between 90
and 65 Ma is consistent with a phase of Neo-Tethys slab rollback that rifted a portion of the
southern Gangdese arc (the Xigaze arc) from the southern Asian margin. Following the continental
collision between India and Asia, crustal thickness increased by ~40 km at ~1.3 mm/a between 60
and 30 Ma to near modern crustal thickness, before the onset of Miocene east-west extension.
Sustained thick crust in the Neogene suggests the onset and later acceleration of extension in
southern Tibet together with ductile lower crustal flow works to balance the ongoing mass
addition of under-thrusting Indian crust and maintain isostatic equilibrium.
*Now at Dept. of Geosciences, Idaho State University, Pocatello, Idaho
83209, USA
Manuscript received 16 Apr. 2020. Revised manuscript received 1 Sept. 2020.
Manuscript accepted 8 Mar. 2021. Posted 9 Apr. 2021.
© The Geological Society of America, 2021. CC-BY-NC.
https://doi.org/10.1130/GSATG461A.1