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.
ASJC Scopus subject areas