Jurassic to Neogene quantitative crustal thickness estimates in southern Tibet from recalibrated Sr/Y and La/Yb trace element geochemical proxies

Abstract:

Sr/Y and La/Yb trace element ratios from intermediate igneous rocks have been empirically calibrated as proxies of crustal thickness. However, there are major discrepancies between these proxies when applied to high ratios found in thickened continental crust. We present new empirical calibrations based on Monte Carlo simulations of individual and paired Sr/Y and La/Yb. Results yield median ± 2σ crustal thickness residuals of 5.0 ± 5.8 km for Sr/Y, 4.8 ± 5.2 km for La/Yb, and 4.5 ± 4.0 km for paired calibrations. We applied these recalibrations to data from Jurassic to Neogene rocks of the Nyainqentanglha region of the Gangdese Mountains in southern Tibet. Results yield estimates with good correspondence between proxies for individual igneous samples of 4.7 ± 8.4 km (2σ), compared to previously calibrations yielding median variations of ~20 km, some producing discrepancies up to >100 km. Paired calibrations yield the least dispersion and the most geologically feasible results. Results suggest the crust in southern Tibet was relatively thin between 180 and 170 Ma, which is consistent with generation of ophiolites that were likely formed in the forearc of the southern Asian margin during southward rollback of the Neo-Tethys oceanic slab. Limited data density precludes thickness estimates between 170 and 90 Ma. The 90 to 70 Ma time interval is characterized by a period of crustal thinning, which is coeval with a phase of Neo-Tethys slab rollback that may have rifted a portion of the southern Gangdese arc (referred to as the Xigaze arc) from the southern Asian margin and led to the opening of an intervening Xigaze backarc ocean basin. Following early Cenozoic continental collision between India and Eurasia, crustal thickness increased from ~40 to ~70 between 60 and 30 Ma. Following attainment of near modern crustal thickness by ~30 Ma, east-west extension initiated across southern Tibet and continues to present day. We propose that crustal thickness and associated high topography reached a critical threshold in gravitational potential energy between 30 and 20 Ma, triggering the switch from contraction to extension in order to evacuate ongoing mass addition during continued northward convergence of India relative to Eurasia.