~55Ma Aged High Topography of the Lhasa Block From Stable and Clumped Isotope Paleoaltimetry: Implications for ~50±25% Crustal Mass Deficit in the India-Asia Collisional System

Abstract:

We reconstruct the ~55 Ma paleo-elevation of the pre- to syn-collisional Linzizong arc by coupling carbonate-derived oxygen stable isotope measurements (d¹⁸O_c) with paleotemperatures derived from the Δ₄₇-‘clumped’ isotope paleothermometer (T(Δ₄₇)). We estimate a pre- to early syn-collisional (~54 ± 2 Ma) paleo-elevation of the Penbo/Linzhou region of >4100 ± 550 meters. This provides the first well-constrained elevation estimate of the pre-collisional Linzizong Volcanic arc in the southern Tibetan Plateau. Our results indicate that high relief at low latitude did indeed prevail on the Asian “Andean” margin prior to collision. We use the most recent plate kinematic constraints to compute the mass flux associated with India-Asia convergence with uncertainty as a function of time. Integration of mass flux as a function of time, together with estimates of the diachronous collision age with uncertainty and corresponding suture zone length with uncertainty allow explicit estimates with uncertainties of pre-collisional crustal mass. Mass balance is estimated using estimates of pre-collisional elevation and crustal thickness with their associated uncertainties relative to the pre-collisional mass. We find that ~50±25% of the collision-related crustal mass cannot be accounted for by the mass preserved in excess crustal thickness (in Himalaya, Tibet, and adjacent Asia), southeast Asian tectonic escape, and exported eroded sediments. This implies large-scale subduction of continental crust, amounting to ~15% of the total oceanic subduction flux since 56 Ma during this continent-continent collision. Contamination of the mantle by direct input of continental crustal materials rather than crust-derived sediments may be more significant than previously thought and may be responsible for crustal geochemical anomalies in mantle-derived melts.