PALEOCENE-EARLY EOCENE HIGH ELEVATION FO THE LINZIZONG ARC IMPLIES LARGE-SCALE SUBDUCTION OF CONTINENTAL CRUST DURING INDIA-ASIA COLLISION

The mass balance of continental crust during the Paleocene-Eocene India-Asia collision has been an unsettled debate for over two decades. We reexamine crustal mass balance in the collisional system using the first robust geochemical paleo-elevation estimate of the pre- to syn-collisional Linzizong arc (~4100 ± 550 meters) as a constraint on Paleocene crustal thickness of the Lhasa block. We reconstruct the ~55 Ma paleo-elevation of the Penbo basin by coupling carbonate-derived oxygen stable isotope measurements (δ¹⁸O_c) with paleotemperatures derived from the Δ₄₇-'clumped' isotope paleothermometer (T(Δ₄₇)). We perform numerical calculations of collisional crustal mass balance using estimates with uncertainties of the pre-collisional elevation and corresponding crustal thickness distribution, and the recently reconsidered diachronous collisional age of India-Eurasia. We find that ~50% of the collision-related crustal mass cannot be accounted for in the excess crustal thickness (in Himalaya, Tibet, and adjacent Asia), southeast Asian tectonic escape, and exported eroded sediments. The unaccounted for mass of Indian continental crust must have been subducted during the India-Asia collision. Evidence of subduction of continental crust to this extent is unprecedented. 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.