THE SEDIMENTARY RECORD OF THE TIMING OF INDIA-ASIA COLLISION: AN EVALUATION OF NEW AND EXISTING CONSTRAINTS

The age of India-Asia collision is quoted at ~65-34 Ma and the degree of dichroneity is disputed. We evaluate constraints from the sediment record where a minimum age to collision has been determined from 1) time of cessation of marine facies, 2) first evidence of Asian detritus on the Indian plate and 3) first evidence of mixed Indian-Asian detritus in the sediment record.

In the Indus molasse (Ladakh) cessation of marine facies is dated at 50.5 Ma (Green et al. ‘08), with the underlying Chogdo Fm considered to show first evidence of mixed Indian-Asian provenance, and to lie in sedimentary contact with the underlying Indian plate (Clift et al ‘01, ‘02), thus constraining collision at >50.5 Ma. However, our mapping and provenance analyses show there is no unequivocal evidence of Indian-derived material in the Chogdo Fm, and that the Chogdo Fm does not lie in sedimentary contact with the Indian plate (Henderson et al., in review). Thus we question the timing of Indian-Asian collision based on these evidences.

In the Tethyan strata (Ladakh), Indian plate limestones of the Paleocene Dibling Fm are overlain by the youngest marine facies of the region - the Kong Fm, and fluvio-deltaic Chulung La Fm. The age of the Kong and Chulung La Fms is given as P5/6 (Fuchs & Willems ‘90) or P8 (Garzanti et al ‘87). Provenance is considered to be either Indian plate ophiolites (Fuchs & Willems ‘90) or Asian TransHimalaya (Garzanti et al ‘87; Critelli & Garzanti ‘94). Our biostratigraphic dating of the Kong Fm gives an age of 53-54 Ma and our provenance data show Asian derivation. Thus collision is constrained by arrival of Asian detritus on the Indian plate by 54 Ma.

In Tingri, Tibet, Indian plate limestones of the Zephure Shan Fm extend to early Eocene, overlain by the marine Pengqu Fm. Age of the youngest marine facies is disputed: dated at 34 Ma (Wang et al. ‘02) or 50 Ma (Zhu et al ’05). Our biostratigraphic dating of the Pengqu Fm gives an age of 50.6-53.5 Ma (Najman et al, in press) and our provenance data indicate Asian derivation, thus indicating that contact between India and Asia had occurred by this time.

We conclude that although the Indus Molasse does not provide constraint to the timing of India-Asia collision as previously thought, data from the Tethyan strata show that collision occurred by 54 Ma in the west, with only extremely limited, if any diachroneity eastward.