STRATIGRAPHIC RESPONSE TO INITIAL INDIAN-ASIAN COLLISION: NEW PALEOMAGNETIC CONSTRAINTS FROM THE EARLY EOCENE GHAZIJ FORMATION, BALUCHISTAN PROVINCE, PAKISTAN

The initial collision of India with Asia has been implicated as a potential causal mechanism in various early Paleogene climatic and biotic changes. Long-term warming at this time may have been linked to changes in the fluxes of organic carbon and volcanic aerosols due to the uplift of passive margin sediments and cessation of explosive volcanism. Short-term warming at the Paleocene/Eocene boundary may have been triggered by formation of warm saline deep water in remnant Neo-Tethyan seas and subsequent destabilization of methane gas hydrates. The earliest Eocene holarctic mammalian origination event may have resulted from the post-collision dispersal of endemic Indian taxa. These hypotheses have been difficult to test due to the lack of a good early-Paleogene stratigraphic record from the leading edge of the Indian subcontinent. Most of that record is now obscured by the deformation and metamorphism of the high Himalaya. The early Eocene Ghazij Formation of northwest Pakistan is a thick, lithologically heterogenous sedimentary package that records a clear transition from shallow marine to continental environments. Although the Ghazij lies near the suture zone, it has undergone no metamorphism and relatively little deformation. New paleomagnetic data from the Ghazij indicate that these sediments were deposited at ~5 N. We compared these results to expected paleolatitude estimates obtained from three independent sources of information: 1) euler poles derived from the Indian Ocean sea floor spreading record, 2) APWP for the Indian plate derived from paleomagnetic analysis of Indian Ocean deep sea cores, and 3) APWP derived from other paleomagnetic data rotated into the Indian plate reference frame. Results show that Ghazij sediments were deposited just before or during a significant decrease in convergence rate between India and Asia. This suggests that these sediments and the coincident transition from marine to continental environments represent the local stratigraphic response to initial Indian-Asian continental contact. Mammalian fossil assemblages suggest that considerable faunal interchange had already been completed by upper Ghazij time.