THE TRANSITION FROM TETHYS TO THE HIMALAYA AS RECORDED IN NORTHWEST PAKISTAN

Early Cenozoic sedimentary rocks exposed in the Kohat Plateau of north western Pakistan record tectonic closure of the Tethys sea and develop ment of a restricted marine basin that formed during Himalayan collisi on between India, Asia, and a series of microplates. In the Paleocene, initial subsidence of the basin was caused by the downward deflection of the Indian plate in response to loading of the Asian plate. The we stern margin of the early Eocene basin was dominated by deposition of shale, sandstone, and conglomerate derived from microplates located to the north and west of the Indian continental margin. The eastern marg in of the basin was a carbonate shelf and sabkha flat, Salt deposition occurred subaqueously in the central parts of the basin. In the late early Eocene, redbeds derived from the northwest were deposited by a f luvial and/or deltaic system. This influx of elastic sediments marks t he earliest record of terrestrial foreland-basin deposition in northwe st Pakistan. During the middle Eocene, the basin was reflooded and a c arbonate shelf developed. Relative sea-level rise may reflect subsiden ce of the Indian plate in response to continued crustal loading in the Himalayan suture zone. Uplift and erosion occurred between the late E ocene and Miocene, possibly related to a peripheral bulge south of the Himalayan suture zone. The main phase of Himalayan foreland-basin flu vial deposition began in the Miocene. Renewed uplift related to final collision of India and Afghanistan during the Pliocene is recorded by a thick sequence of conglomerate in the western Kohat Plateau. Correla tion with Eocene sedimentary rocks from southern Pakistan to northern India delineates the depositional systems that developed as the Tethys sea closed during Himalayan convergence.