PROVENANCE OF THE LOWER TERTIARY MURREE REDBEDS (HAZARA-KASHMIR SYNTAXIS, PAKISTAN) AND INITIAL RISING OF THE HIMALAYAS

The Murree Supergroup of northern Pakistan is represented by an over 6 km thick succession of deltaic redbeds and intercalated impure foraminiferal limestones of latest Paleocene to Middle Eocene (Ilerdian to Lutetian) age, cropping out in the northern part of the Hazara Syntaxis, and by redbeds of younger age (early Middle Eocene to Early Miocene) cropping out in the southern part of the Syntaxis (Rawalpindi area). The quartzolithic composition of the Murree redbeds testifies to a ''collision orogen'' provenance. Detritus was derived from a suture belt including thrust sheets of metasedimentary and subordinately sedimentary rocks, volcanic or volcaniclastic rocks, and ophiolites. Phyllite rock fragments are most common in the Ilerdian sandstones, whereas volcanic and carbonate rock fragments are more abundant in the Lutetian sandstones. Minor quantities of chert and serpentineschist, indicating contribution from uplifted subduction complex sources, as well as siltstone, shale and limestone grains, are invariably present. Detrital modes of sandstones and southwestward progradation of Tertiary clastic wedges both testify to provenance from the proto-Himalayan chain located to the north, and uplifted since the very first stages of the India/Asia continental collision at the end of the Paleocene. The coeval Chulung La redbeds of the Tethys Himalaya instead, consisting of volcanic and subordinate ophiolitic detritus, testify to exclusive provenance from the obducting Trans-Himalayan arc-trench system. This major petrographic difference may be accounted for by the different structural setting of the Chulung La ''piggy-back'' and Murree foreland basins. These two distinct collisional basins have been separated probably since the onset of collision by a fold-thrust belt, beginning to rise in the position occupied today by the High and Lesser Himalayan structural domains. Throughout the Murree Supergroup, main petrographic parameters do not vary greatly, and volcanic, sedimentary, low-grade metasedimentary and ophiolitic detritus persisted until the Early Miocene, indicating slow progressive growth of the chain. Only during the Middle Miocene, when the highly metamorphosed rocks of the High Himalaya were carried southward along the Main Central Thrust (MCT), the mountain range began to rise to dramatic heights, and huge amounts of detritus started to feed the Siwalik foreland basin sandstones and the remnant ocean turbidites of the Indus and Bengal Fans.