Paleogeographic and paleotectonic evolution of the Himalayan Range as reflected by detrital modes of Tertiary sandstones and modern sands (Indus transect, India and Pakistan)

Abstract

Detrital modes of sandstones derived from the Himalayan suture belt record the history of the mountain range since initial collision between India and Asia, which began in latest Paleocene time. Tertiary clastic wedges deposited in fore-arc, foreland, and remnant-ocean basins, and exposed along the Indus transect from northernmost India to the Arabian Sea, represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision. Quartzose “continental-block” and feldspatholithic “magmatic-arc” sandstones were deposited, respectively, on the passive Indian (Tethys Himalayan succession) and active Asian (Indus Group) continental margins during Late Cretaceous–Paleocene time. Closure of the Neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the Indian plate during deposition of sediments dated at foraminiferal zones P6 (Pakistan) to P8 (India). Starting in early Eocene time (deposition of Chulung La Formation and Murree Supergroup), volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding “piggy-back” and foreland basins. Homogeneous petrographic composition within the Eocene–lower Miocene Murree Supergroup, with only slight progressive increase of detrital feldspars, suggests erosion of largely supracrustal rocks involved in thrusting in the north. In middle Miocene time, marked enrichment in medium- to high-grade metamorphic detritus in foreland sandstones (Siwalik Group) reflects rapid uplift of a warm wedge of Indian crust, which was carried southward along the Main Central thrust. This major paleogeographic change was recorded also by quartzolithic remnant-ocean turbidites, which were fed great distances along transverse fracture zones and later accreted in the coastal Makran subduction complex (Panjgur association and Makran Group). Recycled-orogen detritus derived from the elevated Himalayan chain is still accumulating today in the Indus fan. Enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid-crustal rocks along the core of the growing orogen.