SUPERCEDENT, RIVER-COURSE EVOLUTION WITH INITIATION OF A TECTONIC ANEURYSM

The initiation of the Nanga Parbat tectonic aneurysm up through overlying Kohistan-Ladakh island arc rocks in the western Himalaya ~12 myr ago was suggested to have been initiated by the capture of the west-flowing, Indus River. Prior headward erosion by a steeper, south flowing river, which was depositing low-discharge sediment bedforms in the Chinji Siwaliks in the foothills, seems to have allowed diversion of the Indus south at this time. The ancestral Chinji River may have incised more easily along part of the Main Mantle Thrust southwest of Nanga Parbat. This capture and diversion of the superposed Indus to the south produced high-discharge bedforms in the overlying Nagri Siwaliks, and new blue-green hornblende from erosion of the island-arc mafics as Nanga Parbat began to be unroofed. Such capture events are known to typically produce a strong elbow directly downstream from an anomalously steep section. Such a pronounced condition prevails across the Nanga Parbat-Haramosh massif (NPHM) uplift, but whether the steep gradient of the Indus could have existed continuously for such a long time since capture is doubtful unless ongoing uplift is assumed. Instead, massive exhumation caused by the superposition and capture seem to have initiated the tectonic aneurysm, which led to a new antecedence of the Indus as the NPHM uplifted. Such combined superposition/antecedence, or supercedence, may have allowed maintenance of the steep gradient throughout the Quaternary, while the elbow of capture was further displaced by active faulting along the margins of the uplift. In addition, some radial and annular drainage effects also resulted from the growing uplift and glacial erosion during Pleistocene. At the east end of the Himalaya the Tsangpo River was similarly captured by the Brahmaputra to produce the Namche Barwa – Gyala Pelri uplift. Between the two peaks and across the course of the steep Tsangpo, the unusual Hidden Falls (21 m) and Rainbow Falls (33 m) attest to probable ongoing tectonism in that major high-discharge rivers do not long maintain such pronounced gradient knickpoints unless uplift is active.