DIFFERENTIAL UPLIFT AND DRAINAGE BASIN DYNAMICS ON NANGA PARBAT, PAKISTAN

Tectonic processes (rapid uplift of the pop-up structure between the Raikot and Stak faults) shaping the geodynamics of Nanga Parbat massif have generated significant erosion, incision and denudation of the massif. The differential-uplift dynamics of the tectonic-aneurysm, or pop-up tectonic model, coupled with high-elevation geomorphic processes of mass movement, glaciation, river erosion, and catastrophic breakout floods, have generated a landscape that imprints the tectonic and varied geomorphic signatures. Differential uplift accounts for substantial variation in bedrock incision rates, overall denudation measurements, and drainage-basin development on the massif. On the north and northwest side of the massif, steep-gradient rivers drain from high-elevation glaciers. The apparent differential uplift there has produced streams with increasing steepness in the lower parts of the drainage basin rather than the upper parts. These steep-gradient river systems flow directly into the low-altitude Indus master drainage of the region, consequently generate high-energy discharge conditions, move voluminous amounts of sediment off the massif and induce substantial incision along their flow paths. On the south and southeastern slopes of the massif far from the Indus control, and along the Stak-fault hinge, or breaking point of the pop-up aneurysm, gradients are more gentle and stream power and flow shear stresses are lower, causing reduced fluvial erosion and lower denudation rates. Erosional disparities between fluvial and non-fluvial processes are apparent on the south side of the massif as gravity-induced, mass-movement events suggest denudation rates that are an order of magnitude higher than the rates determined from the fluvial processes of the Rupal River in the same drainage basin. More subtle tectonic influences may occur in the general geomorphology of some of the drainage basins that drain the massif the south and southeast parts of the massif. Rupal, Guricot and Sachen basins there may show drainage basin asymmetry predominantly to the south, which suggests preferred stream migration in this direction, possibly in response to south and southeastward ground tilting.