Mechanical Links between Erosion and Metamorphism in Nanga Parbat, Pakistan Himalaya

The mechanics and petrological signature of a collisional mountain belt can be significantly influenced by topographic and erosional effects at the scale of large river gorges. The geomorphic influence on crustal scale processes arises from the effects of both stress localization due to existing topography, and also erosional removal of advected crustal mass. The shear stress concentration and normal stress amplification due to topographic gradients and loads divert strain away from existing topographic loads, while concentrating strain into topographic gaps. Efficient erosional removal of material within topographic gaps with widths of at least the thickness of the brittle crustal layer results in differential advection of crustal material. Concentrated exhumation within a gap leads to thermal thinning of the upper brittle layer of the crust, removing the highest strength part of the continental crust and significantly reducing the integrated crustal strength beneath the topographic gap. A rheological weak spot, triggered by efficient incision, grows in intensity as strain becomes increasingly concentrated within the weak region. The growth of extreme topography of an isolated massif requires that the process of creation of the massif is related to the weakening process and can result from the velocity pattern produced by erosionalrheological coupling. As a result, distinctive thermal/mechanical regions develop within the crust in response to these river-influenced velocity patterns and these regions impose a characteristic signature on material advecting through. The signal is one in which the region of highest topography is bracketed by two high-strain zones between which concentrated advection produces lozenges of sillimanite and dry melt stability approximately 20 kilometers beneath the summit. Above these lozenges is a thermal/mechanical boundary layer containing an active hydrothermal system driven by steep thermal, topographic and mechanical gradients. These thermal mechanical regions are fixed with respect to a crustal reference frame. Passage of rock beneath and through these regions under these conditions produces the distinctive petrology and structure of mantled gneiss domes and is recorded within the moving petrological reference frame. Such erosional-rheological coupling can explain the occurrence of some high-grade gneiss domes in ancient collisional belts as well as the presence of active metamorphic massifs at both ends of the Himalayan orogen.