Graphite reaction weakening of fault rocks, and uplift of the Annapurna Himal, central Nepal

Abstract

Hydrothermal graphite has been added to a regional-scale postmetamorphic fault zone in the High Himalaya. Fault rocks contain graphite veins and cement that locally constitute >50% of the rock. Individual graphite veins are as much as 1 cm wide, and are locally almost pure graphite, with minor muscovite and anatase. Hydrothermal graphite grain size (10-500 μm) is substantially greater than background metamorphosed organic matter (̃1 μm). The graphite is crystallographically disordered, and has maximum refl ectance of ̃3% with weak refl ection pleochroism. This degree of crystallinity is consistent with formation under upper subgreenschist facies conditions, similar to the metamorphic grade of the host rocks that are near the lower boundary of greenschist facies. The graphite formed during hydrothermal mixing of CO2 and CO4, both of which emanate from nearby active springs. The graphite deposition process in these lowgrade rocks contrasts with graphite remobilization and structurally controlled redeposition, without enrichment, during greenschist facies metamorphism, and graphite oxidation and removal during amphibolite facies metamorphism elsewhere in the Himalayan orogen. The reverse fault zone that hosts the graphite formed during tight folding of interlayered carbonate and metapelitic (shale) rocks, and the inhomogeneous nature of these rocks has focused strain into weaker zones that anastomose around stronger rocks, at scales from millimeters to tens of meters. Graphite was emplaced during this fold and fault deformation into weaker zones, further weakening and lubricating the fault zone in a hydrothermally driven feedback system. Differential uplift across the reaction-weakened graphitic fault zone has led to relative uplift of the Annapurna Himal and formation of a large (>2 km) and steep topographic step on the northern side of the high mountains. © 2014 Geological Society of America.