DIFFICULTIES IN CONSTRAINING CRUSTAL DISPLACEMENT PATHS AT NANGA PARBAT, WESTERN HIMALAYAN SYNTAXIS

Nanga Parbat - Haramosh Massif (NPHM) marks the western tip of the Himalaya arc; the syntaxis. In this region, crustal displacements are complicated by the interplay of the "normal" convergence of the Indian plate (~NNW) with the accumulation of the expanding arc as collision progresses (resulting in a strong partitioning of strain with the arc-parallel direction). At NPHM this results in very young plutonism, metamorphism and exhumation. Here, two steep, ~NNE trending, ~conjugate [pro- & retro- geometries], 30-50 km long, crustal-scale, thrust-displacement shear zones have allowed the ~25 km wide core of the massif to be uplifted in a pop-up mechanism but the 3D nature of the flow history is complex, in particular closer to the core. The conjugate shear zones are accompanied by large age gradients in cooling & plutonism that young inwards (i.e. towards the core), and focussed hydrothermal activity. The leading, or pro, thrust, located on the NW side of NPHM is better defined than the retro thrust (the SE side). Foliation orientation, lineation plunge, and sense-of-shear indicators are consistent throughout the pro thrust, and both shear zone margins are are recognisable by abrupt changes in deformation fabric. The retro thrust, however, broadens from a few to >10 km to the south (away from the core of the massif), accompanied by a gradual 180o change in fabric orientation (which the sense-of-shear indicators track), and only the external margin is clear. Yes, these two zones have allowed significant vertical displacement of the upper crust, but good spatial and temporal displacement constraints are tricky; the host rock is a pre-existing high strain fabric that is steeply dipping, ~N-S trending with N-plunging mineral, fold hinge & intersection lineations. It comprises a 10’s km thick shear zone that formed during the earlier main Himalayan N-S convergence. This records a broader range of deformation conditions than that subsequently preserved in the conjugate shear zones, creating difficulties in separating deformation events. Additionally, synkinematic plutonism, especially within the retro shear zone, was localised but recurring throughout the shear zone evolution, providing local foci into which strain was preferentially partitioned thereby further obscuring the finite deformation history