Gerhard Wiesmayr

Kinematic dilatancy effects on orogenic extrusion

Abstract We undertake kinematic modelling to explore the role of volume increase in a slab extruding froman orogenic wedge with constant or decreasing slab width. Using a dilatancy term, we modify the velocity gradient tensor dependent on the stretching-rate factor, kinematic dilatancy and vorticity number. We use this to explore the previously largely ignored role of volume change in kinematic evolution of extrusive flow, considering area change for non-isochoric flow types with no deformation in the intermediate direction. By keeping individual parameters constant for geologically simple scenarios (e.g. finite strain, steady-state flow) we examine the interdependence of the reciprocal parameters (kinematic vorticity and dilatancy number) and note model situations where degrees of freedom are limited. These interdependent parameters thereby provide a set of rules for integrating and modelling real field data. In particular we observe that for extrusion flow with a constant slab (or ‘channel’) width, degrees of freedom in kinematic vorticity and volume change at given finite strains are very restricted. We compare scenarios of low and high strain and low and high volume change on anatexis (related to partial melting of fertile sedimentary rocks and release of water upon crystallization) for different parts of the Himalaya.

Evidence for steady fault-accommodated strain in the High Himalaya: progressive fault rotation of the southern Tibet detachment system in NW Bhutan

Abstract We present fault analyses from the exhumed middle crustal slab of the High Himalaya in eastern Lunana in NW Bhutan. Fault planes from within two-mica, tourma-line-bearing leucogranites, leucogranitic rocks and migmatites indicate a complex brittle fault pattern with two distinct fault groups. A first group of faults (D1) characterized by chlorite, quartz and tourmaline slickenfibres is mainly defined by steeply SSE-dipping oblique-slip normal faults, and by shallowly NNW-dipping normal faults. A second, younger group of faults (D2) characterized by cataclasis products comprises strike-slip faults displaying conjugate patterns and E- and W-dipping conjugate normal faults, all which indicate E-W extension. Cross-cutting relationships amongst the D1 fault group demonstrate that progressively steeper members of the fault group become younger within the NNW-dipping faults and become older within the SSE-dipping faults. These are all post-dated by the D2 fault group. The D1 fault group indicates that the slab experienced ongoing NNW-SSE extension (i.e. flow) via brittle fault accommodation, contemporaneous with fault rotation. This may reflect rotation of the entire upper orogen due to movement over deeply located major ramp structures formed by out-of-sequence thrusting (Kakhtang Thrust) within the High Himalayan Slab of the Bhutan Himalaya.