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.