E-W extension at 19 Ma in the Kung Co area, S. Tibet: Evidence for contemporaneous E-W and N-S extension in the Himalayan orogen

Active faulting in southern Tibet consists of N-S trending extensional faults and linked strike-slip faults, which are an expression of regional E-W extension. A second type of extensional deformation associated with N-S movement is also recognized. This extension is expressed as a series of shear zones and normal faults in the High Himalayas - the Southern Tibetan Detachment System - and mid-crustal rocks exposed in metamorphic domes. Reported constraints on the timing of movements associated with these two phases of extension indicate that N-S extension predates the onset of E-W extension. However, only a few studies have provided clear constraints on the timing of E-W extension and the extent to which the two kinematically distinct domains of extension were contemporaneous is unclear. The Kung Co fault in southern Tibet is a major N-S trending normal fault The associated E-W extension is locally expressed as high-strain ductile deformation. Both field and microstructural observations show that this deformation occurred synchronously with granite intrusion. Previously reported U-Pb zircon dating shows granite crystallization took place at around 19 Ma, implying that ductile E-W extension in the Kung Co area was also active at around 19 Ma. This is the oldest documented example of E-W extension in Tibet and shows that E-W extension was at least locally contemporaneous with N-S extension to the south at shallower crustal levels. Simultaneous mid-crustal N-S extension and upper crustal E-W extension may be explained by southward flow of Tibetan crust with a divergent radial component. (C) 2011 Published by Elsevier B.V.