E–W extension and block rotation of the southeastern Tibet: Unravelling late deformation stages in the eastern Himalayas (NW Bhutan) by means of pyrrhotite remanences

In the Himalayan chain the collision of India into Eurasia has produced some of the most complex crustal interactions along the Himalayan–Alpine Orogen. In NW Bhutan, middle to late Miocene deformation has been partitioned between conjugate strike-slip faulting, E–W extension along the Yadong-Gulu graben and kilometre-scale folding. To better understand the late deformation stages and their implications for the evolution of the eastern Himalayas, the palaeomagnetism in the erosional remnant of the Tethyan Himalayan rocks outcropping in NW Bhutan has been studied. Their position to the south of the trace of the inner South Tibetan Detachment, to the south of the Tibetan Plateau offers a unique possibility to study the Tertiary rotation of the Himalayas. Pyrrhotite is the carrier of the characteristic magnetisation based on 270–325 °C unblocking temperatures. The age of the remanence is ca. 13 Ma indicated by illite 40K/40Ar cooling ages and a negative fold test. Small circle intersection method applied to the pyrrhotite components shows a ca. 32° clockwise rotation with respect to stable India since 13 Ma. We suggest that this clockwise rotation is related to strain partitioning between NE-directed shortening, sinistral-slip along the Lingshi fault, and east–west extension. This represents a field-based explanation and a minimum onset age for present-day eastward motion of the upper-crust of SE-Tibet and NE-Himalayas.