Paleomagnetic evidence for clockwise rotation and tilting in the eastern Tethyan Himalaya (SE Tibet): Implications for the Miocene tectonic evolution of the NE Himalaya

Crustal movement around and away from the Namche Barwa syntaxis is indicated in the Asian velocity field inferred from GPS data and Quaternary fault slip rates. Nevertheless, there is a limited field-based control on the rotational history of the north-eastern Himalayan arc. Exploring the poly-phase nature of deformation, within the Cretaceous diorite dykes and their host-rock (Triassic flysch), in the eastern Tethyan Himalaya (90°-92°E), combined with new remote sensing data and existing thermo-geochronological data, allow us to unravel the kinematic relationship between paleomagnetic remanence vectors and the deformation phases. Decay at 325 °C in high temperature susceptibility curves and in the thermal demagnetization of the SIRM indicate that the characteristic remanent magnetization in the Cretaceous diorite dykes is carried by pyrrhotite. The pyrrhotite component unblocks at 280-350 °C, revealing normal and reverse polarities. It is of post-folding origin with a mean remanence direction of 019°/28° and 001°/20° in the eastern (Qonggyai valley) and western (Nagarze) part of the sampling area, respectively. The ~22 Ma K-Ar age of the last metamorphic event support that the remanence is post-Eohimalayan folding and likely of thermoremanent or thermo-chemical origin. Comparison of the declination with respect to the Early-Miocene reference direction, yields a trend from no apparent rotation in the west to 20° clockwise rotation in the east with respect to the stable Indian plate. This result can be kinematically related to the Middle to Late-Miocene strain partitioning between far-field southeast extrusion of SE Tibet and near-field strike-slip faulting and E-W extension. Furthermore the observed pattern of tilting around horizontal axis may reflect concealed North Himalayan doming.