Eastward escape of Tibet facilitated by mid-crustal upwelling where conjugate faults meet at its western end

Tuesday, 16 December 2014
Nicholas J Van Buer1, Oliver E Jagoutz1, Rajeev Upadhyay2 and Marcel Guillong3, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)Kumaun University, Center for Advanced Study in Geology, Nainital, India, (3)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland
Models of deformation in the India-Asia collision vary between those that emphasize continuity of deformation, distributed horizontally throughout the Tibetan plateau and/or vertically through a weak crust, and those that emphasize the importance of lithospheric-scale faults translating relatively rigid blocks. These models make different predictions for what should happen where the conjugate faults bounding the eastward-escaping Tibetan plateau meet. Our new field and remotely sensed data suggest that the right-lateral Karakoram Fault and the conjugate left-lateral Longmu-Gozha Co Fault, an extension of the Altyn Tagh Fault, are linked by a large-offset (> 40 km), east-dipping, listric normal fault system we refer to as the Angmong Fault. This fault system is marked by kilometer-high, faceted topographic scarps, suggesting recent activity. There is also a huge jump in metamorphic grade across this fault, which separates nearly unmetamorphosed strata with open folds in the hanging wall from upper-amphibolite-grade rocks with tight to isoclinal folds and subhorizontal mineral lineation in the footwall, which crop out in a triangular wedge trailing ~40 km NW of the Angmong Fault. Pseudosection modeling of late-syn-deformational porphyroclast rims and matrix phases suggests footwall pressures around 640 ± 100 MPa, for rocks up to at least 15 km behind the Angmong Fault, in contrast to hanging wall pressures < 100 MPa. LA-ICP-MS zircon U-Pb geochronology on an extensive network of narrow, variably deformed leucocratic dikes that pervade the footwall gives intrusion ages as young as 13.7±0.2 Ma, suggesting that footwall rocks remained hot and were not exhumed until after this time. Our mapping suggests that the ENE-trending Longmu-Gozha Co Fault terminates into the N-S trending Angmong Fault, and all the strike-slip displacement of the former is absorbed by normal displacement of the latter. The Angmong detachment only displaces the upper crust of Tibet eastwards, leaving the ductile mid-crust exhumed in its wake. This suggests that the Longmu-Gozha Co Fault also only displaces the upper crust. Because geophysical data and xenolith sampling suggest a hot, weak mid-crustal layer extends beneath most or all of the Tibetan plateau, similar mid-crustal decoupling likely occurs throughout Tibet.