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

Abstract:

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.