Co-seismic surface ruptures in Qiangtang Terrane: Insight into Late Cenozoic deformation of central Tibet

Recently large earthquakes mostly occurred at the plateau margins along major lithospheric faults. However, a co-seismic rupture has been discovered in central Tibet along the Burgar Co Fault (BGCF), a 150 km-long northwest-trending right-lateral strike-slip fault within the hinterland of the Qiangtang terrane, central Tibet. The activity of the BGCF is marked by a 50 km-long fresh surface rupture from 83 degrees 38'57"E, 33 degrees 58'54"N to 84 degrees 09'50"E, 33 degrees 47'42"N, restrained by a bend with a 17 counter-clockwise angle. Geomorphic offsets measured in the field and from high-resolution satellite images indicate the lateral displacements cluster at 1.4 +/- 0.1 m, 2.7 +/- 0.4 m, 5.4 +/- 0.6 m, 9.2 +/- 0.8 m and 12.5 +/- 1 m. The smallest value (average 1.4 +/- 0.1 m, maximum 1.8 m) is defined as the most recent co-seismic offset of a magnitude Mw = 7 earthquake. Other clusters are interpreted as the cumulative events which demonstrate that earthquakes frequently took place along the BGCF. The seismic behavior of the BGCF together with widespread sinuous north-trending grabens and NE, NW strike slip faults around western and central Qiangtang indicates that the most recent active structures have deformed scattered in the internal part of this terrane. Additionally, the conjugate strike slip fault systems along the Bangong-Nujiang Suture define an intangible southern boundary, altogether implying that the Qiangtang terrane has undergone significant deformation thus can no longer be treated as an integral block. Finally, compared with the continental slip in eastern Qiangtang, an alternative geodynamic model for Qiangtang is inferred: a weak channel flow decoupling the upper and middle-lower crust which facilitates the southeastwards extrusion as an entire block in the east, and continuous deformation as scattered small crustal wedges without expansive upper and middle-lower crust decoupling underground in the western and central areas. The Qiangtang terrane serves as the dominant place for extensive material to migrate eastward, accommodating simultaneous E-W extension and N-S compression.