Rifting and strike-slip shear in central Tibet and the geometry, age and kinematics of upper crustal extension in Tibet
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The youngest deformation structures on the Tibet Plateau are about NNE-trending grabens. We first combine remote-sensing structural and geomorphological studies with structural field observations and literature seismological data to study the Muga Purou rift that stretches at c. 86E across central Tibet and highlight a complex deformation field. ENE-striking faults are dominated by sinistral strike-slip motion; NNE-striking faults have normal kinematics and outline a right-stepping en-echelon array of grabens, also suggesting sinistral strike-slip; along NW-striking fault sets, the arrangement of grabens may indicate a dextral strike-slip component. Thus, in central Tibet, rifts comprise mostly grabens connected to strike-slip fault zones or are arranged en-echelon to accommodate sinistral wrenching; overall strain geometry is constrictional, in which NNE-SSW and subvertical shortening is balanced by WNW-ESE extension. The overwhelmingly shallow earthquakes only locally outline active faults; clusters seem to trace linkage or propagation zones of know structures. The earthquake pattern, the neotectonic mapping, and the local fault-slip analyses emphasize a distributed, heterogeneous pattern of deformation withinadevelopingregionalstructureandindicate that strain concentrationisweakintheuppermost crust of central Tibet. Thus, the geometry of neotectonic deformation is different from that in southern Tibet. Next, we use structural and palaeomagnetic data along the Zagaya section of southern central Tibet to outline significant block rotation and sinistral strike-slip SE of the Muga Purou rift. Our analysis supports earlier interpretations of reactivation of the Bangong-Nujiang suture as a neotectonic strike-slip belt. Then, we review the existing and provide new geo-chronology on the onset of neotectonic deformation in Tibet and suggest that the currently active neotectonic deformation started c. 5 Ma ago. It was preceded by c. north-south shortening and c. east-west lengthening within a regime that comprises strike-slip and low-angle normal faults; these were active at c. 18-7 Ma. The c. east-striking, sinistral Damxung shear zone and the c. NE-trending Nyainqentanghla sinistral-normal detachment allow speculations about the nature of this deformation: the ductile, low-angle detachments may be part of or connect to a mid-crustal dcollement layer in which the strike-slip zones root; they may be unrelated to crustal extension. Finally, we propose a kinematic model that traces neotectonic particle flow across Tibet and specu-late on the origin of structural differences in southern and central Tibet. Particles accelerateand move eastwards from western Tibet. Flow linesfirst divergeasthe plateauis widening.Atc.928E, theflow lines start to converge and particles accelerate; this area is characterized by the appearance of the major though-going strike-slip faults of eastern-central Tibet. The flow lines turn southeastward and converge most between the Assam-Namche Barwa and Gongha syntaxes; here the particles reach their highest velocity. The flow lines diverge southof the cord between the syntaxes. This neo-tectonic kinematic pattern correlates well with the decade-long velocity field derived from GPS-geodesy. The difference between the structural geometries of the rifts in central and southern Tibet may be an effect of the basal shear associated with the subduction of the Indian plate. The boundary between the nearly pure extensional province of the southern Tibet and the strike-slip and normal faulting one of central Tibet runs obliquely across the Lhasa block. Published P-wave tomographic imaging showed that the distance over which Indian lithosphere has thrust under Tibet decreases from west to east; this suggests that the distinct spatial variation in the mantle structure along the collision zone is responsible for the surface distribution of rift structures in Tibet. The Geological Society of London 2011.
