Active tectonics of the Pamir Mountains: kinematics and dynamics

The rules that govern the large-scale active deformation within the continents, and the ways inwhich tectonic motions are accommodated by slip on active faults during earthquakes, are first-order questions, with many of the models established from observations in the India-Eurasia collision zone. In the India/Asia collision, about 30mm/yr of convergence is distributed across a region that is several thousand kilometresin north-south extent[1 -3]. As a result, individual faultscan movevery slowly(order of <1 mm/yr) and have earthquake repeat times on the scale of thousands of years[3]. This means for most faults north of the Himalayan thrust,the timing, frequency and magnitude of past events is unknown.The seismic data record in Central Asia began in 1901[4]and has since sampled four major (>M7) earthquakes in Tajikistan, three of these in the Pamir[5]-[9]. However, geomorphic evidence of Pleistocene/Holocene ruptureselsewhereshows this record is incomplete[10]. ThePamir is alow relief plateau on the western syntaxis of the India-Eurasia collision,composed of a complex set of terranes accreted across the Mesozoic and Cenozoic[11]. It isa site of intracontinental subduction,at both its northern and southern margins [12][13],and is undergoing westward gravitational collapse into the neighbouring Tajik Basin[1].Despite the high north-south convergence velocities in GNSS data, the majority of strain happening in the Pamir appears to be east-west orientated[14][2][15].In addition, there appears to be more east-west strain in the western Pamir than the eastern Pamir[1]. The interior deformationis thought to be taken up by relatively few major faults, consisting of east-west trending thrust/strike-slip faults exploiting terrane sutures, cut bytwo north-south trending strike-slip/normal faultzones[16][17][10].This rich set of behaviours depend on external forces acting on the Pamir;gravitational forceson the topography;and the resistive forces of the faults in the brittle lithosphereand/orthe viscosity of the ductile lithosphere. They also depend on the inherited geologic structuresformed as the Pamir were built. To determine how these structures and forces drive the deformation,the deformation needs to be well characterized. This will be doneusing GNSS measurements of strain, measurements of the long-term average slip rates ofthe faults, and an understanding of their slip vectors. Studyingmajor earthquakeson these faultsgivesus the best chance of extracting their slip rates and slip vectors, and will provide information on magnitude and repeat time, pertinent to seismic hazard assessments. Since most of the major earthquakes will have occurred pre-1901, studyingrecent surface ruptures and dating offset markers (using radiocarbon or optically stimulated luminescence[18]) will be necessary, in addition to cataloginglarger scale offsets in existing geological maps.Aims and ObjectivesThe aim of this project is characterizing motion on major faults in the Pamir Plateau, and using this data to obtain insights into seismic hazard and tectonic kinematics in the region, both of which are poorly understood. I will then attempt to address why we observe this kinematic behavior, and how it depends on the region's geologic structures and forces.