Active faults in zones of continental collision: Quaternary deformation in the Pamir-Tien Shan region, central Asia

PI: Ramón Arrowsmith Department of Geology, Arizona State University, Tempe, AZ 85287-1404

The crustal shortening that drives the development of a mountain belt is not simply accommodated by uniform thickening of continental crust, nor by subduction of one block underneath the other. Instead, the deformation is typically localized into separate zones in which activity varies over time and space. A consequence of such variability is the complex structural relationship and superposition of structural styles preserved in all mountain belts, which poses a major problem in the assessment of the importance of individual orogenic processes. Because of the accumulation of deformation in mountain building, datasets spanning different timescales will provide distinct and sometimes apparently contradictory information about the tectonic activity in the orogen. Observations of the Quaternary deformation preserved in offsets of landforms and young deposits allow one to bridge the gap between the limited historic timescales of geodesy and earthquake seismology and the integrated longer timescale of the geologic record of orogeny. Furthermore, if the contribution to the regional deformation rate by recurrent earthquakes can be determined and compared to geologic estimates of deformation magnitude, the importance of earthquake-related deformation in the absorption of continental collision can be evaluated. Thus, by investigating active deformation in combination with long-term deformation in the Quaternary, space is substituted for time, and the cumulative effect of deformation and its role in longer-term mountain growth can be investigated. In order to illustrate how convergence between two continental plates is absorbed and differentiated, and how mountain fronts at the leading edge of a collision zone are seismotectonically segmented and to provide data to help constrain broader questions regarding the accommodation of India-Eurasia convergence, the annhilation of intermontane basins, and the relatively recent construction of mountain ranges in the Himalayan orogen, an investigation of the Quaternary deformation in the Pamir-Tien Shan convergence zone is propose. Planned work includes: compile published and unpublished geologic data for the Alai Valley region, identify the active faults within the region by field observations of geology and geomorphology and interpretation of remotely sensed imagery, determine their slip rates to various degrees of precision, and define paleoseismic rupture timing and extent. Closely related and coordinated studies by German colleagues at the Universities of Potsdam, Würzburg, and Tübingen will define a Quaternary glacial chronology using cosmogenic and radiocarbon age control and determine the long term exhumation rate for the Trans Alai Range by apatite fission track analysis. Geologic and geomorphic observations of the Trans Alai Range front illustrate that this relatively small sector in a region of continental-scale deformation provides the opportunity to define how convergence between two continental plates is absorbed and differentiated and to illustrate the style of seismotectonic segmentation. Preliminary results show that the central part of the collision zone is characterized be a well-developed thrust belt with slip localized along a narrow zone of faulting. Transitions to the peripheral portions of the range front are accommodated by complex areas in which convergence is kinematically transferred via obliquely slipping transfer faults and thrust systems. This large-scale structural arrangement is also reflected in geomorphic zonation defined by systematic landform responses to the deformation, such as uplifted pediment and terrace surfaces, and areas of important landsliding, and it is further documented in the distribution of historic earthquakes. The activity of faults in the center of this zone of continental collision will be defined with geologic and geomorphic mapping and age control for landforms and deposits. These data will be interpreted using three dimensional mechanical models for fault interaction and will complement recently completed, ongoing, and planned research to the northeast and southwest.