PROJECT SUMMARY
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
ramon.arrowsmith@asu.edu
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 Planned work includes: compile published and unpublished geologic data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.