Estimation of slip rates in the southern Tien Shan using cosmic ray exposure dates of abandoned alluvial fans

Cosmic ray exposure dates (based on in situ-produced ¹⁰Be), in combination with measured heights of fault scarps that cut three abandoned fans, imply a slip rate of less than ∼2 mm/yr on the bounding thrust fault along a segment of the southern flank of the Tien Shan of central Asia. This rate, somewhat lower than those for the segment farther west and for the northern margin of the belt, implies that the distribution of shortening across the Tien Shan must change markedly along the belt. The date of abandonment of the smallest of the three fans is consistent with fan formation since the last glacial maximum, but the other two fans appear to have been formed and abandoned prior to that time. These and other results suggest that advances and retreats of alpine glaciers may not be synchronized with continental ice sheets and that not all prominent geomorphological features in arid Asian regions have formed since the last glacial maximum. In this study we have assessed the relationship between cosmogenic nuclide content of individual clasts and the time since their deposition on a surface. Cosmogenic nuclide accumulation represents the integrated total surface exposure of rocks in source regions during transport and since deposition. Post-depositional processes (erosion, burial, bioturbation, and cryoturbation), all of which decrease the quantity of cosmogenic nuclides produced in a rock in a given time and hence the apparent exposure age, also cause significant scatter among apparent ages of samples from the surfaces. Very low ¹⁰Be concentrations in material from active streambeds and at depth below one of the fan surfaces indicate that exposure prior to deposition onto fan surfaces was minimal (<∼2000 yr); it follows that the dates for individual clasts are lower limits for the time since abandonment of the fans. Because such dates place lower bounds on the time since initiation of scarp preservation, the associated slip rates should represent upper limits.