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 Be-10), in combin ation with measured heights of fault scarps that cut three abandoned f ans, imply a slip rate of less than similar to 2 mm/yr on the bounding thrust fault along a segment of the southern flank of the Tien Shan o f central Asia, This rate, somewhat lower than those for the segment f arther west and for the northern margin of the belt, implies that the distribution of shortening across the Ten Shan must change markedly al ong the belt. The date of abandonment of the smallest of the three fan s 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 re gions have formed since the last glacial maximum, In this study we hav e assessed the relationship between cosmogenic nuclide content of indi vidual clasts and the time since their deposition on a surface, Cosmog enic nuclide accumulation represents the integrated total surface expo sure of rocks in source regions during transport and since deposition, Postdepositional processes (erosion, burial, bioturbation, and cryotu rbation), all of which decrease the quantity of cosmogenic nuclides pr oduced 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 Be-10 concentrations in material from active strea mbeds and at depth below one of the fan Surfaces indicate that exposur e prior to deposition onto fan surfaces was minimal (<similar to 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 l ower bounds on the time since initiation of scarp preservation, the as sociated slip rates should represent upper limits.