APPROACH TO STEADY STATE IN THE INDO-ASIAN COLLISION: A GEOMORPHIC-THERMOCHRONOLOGIC PERSPECTIVE IN THE TIEN SHAN

Recognition of steady state in an orogen commonly requires being able to demonstrate persistence of key attributes, such as thermal state or topographic characteristics. Persistence of a distribution of observable data, such as cooling ages or topographic relief, is usually assessed with respect to time, but in conditions where a range or orogen is propagating laterally, spatial persistence of variables can also indicate steady state for a particular region. Because the time to achieve steady state can vary (longer to achieve a thermal steady state than a topographic one), attributes assignable to different types of steady state may not be present concurrently in any range. For thermochronologic studies, steady state can be inferred if detrital records of cooling-age distributions through time mimic modern ones from the same catchment. For geomorphic studies, it is commonly impractical to determine past distributions of parameters such as mean elevation, relief, or drainage density. In such situations, the approach to steady state may be assessed using the persistence of geomorphic attributes along a range.

We exploit the geomorphology and thermochronology of the Kyrgyz Tien Shan to explore the approach to or attainment of steady state. The existence of a regional, pre-deformational erosion surface provides an invaluable geomorphic and structural marker. Where present, it defines the detailed pattern of structural deformation. Where absent, it commonly indicates that >0.5 km of erosion has occurred, in addition to stripping of any overlying Cenozoic strata (often >1 km). The central Kyrgyz Tien Shan are demonstrably in a pre-steady state condition. In the northern Tien Shan, comparisons of bedrock and detrital apatite fission-track data in the central Kyrgyz Range indicate that a thermal steady state may have been attained during the past 2 My. At least 4 km of incision has occurred there. By examining topographic and geomorphic attributes along the length of the range, from where the erosion surface is still partially preserved in the east to the zone of likely steady state, we define changes in these attributes that depict the topographic evolution of the range toward steady state and indicate the role played by glaciers and fluvial channels in transforming the landscape.