Geologic Versus Geodetic Rates of Convergence in the Southwestern Tien Shan, China

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Research in the southern Tien Shan is defining geologic strain gradients, actual patterns and rates of long-term deformation at time scales ranging from 10,000 -10,000,000 yr, and changes in those rates through time. The research area is characterized by two contrasting styles of structural deformation within an existing geodetic network. In the northern Tarim basin, north-vergent detachment folds presently deform the 9 km of Cenozoic basin fill. In a more proximal position farther north, south-vergent thrust faults cut purported Quaternary conglomerates and have carried their bedrock hangingwalls 15 km southward along very low-angle (perhaps at the surface) detachments. Field data being gathered in this region permit tests of the following hypotheses:1. Geodetic strain rates in the southern Tien Shan are matched by Quaternary rates of shortening in the southernmost detachment folds (northern Tarim basin). 2. Despite striking changes in both style and vergence of Late Cenozoic deform-ation across the Tien Shan margin, rates of shortening since approximately 5 My have remained steady.3. Even though the detachment folds are north-vergent, the more northerly ones formed earliest.If the first hypothesis is true, it suggests that geodetic shortening may be accommodated by large (approximately magnitude 8) earthquakes on low-angle detachments analogous to those in the Lesser Himalaya. Structural and chronologic data are being collected using four primary approaches: geologic mapping across the southern margin of the Tien Shan, chronologies of syn- and pre-tectonic, Upper Cenozoic strata via magnetostratigraphy, surveying and dating of deformed Late Quaternary fluvial terraces, and analysis of reflection seismic profiles. The end product will provide: a test of geologic versus geodetic rates in a contractional fold-and-thrust belt; a reconstruction of the pattern, style, and rates of deformation across the SW Tien Shan; insights into why contrasting deformation patterns developed here; a new model for propagating deformation during intracontinental mountain building; and an estimate of earthquake hazards.