Onset , style and current rate of shortening in the central Tien

We have determined slip rates on the most active reverse faults, reconstructed an extensive preorogenic erosion surface, constructed local and regional cross sections, and dated syntectonic Tertiary sedimentary rock with magnetostratigraphy along an ~northsouth transect that spans the Kyrgyz portion of the west-central Tien Shan. The cumulative late Quaternary shortening rate along this transect is ~12 mm/yr. The shortening is divided across 5 major fault zones, and the most active faults lie in the interior of the range. Using geometric models developed in other regions of basementinvolved deformation, we estimate 35-80 km of shortening during the late Cenozoic. Apparent simultaneous onset of significant basin formation in at least 3 major basins around 12 Ma is interpreted to mark the onset of the current orogenesis. Given the current shortening rate of about 12 mm/yr, measured across active faults and by GPS, we infer that the rate has increased through time. While we propose an acceleration of shortening, our studies indicate that it has always been of similar style, dominated by north-south shortening across ~east-west trending basement-involved reverse faults. Deformation has been localized to ~5 zones that border the largest and deepest Tertiary basins, show the greatest structural relief, and contain the currently most active faults.