[1] Slip rates across active faults and folds show that late Quaternary faulting is distributed across the central Tien Shan, not concentrated at its margins. Nearly every intermontane basin contains Neogene and Quaternary syntectonic strata deformed by Holocene north-south shortening on thrust or reverse faults. In a region that spans two thirds of the north-south width of the central Tien Shan, slip rates on eight faults in five basins range from $0.1 to $3 mm/yr. Fault slip rates are derived from faulted and folded river terraces and from trenches. Radiocarbon, optically stimulated luminescence, and thermoluminescence ages limit ages of terraces and aid in their regional correlation. Monte Carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95% confidence limits. Faults in basins appear to merge at relatively shallow depths with crustal-scale ramps that underlie mountain ranges composed of pre-Cenozoic rocks. The sum and overall pattern of late Quaternary rates of shortening are similar to current rates of north-south shortening measured using Global Positioning System geodesy. This similarity suggests that deformation is concentrated along major fault zones near range-basin margins. Such faults, separated by rigid blocks, accommodate most of the shortening in the upper crust.

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The Tien Shan are the quintessential intracontinental range, situated more than 1000 km north of the suture between India and Asia. Their initiation and growth in the Cenozoic, however, remain poorly understood. In this study we present stratigraphic, detrital fission-track, and magnetostratigraphic results that provide a basis for reconstructing the Cenozoic tectonic evolution of the Kyrgyz Range and adjacent Chu basin in the northwestern Tien Shan. Detrital fission-track thermochronology indicates that the northwestern Tien Shan was tectonically quiescent for much of the Cenozoic. Prior to uplift and exhumation in the late Miocene, the Kyrgyz Range was buried by sediments shed from highlands to the south and/or east. Paired bedrock fission-track and [U-Th]/He ages from a sampling transect of 2.4 km relief demonstrate that rapid exhumation commenced at ca. 11 Ma. Initial thrusting in the hinterland was followed by evaporite accumulation (ϳ0.4 km/m.y.), which coincided with erosion of the pre-11 Ma strata that mantled the Kyrgyz Range. Between 10 and 3 Ma, bedrockexhumation rates decreased to Ͻ0.3 km/ m.y., while sedimentation rates decelerated initially to ϳ0.25 km/m.y. before accelerating to ϳ0.4 km/m.y. at 4-5 Ma. Detrital fission-track results indicate that by 4.5 Ma,

Well‐preserved, actively deforming folds in the Tien Shan of Kyrgyzstan provide a natural laboratory for the study of the evolution of thrust‐related folds. The uplifted limbs of these folds comprise weakly indurated Cenozoic strata that mantle well‐lithified Palaeozoic bedrock. Their contact is a regionally extensive unconformity that provides a persistent and readily traceable marker horizon. Based on the deformation of this marker, preserved fold geometries support simple geometric models for along‐strike gradients in fold amplitude and displacement along the underlying faults, linkage among multiple structures, transfer of displacement among folds and evolution of the folds as geomorphic entities. Subsequent to initial uplift and warping of the unconformity surface, steeply dipping reverse faults cut the forelimbs of many of these folds. Wind gaps, water gaps, recent faulting and progressive stripping of the more readily eroded Cenozoic strata indicate the ongoing lateral propagation and vertical growth of fault‐related folds. The defeat of formerly antecedent rivers coincides in several places with marked increases in erosional resistance where their incising channels first encountered Palaeozoic bedrock. Persistent dip angles on the backlimbs of folds indicate strikingly uniform geometries of the underlying faults as they propagate both laterally and vertically through the crust. Deformation switches irregularly forward and backward in both time and space among multiple active faults and folds with no systematic pattern to the migration of deformation. This distributed deformation appears characteristic of the entire Kyrgyz Tien Shan.

The 24 July 2008 outburst flood at the western Zyndan glacier lake and recent regional changes in glacier lakes of the Teskey Ala-Too range,

[1] The Dzhungarian strike-slip fault of Central Asia is one of a series of long, NW-SE right-lateral strike-slip faults that are characteristic of the northern Tien Shan region and extends over 300 km from the high mountains into the Kazakh Platform. Our field-based and satellite observations reveal that the Dzhungarian fault can be characterized by three 100 km long sections based on variation in strike direction. Through morphological analysis of offset streams and alluvial fans, and through optically stimulated luminescence dating, we find that the Dzhungarian fault has a minimum average late Quaternary slip rate of 2.2˙0.8 mm/yr and accommodates N-S shortening related to the India-Eurasia collision. This shortening may also be partly accommodated by counterclockwise rotation about a vertical axis. Evidence for a possible paleo-earthquake rupture indicates that earthquakes up to at least Mw 7 can be associated with just the partitioned component of reverse slip on segments of the central section of the fault up to 30 km long. An event rupturing longer sections of the Dzhungarian fault has the potential to generate greater magnitude earthquakes (Mw 8); however, long time periods (e.g., thousands of years) are expected in order to accumulate enough strain to generate such earthquakes.

Basement‐cored uplifts bounded by steeply dipping reverse faults are mechanically difficult to explain. Reactivation of strike‐slip faults that aid the formation of new, high‐angle reverse faults in the surrounding crust may provide one origin for these structures. This hypothesis is explored by examining the late Miocene to Quaternary evolution of the Kungey and Zailiskey ranges in the northern Tian Shan. These ranges are cored by the Kemin‐Chilik fault (KCF), an inherited Paleozoic structure with sinistral separation of basement terranes. Range growth in response to northward propagation of the Tian Shan has taken place along a network of steeply dipping reverse and oblique‐slip faults surrounding the KCF. Deformation of a low relief unconformity separating Neogene strata from Paleozoic basement records structural growth in response to fault slip. Deformed river terraces surrounding the ranges are correlated to a well preserved chronosequence in the southern Kungey Range. Cosmogenic10Be dating of this chronosequence combined with offset measurements yields slip rates ranging from 0.07 to 0.37 mm/yr for dip‐slip faults, and 1.1 to 1.5 mm/yr for strike‐slip faults Late Quaternary activity in the Kungey‐Zailiskey ranges is consistent with the longer‐term, outward stepping pattern of range growth. Based on cross sections constrained from the folded unconformity surface, deformed Neogene strata and Quaternary terraces, faults building the Kungey Range are inferred to steepen at depth and emanate from a shear zone co‐located with the reactivated KCF. This geometry is consistent with a slip partitioned system developed by an obliquely slipping reactivated fault at depth.

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