Regional strike-slip faults are widely distributed in continental interiors and play a major role in the distribution of far-field deformation due to continental collisions. Constraining the deformation history of the Talas-Fergana Fault (TFF), one of the largest of such faults in the Himalayan deformed interior, is vital to comprehend the hinterland kinematics of the India -Asia collision. New apatite fission track results from the NW Tien Shan define a rapid exhumation event at c. 25 Ma. This event is correlated with a synchronous pulse in the South Tien Shan, implying that both ranges experienced a simultaneous onset of rapid exhumation. We suggest that strike-slip motion along the TFF commenced at c. 25 Ma, facilitating counter-clockwise rotation of the Fergana Basin and enabling exhumation of the linked horsetail splays. Pamir indentation, located south of the Western Tien Shan, is postulated to be underway by c. 20 Ma. Recently published results suggest synchronous strike-slip deformation in the western Tarim Basin and eastern flank of the Pamir. Based on our results and published data, we are able to connect Tarim and Pamir deformation to the onset of TFF slip. We suggest that this pre-existing regional structure was responsible for transferring Pamir-induced shortening to the NW Tien Shan.

This association is composed of red to light brown, laterally extensive, tabular, laminated or massive mudstone interbedded with thin sandstone beds (Fig. DR2A). Sandstone layers are red to beige, fine-to medium-grained, have upward-fining tendencies and occur with lenticular and tabular geometries of 0.1-0.5 m thick. These beds are arranged in stacked, 1-5 m thick sets that coarsen and thicken upward. Sandstone beds have sharp, erosive bases. This facies association occurs in several of the lithostratigraphic units: lower Mirador and lower and upper Guayabo. Mudstones in the lower Mirador Formation lack the interbedded sandstones. Sporadic occurrences of moderately well developed nodular blocky horizons, 1-2 m thick, indicate the presence of paleosols (Fig. DR2B) in the lower Guayabo Formation. Where present they are always at the top of the bed (Fig. DR2C) and change the color of the bed to light red. In the upper Guayabo Formation, the mudstones are multicolored (dark grey, purple, yellow and beige), laminated and have cm-scale thick organic rich layers (Fig. DR2D). On the basis of grain size, sheet-like geometry, and lack of channelization, these strata are interpreted as overbank deposits produced by flooding or avulsion of the main stream providing mud and sand to the flanking area (Collinson, 1996). The sandstone beds represent crevasse splay and levee deposits. The upward-coarsening trends suggest phases of crevasse lobe migration into floodplain areas (Farrell, 2001) and the upward-fining successions reflect crevasse abandonment. The mudstones represent deposition from low-energy flow or standing water during channel abandonment (Miall, 1996). This interpretation is supported by the lateral continuity of the beds coupled with horizontal lamination. The presence of paleosols reflects subaerial exposure and periods of stability in floodplain areas during deposition of the lower Guayabo Formation. The presence of organic matter in the upper Guaybo may suggest rapid plant accumulation under a relatively humid paleoclimate.

The dextral Talas‐Fergana Fault separates the western from the central Tien Shan. Recent work has shed light on the Cenozoic evolution of the eastern and central Tien Shan; much less attention has been paid to the western Tien Shan. In this contribution we present new thermochronological ages for the Fergana and Alai ranges that, combined with the available data set, constrain the Cenozoic exhumation history of the western Tien Shan. Following a tectonically quiet early Cenozoic period, we suggest an onset of exhumation at ~25 Ma. This early onset was followed by a period of slower exhumation and in some areas minor reheating. A final, strong late Miocene rapid cooling event is well represented in the western Tien Shan as in other sectors of the range. The early onset of uplift of the western Tien Shan dissected the previously continuous westernmost Parathethyan Sea, progressively isolating basins (e.g., Fergana, Tarim, and Alai basins) in the central Asian hinterland. Moreover, the coeval timing of late Miocene uplift along the length of entire Tien Shan implies that neither the Pamir nor Tarim can be the sole driver for exhumation of the entire range.

Key points: Data and modeling show Paleogene and middle Miocene cooling episodes  Cooling episodes separated by stable conditions, slow exhumation or subsidence  Extension pattern is compatible with random rift initiation above a plume This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/2015TC003949 ©2015 American Geophysical Union. All rights reserved. AbstractThe cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes, and evaluating the resource potential of their basin fills. Our apatite fission-track and zircon (U-Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three-phased thermal history since the early Paleocene. The first phase (~65-50 Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15 Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15 Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood-phonolite emplacement and has shaped the present-day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle-driven processes and crustal heterogeneities, rather than the previously suggested north-south migrating influence of a mantle plume.

The least well-documented intramountain basin within the Tien Shan is the Fergana Basin. Cenozoic deformation is localized along thrusts on the northern and southern flanks, and by transpressive deformation associated with the dextral Talas–Fergana Fault on the eastern margin. We use sedimentological and stratigraphic observations from well-exposed Cenozoic outcrops to describe depositional environments, provenance and sources. These results are combined with interpreted seismic reflection lines and geological cross-sections are extended laterally based on outcrop geology to the north and east. Following a tectonically quiet early Cenozoic period, a progressive change in palaeocurrent indicators suggests Oligo-Miocene uplift of the hinterland, coupled with an increase in higher energy facies in the Massaget Formation. A renewed pulse of deformation tilted Massaget strata and deposited a considerable volume of coarse sedimentary rocks (Baktriy Formation). The younger episode moved progressively basinwards, as imaged by growth–strata relationships in the subsurface. Published work shows that the accumulation of an impressive c. 8 km of Cenozoic deposits cannot be accommodated only by lithospheric flexure produced by a tectonic load. We agree with the hypothesis that the thick sediments preserved in the basin are accommodated by lithospheric folding and propose that this is driven by compression associated with south-vergent Pamir subduction.