Flexural deformation and basin-mountain coupling in the northern Kyrgyz Tien Shan: transition from the Issyk-Kul basin to the Kumtor plateau

During the late stage of the India-Asia collision, deformation propagated northwards into the Asian foreland. North of the stable Tarim plate, the Tien Shan range – an old Palaeozoic fold belt – was strongly reactivated. It now accommodates more than one third of the total shortening rate between Stable Eurasia and the Indian continent.In the northern part of the Kyrgyz Tien-Shan Range, the 600 m deep Lake Issyk-Kul occupies a lense-shaped tectonic depression elongated in an E-W direction and bordered on its northern and southern sides by high mountain ranges (> 4000 m high). To the north, the Kungey Alatau range has the structure of an active positive flower structure with the Chon-Kemin – Chilik fault in its middle (location of several Ms > 8.0 historic earthquakes). To the south, the Terskey range forms the frontal scarp of the high and relatively flat Kumtor Plateau whose surface is undulating between 3800 and 5200 m high.Multidisciplinary investigation was performed during several summer campaigns, involving structural geology, paleostress reconstructions, tectono-stratigraphy and paleoseismology in the mountain ranges and lake shore, as well as high-resolution seismic profiling and heat flow measurements in Lake Issyk-Kul. Investigations included also the seismotectonics analysis of a large number of earthquake focal mechanisms determined from the local seismic network.All the results are best integrated in a model of lithospheric deformation by flexural folding and basin-mountain coupling. The Issyk-Kul basin probably formed as a flexural downwarp of the lithosphere rather than as a symmetric ramp basin, as once proposed. South of the Issyk-Kul depression, the Kumtor plateau still show large remains of the pre-Cainozoic flat erosion surface that is widespread in the Central Asia, now strongly uplifted and slightly undulating. Deformation at mountain-basin interface occurs mainly by tilting around a horizontal axis of the pre-Cainozoic basement (up to 60°from the horizontal), and top-to-the south reverse faulting at the southern margin of the basin (basin towards the range), in an opposite sense to what could be expected in the case of a ramp basin. The focal mechanism of a recent earthquake along the southern mountain-basin interface confirms this interpretation (25 December 2006 Ms 5.8)

Flexural deformation and basin-mountain coupling in the northern Kyrgyz Tien Shan: transition from the Issyk-Kul basin to the Kumtor plateau

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A major stage of convergence in the Issyk-Kul basin (Northern Tien-Shan) at the end of the Neogene

The structural features the northern Tien-Shan mountain belt in Kyrgyzstan, including the Issyk-Kul basin, indicate a complex Cenozoic deformation, associated to the Indian-Eurasian collision. The collision caused deformation to propagate inside the continent, resulting in crustal thickening and mountain growth. The present-day shortening occurs at a rate of about 10-15 mm/yr and is oriented roughly N-S in the southern Tien-Shan and up to 2-6 mm/yr and variably oriented in the northern Tien-Shan. Different rate values and orientation could be related to the presence of a Precambrian microcontinent in the northern Tien-Shan. That microcontinent could affect the formation of the Neogene-Quaternary structure of the Issyk-Kul basin. In the Paleogene, more then 3 km of lacustrine sediments was deposited in the subsiding basin. The onset of uplift of the southern Tien-Shan started in the Neogene, when clastic and proluvial sediments, transported from the rising southern ranges were deposited in the basin. In this stage, the basin was much larger and more elongated than at present. It was probably controlled by ENE trending faults.Starting in the late Neogene, clastic material was deposited in a moderately subsiding basin. This indicates the onset of uplift in the Northern Tien-Shan, reaching a peak at the end of the Pliocene-early Pleistocene. In this period, strong N-S oriented contractions caused rigorous deformations inside the Issyk-Kul Cenozoic deposits. The southern and northern edges of the basin were intensely deformed. They are presently exposed and exhibit two major trends of tectonic lineaments, that correspond to transpressive zones, oriented ENE and NW. Pop-up and transpressive flower structures indicate oblique convergence in these zones.Ramp structures developed at the latitudinal edges of the basin and the thrusting of the basement over the basin is accompanied by late Neogene molasse deposition. E-W striking faults controlled the late Neogene structure of the basin.The deformed Neogene sediments are often unconformably overlain by undeformed Quaternary terraces. The Quaternary tectonics in the uplifted parts of the basin is expressed by reverse reactivation of preexisting faults. The border faults shifted toward the internal parts of the basin which was further narrowing in north-south direction, and the borders of the basin were further uplifted. Within the lake, active deformation has been recorded in its southern part, expressed by upright folding with NE trending axis.A major stage of convergence is consequently evidenced, dating from the end of the Neogene, mainly expressed by transpressive movements along a conjugate set of strikeslip zones. This stage of strong deformation is contrasting with the Quaternary tectonic stage of moderate basin inversion. We investigate the relations between the tectonic history of the Himalaya and of the Tibet plateau, and the deformation stages observed for the Issyk-Kul basin