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. |
