BASEMENT-INVOLVED DEFORMATION ACROSS AN ACTIVE INTRACONTINENTAL OROGEN: THE KYRGYZ TIEN SHAN, CENTRAL ASIA

The Tien Shan mountains of central Asia are the most active modern intracontinental orogen on Earth. Located between the strong lithosphere of the Tarim block to the south and the stable Kazakh shield to the north, at the longitude of the Kyrgyz Republic, the Tien Shan absorb nearly half of the total India-Eurasia convergence rate. Although the Tien Shan area was the locus of deformation in Paleozoic orogenies, the major Paleozoic crustal boundaries have not been inherited as Cenozoic range-bounding faults. Preservation of a widespread pre-orogenic peneplain across the Tien Shan coupled with kinematic markers such as progressively deformed fluvial terraces offer insight into the mechanism of basement-involved deformation.

In the Kyrgyz Tien Shan, late Cenozoic crustal shortening has uplifted basement rocks in blocks that range in width from 20-40 km, such as the Kyrgyz and At-Bashi ranges, to the 100-120 km-wide plateau of southern Kyrgyzstan. The smaller, isolated ranges appear to be scaled by the thickness of the elastic portion of the crust. The extensive plateau appears to have grown in part by coalescence of individual fault-bounded ranges through progressive contraction. The ~100 km width of the southern plateau is similar in magnitude to estimates of total late Cenozoic shortening across the Tien Shan, and likely relates to the extent of underthrusting of the Tarim lithosphere.

The intermontane Issyk-Kul basin lies at the leading edge of the growing southern plateau. Geologic mapping and profiles of deformed river terraces crossing the range front reveal that plateau expansion is being accommodated by the growth of a crustal-scale fault-propagation fold. The tilting forelimb extends at least 12 km across strike. Along strike of this boundary, structural style changes abruptly from a homoclinal, north-tilting forelimb on the eastern side, to a series of emergent south-vergent wedge thrusts on the western half of the margin.