Pliocene‐Pleistocene initiation, style, and sequencing of deformation in the central Tien Shan

Abstract

AbstractIn response to the Indo‐Asian collision, deformation of the Tien Shan initiated at ~25 Ma along the northwestern margin of the Tarim Basin. 300 km north, the Kyrgyz Range began deforming ~15 Ma later. Although multiple intervening structures across the Tien Shan are currently active, the sequencing of initial deformation across the orogen's entire width remains poorly known. To determine whether deformation migrated sequentially northward or developed less predictably, we documented deformation patterns within the Naryn Basin in south‐central Kyrgyzstan. Detailed mapping and a published balanced cross section across the Naryn Basin suggest that deep‐seated, relatively steeply dipping thrust faults have disrupted the basin during late Cenozoic deformation. Dating of deformed fluvial terraces with ages between ~10 and 250 ka constrains the rate of deformation across relatively young structures in the Tien Shan interior. Based on geodetic surveys of dated terraces, local rates of relative rock uplift span from 0.3 to 3.5 mm/yr. Folding rates and patterns are temporally persistent at a given site. Moreover, they mimic modern geodetic rates measured from interferometric synthetic aperture radar. Extrapolating these rates into the past suggests that structures within the interior of the Naryn Basin formed in the last 1 Myr, whereas the ranges surrounding the basin initiated at least 1–4 Myr earlier. Hence, within the Naryn Basin itself, deformation has migrated from margins to interior. Similarly, these new chronologies indicate that at least some deformation in the interior of the Tien Shan initiated millions of years later than along either orogenic margin.