CONSTRAINING FAULT GEOMETRY AND KINEMATICS WITH FLUVIAL TERRACES: SOUTHEASTERN ISSYK-KUL BASIN, KYRGYZ TIEN SHAN

The southeastern margin of the Issyk-Kul basin is an abrupt, sinuous rangefront, where extensive north-dipping Quaternary terrace surfaces intersect the basin floor. This northern margin of the Terskey Range is one of only four major range to basin transitions across the Tien Shan at this longitude, and GPS geodesy suggests ~5 mm/yr of shortening there. As the geologic contact at the rangefront is between two unconsolidated late Quaternary units, the geometric relationship between the two is unclear. To resolve ambiguity in subsurface fault geometry we use deformed fluvial terraces as additional strain markers. Each terrace reflects the amount of deformation that it and the underlying rock have experienced since the terrace's abandonment. Hence, terraces of different ages provide an incremental record of strain, and give an important kinematic test that any valid geometric model must pass. The deformed fluvial terrace surfaces were mapped on 1:25 K topographic maps and profiled by Total Station or taken off the topo map if clear.

We have mapped terraces, Tertiary sediment, and the basement/Tertiary unconformity at four primary sites along the southeastern Issyk-Kul basin: the Baskaun, Juku, Chon Kyzyl Su, and Jety Oguz rivers. We have conducted reconnaissance in intervening valleys, and have used satellite images and aerial photos to interpret the geology and geomorphology of this entire basin margin.

The northern margin of the Terskey Range in this area is characterized by Tertiary sediment dipping 20-50° to the north-northeast off of the basement unconformity. The terraces above each of the four rivers we have studied all dip to the north like the Tertiary, and show a pattern of greater dip with greater age, with the oldest surfaces dipping 5-6° steeper than the modern rivers. This tilting spans at least 12 km across strike, and the terraces radiate from a point at the rangefront. The only structural models which reproduce these observations are a north-vergent convex fault which daylights at the rangefront or a north-vergent fault-propagation fold. The sinuosity of the rangefront and length of the tilting limb support the presence of a fault, but the steeper dips of the Tertiary everywhere underlying the terraces and the lack of scarps cutting younger inset terraces suggest that a fold hinge defines the rangefront.