Late Pleistocene slip rate and subsurface fault-plane geometry of the Latgan Fold (Bole Basin, North Tian Shan, NW China) derived from a deformed cosmogenically dated alluvial fan

Abstract

Deformed alluvial fans can indicate the subsurface fault-plane geometry of thrusts. Numerous seismic profiles have illustrated the subsurface fault-plane geometry of thrusts in the Tian Shan piedmonts. However, due to a lack of seismic profiles, the subsurface fault-plane geometry in the Tian Shan is not well-known. This paper focuses on a thrust-related anticline in the eastern Kusongmuxiek Fault (KF), the Latgan Anticline (LA), located in the southern margin of the Bole Basin, northern Tian Shan. Five alluvial fan surfaces, A1-A5 from young to old, are determined along the Dilike River. We used the alluvial fan surfaces and fault attitude to determinate the subsurface fault-plane geometry of the Latgan Fault. The geometric model reveals that the Latgan Fault can be divided into three segments: an upper ramp with a dip of 55°S, a central listric fault, and a lower ramp with a dip of 31°S. The abandonment age of the A4 surface based on its cosmogenic 10Be concentrations and use of a kinematic model of a listric fault fold reveal that the fault dip-slip rate is c. 0.5 mm/a, and the horizontal crustal shortening rate is c. 0.4 mm/a during the Pleistocene. The vertical incision rates of the Dilike River are 0.5–0.6 mm/a in the uplifted area and 0.2 mm/a in the non-uplifted area. This difference may be the result of tectonic uplift. Our field and remote sensing observations and the geometric characteristics of the eastern Keguqin Mountains piedmont thrusts indicate fault propagation into the basin via thick-skinned tectonics. The LA crustal shortening rate and previous studies indicate that the crustal shortening rates of the Bole, Borohoro, and Turfan Units are ~1.7–2.0 mm/a, >4.0 mm/a, and 3.2–4.4 mm/a, respectively. The lower shortening rate of the Bole Unit and higher shortening rates of the Borohoro and Turfan Units indicate that the Bole Unit is the lowest crustal shortening rate unit in the northern Tian Shan.