STRIKE-SLIP DUPLEXING AND ASSOCIATED FAULT “ROUGHENING”: AN EXAMPLE FROM THE AKSAI RESTRAINING STEPOVER ALONG THE ALTYN TAGH FAULT, NW CHINA

Although sophisticated structural models exist for thrust- and normal-fault systems, a similar theoretical understanding of strike-slip systems has yet to be achieved. A critical aspect of this problem is determining how geometric complexities such as bends and stepovers evolve. The active, left-slip Altyn Tagh fault is punctuated by four major restraining bends/stepovers and thus provides an ideal setting to study such complexities. Specifically, we are investigating the structural evolution of the Aksai stepover (93E-94E), which coincides with the ~15 km wide (N-S) and ~75 km long (E-W) Mt. Altyn uplift. This uplift lies between two active traces in the Altyn Tagh system, and our preliminary interpretation is that it is a strike-slip duplex. To determine how horizontal, bend-perpendicular widening of the duplex stack is absorbed, we have conducted structural and neotectonic mapping within a NW-SE trending range that lies N of the uplift. The NE margin of the range is defined by the active, S-dipping Da Long thrust system. This system shows significant along-strike variations in both topography and structural geometry that likely reflect westward-increasing total shortening. For example, the E end of the system is defined by a single, broad anticline within Quaternary(?) conglomerate, whereas the W end comprises a series of S-dipping thrusts that place asymmetric, N-directed hanging wall anticlines cored by Archean gneiss over Miocene(?) and Quaternary(?) deposits. Likewise, both the N-S width and mean elevation of the range increase from E to W. Two key implications follow from this study. First, westward-increasing total shortening within the Da Long system implies that the Mt. Altyn uplift may have undergone clockwise vertical-axis rotation, thereby increasing both bend angle and fault-trace complexity (i.e. roughness) during progressive deformation. This result contrasts with models in which fault-trace complexity diminishes during progressive slip. Second, the Da Long fault system and possibly analogous structures within a flanking range to the south of the uplift may have been important in accommodating the lateral (N-S) widening of the duplex stack within the Aksai stepover. Thus, flanking systems may be a fundamental structural component within such strike-slip duplexes.