QUATERNARY FAULT ACTIVITY IN THE SOUTHERN JUNGGAR BASIN, NORTHWEST, CHINA REVEALED FROM FLUVIAL TERRACES

The southern Junggar basin (SJB) is an active thrust belt in the foreland of the northern Tian Shan, NW China. The uppermost thrust, the Southern Junggar Thrust (SJT), reaches the surface in the forelimb of the Tugulu-Manas-Huoerguosi fold trend. The SJT sourced the 1906 M_w 7.4-8.2 Manas earthquake and acts as an out-of-sequence thrust above anticlinal folds in its footwall. Seismic reflection data constrain the 3-D geometry of the SJT (Stockmeyer et al., 2014) and deeper thrust sheets. Here, we integrate topographic and geochronological constraints with subsurface data to define the Quaternary fault activity in SJB.

Fluvial terraces preserved across SJB are folded above fault bends along the SJT and are uplifted and offset along a well-preserved fault scarp where the SJT is surface-emergent. Field surveys and profiles extracted from a 1-m digital elevation model of folded and faulted terraces along the Huoerguosi and Tugulu structures characterize the Quaternary activity of the underlying thrust sheets. From the south to north, gently north-dipping terraces are abruptly folded and dip to the south where they have been translated across an active synclinal axial surface that is linked at depth to a bend in the SJT where it steps up from an Eocene detachment. North of this folding domain, terraces are undeformed until they are uplifted and warped by a prominent fault scarp where the SJT reaches the surface. North of this scarp, overlying the forelimbs of the structures in the SJT footwall, terraces are undeformed. This latter observation suggests the SJT is the only active structure in this thrust belt. Moreover, both folded and faulted terrace profiles reveal a progressive increase in deformation with relative terrace age, providing a continuous record of deformation on the SJT since the late Quaternary. New post-IR IRSL results provide absolute age constraints on surface deformation in SJB. In addition, we show how integrated models of surface and subsurface earthquake deformation, combined with geochronology, can provide important insights into the partitioning of fault slip at depth between folding and faulting at the surface. Thus, we hope to more accurately relate surface slip to fault slip at depth, providing a method to improve estimates of slip rates and paleoearthquake magnitudes in SJB, and other active thrust belts.