CRUSTAL ARCHITECTURE AND ACTIVE GROWTH OF THE SUTAI RANGE, WESTERN MONGOLIA: A MAJOR INTRACONTINENTAL, INTRAPLATE RESTRAINING BEND

Abstract

The Sutai Range is a structural and topographic culmination at the southeastern end of the Mongolian Altai and a world-class example of an actively forming restraining bend. The range occurs at a major stepover zone along the Tonhil dextral strike-slip fault within a wider region dominated by late Cenozoic transpressional deformation. Analysis of satellite imagery and the results of field investigations reveal that the range is structurally asymmetric with an overall NE tilt due to several major SE-directed thrusts within the core of the range and along its SW margin. The distribution of alluvial sediments shed from the range and stream length asymmetries also indicate a regional NE tilt for the range. Faults that splay off of the main Tonhil Fault bound discrete uplifted blocks that define an oblique-slip duplex at the surface and asymmetric flower structure in cross-section. Outward growth of the range is partly accommodated by growth of foreberg thrust ridges within the adjacent Dariv Basin. Thrust blocks within the centre of the range expose basement schists and mylonitic granite suggesting that the greatest uplift and crustal exhumation has occurred within the core of the restraining bend, although much of the exhumation is likely to be due to older Palaeozoic structural events. The southeast and northwest ends of the range are characterized by smooth unbroken surface ramps (''gangplanks'') that are upwarped towards the centre of the range where maximum Cenozoic uplift has occurred. The geometry and evolution of Sutai Uul and other intracontinental and intraplate restraining bends is fundamentally influenced by the initial width of the stepover zone, the attitude of regional basement structures and extent of brittle reactivation, the direction of SHmax relative to basement structures, and progressive fault and block rotation which may change the kinematics along faults or lead to their abandonment.