FEEDBACKS BETWEEN TOPOGRAPHIC GROWTH AND STRUCTURAL STYLE WITHIN RESTRAINING DOUBLE BENDS DEDUCED FROM THE AKATO TAGH BEND, ALTYN TAGH FAULT, NW CHINA

Double bends provide opportunities to determine the magnitude and timing of motion along strike-slip faults due to the distinctive structural and thermal signatures that may be imprinted upon rocks that migrate through such jogs. To better understand the evolution of restraining double bends, we have investigated the Akato Tagh uplift along the active, left-slip Altyn Tagh fault. This 90-km long range stands ~2 km above its surroundings and is a classic example of a right-stepping double bend: a N90E-striking central fault segment inside the uplift is flanked by N70E-striking segments to the W and E. Where the faults intersect they create “inside corners” in the NE and SW sectors of the range. The topography of the Akato Tagh uplift is asymmetric both parallel and perpendicular to the strike of the bend. The tallest and widest areas form two topographic nodes centered on the inside corners. Bend-perpendicular and bend-parallel motion are partitioned into NS folding and left-slip faulting, respectively. Like the topography, bend-perpendicular shortening appears to be concentrated in the inside corners. The topographic and structural asymmetry of the Akato Tagh has three implications. (1) Focusing of shortening into the inside corners of a double bend may cause the principal fault in the bend to undergo vertical-axis rotation. Fault-perpendicular shortening and exhumation within the bend may therefore decrease during progressive deformation. (2) Migration of crust through the inside corners may produce exhumation ages that systematically increase down-stream from the corners. The lateral age gradients should be a function of the rate at which material migrates through the inside corner. (3) Restraining double bends show marked differences in the style of faulting inside bend uplifts. Such variations may result from different states of stress adjacent to the bends: uplifts flanked by strike-slip faults form where sH=s1, sh=s3, and sv=s2, while thrusts form when sH=s1, sh=s2, sv=s3. This hypothesis predicts that growth of restraining bend topography by early thrusting can convert thrust-dominated bends into strike-slip dominated uplifts. Likewise, changes in the style of faulting could be triggered exclusively by changes in the efficiency of erosion and its resulting effects on the height of the restraining bend.