LEAST-SQUARES RESTORATION OF TERTIARY THRUST SHEETS IN MAP VIEW, TAJIK DEPRESSION, CENTRAL-ASIA

The Tajik depression, located west of the Pamirs and south of the Tien Shan, is a compressional intermontane basin, bounded by basement over thrusts and filled with Mesozoic and Cenozoic sediments. The internal structure is typical of a thin-skinned fold and thrust belt. Kinematic data available in the literature suggest that indentation of the Pami rs into Asia during the Cenozoic collision of India and Asia has been accommodated in various ways within the depression, including westward extrusion, thickening combined with wrenching along N-S folds and thr usts, and counterclockwise rotations. These various deformation proces ses can be analyzed and quantified by reconstruction of the predeforme d state of the depression. However, the combination ol thrusting, wren ching and block rotations implies a nonplane deformation: which cannot be restored propel ly using balanced cross sections alone. We have th erefore developed a numerical method for restoration of stratigraphic surfaces, designed for regions of nonplane compressional tectonics. Th e deformed region is represented in map view as a mosaic of fault-boun ded blocks, overlapping each other along the faults. Blocks are separa tely unfolded and then numerically packed together by least squares mi nimization of overlaps, yielding fields of finite horizontal translati ons and rotations about vertical axes. To analyze the deformation post dating the collision of India and Asia, we have restored a stratigraph ic surface at the base of the Cenozoic. First, in order to test the nu merical method, we restored a map that had previously been restored by a purely manual method. Restoration of a second map, drawn from newly available subsurface data; leads to geometrical inconsistencies: over laps and gaps, which cannot be reduced, remain in the restored slate. After-correction of these inconsistencies, restoration yields a comple x mode of deformation for the depression. Individual thrust slices hav e undergone counter-clockwise rotations about vertical axes, the magni tudies increasing from west to east, up to a maximum of 40 degrees nea t the Pamirs. Horizontal shortening is 150 km (35%) in the center of t he depression and 240 km (85%) in the northeastern part, between the P amirs and the Tien Shan. Strike slip motions, commonly not revealed by balancing cross sections, are associated with thrusting on faults str iking N-S. Independent paleomagnetic data and slip directions on small -scale faults pl provide positive checks on the results of our restora tion.