Least squares restoration of tertiary thrust sheets in map view, Tajik depression, central Asia

Research areas:
DEC 10
The Tajik depression, located west of the Pamirs and south of the Tien
Shan, is a compressional intermontane basin, bounded by basement
overthrusts 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 Pamirs 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 thrusts, and counterclockwise rotations. These various deformation
processes can be analyzed and quantified by reconstruction of the
predeformed state of the depression. However, the combination ol
thrusting, wrenching and block rotations implies a nonplane deformation:
which cannot be restored propel ly using balanced cross sections alone.
We have therefore developed a numerical method for restoration of
stratigraphic surfaces, designed for regions of nonplane compressional
tectonics. The deformed region is represented in map view as a mosaic of
fault-bounded blocks, overlapping each other along the faults. Blocks
are separately unfolded and then numerically packed together by least
squares minimization of overlaps, yielding fields of finite horizontal
translations and rotations about vertical axes. To analyze the
deformation postdating the collision of India and Asia, we have restored
a stratigraphic surface at the base of the Cenozoic. First, in order to
test the numerical 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: overlaps and gaps, which cannot be reduced, remain in
the restored slate. After-correction of these inconsistencies,
restoration yields a complex mode of deformation for the depression.
Individual thrust slices have undergone counter-clockwise rotations
about vertical axes, the magnitudies increasing from west to east, up to
a maximum of 40 degrees neat the Pamirs. Horizontal shortening is 150 km
(35\%) in the center of the depression and 240 km (85\%) in the
northeastern part, between the Pamirs and the Tien Shan. Strike slip
motions, commonly not revealed by balancing cross sections, are
associated with thrusting on faults striking N-S. Independent
paleomagnetic data and slip directions on small-scale faults pl provide
positive checks on the results of our restoration.