Multidisciplinary investigations of the Chaman strike-slip fault along the western Indo-Asian collision boundary, Pakistan
Ul-Hadi, Shams 1974-
MetadataShow full item record
The Chaman left-lateral strike-slip fault bounds the collision zone between Indian and Eurasian plates at the western end of the Himalayan-Tibetan orogen and connects the Makran subduction zone to the Himalayan convergence zone. This western collision boundary received extensive magmatism as Kandahar and Chagai-Ras Koh arcs, which evolved from oceanic to continental arcs before the closure of the neo-Tethyan Ocean and initiation of the left-lateral movement along the Chaman fault as the north-bound translation of the Indian plate continued. Geochemical analyses of the andesitic/basaltic arc rocks exposed west of the Chaman fault show a subduction-related source that was enriched with large ion lithophile elements and depleted with high field strength elements. Younger granitoids intrusions show a source with continental contamination depicting proximity of the Afghan block at the time of emplacement. U-Pb emplacement ages from granitic samples in the Khwaja Amran-Spinatizha area suggest that granitic intrusions within the Chagai-Ras Koh ceased by ~58 Ma as in the Kandahar arc. The fission track ages of ~35 Ma and ~20 Ma from the intrusive rocks of the Chagai-Ras Koh correspond well to the final Indo-Asian collision in the south along the western collision boundary and the initiation of strike-slip movement along the Chaman fault, respectively. Geomorphic history of an active transpression was reconstructed using digital elevation model data integrated with Advance Spaceborne Thermal Emission and Reflection Radiometer and images from GeoEye-1 to understand the interaction of a strand of the Chaman fault and the Spinatizha thrust fault with the evolving landscape within the Chaman basin in western Pakistan. Topographic analysis and geomorphic indices of active tectonics reveal northward growth of the thrust with the presence of three water gaps and two wind gaps. Left-lateral displacement along the Chaman fault and uplift along the Spinatizha fault record slip partitioning resulting from the difference in the Chaman fault azimuth and orientation of the velocity vector of the Indian plate. About 1150 m displaced alluvial fan with a 10Be exposure age of ~ 35 kyr along a strand of the Chaman fault yields a slip rate of ~ 33 mm/yr which agrees with the geologically constrained slip rates along the Chaman fault, but is twice as larger than that inferred from global positioning system measurements (~18 mm/yr). This contrast in slip rates accounts either for; 1) transient variations in rates of elastic strain accumulation, such that the geodetic rates might represent a period of reduced displacement as compared to the averaged Late Pleistocene rate; or 2) fault inversion and strain partitioning within the plate boundary zone. While fault inversion could be the reason of strain variations along mega-structures over longer periods of time (>106 years), transient strain accumulation could explain contrasting slip rates along the Chaman fault which is late in its seismic cycle.