EXPLORING LARGE-SCALE SHEAR FOLD GEOMETRY USING THREE-DIMENSIONAL GOOGLE EARTH MODELING

The Kotah Dome forms a part of the Indian plate in Swat, Pakistan directly south of the Indus Suture zone and Kohistan arc complex. The rocks were metamorphosed to amphibolite facies and deformed in the Late Cretaceous-Eocene during Himalayan orogeny. Within the dome the Middle Permian Swat granitic gneiss is unconformably overlain by Late Permian Marghazar formation that, in turn, is overlain by the Triassic Kashala formation. The Marghazar consists of rift-related quartzo-feldspathic schist and amphibolite that was deposited in extensional basins adjacent to up-thrown blocks of Swat gneiss. These normal faults have influenced subsequent deformation. The goal of this study was to accurately interpret the structural geometry of the Kotah Dome by constructing four intersecting cross sections and combining them into a digital fence diagram using the program Cross Section Model Generator (S. Whitmeyer; www.digitalplanet.org). The fence diagram can be rotated and adjusted for height and look direction to reveal the three-dimensional geometry. The results indicate a southwest-vergent recumbent fold with sheath-fold geometry that extends about 8 km across the dome.

There are two observations that when taken together suggest that the fold formed by distributed shear across a Late Permian normal fault that originally separated an up thrown block of Swat Gneiss from a filled basin composed of Marghazar formation. The first observation is that the fold does not involve the Kashala Formation. Secondly, the Marghazar formation is very thin above the folded layer of Swat Gneiss and much thicker below. The deformation took place during prograde metamorphism and is believed to have occurred when the Indian plate was subducted beneath the Indus Mélange Suture Zone. Based on the vergence of the fold, we suggest that the transport direction of the Indus Mélange Zone was toward the southwest.