CHANGING MECHANICAL RESPONSE DURING CONTINENTAL COLLISION - ACTIVE EXAMPLES FROM THE FORELAND THRUST BELTS OF PAKISTAN

We have used data from teleseismic, seismic reflection and field geolo gic studies, along with both geomechanical and gravity modeling to con trast the tectonics Of four active orogenic wedges in Pakistan: the Ka shmir Himalaya, the Salt Range-Potwar Plateau foldbelt, the Sulaiman R ange and the Makran accretionary wedge. In Makran oceanic crust is sti ll being subducted, and a thick pile of sediments is being accreted an d underplated. Undercompaction and excess pore pressures can explain t he narrow cross-sectional taper and frontal aseismicity of this wedge. Beneath the Sulaiman wedge, continental crust is just starting to be underthrust. Indirect evidence suggests that fine-grained carbonate ro cks found in abundance deep in the stratigraphic section may be deform ing ductilely at the base of the Sulaiman wedge and provide a zone of ductile detachment. The collision has proceeded to a much more mature stage in the Salt Range-Potwar Plateau foldbelt and the Kashmir Himala ya. Isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in both of these wedges, so in that r espect the two foldbelts are similar. However, thick Eocambrian salt b eneath the Salt Range and Potwar Plateau permits that foldbelt to be m uch wider in map view, with a thinner cross-sectional taper and a mixt ure of thrust vergence directions. A major normal fault in basement ca uses the Salt Range to rise in front of the mildly deformed molasse ba sin of the southern Potwar Plateau. Much of the diversity among these mountain belts can be understood in terms of differences in the maturi ty of the collision process in each area, the resulting thickness of t he sedimentary pile encountered at the deformation front, and the pres ence or absence of large contrasts in strength between the various lay ers of the stratigraphic section and basement relief.