Subsurface densities and lithospheric flexure of the Himalayan foreland in Pakistan

Gravity data along a north-south profile from Kohistan to the Punjab plain of Pakistan have been incorporated into recent interpretations of the gross structure of the foreland fold-and-thrust belt of the Himalaya. In northern Pakistan, large deviations from Airy Isostatic equilibrium are observed. An excess of mass characterizes the northern Kohistan arc, and a deficit of mass underlies a broad area extending from southern Kohistan to the Salt Range, while to the south a slight excess of mass seems to prevail in the region of the Sargodha high. This anomalous distribution of mass can be understood if the Indian elastic plate, which is assumed to overlie a buoyant "fluid," is flexed down under the weight of both the overthrust mountains and the sediments eroded off the mountains and deposited in the foredeep basin. In many respects the intracontinental subduction of India beneath the Himalaya is similar to island arc formation, including the seismically active Sargodha high, a basement ridge analogous to the flexural bulge encountered seaward of oceanic trenches. Analysis of Bouguer gravity anomalies along a profile extending from the Sargodha high to the Main Mantle Thrust (MMT) shows that most of the negative-northward gravity gradient can be attributed to crustal thickening. In the Sargodha high area, an additional contribution of about 25 mgal appears to be due to excess of mass at lower crustal or upper mantle levels. The Moho discontinuity is interpreted to bulge up beneath the Sargodha high, then gradually increase in dip from 1 degrees to 3 degrees beneath the Salt Range and Potwar plateau (approximately equal to the change in dip of the basement surface). The Moho is interpreted to change from upwardly convex to upwardly concave beneath southern Kohistan. Finally, north of the Main Mantle Thrust it appears to bend down again, but at a steeper angle of about 15 degrees . Shorter wavelength anomalies, superimposed on the regional Bouguer gradient, are modeled in terms of upper crustal density changes, including those due to: (1) offsets of the basement surface; (2) variable thickness of the Eocambrian evaporite sequence that forms the basal decollement; (3) thrusting and folding of relatively high-density, older parts of the stratigraphic section to higher structural levels, particularly in the Salt Range and northern Potwar plateau; and (4) thickening of the low-density Neogene molasse sequence into the axis of the Soan Syncline, a structural depression between the Salt Range and northern Potwar plateau. Subsurface densities of the overthrust wedge, as well as the definition of the shape of the top surface of the Indian plate interpreted from seismic reflection and drilling data, place bounds on the flexural rigidity of such a plate and the forces that deform it. In northern Pakistan, a steeper Bouguer gravity gradient suggests that the flexural rigidity of the elastic plate (D = 4.0 [+ or - 2.0] x 10 (super 23) Nm) is a factor of 10 smaller than the current values interpreted for the central and eastern Himalaya. Moreover, the maximum flexural stresses are probably concentrated within the crust, which may account for the seismic activity of the Sargodha high and southern Kohistan. At the end of the Indian elastic plate (arbitrarily chosen at the MMT), a large positive vertical shear stress, 9.2 x 10 (super 12) N/m < S (sub 0) < 1.6 x 10 (super 13) N/m, is applied to account for the topographic load north of the MMT. In addition, to fit the gravity constraints it was necessary to apply a large negative bending moment, -1.4 x 10 (super 18) N < M (sub 0) < -0.85 x 10 (super 18) N, at the end of the plate. The negative bending moment can be explained by the combined effect of the northward migration of the Indian plate and the southward differential compressional force generated by the crustal rocks stacked at mid-upper crustal levels beneath the northern Kohistan arc. In addition, buoyancy of the crustal rocks at deeper levels beneath the Kohistan arc may contribute to the negative bending moment. Consequently, in southern Kohistan the surface of the Indian plate is concave up; compressional stresses in the upper part of the plate are probably the primary source of the Hazara seismic zone, where incipient reverse faulting seems to take place. In contrast, the pronounced upward convexity developed along the flexural bulge can account for (1) tensional stress in the upper part of the Indian plate, which is large enough to produce basement normal faults interpreted beneath the Salt Range and Sargodha high; and (2) compressional stress in the lower portion of the crust, which causes the excess of mass and seismicity beneath the Sargodha high.