Subsurface densities & lithospheric flexure of the Himalayan foreland in Pakistan, interpreted from gravity data Public Deposited

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  • Gravity data along a N-S profile from Kohistan to the Punjab plain of Pakistan have been incorporated into recent interpretation 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 ridge. This anomalous distribution of mass can be understood if the Indian elastic plate, which is assumed to overlie an inviscid fluid, is flexed down under the weight of both the overthrust mountains and the sediments eroded off the mountain 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 ridge, an outer topographic rise analogous to the flexural bulge encountered seaward of oceanic trenches. Analysis of Bouguer gravity anomalies along a profile extending from the Sargodha ridge to the main mantle thrust (MMT) show that most of the negative-southward gravity gradient can be attributed to crustal thickening, while short wavelength anomalies are produced by lateral variation of density within the northward thickening sedimentary wedge. In the Sargodha ridge area, an additional contribution of about 25 mgals 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° to 3° beneath the Salt Range and Potwar Plateau (approximately equal to the change in dip of the basement surface). The Moho slope changes from upwardly convex to upwardly concave beneath southern Kohistan. Finally, north of the Main Mantle Thrust (MMT) it bends down again, but at a steeper angle of about 15°. Shorter wavelength anomalies superimposed on the regional 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 gravity, place bounds on the flexural rigidity of such a plate and the forces that deform it. In northern Pakistan the flexural rigidity of the elastic Indian plate (D = 4.0 x 10²³ Nm) is a factor of 10 smaller than the current values interpreted for the central and eastern Himalaya. Because of the small elastic thickness interpreted for the Indian plate in Pakistan (He [approximately] 30 km), the Bouguer gravity gradient is steeper than in the Himalaya of India. Moreover the maximum flexural stresses are concentrated within the crust which may account for the seismic activity of the Sargodha ridge and southern Kohistan. At the end of the Indian elastic plate (arbitrarily chosen at the MMT), a large positive vertical shear stress, S₀ = 9.2 x 10¹² 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 strong negative bending moment, M₀ = -0.85 x 10¹⁸ N, at the end of the plate. The negative bending moment is due to the combined effect of the northward migration of the Indian plate and the southward differential compressional force generated by the crustal rocks stacked beneath the northern Kohistan arc. Consequently, in southern Kohistan the surface of the Indian plate is concave. The upper portion of the elastic plate is therefore under compressional regime, while the lower portion is subject to extentional stress. High flexural stresses are probably the primary source of the Hazara seismic zone where incipient reverse faulting seems to take place. In contrast, the pronounced convexity developed along the flexural bulge can account for 1) tensional stress in the upper part of the Indian plate, large enough to produce basement normal faults interpreted beneath the Salt range and Sargodha ridge; and 2) compressional stress in the lower portion of the crust that cause the excess of mass and seismicity beneath the Sargodha Ridge.
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