| Abstract |
- 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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