| Abstract |
- The Salt Range and Potwar Plateau of northern Pakistan are part of
the thin-skinned, active thrust system related to the ongoing collision of
the Asian and Indian continental blocks. Platform rocks and orogenic
molasse of the Indo-Pakistani shield are deformed in south-verging thrusts
and folds, relative to a northward-converging basement. This results in
three different styles of deformation along a north-south, balanced cross
section in the western Potwar Plateau and Salt Range. This cross section,
as well as structure and isopach maps for the entire Salt Range/ Potwar
Plateau region, were constructed using surface geologic maps, seismic
reflection, drillhole, and gravity data.
In the Northern Potwar Deformed Zone (NPDZ), shortening of 68% is
accomodated by a trailing imbricate stack of high ad low-angle, south-verging
faults. These faults are rooted in a master decollement within an
Eocambrian evaporite sequence (Salt Range Fm.), which overlies a north dipping basement surface at a depth of approximately 8 km. Southward
progression of regional deformation with time appears to be interrupted by
overstep, or back-break, thrusting. The low-angle faults, or a possible
basement uplift, may represent the ramp for a protracted interval of
imbricate faulting that began in middle to late Miocene ([approximately] 15-10 mybp).
Later, the basal detachment propagated rapidly, transporting the NPDZ
sequence and the developing Soan Syncline (southern Potwar Plateau)
southward, without major internal deformation within the Syncline. Within
the last 2.0 my, the older units overlying the Salt Range Fm. have been
exposed along the Salt Range, where the basal detachment ramped to the
surface.
A north-facing, subsurface normal fault, that offsets basement and
controls frontal ramping along the central portion of the Salt Range, is not
present in the western Salt Range. Rather, the older platform rocks and
overlying molasse are carried southward up a basement monocline. This
monocline is a protrusion on the northeast flank of the northwest-trending
Sargodha Ridge, a buried basement arch that may represent a flexural
response to Himalayan tectonic loading. Although the footwall, underlying
the thrust plane, has not been drilled in the Salt Range, seismic control and
two-dimensional gravity modeling suggest the presence of autochthonous
sedimentary rocks, overlain by a thickened layer of evaporites,
In the western Salt Range, the leading edge of the Salt Range Thrust
(SRT) overrode its footwall block about 34 km. Additional shortening may
have been accomodated by a series of imbricate thrusts in the Salt Range
Fm., or the footwall strata may be imbricated. This would add northward
taper to the basement dip, allowing the SRT to be exposed at the surface.
An average shortening rate of approximately 1.3 cm/yr is comparable to the shortening rate for the NPDZ, if thrusting there occurred over a 7-8 my
interval. Overthrust shortening along the SRT in the central Salt Range has
been estimated at between 19 and 23 km. The larger amount of
overthrusting in the western Salt Range, as compared to the central Salt
Range, is consistent with the structurally-salient range front along this
western edge. Minimum overall shortening, from the southern edge of the
Peshawar Basin to the undeformed Jhelum Plain, is estimated at 213 km, a
shortening of about 49%. This is comparable to the amount of fold and
thrust shortening in other deformed forelands.
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