Blackwell Publishing Journal Backfiles 1879-2005
iGravity data are used to investigate the geological structure of an area of about 160000 square miles in the light of the crustal warping hypothesis. The region is in N.W. Pakistan and India, and includes the whole of the alluvial plains of Sind and Punjab, the Salt Range and the Potwar Plateau.A new gravity anomaly is introduced and used by an original method, which leads automatically to the detailed contouring of the basement rock below the region, the basement being assumed to be the upper surface of the Earth's crust. The standard crustal section employed is a two-layer crust with a total thickness of thirty kilometres but a table permits direct comparison with other sections of a six-layer crust, two of which have increased thickness.The basement contours show a ridge about 300 miles long separating the Indus Basin from the Lahore Basin. It is hidden by alluvium except for a few outcrops near its northern end. This ridge has apparently suffered sub-aerial erosion under typical S.W. monsoon conditions, extending in places to a depth of over 3000 feet below sea level. This modification of the basement requires a revision of the contours over the ridge, and two contoured charts show firstly the simple crustal upwarp underlying the ridge and secondly the eroded surface of the basement. A deep valley with its bottom far below sea level cuts through the ridge connecting the Indus and Lahore Basins. In Sind a similar valley leads from the direction of the sea to the Indus Basin, but here interpretation is uncertain.It is concluded that the hypothesis yields results giving depths to the basement of the right order in deeply downwarped areas, but in upwarped areas the possibility of erosion, or other concealing factors, leads to uncertainty of interpretation unless the area is wide enough to include a complete section of the upwarp.
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