Four deep (15 sec twt) seismic reflection lines (PC/9074-86, PC/9044-86, PC/9027-86, PC/9112-86) shot by PetroCanada during 1986 seismic survey are used in the present study for deep structure across the continental margin along with the gravity, magnetic and bathymetry data. The seismic line PC/9074-86 is approximately 88 Km long and runs northeast-southwest through the OGDCL well, PakCan-1.
Free-air gravity and bathymetry data along the seismic line PC/9074-86 is the integral part of this project. Available free-air gravity data shows a linear negative gravity field westward of the present coastline. The areas located closer to the coast show positive gravity up to 15 to 20 mgal. The Indus Canyon, located to the southeast, shows a negative field, which seems to be strongly influenced by the seabed topography. Besides that some independent lows and several anomalous noses are also observed on the map, which are not related to any bathymetric feature, and might have emerged due to density contrasts within sedimentary strata. To the southwest of well PakCan-1, the area is characterised by a strong positive gravity with an approximate gradient of 1.8 mgal Km-1. Relatively steeper gradients in the field can be noticed close to latitude 24° 30' N, longitude 67° 10' E. In order to understand the cause of this strong positive anomaly, bathymetry was compared with the gravity field further to the present shelf break.
In the present study, for the purpose of preparing crustal models along the line PC/9074-86, the crustal velocity is taken as 6.4 Kms-1 (On Nafe and Drake (1963) velocity-density curve this velocity corresponds to 2.8 gm cm-3 density), and mean mantle velocity is taken as 8.1 Kms-1 (the relationship suggested by Christensen (1966) at 10 Kbar pressure provides a density estimate of 3.28 gm cm-3). Furthermore, mean crustal thickness beneath the innermost shelf regions has been assumed 30 Km as the reference thickness (Naini and Talwani, 1983). Dix (1955) Interval velocities are used to deduce averaged interval velocities which are in turn used in computing bulk densities for each rock type (layer) using Gardner relationship (Gardner et al., 1974). All these density assumptions are comparable to those used by Worzel (1965), Scrutton (1979 and 1985), Donato et al. (1983), Naveed (1986 and 1987); and Siddiqui (1996).
Total-intensity magnetic data along the seismic line PC/9074-86 shows that the magnetic field varies from -100 gammas in the southwest to -50 gammas over much of the northeastern area. Within this negative field, low amplitude highs and lows are visible which appear to correspond to seismic structures delineated in the south and south-eastern parts of the study area (Naveed, 1987). The magnetic field does not show a peculiar linear pattern similar to that of typical seafloor spreading anomalies (Kristofferson, 1978); instead, it appears to show a very gentle rise in the basement surface as one moves from southwest to northeast. The overall magnetic field has a longer wavelength indicating some deep-seated source. The short wavelength anomalies, on the other hand, may be representing the supra-basement features.