Crustal structure of the northwestern continental margin of the Indian subcontinent from gravity and magnetic data

Graduation date: 1992The continental margin off the coast of Pakistan between the\ud Murray ridge and the Gulf of Cambay has been studied in this\ud work using gravity, magnetic and bathymetric data. Two dimensional\ud gravity and magnetic models based on free-air\ud gravity and residual magnetic data are developed along a north-south\ud profile off the coast of Karachi. The purpose was to\ud interpret the gross crustal structure of the region. A magnetic\ud map has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E.\ud The gravity model extends to a distance of about 1200 km\ud seaward south of Karachi. The seaward end of the gravity model\ud is constrained by seismic refraction data which suggest the\ud presence of typical oceanic crust. The Moho depth at this end of\ud the profile is about 12 km. At the landward end of the profile A-A'\ud the Moho depth is not constrained by seismic data. The gravity\ud model suggests 27 to 17 km as the possible range of the depth of\ud the Moho and a gradual thinning of the crust from land to sea. In\ud addition, the gravity models as interpreted in this study show\ud grabens at the distances of 350 and 450 km along the profile. If\ud the graben-like structures are rift grabens formed during the\ud rifting of India from Africa then transitional crust can be expected\ud to extend to the 500 km mark along the profile A-A'.\ud Two dimensional models for the magnetic data along the\ud profile were also developed. These anomalies can be interpreted\ud as due to oceanic crust or magnetic bodies embedded in\ud transitional crust. The possibility that the observed magnetic\ud anomalies are due to oceanic crust has been studied in detail in\ud this work. The location of the observed magnetic anomalies with\ud respect to marine magnetic anomaly (28) observed earlier on the\ud Indian Ocean floor, were compared to a marine magnetic time\ud scale. To get a reasonable correlation between the observed and\ud theoretical anomalies requires a considerable amount of\ud adjustment in the spreading rate of individual magnetic blocks.\ud Also on the magnetic map the trend of the lineation of these\ud anomalies is perpendicular to the continental margin instead of\ud being parallel to the continental margin as expected for a rifted\ud continental margin. The presence of horst-and-graben structures\ud in the inland region suggests the rifted nature for the continental\ud margin off Karachi than the sheared nature. This indicates that\ud the lineations should be parallel to the margin. But the magnetic\ud lineations are perpendicular to the continental margin and if they\ud are from oceanic crust then they would suggest that the margin is\ud a sheared margin, which contradicts the extensional structures\ud observed inland. This makes it very unlikely that the source of\ud these anomalies is oceanic crust. However, it is quite possible that\ud the magnetic lineations observed in the map were parallel to the\ud continental margin initially but later on the continent rotated\ud clockwise along a fault landward of the magnetic lineation. This\ud rotation is perhaps responsible for making the lineation\ud perpendicular to the continental margin.\ud One objective of this study was to locate the continent-ocean\ud boundary, but with the available amount of data it is not possible\ud to decide on the most appropriate source for the observed\ud magnetic anomalies. Hence it was not possible to decide exactly\ud on the location of continent-ocean boundary. However, on the\ud basis of gravity and magnetic data it can be said that the\ud continent-ocean boundary lies at a distance of 500 km or greater\ud along the profile