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

The continental margin off the coast of Pakistan between theMurray ridge and the Gulf of Cambay has been studied in thiswork using gravity, magnetic and bathymetric data. Two dimensionalgravity and magnetic models based on free-airgravity and residual magnetic data are developed along a north-southprofile off the coast of Karachi. The purpose was tointerpret the gross crustal structure of the region. A magneticmap has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E.The gravity model extends to a distance of about 1200 kmseaward south of Karachi. The seaward end of the gravity modelis constrained by seismic refraction data which suggest thepresence of typical oceanic crust. The Moho depth at this end ofthe profile is about 12 km. At the landward end of the profile A-A'the Moho depth is not constrained by seismic data. The gravitymodel suggests 27 to 17 km as the possible range of the depth ofthe Moho and a gradual thinning of the crust from land to sea. Inaddition, the gravity models as interpreted in this study showgrabens at the distances of 350 and 450 km along the profile. Ifthe graben-like structures are rift grabens formed during therifting of India from Africa then transitional crust can be expectedto extend to the 500 km mark along the profile A-A'.Two dimensional models for the magnetic data along theprofile were also developed. These anomalies can be interpretedas due to oceanic crust or magnetic bodies embedded intransitional crust. The possibility that the observed magneticanomalies are due to oceanic crust has been studied in detail inthis work. The location of the observed magnetic anomalies withrespect to marine magnetic anomaly (28) observed earlier on theIndian Ocean floor, were compared to a marine magnetic timescale. To get a reasonable correlation between the observed andtheoretical anomalies requires a considerable amount ofadjustment in the spreading rate of individual magnetic blocks.Also on the magnetic map the trend of the lineation of theseanomalies is perpendicular to the continental margin instead ofbeing parallel to the continental margin as expected for a riftedcontinental margin. The presence of horst-and-graben structuresin the inland region suggests the rifted nature for the continentalmargin off Karachi than the sheared nature. This indicates thatthe lineations should be parallel to the margin. But the magneticlineations are perpendicular to the continental margin and if theyare from oceanic crust then they would suggest that the margin isa sheared margin, which contradicts the extensional structuresobserved inland. This makes it very unlikely that the source ofthese anomalies is oceanic crust. However, it is quite possible thatthe magnetic lineations observed in the map were parallel to thecontinental margin initially but later on the continent rotatedclockwise along a fault landward of the magnetic lineation. Thisrotation is perhaps responsible for making the lineationperpendicular to the continental margin.One objective of this study was to locate the continent-oceanboundary, but with the available amount of data it is not possibleto decide on the most appropriate source for the observedmagnetic anomalies. Hence it was not possible to decide exactlyon the location of continent-ocean boundary. However, on thebasis of gravity and magnetic data it can be said that thecontinent-ocean boundary lies at a distance of 500 km or greateralong the profile.