The continental margin off the coast of Pakistan between the
Murray ridge and the Gulf of Cambay has been studied in this
work using gravity, magnetic and bathymetric data. Two dimensional
gravity and magnetic models based on free-air
gravity and residual magnetic data are developed along a north-south
profile off the coast of Karachi. The purpose was to
interpret the gross crustal structure of the region. A magnetic
map 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 km
seaward south of Karachi. The seaward end of the gravity model
is constrained by seismic refraction data which suggest the
presence of typical oceanic crust. The Moho depth at this end of
the profile is about 12 km. At the landward end of the profile A-A'
the Moho depth is not constrained by seismic data. The gravity
model suggests 27 to 17 km as the possible range of the depth of
the Moho and a gradual thinning of the crust from land to sea. In
addition, the gravity models as interpreted in this study show
grabens at the distances of 350 and 450 km along the profile. If
the graben-like structures are rift grabens formed during the
rifting of India from Africa then transitional crust can be expected
to extend to the 500 km mark along the profile A-A'.
Two dimensional models for the magnetic data along the
profile were also developed. These anomalies can be interpreted
as due to oceanic crust or magnetic bodies embedded in
transitional crust. The possibility that the observed magnetic
anomalies are due to oceanic crust has been studied in detail in
this work. The location of the observed magnetic anomalies with
respect to marine magnetic anomaly (28) observed earlier on the
Indian Ocean floor, were compared to a marine magnetic time
scale. To get a reasonable correlation between the observed and
theoretical anomalies requires a considerable amount of
adjustment in the spreading rate of individual magnetic blocks.
Also on the magnetic map the trend of the lineation of these
anomalies is perpendicular to the continental margin instead of
being parallel to the continental margin as expected for a rifted
continental margin. The presence of horst-and-graben structures
in the inland region suggests the rifted nature for the continental
margin off Karachi than the sheared nature. This indicates that
the lineations should be parallel to the margin. But the magnetic
lineations are perpendicular to the continental margin and if they
are from oceanic crust then they would suggest that the margin is
a sheared margin, which contradicts the extensional structures
observed inland. This makes it very unlikely that the source of
these anomalies is oceanic crust. However, it is quite possible that
the magnetic lineations observed in the map were parallel to the
continental margin initially but later on the continent rotated
clockwise along a fault landward of the magnetic lineation. This
rotation is perhaps responsible for making the lineation
perpendicular to the continental margin.
One objective of this study was to locate the continent-ocean
boundary, but with the available amount of data it is not possible
to decide on the most appropriate source for the observed
magnetic anomalies. Hence it was not possible to decide exactly
on the location of continent-ocean boundary. However, on the
basis of gravity and magnetic data it can be said that the
continent-ocean boundary lies at a distance of 500 km or greater
along the profile. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29074 |
| Date | 05 August 1991 |
| Creators | Soofi, Muhammad Asif |
| Contributors | Couch, Richard W. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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