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Book Chapter

The Little Murray Ridge

By
R.S. White
R.S. White
Bullard Laboratories, Department of Earth Sciences, University of Cambridge
1
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Published:
January 01, 1983

Abstract

The prominent basement ridge in the center of the seismic profile is the Little Murray Ridge, which crosses the Gulf of Oman and passes obliquely beneath the Makran Continental margin a short distance to the east of this line. It is a volcanic ridge now largely buried beneath sediment, although elsewhere projecting through the sea floor. Parallel to this basement ridge and to the southeast lies the Murray Ridge proper which forms a major bathymetric feature charted by Barker (1966). The Murray Ridge marks the boundary between the Arabian plate to the north and the Indian plate to the south (see index map). It joins two active left-lateral fault systems, the Owen Fracture Zone at its southwestern end (Whitmarsh, 1979) and the Omach-Nal Fault zone in Pakistan at the northeastern end. A triple junction between the Arabian, Eurasian, and Indian plates has formed at the northeastern end of the Murray Ridge close to Karachi (Jacob and Quittmeyer, 1979).

The illustrated profile runs from the Makran continental margin in the north to the edge of the Murray Ridge in the south. At the northern end it crosses an accretionary sediment prism formed by subduction of the thickly sedimented Arabian plate beneath the Eurasian continental plate to the north. Wdescribe some of the features, firstly of the accretionary prism, secondly of the Little Murray Ridge, and lastly of the sediments deposited over the basement ridge.

This profile is a single channel display with no processing other than a linear ramp time variable gain triggered from the sea floor to enhance the deep reflections. The profiling system comprised a single 2.6 liter (160 cu in) airgun fired once every 35 m (115 ft) along the profile into a Geomechanique streamer. Toward the bottom of the seismic section at the northern and southern ends of the line, the water multiple appears (labelled "wm" on profile), but it arrives sufficiently late not to interfere with arrivals of interest to us. At the sea floor, the vertical exaggeration of the time sections is about 10:1. As depth into the sediment increases, the increase in seismic velocity causes greater vertical exaggeration. In the line drawing interpretation, the profile was migrated into a true-scale depth section using velocities from sonobuoy profiles (White and Klitgord, 1976; White and Louden 1982).

The series of ridges and intervening basins in the sea floor seen on the northern 40 km (25 mi) of the profile represent the seaward part of the Makran accretionary prism. They are generated as packets of sediment are scraped off the oceanic Arabian plate which is subducting northward beneath the continental Eurasian plate at a rate of about 50 mm/yr (2 in/yr). The accretionary prism extends several hundred kilometers northward becoming exposed on land in the Makran. Sediments on the subducting plate are several kilometers thick, preventing the formation of a topographic trench at the subduction zone and causing the front of the accretionary prism to migrate seaward as great amounts of material are added to it.

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Contents

AAPG Studies in Geology

Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces

A. W. Bally
A. W. Bally
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American Association of Petroleum Geologists
Volume
15
ISBN electronic:
9781629810188
Publication date:
January 01, 1983

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