R.S. White, 1983. "The Little Murray Ridge", 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
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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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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.