Active Margins, Part 4—Makran Fold Belt, Profile N 1804
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Active Margins, Part 4—Makran Fold Belt, Profile N 1804", 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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Profile N 1804 across the offshore part of the Makran fold belt in the northern Gulf of Oman was selected to show the structure of an accretionary wedge produced by subduction of a basement covered with thick sediments.In the framework of global tectonics, the Gulf of Oman is interpreted as the second largest oceanic gap (next to the Sunda arc) in the collision front between Eurasia and the Gondwana continents. This gap is interpreted as a remnant of the Tethys, between India and Arabia. The Makran fold belt represents the accretionary wedge of a subduction complex, which extends from the base of the continental slope of Pakistan and Iran to the northern border of Baluchistan.
There is no direct evidence for the age of the basement below the Gulf of Oman and its oceanic nature is not yet fully confirmed. All available data from regional geology and offshore seismic, however, suggest that the oceanic basement ranges in age from Late Cretaceous to early Tertiary. It is covered with 4 to 6 km (2.5 to 3.7 mi) of flat and level sediments.
The top of the basement can be traced on seismic below the accretionary wedge of folded and overthusted sediments in the Pakistan slope approximately as far as the shelf edge, where it is observed at a depth of around 15 km (9.3 mi).
Evidence from earthquake seismic indicates that crustal subduction takes place further inland below the Hamon-Rud basin in northern Baluchistan, a fore-arc basin in front of the Tertiary volcanic arc.In contrast to the rather chaotic accretionary prisms of the Java and Japan trenches, seismic sections across the frontal part of the Makran fold belt show an orderly set of overthrusts and thrustfolds which resemble the Canadian foothills type. Moving away from the thrust front in a landward direction, the anticlinal ridges become narrower and diapiric, and the synclines grow wider and deeper.
Based on the age of the volcanics in northern Baluchistan, it is assumed that subduction began in Eocene time. Line N 1804 indicates continuous deformation into Recent times. Individual thrusts produce elongated ridges with a local relief often in excess of 1,000 m (3,281 ft). Undisturbed, ponded sediments in synclinal areas are very thin. The reflection band about 1 sec below the seabed is interpreted as gas hydrate, a common feature in subsea fold belts.
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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.