R.S. White, 1983. "The Makran Accretionary Prism", 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 Makran continental margin of Pakistan and Iran forms the seaward part of a folded and faulted accretionary sediment prism which extends several hundred kilometers inland across the onshore Makran. The accretionary prism is formed as the 6 to 7 km (3.7 to 4.3 mi) thick pile of sediments overlying oceanic crust beneath the Gulf of Oman is scraped off the Arabian plate. The convergence rate between the subducting Arabian plate and the continental Eurasian plate to the north is about 50 mm/yr (2 in/yr). Although there is no well developed Benioff zone, the seismicity appears to deepen toward the north (Jacob and Quittmeyer, 1979), in a manner consistent with a shallow-dipping subduction zone. Between 400 to 500 km (249 to 311 mi) north of the coast is a chain of Cenozoic volcanic and plutonic rocks of andesitic to rhyolitic composition which may represent a volcanic arc (Farhoudi and Karig, 1977). The onshore Makran comprises a thick series of uplifted, faulted flysch deposits (Hunting, 1960; Ahmed, 1969) originally accumulated in an accretionary wedge. Subduction has been active since the Late Cretaceous (McCall and Kidd, 1982).
The submerged front of the accretionary prism has been traced over 900 km (559 mi) from the Straits of Hormuz in the west to near Karachi in the East (White and Klitgord, 1976; White and Ross, 1979; White, 1982). The Oman line marks the western limit and the seismically active left-lateral Chaman and Ornach-Nal fault systems form the eastern edge of the accretionary prism. This particular subduction zone forms one end-member of the many different expressions of convergent margins; in this case the sediment pile on the downgoing plate is very thick, generating a rather open imbricated stack of folded fault slices as material becomes incorporated into the accretionary prism.The profile illustrated here runs approximately from north to south across the seaward position of the accretionary prism. A detailed grid of seismic reflection profiles was made within a box extending from the Gulf of Oman abyssal plain onto the Makran continental margin, and this profile is a typical example chosen from the survey lines (heavy line on figure showing bathymetry gives location). Flat lying sediments from the Gulf of Oman first become folded in the frontal fold at the seaward margin of the accretionary prism. They are subsequently uplifted along presumed imbricate thrust faults to form a series of sediment ridges parallel to the coast and seen in cross section on our profile. The fold ridges are well lineated and are continuous across our detailed survey area. Between the ridges are ponded slope sediments which record continuing tilting as further thrust slices are accreted onto the front of the offscraped wedge. In the following sections we first describe the acquisition and display of the profile, then discuss the presence of prominent gas reflectors which if they go unrecognized could be erroneously interpreted as revealing structural detail, and lastly describe the structure of this accretionary prism, moving shoreward across the accretionary prism from the initially undeformed abyssal plain sediments onto the accreted sediment pile.
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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.