Summary of the Geology of the Makran Coast
J.C. Harms, H.N. Cappel, D.C. Francis, T.J. Shackelford, 1983. "Summary of the Geology of the Makran Coast", 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 following summary of the geology of the Makran coastal area is taken, with minor modification, from Harms and others (1982).
Makran comprises the southern part of Pakistan and Iran between Sonmiani Bay, near Karachi, and the Straits of Hormuz (Figure 1). The area consists of a great festoon of folded and faulted Tertiary sediments extending 800 km (497 mi) from the Las Bela fold belt on the east to the Oman Line on the west (Figure 2). These eastern and western boundaries separate Makran from older terranes with deformational styles and histories distinct from that of Makran (Farah and Dejong, 1979). To the east the Las Bela fold belt represents deformation associated with the collision of India with Asia whereas to the west the Zagros Mountains of Iran represent the convergence of the Arabian and Iranian plates. In both cases, blocks of continental crust have moved northward against other continental plates, closing former deep oceanic seaways. In contrast, along Makran, it appears that only oceanic crust has been subducted beneath a continental margin composed of small plates and complex ophiolite zones.
Coastal Makran and the area to the north is an accretionary wedge of deformed sediments ranging in age from perhaps Late Cretaceous to Recent, piled up at an oceanic subduction margin. The structure and depositional setting has been compared to "a typical arc model" composed of upper-slope deposits followed by lower-slope and trench deposits, progressively deformed by continuing subduction (Farhoudi and Karig, 1977). However, as an arc-trench system, Makran is hardly typical; indeed it is perhaps largely anomalous in its characteristics, as pointed out by Jacob and Quittmeyer (1979). The arc-trench gap is on the order of 500 km (311 mi), far wider than most systems. A possible Benioff zone is only weakly developed; shallow focal mechanism solutions indicate instances of tension in the downgoing oceanic slab. Volcanic centers are few and widely spaced along the arc feature. Additionally, a very large part of the accretionary prism is exposed, and a significant volume of post-middle Miocene sediments are shallow shelf deposits, not trench and slope deposits as alleged by Farhoudi and Karig (1977).
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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.