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Book Chapter

The Makran Accretionary Prism

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

Abstract

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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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

GeoRef

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