The Makran region differs from the other collisional (subducting) segments of the Arabian Plate in that here oceanic, rather than continental, crust is being subducted beneath the Eurasian Plate. East of Oman the Arabian Plate consists of oceanic crust that floors the Gulf of Oman and the adjacent Arabian Sea map. The oceanic crust of the Arabian Plate extends eastward to the Owen Fracture Zone along the Indian Plate boundary, southward along the Gulf of Aden spreading center, and northward where it descends beneath the Eurasian Plate at the trenchlying offshore from the Makran.
The diagram above relates the features seen at the Earth's surface to the position of the downgoing slab of the Arabian Plate (top of the plate is shaded in the lower half of the figure). The upper and lower parts of the diagram are at different vertical exaggerations, in order to show the pertinent features of both surface and subsurface regions of the plate (from Jacob and Quittmeyer, 1979).
The angle of descent of the Arabian Plate appears steeper than it really is, owing to the vertical exaggeration. The circles and triangles indicate locations of major earthquakes within 200 km of the line of the cross section. Most are related to bending of the plate or to slip on the interface between the two plates. Shallow-focus quakes beneath the volcanoes are probably due to movement of magma from deeper to shallower levels. The downgoing plate reaches the depth (~100 to 150 km) where melting begins, and where magma can be generated to feed volcanoes, farther inland here than in most subduction zones (e.g., Chile, Japan, Aleutian Islands, Guatemala). The reason lies in the age and density of the crust of the descending Arabian Plate, which is comparatively young and light; mature oceanic crust, as in the Aleutians, has become cooler, thicker, and denser and thus sinks faster. In addition, there are only a few major volcanoes in the Makran region because the rate of subduction is slow and new material is not generated rapidly enough to feed more; locations of volcanoes Bazman (BAZ) and Taftan (TAF) are projected into the line of the cross section.
The Arabian Sea floor and its cover of sediments, supplied primarily by the Indus River, is moving northward under the continent. As it does so, the sediments are scraped off and added to the bottom of the earlier accreted pile. In this way the older sediment packages are lifted up and eventually reach sea level (accretionary wedge in the diagram above); they are then subjected to erosion and the sediment is transferred back to the ocean floor by rivers. The remainder is incorporated into folded packages that are separated by thrust faults; the packages become progressively older inland, toward the original trench-slope break. Along that zone remnants of oceanic crust (ophiolite), which were involved in the original collision, mark the suture between the Arabian and Eurasian Plates; they have been driven landward over the Eurasian Plate margin during accumulation of the accretionary wedge. Seaward from the volcano line is a downwarped basin (Jaz Murian in this instance) that is the forearc basin, a sediment trap for eruptive material from the line of volcanoes and sediments from the accretionary wedge.
An ancient analogue to this complex is California. The Coast Ranges are of accreted material, the Central Valley is the forearc basin, and the Sierra Nevada represents the roots of the volcanic line.
|The Makran coastal region, at right in this westward viewtoward the Straits of Hormuz (H), consists of Indian Ocean floor that is being subducted under the Eurasian continent. The long ridges of the Makran are accreted submarine fan deposits derived from the sediment of the Indus River delta (off frame at lower right). They were deposited in the ocean trench that parallels the coast; as subduction continues, they have been jammed under the earlier sediment piles. J- Jaz Murian depression (J) is a forearc basin; volcanoes lie to the right (north).|
North of the Makran region is the Helmand block (H), a continental block that collided with Eurasia before the arrival of the Arabian Plate. Lut block (L) at left is another continental block. The complex transform boundary between the Indian plate (IP) and Eurasian plate extends up the right side of the photo. The few volcanoes (V) related to the subduction of the Indian Ocean floor beneath the Makran coast are visible north of the Jaz Murian forearc basin (J).
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