Abstract We present a numerical model of the geothermal field of the Makran accretionary prism and of the slab being subducted below it. Calculated heat flow density values for the sea floor of the abyssal plain and the shelf slope are compared with in situ measured and bottom simulating reflector (BSR)-derived heat flow density values. The result suggests a predominance of conductive heat transport within the accretionary complex. Little evidence is found to suggest that fluid flow or frictional heat modifies the observed geothermal field to any great extent. We also studied the geothermal field associated with the decay of the potential gas hydrate layers (indicated by the presence of BSRs), as gas hydrate layers are being tectonically uplifted out of the gas hydrate stability field into shallower and warmer sea water. Theoretical considerations suggest a complete disappearance of gas hydrates at a water depth of about 750 m. The observed presence of numerous gas seeps almost exclusively at water depths of less than 800 m suggests that gas hydrate layers in the Makran accretionary prism act as a very effective cap rock to upward-directed flow of fluids containing notable amounts of dissolved gas from within the prism to the sea floor.

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.

Abstract The Iranian Makran has been entirely mapped geologically on a scale of 1:250 000, except for a narrow coastal strip, which exposes the very youngest Cenozoic sediments of the main Makran accretionary prism. The geology of the Makran is less widely known than the geology of Oman, because it has been published in detail only in reports of the Geological Survey of Iran. There is no extension of the geological formations of Oman into the Makran, the only extension of Oman ophiolitic formations into Iran being at Neyriz and Kermanshahr, hundreds of kilometres to the NW. This summary is based on field mapping, photo-interpretation being used only to connect traverse lines. The oldest rocks are metamorphic rocks, which form the basement to the Bajgan-Dur-kan microcontinental ‘sliver’, a narrow block that extends hundreds of kilometres from the Bitlis Massif in Turkey, through the Sanandaj-Sirjan Block of the Zagros, to north of Nikshahr in the east of the Makran. Other metamorphic rocks form the Deyader Complex near Fannuj on the southern margin of the Jaz Murian Depression. These include blueschists, and are thought to form the tip of the Tabas Microcontinental Block, largely exposed north of the depression. There is also a small microcontinental block to the east, the Birk Block, which exposes only Cretaceous platform limestones and Permian sediments. The Bajgan Metamorphic Series are overlain, with a tectonized unconformable contact, by highly deformed and disrupted platform carbonates of Early Cretaceous to Early Paleocene age (Dur-kan Complex), containing tectonic inliers of Carboniferous, Permian and, rarely, Jurassic age. Ophiolites occur in two structural positions. South of the Bajgan-Dur-kan Block, the tectonic Coloured Melange of the Zagros continues eastwards inland of the Bashakerd Fault; this includes two layered ultramafic complexes, one with chromitites. The blocks forming the melange include radiolarites and deep-water limestones of Jurassic to Early Paleocene age. Ophiolites developed north of the microcontinental block form three distinct igneous complexes, two layered and one with intermediate sheeted dykes. Intercalated in the volcanic rocks of these ophiolites are radiolarites and deep-water limestones ranging in age from Jurassic to Paleocene time. There are small developments of Cretaceous sediments carrying rudists in the extreme NW of the inner ophiolite tract. In the NE, ophiolites are developed in the Talkhab Melange. All these ophiolites represent former, largely Cretaceous, tracts of deep ocean. The Cenozoic rocks form two immense accretionary prisms. The main Makran prism includes Eocene-Oligocene and Oligocene-Miocene flysch turbidite sequences, estimated as individually > 10 000 m thick. Above these sequences, there is an abrupt passage up without any apparent unconformity, through reefal Burdigalian limestones, and locally a harzburgite conglomerate development, into neritic sequences with minor turbidites, extending into the Pliocene units. The Saravan accretionary prism to the east repeats tectonically three thick flysch turbidite sequences of Eocene-Oligocene age, but younger sediments are restricted here to minor Oligocene-Miocene conglomerates, unconformable on the above sequences. There is a line of OHgocene(?) granodiorite bodies within the Saravan accretionary prism. Intense folding and development of schuppen structure, dislocation and melanging of the sediments affected the entire region in Late Miocene-Early Pliocene time. Post-tectonic uplift was followed by scattered developments of fanglomerates beneath the fault scarps. The Neogene deformation has obscured earlier deformational events. There is unconformity beneath Eocene sediments representing a mid-Paleocene disturbance. There is also evidence of a discontinuity in mid-Oligocene time. Pliocene-Pleistocene fanglomerates are unconformable on folded rocks. There are discontinuous developments of Eocene-Oligocene neritic sediments unconformably above the older rocks (ophiolites, platform limestones, metamorphic rocks), and to the north of the southern edge of the Jaz Murian Depression, the northern limit of the Makran, there is evidence of the survival here of a very shallow sea through Neogene time and the formation of small patches of reefal Oligocene-Miocene limestones, and Eocene to Pliocene shallow-water clastic sediments. A 150 km wide tract separates the coast from the trench, the total Cenozoic accretionary prism being 500 km wide. Extension from the Murray Ridge affects the extreme east of the region. The Saravan accretionary prism, it is suggested, faced a gulf, comparable with the Gulf of Oman, and this Saravan Gulf filled up and closed up by Early Oligocene time. Seismological evidence suggests that there is now active continental collision continuing along this suture.

Abstract The prominent basement ridge in the center of the seismic profile is the Little Murray Ridge, which crosses the Gulf of Oman and passes obliquely beneath the Makran Continental margin a short distance to the east of this line. It is a volcanic ridge now largely buried beneath sediment, although elsewhere projecting through the sea floor. Parallel to this basement ridge and to the southeast lies the Murray Ridge proper which forms a major bathymetric feature charted by Barker (1966). The Murray Ridge marks the boundary between the Arabian plate to the north and the Indian plate to the south (see index map). It joins two active left-lateral fault systems, the Owen Fracture Zone at its southwestern end (Whitmarsh, 1979) and the Omach-Nal Fault zone in Pakistan at the northeastern end. A triple junction between the Arabian, Eurasian, and Indian plates has formed at the northeastern end of the Murray Ridge close to Karachi (Jacob and Quittmeyer, 1979). The illustrated profile runs from the Makran continental margin in the north to the edge of the Murray Ridge in the south. At the northern end it crosses an accretionary sediment prism formed by subduction of the thickly sedimented Arabian plate beneath the Eurasian continental plate to the north. Wdescribe some of the features, firstly of the accretionary prism, secondly of the Little Murray Ridge, and lastly of the sediments deposited over the basement ridge. This profile is a single channel display with no processing other than a linear ramp time variable gain triggered from the sea floor to enhance the deep reflections. The profiling system comprised a single 2.6 liter (160 cu in) airgun fired once every 35 m (115 ft) along the profile into a Geomechanique streamer. Toward the bottom of the seismic section at the northern and southern ends of the line, the water multiple appears (labelled "wm" on profile), but it arrives sufficiently late not to interfere with arrivals of interest to us. At the sea floor, the vertical exaggeration of the time sections is about 10:1. As depth into the sediment increases, the increase in seismic velocity causes greater vertical exaggeration. In the line drawing interpretation, the profile was migrated into a true-scale depth section using velocities from sonobuoy profiles (White and Klitgord, 1976; White and Louden 1982). The series of ridges and intervening basins in the sea floor seen on the northern 40 km (25 mi) of the profile represent the seaward part of the Makran accretionary prism. They are generated as packets of sediment are scraped off the oceanic Arabian plate which is subducting northward beneath the continental Eurasian plate at a rate of about 50 mm/yr (2 in/yr). The accretionary prism extends several hundred kilometers northward becoming exposed on land in the Makran. Sediments on the subducting plate are several kilometers thick, preventing the formation of a topographic trench at the subduction zone and causing the front of the accretionary prism to migrate seaward as great amounts of material are added to it.

Abstract Two long refraction lines parallel with the structural strike show that the Moho of the subducting oceanic Arabian plate dips north beneath the Makran accretionary prism at an angle of less than 2°. The downgoing plate carries a thickness of about 7 km of sediment into the accretionary wedge. The accreted sediment rapidly consolidates with a concommitant increase in seismic velocity. Slope sediments are trapped in narrow, well-lineated slope basins formed between uplifted fold ridges, and the dip of sediment layers within these slope basins records the progressive tilting of the margin towards the coast. We document the shapes of channels along which downslope sediment movement occurs. Decoupling layers within the sediments, caused by high pore pressures generated by the non-expulsion of pore water from overpressured shale sections, control the deformation in the frontal 70 km of the accretionary prism.