Abstract Petrographic and geochemical data for new basalt and peridotite samples recovered from sampling sites at the Southern Murray Ridge help to constrain models for the evolution of the Owen-Murray Ridge system, which forms the northwestern boundary of the Indian plate. Trace elements immobile during alteration (Ti, Zr, Nb, Y and rare earth elements) suggest that the altered microphyric metabasalt has affinities to magmatism of active margins (island-arc tholeiite sensu lato). It is distinctly different from mid-ocean ridge basalt, back-arc-basin basalt, or intra-plate Deccan Trap basalt. The sampled serpentinized harzburgite or clinopyroxene-poor lherzolite was deformed under mantle conditions and is similar to the mantle section of nearby ophiolite sequences. This association of rocks suggests that an ophiolite melange was sampled. However, results from sampling station 462 NIOP indicate that the Murray Ridge complex also contains igneous rocks with Deccan Trap affinity. For the emplacement of the island-arc tholeiite we assume an origin in a convergent supra-subduction setting, related to the closing of a Late Cretaceous Neo-Tethyan ocean basin between the Arabian and Indian plates to the south and the Eurasian plate to the north. Since Neogene time, the Murray Ridge-Dalrymple Trough has been underlain by attenuated (?)continental crust and characterized by extensional rift tectonics.

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 The margins of the Gulf of Oman Basin range from convergent at the north to translation at the west and east, and passive at the south. The basin's northern margin has been a site of continuous subduction since Cretaceous time, which has led to the creation of an 800 km long and 650 km wide accretionary wedge, most of which is above sea level. Strata in the centre of the Gulf of Oman Basin display minor deformation resulting from the northward tilting of oceanic crust. A basin-wide unconformity dividing these strata in two was the result of erosion during Early Oligocene time when bottom water circulation was enhanced during a climatic deterioration. The morphology of the basin's south margin is due to Early Triassic rifting, deposition during Jurassic-Early Cretaceous time, early Late Cretaceous ophiolite obduction and Late Cretaceous-Cenozoic deposition. The western side of the accretionary wedge, along the north side of the Gulf of Oman Basin, is in sharp contact with the western translation margin. Structures along this margin are the result of post-Eocene convergence of the Lut and Central Iran microplates. The eastern end of the accretionary wedge, however, is not in contact with the eastern transform margin, but is separated from it by a north-trending trough. The landward extension of this trough is defined by the north-trending Las Bela Valley. The eastern side of the accretionary wedge turns northward at 65°30'N along the west side of the trough and becomes aligned with the north-trending Ornach-Nal Fault along the west side of the Las Bela Valley. Similarly, the Murray Ridge complex turns northward at 25°N and becomes aligned with the north-trending Surjan Fault on the Las Bela Valley's east side. The Ornach-Nal and Surjan faults merge at the apex of the Las Bela Valley with the north-trending Las Bela-Chaman Structural Axis. Differences between the eastern and western sides of the accretionary wedge may be due to the presence of the Ormara microplate on the eastern end of the wedge, a plate that is being pushed ahead of the Arabian plate. The morphology of the Murray Ridge complex is the result of transtension and secondary compression along the Indian-Arabian plate boundary. We infer that most of the relief of the Murray Ridge complex resulted from a change in plate geometry in Early Miocene time. Subsequent tectonic Pliocene-Quaternary events have enhanced this relief.

Abstract In the northern Arabian Sea the Arabian, Eurasian and Indian Plates are in tectonic interaction with one another. We present interpretations of multichannel seismic profiles across the Makran sub-duction zone (which is part of the Eurasian-Arabian Plate boundary) and the transtensional Murray Ridge and Dalrymple Trough (which are part of the Arabian-Indian Plate boundary). We distinguish four megasequences in the sedimentary succession, which we correlate over the entire study area. Regional unconformities separate the megasequences and enable us to establish a common history of the region before Late Miocene time (c. 20 Ma). The Early Pliocene (c. 4.5 Ma) reopening of the Gulf of Aden caused a reorganization of the plates and subsequent tilting of the oceanic crust of the Arabian Plate toward the Makran subduction zone. This event is documented by the regional M-unconformity. Since that time, sedimentation on the Oman Abyssal Plain has been permanently separated from the Indus Fan by the Murray Ridge, on the northern end of which there has been no significant sedimentation.

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 Indus River system is one of the largest rivers on the Asian continent, but unlike the Ganges–Brahmaputra system, the drainage of the Indus is dominated by the western Tibetan Plateau, Karakoram and tectonic units of the Indus Suture Zone, rather than the High Himalaya. The location of the river system relative to the Indus Suture Zone explains the deep exhumation north of that line in the Karakoram, compared with the modest erosion seen further east in Tibet. The modern Indus cuts Paleogene fluvial sedimentary rocks of the Indus Group located along the Indus Suture Zone in Ladakh, northern India. After the final marine incursion within the Indus Group in the early Eocene (<54.6 Ma), palaeo-current indicators changed from a north-south flow to an axial, westward pattern, synchronous with a marked change in sediment provenance involving erosion of South Tibet. The Indus probably was initiated by early Tibetan uplift following the India-Asia collision. The river has remained stationary in the suture since Early Eocene time, cutting down through its earlier deposits as they were deformed by northward folding and thrusting associated with the Zanskar backthrust at c. 20 Ma. The Indus appears to have been located close to its present position within the foreland basin since at least Mid-Miocene time (c. 18 Ma), and to have migrated only c. 100 km east since Early Eocene time. In the Arabian Sea Paleogene fan sedimentation was significant since at least Mid-Eocene time (c. 45 Ma). Sediment flux to the mid fan and shelf increased during Mid-Miocene time (after 16 Ma) and can be correlated with uplift of the Murray Ridge preventing sediment flow into the Gulf of Oman, tectonic uplift and erosion in the Karakoram and western Lhasa Block, and an enhanced monsoon triggered by that same uplift. Sedimentation rates fell during Late Miocene to Recent time. The Indus represents 18% of the total Neogene sediment in the basins that surround Asia, much more than all the basins of Indochina and East Asia combined (c. 11%). Unlike the rivers of East Asia, which have strongly interacted as a result of eastward propagating deformation in that area, the Indus has remained uninterrupted and represents the oldest known river in the Himalayan region.

Abstract In 1997 Lasmo Oil Pakistan Ltd (Lasmo) gained a significant position in the offshore Indus Basin with the award of the Indus A and B Blocks. The main hydrocarbon play comprises Miocene shelf-delta sands interbedded with intraformational shale seals and sourced by gas-prone offshore equivalents. Approximately 12000 km of seismic data have been interpreted in the detailed evaluation of these blocks. However, only four wells have tested the preferred play type and no core or rock data were available to provide further insights into facies or age dating. Log data from two key wells in the offshore Indus area record the initial infilling of the basin by shale-dominated basinal or outer shelf sediments, followed by stacked thin-bedded sandstone-shale sequences of a shelf-delta nature. A zone of progradational sequences marks the transition between the two, but no other workable stratigraphic divisions were apparent. Regional seismic correlation established the diachronous nature of the prograding shelf package and this was matched by distinct bands of seismic progrades. A series of simple palaeogeographies of the prograding shelf margin were developed showing initial sediment input from the north and rapid progradation towards the south and west. The Oligo-Miocene basin fill of the offshore Indus Basin appears to be a 'one-step' fill process of a significant depocentre created between the Karachi Platform and the Murray Ridge. Canyons are a very distinct feature on seismic profiles and two main phases of development are apparent. The earlier phase is interpreted to be of Early Miocene age. Downcutting at this time rarely exceeded 400 m. The second phase of canyon development occurred during Plio-Pleistocene time, and these younger canyons often dominate the shallow section, with multiple phases of downcutting sometimes exceeding 1000 m. Where drilled, canyons of both ages have been found to be shale prone. These drilled canyons are interpreted to be on the palaeo-slope where erosion and sediment by-pass occurred during the active phase, and were subsequently filled by fine-grained deposits after abandonment. The two phases of canyoning are considered to relate to phases of tectonic activity in the collision zone between the western margin of the Indo-Pakistan plate and the Eurasian plate.