Abstract Since the breakup of the former Soviet Union, the Greater Caspian region has become available to foreign investors for developing existing hydrocarbon resources and for oil and gas prospecting. Having a large territory and a long history of oil production, the Greater Caspian has attracted many oil companies, large and small. However, a very significant uncertainty in the amount of proven and potential hydrocarbon reserves in the region exists mainly because of differences between Soviet and Western methods of reserve estimation. More definite reserve estimates are important for decision making in many spheres, including business and politics. In our study, we focus on compiling information about each geological basin in the region and evaluating reserve estimates that came from different sources, such as state government, exploration and production companies, and independent consultants. We then compare the Greater Caspian hydrocarbon reserves with other oil-producing countries to assess the market share of the Central Asian countries and to estimate their potential. Our results show that the Greater Caspian proven reserves account for only 3% of world oil reserves and 7% of gas.

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Seismic sections across the southern margin of the Caribbean reveal structures related to the convergence of the Caribbean and South American plates. The South Caribbean Deformed Belt and its eastward extension, the Curacao Ridge, is a zone of intensely deformed Cretaceous and Tertiary sediments that lies along the southern edge of the Colombia and Venezuela Basins. Undeformed sediments of the Caribbean basins abut the deformed belt abruptly to the north. To the south, the South Caribbean Deformed Belt gives way to older deformed belts of the Netherlands and Venezuelan Antilles Ridge and to the continental margin of Colombia and Venezuela containing pre-Tertiary structures. Along most of the South Caribbean Deformed Belt an apron of sediments progrades northward across the deformed belt suggesting active deformation at the northern edge of the belt and progressively older Tertiary deformation to the south. Caribbean oceanic crust extends southward beneath the deformed belt and southward-dipping reflections occur within the deformed belt possibly indicating slices of oceanic crust incorporated within it. Bottom simulating reflectors along parts of the deformed belt indicate the presence of gas hydrates. The chemical phase relationships of gas hydrates and the depth of the bottom simulating reflections indicate a thermal gradient of approximately 0.04 degrees/meter.

Abstract Sediment and upper crustal structure derived from sonobuoys, together with other geophysical evidence, divides the continental margin of western India into two provinces: the western basin and the eastern basin. The western basin exhibits well-developed sea-floor spreading type magnetic anomalies, whereas the eastern basin has no significant correlatable anomalies. The two basins are divided by the Chagos-Laccadive and the Laxmi ridges. The Laxmi ridge has a velocity structure similar to that of the eastern basin; however, the total crustal thickness of the ridge (more than 20 km) is much greater than that of the eastern basin. We propose that the western margin of India has a two phase evolutionary history: a phase of rifting followed by a phase of sea-floor spreading in early Tertiary time.

Abstract Recently acquired deep crustal refraction measurements made aboard R/V Vema in January and February, 1976, are combined with new and existing seismic reflection, shallow-refraction, gravity, magnetic and other data, to present a picture of the structure of the Magnetic Quiet Zone south of Australia from its surface morphology down to the mantle. We deduce that the quiet zone is bounded by primary discontinuities in the earth's crust that extend to great depths beneath the surface. The inner (landward) boundary is marked by a prominent magnetic trough, a steplike change in crustal thickness and velocity structure, and commonly an isostatic gravity high. Its outer (seaward) boundary is marked by anomalously shallow basement topography, and represents the sharp boundary with normal oceanic crust produced at the Southeast Indian Ridge. The crust within the quiet zone lies at a relatively uniform depth, but has a variable velocity structure. Computed seismic results indicate "continental" and "oceanic" sections and still others that are neither "oceanic" nor "continental." No systematic gradation from one kind to the others was observed. Rather, the quiet zone appears to be an inhomogeneous amalgamation of different crustal types. We propose that this type of crust may be unique, being neither continental nor oceanic. We suggest that the evolution of this margin took place in two stages. In the first stage, a continental rift valley about 150 to 200 km wide was formed. Its width stayed nearly constant while predominantly vertical motions took place within it and the unique rift crust evolved during a period of several tens of millions of years. During the second stage, on the other hand, horizontal motions dominated. The rift valley broke apart at its axis and seafloor spreading started. Thus, in contrast to a continuous evolution of the margin as many previous authors have suggested, we propose a two-stage evolution: the first in which the width of the rift valley remained constant but the crustal composition evolved, and the second in which the geometry changed (that is, drift took place) but the composition of the new oceanic crust remained relatively constant.

Abstract Three crossings of the Mid-Atlantic Ridge with the seismic profiler between Buenos Aires and Capetown: Buenos Aires and Dakar; and Dakar and HaTTfax have shown several important features ot the sediment distribution. The total accumulation is remarkably small, averaging 100–200 m. On the northern and middle crossings, the sediments are mainly in pockets, and intervening areas fire almost or entirely bare. A large percentage ot the pockets have almost level surtaces. These facts suggest that the sediments deposited on the ridge flow easily after reaching bottom here. Where impounded, the ridge sediments apparently develop cohesiveness and will not allow easily it subsequently tilted. The sediments are unstratified and remarkably transparent acoustically. Certain areas, particularly on the lower flanks ot the ridge, contain distorted sedimentary bodies that apparently indicate postdepositional tectonic activity. The basement surface on which the sediments rest is unitormly rough trom the crust of the ridge out to the lower flanks and continues so underneath the basin sediments. It is the upper surface of the intermediate layer (seismic velocity about 5 km/sec) that constitutes the upper 1-3 km of the oceanic crust. On the southern crossing the sediment layer tends toward uniform thickness across most of the section. This is-evidence that the ridge sediments here are mainly pelagic, and that the amount of sediment seen on the; record represents the total deposited. Sediment tores and ocean-bottom photographs provide additional information about sediment composition and distribution where the sediment cover is too thin to be measured by the profiler. The photographs also prov ide information about the presence ot currents capable of altering sediment distribution. The results suggest that the total accumulation in the oceans is small compared to that which would be inferred from any of the currently accepted estimates of-Cenozoic rates of deposition, but that the relative amounts of carbonate and red clay conform to the accepted ratio of their respective rates.

Abstract The magnetic anomaly lineation pattern in the North Atlantic Ocean (between the latitudes of 15° N. and 63° N.) has been examined in ligln of the hypotheses of sea-floor spreading and plate tectonic . There is no evidence of significant subduction or deformation along the,margins of the. Atlantic since the Late Triassiez asd thus the sea-floor spreading that has occurred since that, time has resulted in, continental drift only. The rate and direction of drift between Europe and North America and between Africa and North America have differed at all times since the Late Triassic. Although Eurasia may have been rifted from North America in the Jurassic, the major phase of drift did not begin until the Late Cretaceous. Separation varied from 5.0 to 4.0 cm/yr (at a latitude of 45° N.) from the Cretaceous until 53 m.y. ago. The rate of separation slowed about 53 m.y. ago. The average rate was slightly less than 2 cm/yr for ths intervals from 53 m.y. to 38 m.y. ago and from 38 m.y. to 9 m.y. ago. The sediment , discontinuity found by others at about the location of anomaly 5 on both flanks of the Mid-Atlantic Ridge, north of the Azores, thus cannot be explained by a discontinuity or drastic slowing in the rate of spreading. From 9 m.y. to the preset, separation has been at a rate somewhat greater than 2.0 cm/yr. The initiation of rifting between Africa and North America may have occurred 200 m.y. ago. However, we have assumed that the active ,phase of drift did not begin until 180 m.y. ago. The separation proceeded at an avenge rate .jof 4:0 cm/yr from 180 m.y. to 81 m.y. ago; 3.4 cm/yr from 81m.y. to 63 m.y. ago; 2.4 cm/yr from 63 m.y. to 39 m.y. ago; 2.0 cm/yr frdm f 38 m:y. to 9 nj.y. ago; and 2;8 cm/yr 9 m.y. ago to the present (the rates are computed for a latitude of 36° N.). We have fitted together lineations of the same age but from opposite, sides of the ridge axis in the same fashion that previous workers have fitted together continental margins. Each fit is described by a pole and angle of rotation about the pole. Each fit gives the paleogeographic relations of the respective continents ana oceanic plates for the particular age of the lineation. We conclude from these paleogeographic reconstructions that there was probably no Late Cretaceous (81 m.y. to 63 m.y. ago) sea-floor spreading, in tlfe Arctic, but that the relative motion between Erurasia and Nopth America in the Arctic region was compres stonal fturing this interval. This compression may have been accommodated by subduction at Bowers Ridge (which appears to be an inactive island-arc trench system) and subduttion in eastern Siberia. It also may have been accommodated by compressional deformation in the Brooks Range, the Verkhoyansk Mountains, and the Sverdrup Basin (in central northern Canada). All the spreading in the Arctic region that has occurred since the Late Cretaceous has taken.place in the last 63 m.y. The locus of this spreading had been the Mid-Arctic Ridge which lies between the Lomonosov Ridge and the Eurasian continental shelf. The effect of this spreading has been jto separate the pre-existing Lomorjosov Ridge from the , Eurasian ccontinental shelf. The Alpha Cordillera has not been the locus of sea-floor spreading in the Cenozoic. The exact pattern of the separation of Greenland from North America is not known. There may have been minor rifting in The Labrador Sea durine the Jurassic. However, the major phase of drift .occurred from the Late Cretaceous to the late pocene. The final separation of Eurasia (Spitsbergen), Greenland, and North America did not occur until the middle Eocene. The pattern of magnetic lineations suggests that the well documented counterclockwise rotation of the Iberian Peninsula occurred between the Late Triassic and the Late Cretaceous, and that there has been little if any counterclockwise rotation subsequent to time We have used the derived poles and the angular rates of rotation to compute isochrons Which give the age of the. basement in the North atlantic. The basement ages agree wellwith other data such as those obtained as the result of JOIDES-drilling. The isochrons sometimes give greater ages which can be reconciled with the, drilltig / results by.involving subsequent volcanism, but in, no case do the isochrons give smaller ages. The Keathey sequence of magnetic anomalies which lie just seaward bf the quiet zone and southwest of Bermuda in.the western Atlantic atjd north-west of Dakar in the eastern Atlantic, has been gjven an age of about 130 to 155 m.y. Comparison of the isochrons with, the magnetic lineations indicate that two important shifts of the ridge axis may have occurred The first,, in the region South ,of the New England Seamounts ana the Canary Islands was a 200- km eastward jump or migration that took place prior to 155 m.y. ago; the second in the region north-, of the New England Seamounts and Canary Islands but south of the Azores was a more complex westward . Shift of 150 km maximum extent that occurred between 135(?) m.y. ana 72 m.y. ago. We have also computed a pattern .of synthetic fracture zones or flow lines. Previous Workers have proposed that the South Atlas fault, the western Canary Islands, and the.NewEngland Seamounts lie along a fundamental fault or fracture zone. We note that these features are approximately. parallel one of these synthetic flow lines. The seaward escarpmentbounding the southern Bahamas as well is several well-surveyed fracture zones and other bathymetric-features are parallel to the synthetic fracture zones.