Showing papers on "Continental margin published in 1979"

The Mesozoic South Atlantic Ocean and evolution of its continental margins

Abstract: Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas-Falkland fracture zone and the rifted margins of Argentina and southern Africa south of the Rio Grande Rise and Walvis Ridge, respectely. These margins have associated with them, for the most part, linear magnetic anomalies that can be modeled as edge effect anomalies separating oceanic from continental basement. Coincident with the magnetic anomalies are gradients in the isostatic gravity anomaly. We have taken the location of these geophysical lineaments on the African margin and rotated them clockwise to fit the anomalies on the Argentine margin. This fit, which gives us a new pole of total closing for the South Atlantic Ocean, obviates, for the most part, the gaps and overlaps observed in other reconstructions. The improved fit thereby suggests rigid plate behavior and minimum stretching of continental crust during the early opening of the southern South Atlantic Ocean.

Seismic Evidence for Widespread Possible Gas Hydrate Horizons on Continental Slopes and Rises

Abstract: Anomalous reflections in marine seismic reflection data from continental slopes are often correlated with the base of gas hydrated sedimentary rocks. Examination of University of Texas Marine Science Institute reflection data reveals the possible presence of such gas hydrates along the east coast of the United States, the western Gulf of Mexico, the coasts of northern Colombia and northern Panama, and along the Pacific side of Central America in areas extending from Panama to near Acapulco, Mexico. Suspected hydrates are present in water depths of 700 to 4,400 m and extend from 100 to 1,100 m subbottom. Geometric relations, reflection coefficients, reflection polarity, and pressure-temperature relations all support the identification of the anomalous reflections as the base of gas hydrated sediments. In most places, gas hydrate association is related to structural anomalies (anticlines, dipping strata), which may allow gas to concentrate and migrate updip into pressure and temperature conditions suitable for hydrate formation. The gas hydrate boundary can be used to estimate thermal gradients. In general, thermal gradients estimated from the gas hydrate phase boundary are higher than reported thermal gradients measured by conventional means.

Geometry of Subducted Slabs Related to San Andreas Transform

Abstract: Development of the San Andreas transform by rise-trench encounter in coastal California influenced the structural evolution of a large region within the adjacent continent. Continuation of arc magmatism and tectonism depends upon the presence of a subducted slab of lithosphere at depth beneath an arc-trench system. The lack of subduction at the transform plate boundary along the California continental margin led to the growth of a slab-free region beneath the part of the continental block adjacent to the San Andreas transform. Geometric analysis based on ideal assumptions predicts that generation of a lengthening transform by rise-trench encounter will also generate an expanding triangular hole or window in the slab of lithosphere subducted beneath the continent. One leg of the slab-window is the adjacent transform, but the orientations and lengths of the other two legs depend upon the relative motions of the three plates involved. By inference, arc volcanism and tectonism cannot persist across the no-sla...

Petrology and Provenance of Neogene Sand from Nicobar and Bengal Fans, DSDP Sites 211 and 218

Abstract: The Bengal-Nicobar submarine fan complex is part of a linked sedimentary chain consisting of molasse, deltaic, and flysch deposits resulting from the sequential closing of a remnant ocean basin. Ultimate sources for turbidite sand from this fan complex are the uplifted gneissic, sedimentary, and metasedimentary terranes of the Himalayas. Detailed point-counts of lithic grains, as well as standard QFL percentages, of 22 Neogene sand samples from DSDP sites 211 and 218 reveal very uniform compositions. Typical QFL percentages are: 55-30-15. Plagioclase/total feldspar is typically near 0.7. Lithic types are dominated by quartz-mica tectonite. quartz-mica aggregate, polycrystalline mica, and other sedimentary and metasedimentary varieties. Andesitic volcanic lithic grains are absent. The indicated provenance ("tectonic highlands") for Bengal-Nicobar sands contrasts markedly with that of sand and sandstone derived from magmatic arcs and rifted continental margins. Lithic populations of magmatic arc sand and sandstone are dominated by volcanic grains. whereas lithic populations of rifted continental margin sand and sandston are dominated by polycrystalline quartz and sedimentary grains. Two triangular plots of lithic grains distinguish the provenance of sandstone derived from major tectonic settings. Detailed point-counts of lithic grains from thick sedimentary accumulations of unknown tectonic setting are a powerful paleogeographic tool when used in the manner outlined here.

Geological and Geophysical Investigations of Continental Margins

Abstract: Knowledge of continental margins advanced rapidly during the 1970s. Multichannel seismic reflection whose cost formerly restricted its use largely to the immediate vicinity of shallow-water prospects has become more common in deeper waters. The use of the technique by government and academic groups helped solve basc structural and evolutionary problems of rocks of the deeper offshrore. Better sources and more sophisticated processing yielded better and deeper resolution of the data. To better disseminate new knowledge of continental margins, AAPG held three meetings in 1977 to review the current status of knowledge. The papers presented at those meetings are contained in this volume. There are 32 chapters divided into the following sections: Rifted Margins; Convergent Margins; Small Basin Margins; and Resources, Comparative Structure, and Eustatic Changes in Sea Level.

The Southern Uplands of Scotland: A Lower Palaeozoic accretionary prism

Abstract: The Southern Uplands contain 10 or more distinct deep-water stratigraphic sequences consisting of basalt and chert and/or graptolitic shale, below thick greywackes. These sequences are separated by major reverse strike faults. Adjacent sequences differ in sedimentary characteristics and age, indicating that they were originally deposited far apart. Within each fault-bounded tract the individual sequences are disrupted and repeated by further, less important, strike faults and by folding. In the N (away from the site of the Iapetus Ocean) the sequences are older than in the S, but the majority of the beds within each sequence young to the N. This structure is compared with modern accretionary prisms, which develop on active continental margins where the oceanic plate has a thick sedimentary cover.

Geometry of triple junctions related to San Andreas Transform

Abstract: Understanding the evolution of the San Andreas transform is a key not only to broad aspects of regional geology but also to the development of specific structural provinces. Though stable in the regional sense, the paired triple junctions at the ends of the transform have had transient unstable configurations wherever the trends of the prior trench and the newly developing transform were locally not collinear. The resulting instabilities were mainly of kinds inferred to induce extensional tectonics within a nearby region. Passage of the Mendocino fault-fault-trench triple junction northward along the central California coast coincided well with pulses of initial subsidence in Neogene sedimentary basins near the continental margin and with eruptions at local volcanic centers in the Coast Ranges. Passage of the Rivera ridge-trench-fault triple junction southward was associated with the rifting events that formed the California Continental Borderland and the Gulf of California.

Evidence for late Precambrian plate tectonics in West Africa

Abstract: In the Gourma and Iforas regions (Mali) rifting occurred around 800–850 Myr ago along the eastern margin of the West African craton with a triple point in Mali, the Gourma being interpreted as an aulacogen. Oceanic closure around 600 Myr led to a collision between the passive continental margin of the West African craton and an active continental margin to the east displaying island arc and marginal trough volcano–clastic assemblages bordering a deformed continental mass intruded by a high-level batholith. The suture is marked by a string of positive gravity anomalies corresponding to the emplacement of ultrabasic and basic rocks including perhaps ophiolites. East–West shortening was accompanied by the translation onto the West African craton foreland of nappes including internal nappes displaying high pressure–low temperature metamorphism assemblages.

Rifting and Subsidence of the Northern Continental Margin of the Bay of Biscay

Abstract: In the northeast Atlantic, DSDP drilling results, combined with intensive geophysical surveys, permit a proposed model of the structural evolution of a starved, passive continental margin. Environment and tectonics of the rifting phase have been established. Active rifting took place in Early Cretaceous time in a pre-existing marine basin in contrast to many subaerial rift systems. The overall tectonic style is characterized by a series of tilted fault blocks bounded in many cases by listric faults. The rotation of the blocks (20-30°) along listric faults reduced the thickness of the upper continental crust from 6 to 8 km to 4 to 5 km. Close to the near horizontal base of the listric faults, a strong horizontal reflector corresponding to the 6.3 to 4.9 km/s refraction interface has been interpreted as the boundary between the upper brittle and the lower ductile continental crusts. The Moho discontinuity, 25 km deep in the vicinity of the shelf break, is 12 km deep in the lower part of the margin. In this area the ductile part of the crust (6.3 km/s) is only 3 km thick. Drill, dredge, and seismic reflection data allow reconstruction of the topography of the sea floor at the end of rifting in Aptian time. In the axis of the rift system, submarine troughs 2.5 km deep existed. The mechanism of riftingis discussed. The thinning of the continental crust cannot beexplained by the 10 to 15 per cent of extension estimated for the upper brittle part. It is suggested that the ductile part of the crust is thinned by creep in response to tension in the continental plate. Knowing the topography of the sea floor at the end of rifting and the present depth of the Aptian datum, the absolute amount of subsidence can be determined on a transect of the margin after the beginning of accretion (late Aptian time). This value decreases continuously from the oceanic/ continental crust boundary (4000 m) to the shelf break. For each point of the margin, the subsidence versus time curve is an exponential, the time constant of which increases with depth. The post-rifting subsidence is essentially an isostatic adjustment to cooling of the lithosphere in which the continental crust previously has been thinned during the rifting process.

Subduction of continental lithosphere: Some constraints and uncertainties

Abstract: Two effects that contribute to the subduction of continental lithosphere are the negative buoyancy of the relatively cold mantle part of continental lithosphere and the pull of a downgoing slab of oceanic lithosphere on continental lithosphere trailing behind it. The mantle part of the lithosphere could continuously subduct about 10 km of continental crust, if the upper and lower crust could be detached from one another. This estimate, however, could be in error by as much as a factor of three, given the range of plausible values for the requisite parameters. The second effect is more difficult to estimate because of our ignorance of what proportion of the gravitational force acting on the downgoing slab is transmitted to the surface lithosphere. For various assumptions, subducted oceanic lithosphere could be expected to pull a short (tens of kilometres) or long (hundreds of kilometres) length of intact continental crust into the asthenosphere. Although we favor subduction of only a short length of intact continental crust, we cannot prove that a large amount of crust is not subducted. These calculations are for continental margins that meet subduction zones flush. If continental crust can remain intact, peninsulas and microcontinents might be subducted completely.

The spring transition in currents over the Oregon Continental Shelf

Abstract: Current meter moorings maintained over the Oregon continental shelf in 1973 and 1975 clearly show the difference between winter and spring oceanographic regimes and the rapid transition between the regimes. In winter the mean alongshore current is northward at all depths and strongest near shore; there is no mean vertical shear, no mean offshore density gradients, and mean sea level is high. In spring the mean alongshore current is weak near the bottom and strongly southward at the surface with a maximum over the mid-shelf; the strong vertical shear is balanced by strong offshore density gradients, and mean sea level is low. In both 1973 and 1975 the transition between these regimes occurs within about a week during a strong southward wind event. In many respects the transition event does not differ significantly from previous southward wind events, but the lateral density gradients established during the transition event exceed those of earlier events. The transition appears to be the result of a large cumulative offshore Ekman transport caused by local wind stress rather than by propagation of effects generated elsewhere. The lateral density gradients and the vertical shear established during the transition event subsequently persist, even during northward wind events with moderate onshore Elanan transport.

U.s. Geological survey core drilling on the atlantic shelf.

Abstract: The first broad program of scientific shallow drilling on the U.S. Atlantic continental shelf has delineated rocks of Pleistocene to Late Cretaceous age, including phosphoritic Miocene strata, widespread Eocene carbonate deposits that serve as reflective seismic markers, and several regional unconformities. Two sites, off Maryland and New Jersey, showed light hydrocarbon gases having affinity to mature petroleum. Pore fluid studies showed that relatively fresh to brackish water occurs beneath much of the Atlantic continental shelf, whereas increases in salinity off Georgla and beneath the Florida-Hatteras slope suggest buried evaporitic strata. The sediment cores showed engineering properties that range from good foundation strength to a potential for severe loss of strength through interaction between sediments and man-made structures.

The seaward margin of Belize barrier and atoll reefs : morphology, sedimentology, organism distribution, and late quaternary history

Abstract: The geological setting of Belize reefs. The geophysical anatomy of the southern Belize continental margin and adjacent basins. The morphology, sediments and organisms of the deep barrier reef and fore--reef. The Perireefal sediments. The composition and age of limestones from the reef front, wall and fore--reef. Petrography of limestones from the wall and fore--reef. Comparative anatomy, organism distribution and late quaternary evolution of modern reef margins. Sedimentation and diagenesis on the deep seaward margin of modern reefs

The Weddell gyre

Abstract: South of 50°S in the Atlantic sector of the Southern Ocean the water at all depths circulates mainly round an elongated gyre, westwards along the continental margin, northwards along the Antarctic Peninsula, and eastwards across the Atlantic Ocean. At its eastern end, in 20 to 30°E, the gyre is completed by southward movement in the deep and bottom layers, but there seems to be little evidence of southward movement near the surface. The boundary region between east and west winds, generally known as the Antarctic divergence, lies in 65 to 70°S, but there are stronger meridional gradients of atmospheric pressure and Ekman water drift some 5° farther north. The deep-water isotherms curve upwards near the maximum Ekman divergence, giving an indication of strong upwelling, but the near-surface isotherms curve downwards, indicating eastward flow. It seems likely that the low temperatures at all depths may be due to eastward flow as well as upwelling. The Antarctic Peninsula geographically contains the western end of the gyre; the factors that control the eastern end are not so clear. Recent studies indicate that the total outflow from the Weddell Sea, mainly in the bottom and deep layers, may be nearly as large as the Gulf Stream transport. Biological as well as physical problems require more information about the horizontal and vertical movements, the exchanges between the westward and eastward currents, and what happens at the eastern end of the gyre.

Long-term observations of bottom current and bottom sediment movement on the mid-Atlantic continental shelf

Abstract: Long-term in situ observations made at three locations on the mid-Atlantic continental shelf during 1975--1976 clearly show intermitten movement of bottom sediment caused by currents, waves, and other forcing mechanisms. In winter, storm-associated bottom currents greater than 30 cms/sup -1/ resuspended and transported sediments. Net water particle excursions during storms were about 20--30 km longshelf and 5--10 km cross-shelf. Wave-induced bottom currents also resuspended sediments during periods of low mean flow. Sediment motion was observed in summer, although bottom conditions were generally tranquil. Significant changes in suspended matter concentration were observed that were only partially related to bottom currents. These changes may have been caused by biological activity or advection. Bottom currents on the mid-Atlantic region of the continental shelf were characterized by a coherent, primarily cross-shelf tidal flow of 5--10 cm s/sup -1/ and a low-frequency longshelf component of 5--20 cm s/sup -1/. The longshelf current was coherent over length scales of 100 km at tidal frequencies and for motions with periods greater than 50 hours. For these longer periods the longshelf flow was coherent with wind stress, which implies that winds were a mojor driving force of the longshelf current. The cross-shelf current was not coherent at stationsmore » separated by 100 km except at tidal frequencies. Packets of high-frequency internal waves were observed during stratified conditions in summer with bottom currents as large as 20 cm s/sup -1/.« less

Geophysical review of the continental margins of eastern and western Canada

Abstract: The evolution and geophysical features of the continental margins of eastern and western Canada are reviewed in light of recent plate-tectonic concepts. The two margins are very different in age, s...

Petroleum Source Beds on Continental Slopes and Rises: Resources, Comparative Structure, and Eustatic Changes in Sea Level

Abstract: Continental slopes commonly are sites of high marine organic productivity and frequently contain reducing bottom conditions, quiet water, and intermediate sedimentation rates, all of which favor deposition of organic-rich sediments. These deposits typically have high percentages of aquatic organic matter with high petroleum yields as contrasted to relatively organic-lean shelf deposits which contain primarily low-yield terrestrial organic matter. Conversion of organic matter in potential source beds to oil and gas requires a combination of temperature and time. These variables are controlled primarily by the geothermal gradient, the rate of burial, and the age of the source interval. Most divergent margins need between 2 and 4 km of overburden for oil generation and from 3 to 7 km for gas generation. Typically cooler and younger convergent margins and deltaic margins must have even greater burial depth to achieve the same results. Continental margins, including present slopes and rises, can contain oil and gas source beds when minimum requirements of organic content, kerogen type, and thermal maturity are met. Migration and accumulation are most efficient, however, where reservoir sequences prograde over source beds in areas of structural complexity. Preservation of trapped petroleum requires effective seals and minimal structural readjustment after accumulation. All these conditions can be found on present slopes and rises, although they are not common, and must be considered as part of any economic evaluation of these largely untested deepwater realms.

Mamonia Complex, southwest Cyprus: Evolution and emplacement of a Mesozoic continental margin

Abstract: The Mamonia Complex in southwest Cyprus comprises an assemblage of Upper Triassic to Lower Cretaceous sedimentary rocks, Upper Triassic mafic igneous rocks, and subordinate metamorphic rocks, which together lie adjacent to the southwest margin of the Troodos ophiolitic complex and its Upper Cretaceous sedimentary cover. The sedimentary facies of the Mamonia Complex record the progressive development of a Mesozoic passive continental margin. Initial crustal subsidence associated with deltaic and turbiditic terrigenous clastic sedimentation was followed by continental rifting and then the genesis of marginal oceanic crust in Late Triassic time. Subsequent pelagic and hemipelagic deposition during Jurassic and Early Cretaceous time reflects passive continental margin subsidence. Shallow-water calcareous material derived from an adjacent carbonate platform, probably now located in southern Turkey, was deposited on the margin by gravity flows. After initial rifting, coarse terrigenous clastic input was minimal until a major influx of terrigenous material in Early Cretaceous time, possibly contemporaneous with an episode of renewed ocean-floor spreading in the area. Allowing for the 90° Tertiary rotation of Cyprus, the Mamonia rocks formed part of the northern margin of a small ocean basin during Jurassic and Cretaceous time. The Troodos Complex is likely to represent a younger surviving fragment of the same basin. By Late Cretaceous time, ocean-floor spreading had ended, followed in the Maastrichtian by disruption, folding, and emplacement of the Mamonia sedimentary sequences onto the Troodos ophiolitic rocks. The Mamonia sedimentary rocks are now arranged in subhorizontal sheets which, on the basis of fold vergence and facing directions, were emplaced toward the present northeast – that is, prior to paleorotation, toward the original southeast. The displaced sheets probably represent down-margin gravity slides. The evidence is compatible with both the Mamonia and Troodos Complexes having a local (“external”) origin in contrast to the currently favored “internalist” hypothesis involving long-distance transport.

Reinterpretation of the geology of Seram: implications for the Banda Arcs and northern Australia

Abstract: Reconnaissance field traverses in Seram have led to major revisions in the stratigraphy, structure, tectonic history and geological maps. The island is composed of 4 principal stratigraphical-structural elements: (1) metamorphic continental of uncertain structural status and palaeogeographical affinity, (2) an entirely marine early Triassic-Miocene imbricate succession regarded as para-autochthonous, (3) an allochthon composed of several different thrust sheets, including metamorphic rocks, Triassic limestone and a late Miocene olistostrome, (4) a Plio-Pleistocene post-orogenic autochthon. The apparently over-thrust slices of metamorphic basement complex can be interpreted as derived from either the Asian or Australian craton. The Australian shelf, slope and rise sediments, possibly including some oceanic sediment, are regarded as para-autochthonous. Remarkably close correlation is demonstrated between the stratigraphical breaks reported from the Mesozoic-Cenozoic succession of the NW Australian shelf, from Misool, and the para-autochthonous rocks of Seram and Timor. This emphasizes the presence of the Australian craton underlying these 3 islands. Close correlation is also found between the allochthonous rocks of Seram and Timor. Some of these thrust sheets are interpreted as having been derived from the Asian continental margin. The ultrabasic rocks of SW Seram and Ambon seem to form the highest thrust sheets. The main period of orogenesis, involving over-thrusting, olistostrome emplacement and imbrication of the underlying Australian cover-rock sequences occurred in the late Miocene-early Pliocene (N.18). The structural position of the volcanic rocks of Ambon is uncertain. A tentative interpretation is that they are in situ , having been extruded from deep-seated fractures that penetrate the 9Asian9 thrust sheets and the underlying Australian continental basement.

Tectonics of the western Gulf of Oman

Abstract: The Oman line, running northward from the Strait of Hormuz separates a continent-continent plate boundary to the northwest (Persian Gulf region) from an ocean-continent plate boundary to the southeast (Gulf of Oman region). A large basement ridge detected on multichannel seismic reflection and gravity profiles to the west of the Oman line is probably a subsurface continuation of the Musandam peninsula beneath the Strait of Hormuz. Collision and underthrusting beneath Iran of the Arabian plate on which this ridge lies has caused many of the large earthquakes that have occurred in this region. Convergence between the oceanic crust of the Arabian plate beneath the Gulf of Oman and the continental Eurasian plate beneath Iran to the north is accommodated by northward dipping subduction. A deformed sediment prism which forms the offshore Makran continental margin and which extends onto land in the Iranian Makran has accumulated above the descending plate. In the western part of the Gulf of Oman, continued convergence has brought the opposing continental margin of Oman into contact with the Makran continental margin. This is an example of the initial stages of a continent-continent type collision. A model of imbricate thrusting is proposed to explain the development ofmore » the fold ridges and basins on the Makran continental margin. Sediments from the subducting plate are buckled and incorporated into the edge of the Makran continental margin in deformed wedges and subsequently uplifted along major faults that penetrate the accretionary prism further to the north.« less

Paleogene anatexis along the Gulf of Alaska margin

Abstract: Early Tertiary plutons of biotite tonalite, granodiorite, and granite are found in a curvilinear 2,000-km-long belt along the margin of the Gulf of Alaska. These plutons intrude flyschoid rocks that were accreted to the continental margin during Late Cretaceous and/or early Tertiary time. Field, petrologic, age, and Rb-Sr data on plutonic and metamorphic rocks in eastern Chugach Mountains suggest that the granitic magmas were produced by partial melting of deeper parts of the accretionary prism after it was deformed against the continent. Such magmas may be a common product of heating at the termination of certain major accretionary episodes; they are not subduction-related magmas from subcrustal sources.

Late Quaternary extent of the West Antarctic ice sheet: New evidence from Ross Sea cores

Abstract: Ross Sea cores contain diatom floras and have sediment characteristics which suggest that grounded ice filled the Ross Sea embayment to the continental shelf margin during the last, and previous, glacial advances. Each successive advance reworked sediments from the previous interglacial period with older material and compressed them into a firm deposit with reworked (mixed and fractured) microfossils. As the grounding line retreated past each core location, subglacial meltwater, tidal pumping, and marine bottom-water flow winnowed light, less heavily silicified diatoms, leaving a lag of Eucampia balaustium in well-sorted, sandy sediment. Warm-water, open-marine conditions initially prevailed after deglaciation on the northern part of the continental shelf during summers. Open-marine conditions were gradually replaced by the pack-ice cover that today characterizes the Ross Sea continental shelf region.

Sequential development of the Appalachian orogen above a master decollement — A hypothesis

Abstract: Surface geology and seismic-reflection data suggest that rather than having a massive rooted central core the southern part of the Appalachian orogen from the Appalachian Plateau to the Atlantic continental shelf is underlain by an eastward-dipping decollement zone. We infer that this decollement zone was a long-lived structural element, intermittently growing from east to west during late Proterozoic to late Paleozoic time. Onshore displacement along the detachment surface was episodic through this long period of time, so that reliable estimates of total shortening for the entire orogen are not possible. Palinspastic restoration of only the western margin of the orogen, the area west of the Brevard zone, suggests a minimum of 280 km (175 mi) of Alleghenian displacement. Total accumulated shortening through time within the entire orogen must be considerably greater, probably exceeding hundreds of additional kilometres.