Showing papers on "Continental margin published in 1971"

Thermal Effects of the Formation of Atlantic Continental Margins by Continental Break up

Abstract: Summary The thermal history of Atlantic continental margins resembles that of the oceanic crust as it spreads away from a mid-oceanic ridge, since the margin was formed when a ridge began spreading beneath a pre-existing continent. During break-up the thickness of the continental crust along the new margin was reduced by subarea1 erosion and subcrustal processes. Afterwards the continental shelf subsided, probably due to thermal contraction of the lithosphere. The observed subsidence rate on the Atlantic and Gulf coasts of the United States declined exponentially with a time constant of about 50My, as it does for ridges. Except for the Florida peninsula, deviations of the observed sedimentation from a smooth curve with respect to time could be associated with eustatic changes and variations in the supply of sediments. The subsidence rate of basins in the mid-continent of North America also decreases with a 50 My time constant. In Kansas a subcrustal process must have thinned the crust and initiated subsidence as a sequence of thinly bedded sediments beneath the basin is uneroded.

Origin and Emplacement of the Ophiolite Suite: Appalachian Ophiolites in Newfoundland

Abstract: The ophiolite suite is probably generated by axial plate accretion at oceanic ridges, and by diffuse slow spreading in marginal basins behind and within island arc complexes. The dunite/harzburgite component may represent the highest parts of a depleted upper mantle shell, about 16 km thick, from which a basaltic partial melt (approx. 30%) has been withdrawn to form the gabbros, dike swarms, and pillow lavas. Ophiolites have complex internal igneous, structural, and metamorphic relationships that are probably related to processes involved in their generation at ridges and in marginal basins, having no significance in terms of processes within the orogenic belt in which they are finally emplaced. Varying plate accretion rates may have a great influence on the structural and metamorphic patterns of ophiolites. The autochthonous sedimentary cap of ophiolites developed by axial accretion may, rarely, consist of continental margin non-volcanic flysch and salt, but usually comprises chert, argillite, and deep-water limestone: that of ophiolites generated in marginal basins probably consists, in addition to these facies, of andesitic flysch derived from adjacent island arcs. Ophiolites are probably emplaced, in orogenic belts, at consuming plate margins either beneath and behind oceanic trenches (subduction zones) or by thrusting onto continental margins (obduction zones) when a continental margin meets a subduction zone. Metamorphic relationships within and around ophiolite complexes are numerous, probably reflecting processes involved in both genesis and emplacement, and may involve (1) thermal and hydrothermal alteration at the site of origin, (2) blueschist metamorphism in subduction zones, (3) regional high-temperature metamorphism in island arcs, (4) high-temperature aureoles developed around peridotites injected into oceanic crust at time of origin, into the transition region between marginal basins and island arcs or into the regionally metamorphosed assemblages of island arcs, (5) garnet-amphibolites developed along the sole thrusts of obducted peridotites, which may represent transported high-temperature arc metamorphics, oceanic crust, or metamorphic rocks generated during obduction, (6) rodingites developed during low-temperature peridotite diapirism and serpentinization on ridge flanks or during ophiolite emplacement, and (7) amphibolites and eclogites, in subduction melanges, representing transformed oceanic crust ripped up from the downgoing plate beneath trenches. Obducted ophiolites often indicate a narrow time span between origin and emplacement, possibly due to the opening and closing of marginal basins. Evolving ridge/transform/trench and trench/trench/trench triple junctions have great significance for developing diachronous events associated with ophiolite emplacement along continental margins and island arcs. Early Ordovician ophiolites in the Newfoundland Appalachians have complex and variable structural relationships with rocks of the Paleozoic continental margin. Immense obducted ophiolite slices carry transported garnet-amphibolites, and are believed to have developed in a marginal basin behind early Paleozoic island arcs on the northwestern (present) margin of a proto-Atlantic Ocean.

Andean orogeny and ocean floor spreading.

Abstract: The periodicity and eastward migration of the Andean orogeny is explained in terms of periodic loss of marginal continental crust by crustal subduction.

Crustal structure of the Melanesian Area

Abstract: Seismic refraction studies in the Melanesian Borderland (the area between Australia on the west and New Zealand and Tonga on the east) show extreme diversity of crustal structure. The Lord Howe rise and the Norfolk ridge are topped by thick sediments and have deep crustal roots and thick layers of material with the same compressional wave velocity as the Australian continental crust. The Kermadec and Lau ridges, on the other hand, have structure and velocities typical of normal island arcs. The South Fiji basin structurally resembles an oceanic area with additional sedimentary fill. The western part of the Fiji plateau has thin sediments and has the structure of an area of normal deep-ocean basin that has been uplifted two kilometers. All the features are compatible with the hypothesis that the area has been disrupted and fragments of continental material have been separated from the Australian mass.

Early Paleozoic ophiolite complexes of the Newfoundland Appalachians as mantle‐oceanic crust sequences

Abstract: The layered ultramafic-mafic complexes of the Northwest platform and the Fleur de Lys orthotectonic zones of the Newfoundland Appalachians are comparable to layered Tethyan ophiolite suites of the Piedmont Alps, the Dinarides, and the Oman. They can be interpreted as oceanic crust and mantle: the peridotites, with high-pressure mineral assemblages, kaersutite, and titaniferous phlogopite, represent the upper mantle; the overlying gabbros and ‘sheeted’ diabases represent layer 3 of the oceanic crust; and pillow lava, chert, and the greywacke unit represent layers 2 and 1. As oceanic lithosphere the ophiolites may represent an intrasialic small ocean basin rather than a major ocean basin such as the Atlantic or Pacific. The early Ordovician thrust emplacement of the ophiolite sheets followed tectonism and metamorphism of a thick sequence of late Proterozoic-Cambrian clastic sediments (Fleur de Lys supergroup) that was deposited as a marginal wedge during the early stages of the development of the Appalachian geosyncline within a composite ‘North American-European’ continental plate. Their emplacement may reflect an early phase in the closing of an Appalachian ocean basin by underthrusting of the North American plate along a southeasterly (present geographic coordinates) dipping Benioff zone. Final closing of the ocean basin during the late Ordovician was effected by subduction of oceanic crust, also along a southeasterly dipping Benioff zone, during the northwest drift of the European plate. Alternatively, the ophiolites may have been emplaced directly onto the Fleur de Lys rocks either (1) at the time of inception of an ocean-forming ridge within a geosyncline that was initiated on continental sialic basement, or (2) during an ocean-closing phase of drift while a ridge was positioned relatively close to the continental margin, or was newly developing within the margin. The Newfoundland ophiolites may represent the oldest known oceanic lithosphere within the North American continent.

Evolving subduction zones in the Western United States, as interpreted from igneous rocks.

Abstract: Variations in the ratio of K2O to SiO2 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.

Focal mechanism of a shock in the middle of the Nazca Plate

Abstract: Body and surface waves for a shallow shock that occurred on November 25, 1965, in the middle of the Nazca plate indicate a double-couple dip-slip source with a horizontal pressure axis in the east-west direction. This suggests that the Nazca plate is being compressed in the direction of plate motion. A focal depth of 9 km below the ocean bottom was precisely determined from a combination of body- and surface-wave data. For a dip-slip source, the azimuthal radiation pattern for Rayleigh waves is strongly frequency dependent, and this characteristic can be used for an accurate focal depth determination. The maximum effect of the continental margin on surface-wave amplitude was estimated assuming conservation of energy without reflections or changes in mode. Love waves are potentially more affected than Rayleigh waves by the continental margin. The effects on Rayleigh- and Love-wave amplitudes due to the continental margin were measured between Galapagos and Quito.

Mesozoic Geology and the Opening of the North Atlantic

Abstract: The bearing of Mesozoic geology on the time and character of the opening of the North Atlantic is investigated. Of the alternative initial fits of the continents, that along the boundaries of the Quiet Magnetic Zones is preferred. Igneous and tectonic activity along the continental margins, the nature and distribution of epicontinental sediments, and several features of fossil distribution are briefly reviewed and the following interpretation tentatively proposed. Although some tension and subsidence occurred in the late Triassic, it is believed that the continental fragments did not begin to move apart until the Jurassic. Based mainly on the collapse of the Mediterranean carbonate platform, faunal divergence between Europe and Africa, and igneous episodes in the Atlantic region-the earliest likely date of opening of the southern North Atlantic is late Lower Jurassic, although it could possibly have been late Upper Jurassic, a time of tectonic disturbance on the eastern margins. Rifting extended northward...

Geologic framework and petroleum potential of the Atlantic Coastal Plain and continental shelf

Abstract: The Atlantic Coastal Plain and Continental Shelf of North America is represented by a belt of Mesozoic and Cenozoic rocks, 150 'to 285 miles wide and 2,400 miles long, extending from southern Florida to the Grand Banks of Newfoundland. This belt of Mesozoic and Cenozoic rocks encompasses an area of about 400,000 to 450,000 square miles, more than threefourths of which is covered by the Atlantic Ocean. The volume of Mesozoic and Cenozoic rocks beneath the Atlantic Coastal Plain and Continental Shelf exceeds 450,000 cubic miles, perhaps by a considerable amount. More than one-half of this is far enough seaward to contain marine source rocks in sufficient proportion to attract exploration for oil. A larger fraction, perhaps three-quarters of the volume, may be of interest in exploration for gas. The Coastal Plain consists of land between the crystalline rocks of the Piedmont province of the Appalachian Mountain System and mean low tide from southern Florida to the tip of Long Island plus a few small offshore islands and Cape Cod. This is an area of more than 100,000 square miles. The continental shelf extends from mean low tide to the break marking the beginning of the continental rise, which is somewhat less than 600 feet in depth at most places. It is a gently sloping platform, about 350,000 square miles in area, that widens from less than 3 miles off southern Florida to about 285 miles off Newfoundland. The Blake Plateau occupies an area of about 70,000 square miles between the 500and 5,000-foot bottom contours from the vicinity of Cape Hatteras to the northernmost bank of the Bahamas. It has a gentle slope with only minor irregularities and scattered patches of Holocene sediments. Gravity and magnetic anomalies along the Atlantic coast primarily reflect compositional differences in the earth's crust at great depths, but they are also related to some extent to the structure and composition of the Coastal Plain sedimentary rocks and shallow basement. Four alternating belts of predominantly positive and predominantly negative Bouguer gravity anomalies extend diagonally across the region from southwest to northeast These correspond roughly with the continental rise and slope, the continental shelf and Coastal Plain, the Appalachian Mountain System front, and the Piedmont Plateau-Blue Ridge-Appalachian Basin region. Long, linear, northeastward-trending magnetic anomalies roughly parallel the Appalachian Mountain System and the edge of the continental shelf. These trends are interrupted along the 40th parallel, about 50 miles south of New York, by a linear anomaly, suggesting a transcurrent fault, more or less alined with a string of seamounts extending down the continental rise to the abyssal plain. The trends parallel to the Appalachians terminate in Florida against a southeasterly magnetic trend thought by some to represent an extension of the Ouachita Mountain System. One large anomaly, known as the slope anomaly, parallels the edge of the .continental shelf north of Cape Fear and seemingly represents th** basement ridge located previously by seismic methods. Structural contours on the basement rocks, as drawn from outcrops, wells, and seismic data, parallel the Appalachian Mountains except in North and South Carolina, where they bulge seaward around the Cape Fear arch, and in Florida, where the deeper contours follow the peninsula. Tl * basement surface is relatively smooth and dips seaward at rrtes ranging from 10 feet per mile inland to as much as 120 fret per mile near the ocean. A decided steepening of the slope is apparent below a depth of 5,000 feet in most of the area. Tve principal structural features are the Southwest Georgia embayment, South Florida embayment, Peninsular arch, Bahama uplift, Southeast Georgia embayment, Cape Fear arcl, Salisbury embayment, Blake Plateau trough, Baltimore Canyon trough, Georges Bank trough, and Emerald Bank trough. Triassic, Cretaceous, and Tertiary rocks crop out roughly parallel to the present Atlantic coastline. Triassic outcrops are confined to scattered down-faulted basins within tl * piedmont. Lower Cretaceous outcrops are recognized in tl«> Salisbury embayment of New Jersey, Delaware, Maryland, a^d Virginia, and may be represented farther south as thin clastic beds mapped with the basal Upper Cretaceous. Upper Cretaceous rocks crop out almost continuously along the Far Line from eastern Alabama to the north flank of the Cape Fear arch in North Carolina and from Virginia to New York. Tertiary rocks crop out in broad patterns throughout the Costal Plain except on the Cape Fear arch and where masked by a veneer of alluvial deposits. The Cretaceous and Tertiary rocks exposed from southern Georgia northward to Long Island are mainly continental elastics interspersed with some thin lignitic layers and marl beds. Seaward, these rocks become marine in character and thicken to more than 10,000 feet at the coastline. Cretaceous rocks do not crop out in southern Georgia and Florida, and Tertiary rocks are only partially exposed. Both are predominantly marine carbonates in the subsurface and exceed 15,000 feet in thickness in the Florida Keys and Bahama Islands. The subsurface correlations of the Mesozoic and Cenozoic rocks beneath the Coastal Plain are traced along eight cross PETROLEUM POTENTIAL, ATLANTIC COASTAL PLAIN AND CONTINENTAL SHELF sections. One section extends subsurface correlations from the marine carbonate facies beneath the Florida Keys northward into the mixed marine and continental clastic facies beneath Long Island. The other sections trace units of the predominantly clastic outcrops downdip into marine facies along the coast. The pre-Mesozoic basement rocks beneath the Coastal Plain are primarily igneous and metamorphic rocks of Precambrian and Paleozoic age. Some Paleozoic sedimentary rocks ranging from Early Ordovician to Middle Devonian in age are in the basement in northern Florida. The oldest rock recovered from the sea bottom along the Atlantic coast has come from the Paleozoic granite pinnacles at a depth of about 30 feet on Cashes Ledge near the middle of the Gulf of Maine. Triassic(?) rocks, which consist of red arkose, sandstone, shale, tuff, and basalt flows, in places intruded by diabase, are present in downfaulted basins in the basement. Rocks of Late Jurassic or Early Cretaceous (Neocomian) age are present beneath southern Florida. There the sequence, as much as 1,100 feet thick, consists principally of limestone, dolomite, and anhydrite with a marginal clastic facies at the base where it rests on igneous basement. Equivalent rocks about 900 feet thick are present at Cape Hatteras, N.C., and extend northward along the coast into New Jersey. In Florida, the Lower Cretaceous rocks, subdivided into rocks of Trinity, Fredericksburg, and Washita age, are predominantly carbonates and exceed 6,700 feet in thickness beneath the Florida Keys. Northward along the coast, the rocks wedge out on the Peninsular arch and then reappear as a thin clastic unit across Georgia and South Carolina. They are missing from the higher parts of the Cape Fear arch in North Carolina but are present on the east Sank as a thickening wedge of mixed clastic and carbonate rocks more than 2,800 feet thick at Cape Hatteras and 2,600 feet thick in Maryland. Lower Cretaceous rocks probably extend into northern New Jersey but do not reach Long Island. Lower Cretaceous submarine outcrops are present in the Blake Escarpment. Upper Cretaceous rocks, which can be subdivided into rocks of Woodbine, Eagle Ford, Austin, Taylor, and Navarro age, are about 1,200 to 3,000 feet thick in wells along the coast. In Florida, they are almost totally marine carbonates. These grade northward along the coast into mixed marine carbonates and elastics in North Carolina and then into marine and continental elastics beneath Long Island. Rocks of Taylor and Navarro age have been dredged from Oceanographer and Gilbert Canyons off Georges Bank and rocks of probable Woodbine age from the Blake Escarpment. In addition, cobbles of chalk containing Cretaceous Foraminifera have been found in a core from the floor of Northeast Providence Channel, 11,096 feet beneath the sea between the Bahama Islands, and reworked Cretaceous Foraminifera have been identified in a core of coarse glauconitic sand on the continental rise, 155 miles southwest of Cape Hatteras. Tertiary rocks and thin Quaternary deposits are present along the Atlantic coast. The thickness of Tertiary rocks along the coast ranges from 4,300 feet in southern Florida to 130 feet on Long Island. In general, the Tertiary rocks are predominantly carbonates along the southern half of the Atlantic coastline and are mostly sandstone and limy shale along the northern half. Marl of early Miocene age crops out on the fishing banks known as Black Rocks off the coast of North and South Carolina. The Ocala Limestone of late Eocene age is not far beneath the sea bottom where artesian submarine springs issue along the east coast of Florida. Short cores and dredgings of Tertiary rocks, mostly Late Eocene (Jackson) and younger in age, have been recovered at n^re than 3 dozen localities concentrated lor the most part between Georges Bank and the Hudson Canyon and ir the Blake Plateau-Bahama Banks region. Pleistocene silts an^ clays have been found in many cores, and gravel afcd boulde-s of glacial origin have been dredged north of New York City. Tertiary strata beneath the continental shelf have been penetrated by two test holes about 10 miles off Srvannah, Ga. The test holes, which stopped in the Ocala Limestone, revealed that rather uniform thicknesses of Oligocene, lover Miocene, and middle Miocene strata extend from the shore seaward for at least 10 miles; that the upper Miocene rocks and the Pleistocene and Holocene deposits decrease in thickness seaward; and that only the Oligocene rocks exhibit a pronounced facies change, from carbonates to elastics in a seaw

Small Plate Tectonics in the Northeastern Pacific

Abstract: Lithospheric plate motions in the northeastern Pacific were complicated at about 2.5 m.y. B.P. by the movement along a major northeast-trending fault cutting Cascadia Basin. An estimate of the slip rate along this fault gives critical information on the relative motions of four geometrically interdependent blocks. The fault is presently inactive. Seventy km of slip along this fault during 2 m.y. or less gives an average slip rate of about 3-5 cm/yr or greater, and resulting plate motions suggest a significantly greater rate of net subduction along the continental margin off Oregon than off Washington and Vancouver Island. Subduction rate off Oregon is less sensitive to slip rate along this fault than is subduction off Washington.

The deep structure of the Iceland-Faeroe Ridge

Abstract: Two long seismic refraction lines along the crest of the Iceland-Faeroe Ridge reveal a layered crust resembling the crust beneath Iceland but differing from normal continental or oceanic crust The Moho was recognised at the south-eastern end of the lines at an apparent depth of 16–18 km A refraction line in deeper water west of the ridge and south of Iceland indicates a thin oceanic type crust underlain by a 71 km/s layer which may be anomalous upper mantle

Continental Margin in Chile—Is Tectonic Style Compressional or Extensional?

Abstract: Sea-floor spreading and continental drift seem to imply, for the Pacific segment of South America, upthrust of the continent and compression along its margin. Geologic evidence, however, indicates extension in this area since at least the Miocene. Andean folding has migrated east, away from the Pacific and toward the continent, whereas the tectonic development near the continental margin, which is backland of Andean orogeny, is retrograde and has resulted in a breakdown of the crust mainly along old lineations. There extensional stress dominates the upper crust over an area 300-400 km wide. The observed extensional deformation is not necessarily incompatible with the forces and stress conditions as postulated for an active continental margin. Compression farther inland away from this margin, and continentward migration of fold belts appear to be related directly to the destructive processes that occur in the continental border area.

Timing and Coordination of Orogenic, Epeirogenic, and Eustatic Events

Abstract: The Devonian-Mississippian Antler Orogeny is analyzed and reasons are advanced to show that its movements were felt all around western and Arctic North America. The Antler Orogeny began in the Franklinian Geosyncline during late Early Devonian time and became pervasive in all Arctic regions by the beginning of the Late Devonian. Antler erogenic events migrated southward during the Devonian and reached a climax along the whole span of continental margin, from Ellesmere Island to California, at about the beginning of the Mississippian. The Acadian Orogeny of eastern North America involved deformation of similar timing, suggesting that the two named orogenies are mirror-image events of a single master orogeny. The timing of the three other major North American orogenies of the Paleozoic and Mesozoic is analyzed more briefly. During the Middle Ordovician to Cretaceous interval, the cratonic interior of North America underwent four major onlap-offlap cycles of sedimentation called sequences. Evidently orogeny, epeirogeny, and eustasy act in concert in response to the same driving mechanism, because the timing of orogeny in the geosynclines and of the spread of marine waters to their maximum extent on the cratonic interior is found to coincide with remarkable accuracy during three of the four orogenies, and to be permissively in accord with this correspondence of timing in the fourth. The name “Antler Effect” is suggested for this fundamental relation. Because eustatic sea-level fluctuations are inadequate by themselves to explain the cyclic nature of Permo-Carboniferous sediments of the North American cratonic interior, consideration of the Antler Effect suggests pulsating orogenic movements at the continental margins during that time. In terms of plate tectonic theory, the North American and the European and African plates, juxtaposed during the Carboniferous-Permian, still must have been subject to the driving forces that brought them together, resulting in alternating compressional and relaxatory movements, offering an explanation of the diastrophic theory for the origin of cyclothems. Interpretation of orogeny by investigation of the cratonic sedimentary sequences that are deposited in concert requires a view of orogeny as a long-lasting series of tectonic events separated by short anorogenic times in the geosynclines—a concept at odds with those offered up o t this time. If orogeny is the result of plate convergence, then transgression of epicontinental seas occurs when plates move. Short anorogenic interludes, corresponding to times of regression, appear to occur when the active vectors of the plate-driving force are reorienting. Eustatic sea-level fluctuations, resulting from the activity of oceanic rises, and epeirogenic warping of the continents, acting together, can explain the timing and paleogeographic patterns of the transgressive-regressive cycles on the continents. Because orogeny can occur simultaneously on opposite sides of a continental block, the driving mechanism for plates must involve processes which are independent of the plates themselves.

Structure of Continental Margin off Punta del Este, Uruguay, and Rio de Janeiro, Brazil

Abstract: The results of 33 seismic refraction profiles recorded along two lines, one west-east between Punta del Este and the Rio Grande rise and the other southeast-northwest between the Rio Grande rise and Rio de Janeiro, are presented in two structure sections, with continuous seismic reflection profiles recorded parallel with the refraction lines. Plots of magnetic and gravity data recorded along the reflection lines are shown with the seismic data. These data, supplemented by previously reported measurements on the northern Argentine shelf and in the Argentine basin, are compiled in a generalized structure map. The seismic section across the Uruguayan continental margin is similar to one previously measured eastward of Rio de la Plata. The continental margin south of Rio de Janeiro contains a sediment-filled shelf embayment and a marginal plateau which extends 450 km out to sea. Possible velocity-age correlations are given for the seismic sections.

Transitional Tectonics and Late Cenozoic Structure of the Continental Margin off Northernmost California

Abstract: The continental margin north of the Cape Mendocino triple junction is a region of tectonic transition between structures related to underthrusting on the continental slope and those related to right-lateral strike-slip along the coast. Underthrusting of late Cenozoic rocks of the continental slope by the Gorda lithospheric plate is seen by the presence of magnetic anomaly 3, age 5 m.y., beneath the slope. Folds and faults mapped by reflection profiling on the continental slope are parallel to it, and the faults have dip separations predominantly with west side down. These structures are best explained as resulting from oblique underthrusting, with structural trends controlled by the slope direction. Northwest-trending faults occur along the coast and reported earthquake mechanism solutions show right-lateral first motions. These faults and related folds affect Mesozoic rocks and parallel the San Andreas fault system which lies to the south, most probably sharing its origin—that of shear interaction between the Pacific and North American lithospheric plates.

Electrical Conductivity Structure by Geomagnetic Induction at the Continental Margin of Atlantic Canada

Abstract: Summary Geomagnetic variations measured at 10 stations in Atlantic Canada show significant laterally inhomogeneous induction. Transfer functions giving the systematic dip angle and direction of the variation field lines have been computed for periods from 20s to 120min and the results numerically modelled. High electrical conductivity exists starting at a depth of 15 km or less under the continental shelf off Nova Scotia and Newfoundland and perhaps under the Bay of Fundy. The apparent coast effect with a maximum at 30-min period results from the contrast between the highly conducting shelf structure and the more resistive inland rocks. The most likely explanation of the high conductivity is that there is highly saline interstitial water in the lower part of a 10km sedimentary section associated with evaporite, salt layers, or that part of the crust is hydrated in this area. Strong electric currents flow in the various arms of the Gulf of St Lawrence for short period (10 s to 10 min) inducing fields. Numerical models show that they can be explained by local induction in the shallow sea water. The details of the current flow indicate that significant conductive channelling must occur. One station on the north shore of the St Lawrence River has large long period (30 min) anomalous vertical fields. They probably result from a contrast in deep conductivity between the Appalachian and Canadian Shield geological provinces.

Evidence of foundered continental crust beneath the central Tyrrhenian Sea.

Abstract: THE dredging of schists, phyllites and marbles from the faulted margin of a tilted crystal block in the central Tyrrhenian Sea shows that the acoustical basement beneath the centre of this sea basin includes a sequence of rocks similar or perhaps identical to the Palaeozoic and Triassic schists and phyllites of the adjacent Apennine, Calabrian and Sicilian chains, the Pontian Islands1 and Sardinia. Even the low to medium grade metamorphism observed must have occurred beneath the Earth's surface and following metamorphism and deformation we infer that these rocks were uplifted, denuded by subaerial erosion and finally foundered more than 3,000 m below sea level. The Neogene subsidence is still continuing. The metamorphic rocks obtained from the Tyrrhenian acoustic basement appear to support the former existence of the Tyrrhenides and indicate that this ancient upland was underlain by continental crust.

Submarine Pingos in the Beaufort Sea

Abstract: Numerous underwater mounds found on the continental shelf of the Beaufort Sea are thought to be pingos (hills that have a central core of ice) which have formed in the marine environment subsequent to oceanic transgression.

Phanerozoic history of Western Australia related to continental drift

Abstract: From north to south, the sedimentary basins of Western Australia change from broad platforms of wholly marine strata that span the entire Phanerozoic (Bonaparte Gulf and Canning Basins) through the intermediate Carnarvon Basin to rifts of nonmarine Permian and Mesozoic strata (Perth Basin). These contrasts in age, facies, and structure reflect different positions of the basins in Gondwanaland: the Bonaparte Gulf and Canning Basins have lain at the continental margin facing an open ocean during the entire Phanerozoic, whereas the Perth Basin lay in the interior of Gondwanaland until India and Australia moved apart in the Cretaceous. The Eucla Basin came about by events connected with the dispersal of Antarctica and Australia in the Eocene. The northwest part of the Australian Block (Timor and the Timor Sea) was deformed in the Miocene when Australia collided with southeast Asia.

Sea Bottom Velocity Profiles on the Continental Shelf south-west of England

Abstract: THE peak velocity of tidal currents on the Continental Shelf around the British Isles varies greatly from place to place, nowhere more markedly than in the south-west (Fig. 1). In this area the grade of the superficial sediment and the geometry of the sea floor are both highly variable1,2 and grain size tends to decrease as tidal current strength weakens1,3 along certain sediment transport paths1. It is usually considered4–6 that the bulk of this sediment can be redistributed by tidal currents only when they are augmented by storm-induced oscillatory water movement. But it is still thought that the regular, essentially horizontal and reversing, mass water movements associated with tidal flow exert the principal, long term control on the areal distribution of sediment. We are interested in quantifying this dependency, and hope to show whether any relationship connects the median sediment grade Md and the peak horizontal shear stress τ0max exerted by tidal flow at the sediment/water interface.

Geophysical investigations on the continental shelf and slope north of Scotland

Abstract: Synopsis A gravity, magnetic and seismic reflection survey of the continental shelf and slope around the Orkneys, Hebrides and Shetlands was completed by Durham University on RRV John Murray in 1968. The Bouguer gravity anomaly map shows a large gravity ‘high’ reaching 94 mgal which is continuous for 155 miles (250 km) and tends approximately NNE. across the shelf. This is interpreted as being due to a seaward extension of Lewisian basement rocks exposed on the Scottish mainland. Large amplitude gravity ‘lows’ also occur on the shelf and slope. A gravity ‘low’ 56 miles (90 km) west of the Shetlands has been interpreted as being due to a deep sedimentary basin infilled by Mesozoic–Tertiary sediments and bounded on its SE. margin by a large normal fault. Recent sediments form the continental slope which gently truncates the older Caledonian structures observed on the shelf.

Aeromagnetic survey of the Arctic Ocean: Techniques and interpretations

Abstract: Approximately 147000 km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas, for the greater part with a digitally recording nuclear precession magnetometer designed and built by Wold (1964). The digital recording feature of the system facilitated numerous data processing and analytical techniques which are described herein. These include: noise filtering coordinate conversion, removal of the regional field, second derivatives, downward continuations, polynomial fits of varying degrees to profiles and surfaces, numerical approximations, and depth to source calculations. Using these data and interpretative techniques some inferences could be made about the geologic structure and evolution of the Arctic Ocean Basin. Salient amongst these are: both gravity and magnetic data suggest that there is a 2 1/2 km basement uplift in the eastern Chukchi Shelf associated with the Tigara structure which truncates the western end of Lisburne Peninsula. A 30–40 km wide basement root encircles the Chukchi Rise and extends over 30 km into the mantle. Within the Canda Basin there is a thickening of sediments from the Asian continental margin toward the Canadian Arctic Archipelago. Sediment thickness in the Makarov Basin is 1–1 1/2 km. There appears to be only about a 1/2 km sediment cover in the Fram and Nautilus Basins. The absence of large amplitude magnetic anomalies over these basins is attributed to a 10 km elevation of the Curie isotherm. The Alpha and Nansen ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 mybp when the locus of rifting shifted to the Nansen Cordillera. The absence of significant magnetic anomalies over the Lomonosov Ridge reinforces the hypothesis that it is a section of the former Eurasian continental margin that was translated into the Arctic Basin by sea-floor spreading along the Nansen Cordillera axis.

Bottom currents on the New England Continental Rise

Abstract: Current meters were placed approximately 2 meters off the bottom at depths from 2900 to 5000 meters on the continental rise near the New England seamount chain. Current velocities greater than 15 cm/sec were recorded in a west-southwest direction, parallel to the regional contours. The greatest velocity recorded was 26.5 cm/sec toward the southwest. Spectrum analyses of the time-series curves have resolved periodic motions superimposed on secular, contour-following trends. An interaction between the contour current and the Gulf Stream is also apparent. These data suggest that the contour current, a Western Boundary Undercurrent, is competent to transport and erode continental rise sediments. A significant sediment dispersal system parallel to the regional slope is confirmed to be operative on the present-day continental rise.

Diapirs of the Continental Margin of Angola, Africa

Abstract: A geophysical study of the continental slope and rise off Angola shows the presence of two basement ridges under the sedimentary prism. These ridges are the continuation of onshore basement highs, or parallel to them. The continental rise deposits are built up behind the outermost of these ridges; the deposits of the onshore Cuanza basin and its northward offshore equivalent behind the innermost one. The sedimentary sequences in both zones have a parallel history of evaporite deposition followed by diapirism. Two separate phases of salt tectonics can be distinguished which propagated eastward with time from their western terminal basement ridges. The relative ages of the onshore and offshore sequences cannot be established at this time. The offshore diapir province has a width of more than 300 km and extends possibly from the Congo Canyon in the north to near the Walvis Ridge in the south.

A Review of Geophysical Studies in the Canadian Cordillera

Abstract: Geophysical studies of the crust and upper mantle have been conducted in the Canadian Cordillera for over two decades, but only recently have sufficient data been collected to permit a synthesis and a correlation with the major geological units. The studies have included gravity, heat flow, and magnetotelluric observations, geomagnetic depth sounding, and high level aeromagnetics as well as both small and large scale refraction and reflection seismic surveys. It now appears that major crustal units may be recognized geophysically: (i) Seismic and gravity data suggest that the Plains and Rocky Mountains are underlain by two units of the North American craton with a crustal section 45-50 km thick. The northern unit appears to terminate at the Rocky Mountain Trench while the southern unit may extend to the Omineca Geanticline. (ii) The combined geological and geophysical data suggest that the Rocky Mountain Trench and possibly the Kootenay Arc near the 49th parallel mark the edge of the Precambrian continental margin and that the western Cordillera was formed by a complex succession of plate interactions with repeated reactivation of block boundaries. (iii) A combination of magnetic and heat flow data suggest that the region between the Rocky Mountain Trench and the Fraser Lineament is part of the Cordilleran Thermal Anomaly Zone recognized by Blackwell in the United States. (iv) Seismic data in Central British Columbia suggest that the Pinchi Fault system is a boundary between two crustal blocks. (v) The crustal thickness of the Coast Geanticline appears to increase gradually to the west to approximately 40 km and, at least in southern British Columbia, does not have a root zone below the mountains. (vi) The crustal section beneath Vancouver Island is abnormally thick and there is some paleomagnetic data which suggest that the Island may not have been formed in its present position, contiguous to the Cordillera. The crustal section for the northern part of the Insular Trough is significantly thinner. (vii) The active spreading of the Juan de Fuca Rise - Explorer Trench is now well documented. The geophysical data suggest active subduction of the Juan de Fuca plate beneath Oregon, Washington, and southern Vancouver Island. However, further north there is no evidence for subduction.