Showing papers on "Continental margin published in 1983"

Role of the sub-continental lithosphere in magma genesis at active continental margins

Abstract: Volcanic are basalts can usually be distinguished from basalts erupted in other settings by their selective enrichments in LIL elements and also, in some cases, in light rare-earth elements and P. Ibis enrichment component is independent of the nature of the overlying mantle wedge and is assumed to be subduction-derived. However, basalts erupted at active continental margins carry an additional geochemical component, which is not observed in basalts front oceanic island arcs. This component, which contains Nb, Ta, Zr and Hf in addition to the elements mentioned above, cannot realistically have a subduction or crustal origin and is thus assumed to be derived from trace element enriched metasomatized sub-continental lithosphere. The relative contributions of the subduction and lithosphere components to the composition of active continental margin basalts can be calculated from their MORB-normalized geochemical patterns. In a typical basalt, from Central Chile, the subduction zone can be shown to make the dominant contribution to its content of Rb, K, Ba, 71 and Sr, whereas the lithosphere contributes the greater proportion of its content of Ce, Sm and P and all its content of Ta, Nb, Zr, Hf, Ti, Y and Yb.

A Model for Development of Red Sea

Abstract: Although motion between Arabia and Africa is presently occurring along the entire length of the Red Sea, the morphology and tectonics that result from this motion vary greatly along its length. South of 21°N, the main trough is bisected by a deep axial trough which has formed by sea-floor spreading during the past 4 m.y. and is associated with large-amplitude magnetic anomalies and high heat flow. North of 25°N, an axial trough is not present and the floor of the main trough has an irregular faulted appearance. The magnetic field in the north is characterized by smooth low-amplitude anomalies with a few isolated higher amplitude magnetic anomalies commonly associated with gravity anomalies and in many places probably due to intrusions. Between these regions, the axial trough is discontinuous with a series of deeps characterized by large-amplitude magnetic anomalies alternating with shallower intertrough zones which lack magnetic anomalies. It is argued that the different regions represent successive phases in the rifting of a continent and the development of a continental margin. An initial period of diffuse extension by rotational faulting and dike injection over an area perhaps 100 km (60 mi) wide is followed by concentration of extension at a single axis and the initiation of sea-floor spreading. The main trough in the southern Red Sea, away from the deep axial trough, formed during the Miocene by the same processes of diffuse extension that are still active in the northern Red Sea. This model explains the available geologic and geophysical data and reconciles previous models for the formation of the Red Sea which emphasize either the evidence for considerable motion between Arabia and Africa or the evidence for down aulted continental crust beneath much of the Red Sea. The initial pre-sea-floor spreading stage results in considerable extension (160 km or 100 mi) at 25°N in the Red Sea), can last for several tens of millions of years, and is an important factor in the development of the continental margin. Such an extended phase of rifting and diffuse extension must be taken into account in studies of sedimentation, subsidence, and paleotemperatures.

Marine geology of Korean seas

Abstract: Preface. Introduction. Korean Peninsula. Introduction. Kyonggi and Yongnam Massifs. Okchon Fold Belt. Taebaeksan Basin. Imjingang Belt. Orogenic Events. Cretaceous Non-Marine Basins. Pohang Basin. Jangki Group. Quaternary Volcanism in Cheju Island. Sedimentation and Tectonic History. Yellow Sea. Physiography. Geologic Setting. Northern Yellow Sea Basin. Southern Yellow Sea Basin. Basin Evolution. Shallow Structure. Surface Sediments. Dispersal of Fine Sediment in the Western Part. Dispersal of Fine Sediment in the Southeastern Part. Mass Physical Properties. Tidal Flats. Reclamation Effect on Sedimentation: Daeho Area. Transgressive Holocene Sequence Stratigraphy. South Sea and East China Sea. Geologic Setting. Sedimentary Basins. Coastal Embayments. Surface Sediments. Late Quaternary Transgressive Deposits. East Sea. Physiography. Crustal Structure. Magnetic and Gravity Anomalies. Heat Flow. Age and Type of Crust. Stratigraphy. Tectonic Evolution. Surface Sediments. Late Quaternary Sediments. Late Quaternary Paleoceanography. Eastern Continental Margin. Physiography. Geologic Structures. Seismic Stratigraphy. Sedimentary Basins. Evolution History. Surface Sediments. Late Quaternary Sediments. Ulleung Basin. Physiography. Crustal Structure. Gravity and Magnetic Anomalies. Seismic Stratigraphy. Tectonic Evolution. Late Quaternary Sediments. Late Quaternary Sedimentation. References. Subject index.

Back arc thrusting in the Eastern Sunda Arc, Indonesia: A consequence of arc-continent collision

Abstract: The structure of the eastern Sunda back arc region is dominated by two large north directed thrusts, the Wetar and Flores thrusts, and one or more minor thrusts, which may represent early stages of subduction polarity reversal of the arc. The relationships between thrusting and magmatic activity, surface slopes, cross-arc faulting, uplift, forearc structure, and collision by the Australian continental margin provide constraints on the likely driving mechanisms of thrust formation. Driving mechanisms are considered in two broad categories. One set of possible mechanisms originates within the upper plate and includes gravity sliding, gravity spreading, and magmatic intrusion. The other set focuses on stress propagation between the upper and lower plates, and for this arc system the major mechanism is that of continental collision. While no one mechanism explains the range of observations we see in the back arc region, all of these aspects play some role in the development of the thrusts. We can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting. In addition, the Wetar thrust shows a negative relation between thrusting and volcanism, and side-looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism. Collision between the arc and the Australian continent provides the clearest driving mechanism, but it does not explain the discontinuous nature or the particular location of the thrusts. We suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts. Thrusting was initiated in those areas where the crust of the forearc region was thick, a factor that facilitated stress propagation across the arc, and where the crust of the back arc was thin, which facilitated back arc thrusting. Thrusting concentrated at the base of the slope where the slope stress is maximum. The early thrusting may have been helped also by thermal weakening of the crust due to volcanism. As convergence proceeded, volcanism waned then ceased in the eastern part of the arc. The crust then strengthened, and thrusting required higher stress to maintain and propagate. This later stage should show higher seismic energy release, which appears to be the case for the Wetar thrust region. Displacement on the Flores thrust appears to be matched by deflection of the volcanic arc. We have estimated shortening in the back arc from interpretation of reflection profiles, and for the Flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc. Compressional deformation continues west of the collision zone, across the Bali Basin and into Java. It is reported in NE Sumatra as well. This deformation involves much less convergence than the eastern Sunda thrusts, and it may be related to magmatic processes, to subduction of oceanic plateaus, such as the Roo Rise, or, in the case of the Bali Basin, to efficient lateral propagation of the Flores thrust.

Seismic refraction profiles between Cyprus and Israel and their interpretation

Abstract: Summary. A long seismic refraction profile was carried out between southern Israel and Cyprus. The seismic energy was generated by 33 sea shots each of 0.8 t explosives and was recorded by land stations in Israel and Cyprus and by ocean bottom seismographs deployed along the profile. The results showed that the continental crust of southern Israel thins towards the Mediterranean underneath a northward thickening sedimentary cover. Cyprus is underlain by a 35 km thick continental crust thinning south-wards and extending to Mt Eratosthenes. Between Mt Eratosthenes and the Israel continental shelf the crystalline crust is composed of high velocity (6.5 km s-1)material and is about 8 km thick. It is covered by 12–14 km of sediments and may represent a fossil oceanic crust.

Climatology of the Southeastern United States Continental Shelf Waters

Abstract: Data from 2872 hydrographic stations have been used to determine the oceanographie climatology of the southeastern United States continental shelf waters. The data were sorted by each degree of latitude and by depth into three zones (0–20 m, 21–40 m, 41–60 m). Inner shelf water temperatures were similar to adjacent land air temperatures, while outer shelf temperatures were moderated by the Gulf Stream. Minimum and maximum water temperatures occurred in Georgia and South Carolina inner shelf water. Bottom temperatures were unusually low off Florida in the summer probably because of shelf break upwelling. Surface salinity was lowest adjacent to the rivers and reached minimums in the spring at the time of high runoff. An exception to this occurred in the fall, when strong southward winds apparently advected low salinity coastal water southward and offshore flow was restricted. Heat flux was calculated from changes in monthly mean depth-averaged inner shelf water temperatures. Heating occurred from March through July with maximum rates of 103 W m−2. Cooling occurred from October through February with maximum rates of −90 W m−2. Bulk stratification was estimated from the difference in near-surface and near-bottom monthly mean density. In the spring, stratification increases in inner shelf areas because of decreasing winds and increasing heat flux and runoff. By summer the whole shelf is highly stratified reflecting the contrast between high surface water temperatures and cooler bottom waters. Highest bulk stratification is found over the outer shelf. Stratification decreased with the approach offall with the associated cooling and high winds. Mean flow at midshelf was northward and appears to be produced by an along-shelf slope of sea level of oceanic origin. Data are available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W. Washington, D.C. 20009. Document C82-002; $2.50. Payment must accompany order.

Sedimentological and structural evolution of the Arabian continental margin in the Musandam Mountains and Dibba zone, United Arab Emirates

Abstract: The northernmost extremity of the Oman Mountains, the Musandam Peninsula, is composed of an allochthonous sequence of Permian to middle Cretaceous shelf carbonates. These are separated from ophiolitic rocks to the south by a northeast-southwest-trending belt known as the Dibba zone. This structurally complex belt is composed of allochthonous slope- and basin-facies sediments, Haybi volcanic rocks, “Oman Exotic” limestones, sub-ophiolitic metamorphic rocks, and ultra-mafic slices. Lithofacies correlation confirms that fragmentation and rifting of a vast east-facing carbonate platform occurred in Middle to Late Triassic time, resulting in the establishment of a shelf edge and small ocean basin. This margin was typified from northwest to southeast by ooid-skeletal lime sand shoals or small bioherms on the shelf edge, a bypass foreslope of well-laminated periplatform ooze, and basin-margin accumulations of carbonate turbidites or debris flows. The shelf edge now coincides with the northern boundary of the Dibba zone and appears to have remained stationary from Middle Triassic to Late Jurassic time. Periods of arrested carbonate sedimentation on the platform either because of exposure (Late Triassic and middle Cretaceous) or because of drowning (Late Jurassic) are represented in the basin by starved sedimentation and deposition of radiolarian cherts. A spectacular platform margin collapse occurred in the middle and Late Cretaceous, as represented by massive conglomerates, with clasts as young as Albian, above an unconformity that progressively removed all of the Lower Cretaceous. The Tethyan basinal and ophiolitic rocks of the Dibba zone were emplaced from the east-southeast during the Turonian to lower Maastrichtian. After emplacement of these allochthonous units, compressional deformation in the mid-Tertiary resulted in large-scale, open, “whaleback” folds with wavelengths as much as 15 km, generally with north-south axes. In places, these folds, which affect the complete shelf and allochthonous sequences, are overturned toward the west, and thrusting has caused previously lower tectonic units of the Late Cretaceous stacking order to be thrust over previously higher tectonic units, thus reversing the Late Cretaceous tectonostratigraphy. The maximum amount of translation on the later thrusts is in excess of 5 km on the Hagab thrust, where the complete shelf carbonate sequence of the Musandam Mountains has been thrust west-northwest over the previously higher Hawasina and Haybi thrust sheets. The Tertiary folding and thrusting can be correlated in time and space with the Zagros fold belt of southwestern Iran.

Accretionary Tectonics of the North American Cordillera

Abstract: Continental geology stands on the threshold of a change that is likely to be as fundamental as plate-tectonic theory was for marine geology. Ongoing seismic-reflection investigations into the deep crustal structure of North America are verifying that orogenic zones are underlain by low-angle faults of regional extent (Brown et al 1981). The growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments. Paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture-bounded fragments, particularly within the North American Cordillera. Such interdisciplinary studies are leading to a consensus that the Cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in Mesozoic and early Cenozoic time. Such crustal growth is referred to as accretionary tectonics. In this paper, we review some of the important concepts in accretionary tectonics, discuss the nature of the materials accreted between central Alaska and southern California in Jurassic and Cretaceous time, and consider the general relations between Cordilleran accretion and the movement of lithospheric plates. The concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest. With the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen, along with mechanisms for crustal attrition. Accretion mechanisms include the growth of imbricated sedimentary prisms along inner-trench walls, slicing off of submarine topographic irregularities within subducting plates, and collision of continents and volcanic arcs by ocean-basin closure. Tectonic attrition mechanisms include rifting, transform faulting, and strike-slip or underthrust removal of inner-trench wall materials coincident with or in place of accretionary prism growth. Growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments. An important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins. Since attrition by nature leaves little material evidence of having operated, one of the major problems confronting Cordilleran geologists lies in the recognition of such attrition within the ancient record, particularly when interspersed with accretionary events. The spectrum of accretion and attrition mechanisms viewed at cm yr^-1 plate-transport rates over time scales of 100 m.y. leads one to suspect a highly mobile history for continental-margin orogens. The serial arrangement of subducting, transform, and rifting links along the modern Cordillera plate-juncture system and both serial and parallel arrangements in the western Pacific systems show the complex interplay of such mechanisms through space. Similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the Cordillera, which presently resemble a collage of crustal fragments (Davis et al 1978). Recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust, and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and Earth history. The first problem to be considered is the recognition of native North American crust from exotic fragments that have been accreted to its edge.

Model for the origin of the Yakutat block, an accreting terrane in the northern Gulf of Alaska

Abstract: A composite oceanic and continental terrane, the Yakutat block, is currently colliding with and accreting to North America in the northern Gulf of Alaska. Marine geophysical data indicate that the block has moved with the Pacific plate during Pliocene and Quaternary time, but are indeterminant about its previous movement history. A model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the Kula-Farallon spreading center beneath North America. In this model, the spreading center that formed the basaltic basement of the Yakutat block was offset from a spreading center to the south by a left-lateral transform fault 400–500 km long. Subduction of the northern spreading center about 45 m.y. ago sliced off part of the North America continental margin, attached it to the Kula plate, and initiated northward movement of the composite terrane as the Yakutat block. During Eocene and Oligocene time, migration of the southern spreading center toward North America truncated the Yakutat block on the south and left a fossil fracture zone that is now seen as the Transition fault. The southern spreading center subducted about 25 m.y. ago, and the Yakutat block has since traveled with the Pacific plate to southern Alaska.

Circulation on the newfoundland continental shelf

Abstract: The circulation on the Newfoundland Continental Shelf derived from a review of different data sources generally agrees with the classical description of the flow in this area given by Smith et al. (1937). Hydrological, surface and bottom drifter, satellite‐tracked buoy, ships drift, current meter and sea‐level observations are used to estimate mean flows, transports, and fluctuating currents and to define the circulation pattern on the shelf and slope. The review suggests several aspects of the oceanography and dynamics of the Newfoundland Continental Shelf that merit further attention.

The Shelfbreak : critical interface on continental margins

Abstract: The shelfbreak is that point where the first major change in gradient occurs on the outermost edge of the continental shelf. Although this environment delimits the boundary between two principal and well-defined provinces, the continental shelf and slope - and thus is of the first order of importance on continental margins - it has received surprisingly little specific attention in either modern oceans or in the rock record. This volume, the first compendium dedicated specifically to the shelfbreak, was derived from an SEPM Research Symposium convened at the joint Annual Meeting of SEPM and AAPG on June 2, 1981. The material is organized in a manner to illustrate examples of the shelfbreak in both modern oceans and the rock record.

Tectonics of the Acadian Orogeny in New England and Adjacent Canada

Abstract: Paleogeographic analysis of post-Taconian rocks in New England and adjacent parts of Canada has revealed the existence of two volcanic arcs which shut off at the time of the Acadian Orogeny. One was built on arc basement previously accreted to North America during the Taconic Orogeny, the other on Precambrian continental basement of Avalonia. In the intervening Merrimack-Fredericton Trough, metamorphosed and poly deformed turbidites and black shales record deep water conditions in Silurian. Following McKerrow and Ziegler (1971), this tract is interpreted as the site of an ocean which closed in Siluro-Devonian by simultaneous subduction beneath both continental margins. In the Molucca Sea in Indonesia, a comparable arc-arc collision is in an early stage of development; Moore et al. (1982) suggested that an accretionary prism built against one arc is overthrusting its counterpart, which developed on the other side of the ocean. An identical geometry, with an Avalonian accretionary prism overriding the conve...

Transport and fate of river discharge on the continental shelf of the southeastern United States

Abstract: The authors analyzed 8 years of wind and salinity data from a frontal zone in a region of the inner continental shelf off the southeastern United States. The changes in low-salinity water stored in the frontal zone have been parameterized by analyzing the monthly rate of change in freshwater content. When the rate of change in freshwater content was negative, this was interpreted as a loss of low-salinity water from the frontal zone. When this parameter was compared with seasonally averaged alongshore wind stress, the rate of loss was independent of the alongshore wind stress magnitude until threshold of about 0.1 dyne cm/sup -2/ was reached. Above the threshold there was a clear relationship between northward alongshore wind stress and rate of loss of freshwater from the inner shelf. Experimental evidence suggests that horizontal currents in the inner-shelf frontal zone have cyclonic shear with increasing depth. When wind stress is northward and offshore, near-surface low-salinity water is transported offshore by Ekman transport while near-bottom high-salinity water is transported shoreward. 9 figures, 2 tables.

The Scottish continental slope current

Abstract: A four-month record from a current meter mooring on the continental slope west of Scotland displayed a steady northward current of 0.16 m s−1 which appears to be associated with a core of warm light water normally observed over the shallow side of the slope. Although the tides change considerably between deep water and the continental shelf, the horizontal velocity gradients are not sufficient to explain the observed average current. One possible explanation, which displays the tendency to enhance the current over the shallow side of the slope, is a boundary current driven by a north-south pressure gradient, with pressure increasing towards the north. Such a pressure gradient could be a surface level gradient which contributes to a large term in the vorticity equation in regions with large bottom slopes.

Mississippian Continental Margins of the Conterminous United States

Abstract: The paleogeography paleotectonics and paleoceanography of continental margins and shelfedges around the present western southern and eastern sides of the conterminous United States are reconstructed for a brief span about 1 5 m y of Mississippian time The time is that of the middle Osagean anchoralis latus conodont Zone latest Tour naisian Mamet foram zone 9 At this time a shallow tropical sea covered most of the southern North American continent and was the site of a broad carbonate platform Bordering this platform were three elongate foreland troughs each containing several bathymetrically distinct starved basins on their inner continentward sides The foreland troughs were bordered on their outer sides by orogenic highlands or a welt that formed in response to successive collisions orconvergences with North America by Africa and Europe to the east by an oceanic plate to the west and by South America to the south During a eustatic rise of sealevel that accompanied the orogenies and culminated during the anchoralis latus Zone the carbonate platform prograded seaward while the troughs subsided and carbonate sediments were transported over the passive shelfedges to intertongue with thin carbonate foreslope deposits and thin 10 m phosphatic basinal sediments Simultaneously thick 500 m flysch and deltaic terrigenous sediments such as the Antler flysch on the west and the Borden deltaic deposits on the east were shed into the outer parts of the foreland basins from active margins along orogenic highlands This Mississippian reconstruction provides a unique opportunity to compare and contrast passive and active shelfedges of aPaleozoic continent during a high stand of sealevel The passive shelfedges can be recognized and mapped by application of a six part sedimentation and paleoecologic model developed for the shelfedge of the Deseret starved basin in Utah Idaho and Nevada

Geochemical profiles in the central Arctic Ocean: Their relation to freezing and shallow circulation

Abstract: Temperature, salinity, nutrient, tritium, and oxygen isotope data collected during the Lomonosov Ridge Experiment along a drift track between the Makarov and Fram Basins over the Lomonosov Ridge are presented. The relationship of these quantities to the processes that maintain the halocline, in particular to the production of more saline waters by addition of brines formed during the freezing of seawater, is described. The results support the idea that the wide continental shelves of the Arctic Ocean play an important role in maintaining the halocline. productivity, and to the chemical and physical effects of freezing sea water. A comparison of this ocean, with its low productivity (at least during winter), with other oceanic regions should help to elucidate some of the effects of biological activity on marine chemistry, for example, trace metal cycling and speciation. Aagaard et al. ( 1981) discussed the way in which the cold waters of the halocline which lie immediately below the surface mixed layer and extend to the top of the warm Atlantic layer could be produced on the wide continental margins surrounding the Arctic Ocean. The processes in- volve freezing with release of highly saline brines having a temperature close to the freezing point of surface sea water. This water flows over the shelves and into the central basin, sinking then to the appropriate density surface. The distribu- tions of various chemical parameters will be affected in predictable ways by these processes. As a part of the Lomonosov Ridge Experiment (LOREX), which ran from late March until the end of May 1979, a program was undertaken aimed at describing more accurate- ly the hydrography of the central Arctic Ocean and studying the geochemistry of selected trace elements in this unusual environment. To this end, water samples were collected from a drifting ice station (LOREX satellite camp 1) for the measurement of salinity, silicate, phosphate, /580, trace metals (Moore, 1981, 1983), carbon-14, tritium, helium-3, caesium-137, caesium-134, strontium-90, plutonium, ameri- cium (H. D. Livingston et al., manuscript in preparation, 1983), alkalinity, and pH. Ice cores were collected to investi- gate the possibility of deducing the extent of temporal changes in the tritium content of the surface waters.

Faeroe-Shetland Escarpment and Continental Margin North of the Faeroes

Abstract: The Escarpment is re-interpreted as the buried feather-edge of a thick pile of early Eocene flood basalts, overlying a thinner but more widespread layer of basalts of late Palaeocene age. The Escarpment does not, therefore, define the continent-ocean boundary in the southern Norwegian Sea. In the Faeroe-Shetland Trough and More Basin the basalts overlie several kilometres of sediments ranging in age from Palaeocene to at least as old as early Cretaceous, resting in turn on thin crust, with the Moho at around 15 km. The continent-ocean boundary north of the Faeroes underlies a belt of north-dipping ‘smooth’ intra-basalt reflections seen on multichannel reflection data, which is interpreted as oceanic layer 2, formed during subaerial spreading immediately preceding anomaly 24 time.

The West Spitsbergen Current: Transport, Forcing, and Variability.

Abstract: : The West Spitsbergen Current is commonly considered to be the northern extension of the Norwegian Atlantic Current, and it forms one of the branches of the circulation pattern of the Greenland-Norwegian sea system. It flows northward along the continental margin west of Spitsbergen, entering the Arctic basin on the east side of the Greenland-Spitsbergen passage (Fram Strait). Previous work (Aagaard and Greisman, 1975) has shown that it provides the major component in the mass and heat balances of the Polar Basin. Beginning in summer 1976, moored current and temperature recorders have been deployed in the WSC at 79 N, and the data records from 1976-79 and 1980-81 provide the basis for this work. The data have been used to estimate the volume and heat transports by the current and to define the structure and variability of the flow. In addition to the presentation of basic results, there is a discussion of the dynamics of the current with emphasis on explaining the length and time scales of the observed variability and on ascertaining the nature of the forces driving the flow.

Simple Bouguer gravity anomaly field and the inferred crustal structure of continental Ecuador

Abstract: The simple Bouguer gravity anomaly field of continental Ecuador corresponds closely to the physiographic provinces of the country. The Sierra, which includes the Andes and their foothills, is characterized by a pronounced low with values to −292 mgal, which reflects the deep Andean root. Bouguer anomalies over the Oriente become less negative away from the Sierra, chiefly in response to progressive thinning of continental crust eastward. The Costa, between the Sierra and the Pacific shore, in the north has the most positive on-land Bouguer anomalies (+162 mgal) so far known in the Western Hemisphere. This part of the Costa is underlain by an ancient oceanic plate now welded to the northwestern corner of the otherwise continental South American plate.

The Mesozoic marginal basin of central Peru

Abstract: The west Peruvian Trough1,2 extends some 1,300 km from north of Trujillo to about 150 km short of Arequipa (Fig. 1). It received upwards of 9,000 m of material during the Mesozoic and within the Cretaceous rocks there is a well defined west-to-east facies change whereby thick, volcaniclastic turbidites and pillow lavas (the ‘eugeosyncline’ of Cobbing3) thin eastward to be replaced by a mixed clastic shelf sequence (the ‘miogeosyncline’). We report here chemical analyses of the volcanic rocks and data on the association of cherts, volcaniclastic turbidites, dyke and sill complexes and large gabbro masses as well as geophysical evidence which suggests that the ‘eugeosyncline’ is an extensional marginal basin similar to that seen in rocks of the same age in Southern Chile4. The Coastal Batholith is thought to have been emplaced within this marginal basin during the Upper Cretaceous5, and the evolution of this ‘ensialic’ new crust therefore has important implications for the growth of the continental margin of South America, as well as for batholith genesis.

Late Proterozoic lavas of the Central Arabian Shield—evolution of an ancient volcanic arc system

Abstract: A petrological assessment of 11 volcano-sedimentary belts in the Central Arabian Shield (CAS) using 118 new analyses is presented. The same samples have been used in a Rb-Sr dating programme also reported in this volume. The oldest volcano-sedimentary belts (‘sequence’ C; >900 Ma) are chemically immature bimodal suites of low-K tholeiites and sodic dacite/rhyolite depleted in lithophile elements. These lavas have chemical characteristics similar to immature island arcs such as the Tonga–Kermadee and Lesser Antilles arcs. Younger lavas of ‘sequence’ B (900–700 Ma) are slightly more mature in composition, being predominantly calc-alkaline and low-K arc tholeiite series with higher lithophile element contents. They are comparable with Pacific island are averages. The youngest voluminous lavas (‘sequence’ A; 700–570 Ma) are generally calc-alkaline or high-K, calc-alkaline series lavas with moderately high lithophile element abundances. They are comparable to volcanic arcs such as Central America and Indonesia, which are transitional between island arcs and continental margin volcanic arcs. The cratonization process in the CAS thus involved the superimposition of three volcano-sedimentary sequences, formed in progressively maturing volcanic arcs. Early-formed, immature island arcs were succeeded by chemically more mature island arc deposits and then by deposits formed in volcanic arc(s) transitional between island arcs and continental margins. The evolution ceased at this stage and probably did not advance to the equivalent of an Andean continental margin.

Unstable Progradational Clastic Shelf Margins

Abstract: In some continental margin basins such as the northwestern Gulf of Mexico and the Niger Delta large scale slump ing of the continental slope disturbs the topset foreset geometry of the prograding shelf margin and thereby inhibits recognition of ancient shelfedges As a result concepts of shelf margin dynamics have been underemphasized in explaining the structure and stratigraphy of such basins Nonetheless ancient unstable clastic shelf margins can be approximately located by criteria such as isopach maxima timing of growth faulting and the stratigraphic top of geopressure Gravity sliding of the continental slope creates a strongly extensional regime along the shelf margin resulting in growth faulting and greatly enhanced subsidence rates The corresponding compressional regime along the lower slope is important in initiating salt and shale structures if the shelf margin progrades over these structures diapiric activity can greatly complicate the style of growth faulting High subsidence rates result in greatly expanded progradational cycles which serve to distinguish shelf margin deltaic sequences from deltas of the more stable shelf platform Rapid fault movement along the shelf margin can hydraulically isolate shallow water sandstones and juxtapose them against dewatering slope shales thus allowing the development and maintenance of excess fluid pressure These deep water shales are probably a major source of both hydrocarbons and brines instrumental in diagenesis of geopressured deltaic sandstones

Metamorphism and tectonics of the Himalaya

Abstract: Metamorphism connected with the main growth and deformation stages of the Himalaya ranges from mid-Cretaceous to the Quaternary. Northward subduction gave rise in the early to mid-Cretaceous to an island are complex with greenschists, amphibolites, granulites and blueschists in the W (Kohistan-Ladakh) and an Andean-type margin with greenschist-amphibolite grade metamorphism in the E (India and Tibet). Ophiolitic nappes were thrust southwards over the Indian continental margin in the Palaeogene during the final stages of closure of the southern arm of Tethys. The Indus-Zangbo Suture bifurcates westwards and between the two sutures lies the Kohistan-Ladakh island arc which was converted to an Andean-type arc in the Palaeocene-Eocene. The Karakorum Range underwent northward subduction at least during the late Cretaceous to give rise to a complex and prominent calc-alkaline batholith. Post-collisional southward thrusting of crustal slabs over the Indian continental margin took place in the early Miocene with production of high-grade metamorphism, inverted isograds and crustal melt granites. Later southward thrusting in the Pliocene to Quaternary gave rise to localized low-grade recrystallization against the thrusts.

Continental margins, orogenic belts, and intracratonic structures

Abstract: Angular bends (promontories and embayments) in rifted and passive continental margins are interpreted to be the precursors of curves (recesses and salients, respectively) in orogenic belts. Convergence at an irregularly shaped continental margin results in along-strike diachroneity of closing (and thus of orogeny) and in a variable trajectory of stress into continental crust. Both timing and sense of movement of intracratonic structures apparently have complex relationships to the progress of convergence at irregular continental margins.

Comparison of terrane accretion in modern Southeast Asia and the Mesozoic North American Cordillera

Abstract: The western part of the North American Cordillera has been divided into numerous suspect terranes. Some of these terranes have traveled hundreds to thousands of kilometres between the time of their formation and their final incorporation into the accretionary belt of the Cordillera. Most of the terranes appear to be relics of island arcs, oceanic plateaus and islands, continental margin fragments, and complex accretionary terranes, the latter including melange belts, ophiolite fragments, and thrust-faulted forearc provinces. Because the terranes have complex histories and diverse stratigraphies, it has been difficult to envisage an actualistic setting for the Mesozoic Cordillera. Here we propose an analogy with the tectonic setting of the Indo-Pacific region, from the Tonga trench on the east to eastern Indonesia on the west. This region comprises several distinct island arcs, several large oceanic plateaus, numerous accretionary terranes, melange zones, ophiolite fragments, and a variety of continental fragments, all moving toward the central collision zone. Like the Cordillera, this part of the Indo-Pacific region is undergoing oblique convergence, driven by relatively high rates of movement between the oceanic plates and the collision zone. Also analogous to the Cordillera is the formation of a foreland fold and thrust belt, developing between the collision zone and the Australian continent, and a zone of basement-rooted foreland folds, suggestive of an early Laramide style of deformation.

Role of Submarine Canyons on Shelfbreak Erosion and Sedimentation: Modern and Ancient Examples

Abstract: Heads of submarine canyons may occur anywhere on continental margins from river mouths to continental slopes producing a distinctive interface between shallow and deep marine environments Inception of most canyons is subaer ial fIuvially cut during lowered sealevel Submarine mass flow also commences canyon formation Submarine erosion shapes all canyons and is especially effective in the headward region Sliding and slumping are volumetrically most important as erosive agents but sand spillover bioerosion sand flow sand creep and debris flow all playa part Fluctuating channelized currents and low velocity turbidity currents also erode and transport sediments Canyons alter shelfbreak circulation and sedimentation They remove detritus from fluvial outflow longshore trans port and cross shelf drift and may influence the position of rip currents On narrow shelves surface waves diverge over canyon heads providing a transport corridor for the return of turbid water Suspensates downwell along canyons as high density nepheloid layers Channelized currents winnow fines in upper canyon heads focused internal tides and waves may actually break producing more extensive erosion Although research on modern canyon systems has rapidly increased detailed studies of ancient canyons remain sparse An Eocene example from Southern California contains a tripartite fill representing progressive detachment from a nearshore source during a eustatic sealevel rise Suspensate fallout tractional flow and mass flow processes formed a basal amalgamated pebbly sandstone overlain by planar to convolute laminated sandstone topped by var iegated cut and fill mudstone channels This tributary system fed the main canyon filled with fining and thinning upward complexes The Pigeon Point and Carmelo Formations of coastal California and Tethyan submarine canyons of Czechoslovakia display similar fining upward canyon fills Contrasting fill sequences includecoarse grained units that dominate French Maritime Alps and New Zealand canyon complexes and shales that plug canyons in the Gulf Coast Sacramento Valley and Israel Shelf size and gradient rates of eustacy tectonism and subsidence and sedimentary source input and migration interact to create this diversity of fills in ancient submarine canyons Quantified analyses of canyon formation maintenance and fill and application of sedimentary hydrodynamics to observed mass transport processes