Showing papers on "Continental margin published in 1974"

Possible Ancient Continental Margins in Iran

Abstract: The ophiolites of the Alpine folded region of Iran are examined as an indication of the extent of ancient oceanic realms bordered by ancient continental margins. They are grouped into four geographically and geologically distinct zones, differing from each other in composition, structure, and age. The possibility of these four zones marking former continental margins is then checked against the background of the general structural evolution of Iran. It is concluded that during Paleozoic time Iran was an extension of the Arabian platform, and thus a part of Gondwanaland, possibly bordered by a “Paleo-Tethys” in the north, along the present northern foot of the Alborz Range. Closing of the “Paleo-Tethys,” short of a possible modern relict in the South Caspian depression, may have been related to Hercynian orogenic processes in the ScythoTuranian plate to the north and was completed by Liassic time. A rift in the Arabian-Iranian platform along the “Main Zagros Thrust line” in the early Mesozoic or late Paleozoic was followed by the formation of a “Neo-Tethys” in the south, possibly interrelated and simultaneous with the closing of the “Paleo-Tethys” in the north. Further breakup of Iran led to the formation of several branch troughs of the “Neo-Tethys” and temporary isolation of a “Central-and-East Iranian Microcontinent” in the late Mesozoic. Closing of the “Neo-Tethys” in the early Maestrichtian was followed by reintegration of the “microcontinent” and folding of central and north Iran during the Paleocene paroxysm of the Alpine orogeny.

The geology of continental margins

Abstract: The continental margins of the world constitute the most impressive and largest physiographic feature of the earth's surface, and one of fundamentally great geological significance. Continental margins have been the subject of increasing attention in recent years, an interest focused by a body of new data that has provided new insights into their character. This interest was further stimulated by the realization that, in addition to the abundant living resources, continental margins contain petroleum and mineral resources that are accessible with existing technology. This realization, along with their basic geological importance, has provoked further research into the nature of continental margins throughout the world. A summary of these findings, as related to both recent and ancient continental margins, is the subject of this book. At various times in the past we had been approached individually to prepare a basic reference to continental margins; we then proposed to do such a volume jointly. However, the stimulus for the present volume eventually arose from a Penrose Conference arranged through the Geological Society of America. This conference was attended by specialists of numerous disciplines and from throughout the world, many of whom insisted that such a volume would be both timely and useful. Consequently, we agreed to undertake the task of assembling this book, with the objectives of making it available as soon and as inexpensively as possible.

The development of continental margins in plate tectonic theory

Abstract: Atlantic-type continental margins are formed by rifting and subsequent breakup of continents by sea-floor spreading. Large scale horizontal displacements of continental blocks on lithospheric plates are well-described by current plate tectonic theory. However, the rifting process itself entails vertical tectonics which operate prior to breakup, and such processes are not well understood within the framework of plate tectonic theory. The qualitative models, at present described in the literature involve a sequence of events illustrated by type examples: the East African Rift; the ethiopian-Afar Rift; the Red Sea Rift; and the Gulf of Aden. The thinning and subsidence of the continental crust during rifting have been considered to be the result of crustal stretching. However the writer considers the evidence ambiguous and the theory inadequate. The model presented here involves the response of the crust and the lithosphere to thermal processes implicit in plate tectonic theory. Firstly, uplift is caused by thermal expansion and phase-boundary migration in the lithosphere. which leads to crustal thinning by erosion. Secondly, subsidence is caused by metamorphism in the deep crust. These events occur out of phase and result in two cycles of uplift/erosion/and subsidence during continental margin formation. From this generalised model, theoretical time-stratigraphic and structural cross sections may be constructed to suit the different continental margins. These cross sections are composed of different litho-tectonic elements, namely: pre-rift, rift valley, and post breakup, which are in general separated by two angular unconformities. These features may be readily identified on Atlantic continental margins with only a minimum of seismic and well control.

Hot Spots and Continental Break-up: Implications for Collisional Orogeny

Abstract: Orogenic belts resulting from the collision of continents are characterized by a lengthwise pinching and swelling from narrow zones of high strain, where sutures may be cryptic, to wider zones of lower intensity strain, where former oceans have incompletely closed. This results from the collision of irregularly shaped continental margins whose jaggedness is a consequence of continental break-up along zones of extension that link rift-valley systems developed on hot spots. The progressive convergence of irregular continental margins proceeds from projection points of initial impingement along lengthening suture zones. This leads to complex diachronous relations between basement nappes, sideways-feeding flysch fans, and sideways-driven splinters of continental crust.

Lithogenesis and Geotectonics: The Significance of Compositional Variation in Flysch Arenites (Graywackes)

Abstract: Flysch arenites graywackes vary greatly in their framework mineralogy and volatiles free chemistry Three distinct groups are recognized on the basis of abundance 1 quartz poor graywackes 15 quartz average 58 SiO K OjNa O 1 2 quartz intermediate graywackes 15 quartz average 68 74 SiO K OjNa O 1 and 3 quartz rich graywackes 65 quartz average 89 SiO K OjNa O 1 Mineralogical data from modern deep sea sands which are regarded as the contemporary analogues of flysch arenites together with data from sands of rivers that debouch into the deep sea provide the basis for an actualistic hypothesis to explain the compositional variation of ancient graywackes in terms of their geo tectonic settings This hypothesis is supported by chemical comparisons Quartz poor graywackes are indicative of magmatic island arcs Their average chemical composition ap proximates the average composition of these arcs and that of tholeiitic andesite Quartz intermediate gray wackes are indicators of Andean type continental margins and approximate the upper levels of continental crust in composition Quartz rich graywackes indicate Atlantic type continental margins Chemically they are related to the sand fraction of continental platform cover

Oceans, New Frontier in Exploration

Abstract: Attractive hydrocarbon prospects of the offshore do not end or necessarily diminish at the edge of the continental shelf. Large geosynclinal basins along continental margins offer interesting targets for exploration in deeper water. Continental margins are broad boundary zones between ocean basins and continents. Two basic types are distinguished: Atlantic-type margins, where collapse and extension tectonics combined with regional subsidence led to the formation of marginal geosynclines; and Pacific-type margins, following the trend of active orogenic systems, where repeated phases of tectonic and related magmatic activity created a complex array of basins and uplifts. Seismic profiles from the offshore of the African continent and northwest Australia illustrate the tectonic framework of Atlantic-type margins. The Sunda arc is shown as a typical example of Pacific-type margins.

Continental Shelf Sedimentation

Abstract: In one of the first models of clastic sediments on continental shelves, Douglas Johnson (1919, p. 211) saw the shelf water column and the shelf floor as a system in dynamic equilibrium, in which the slope and grain size of the sedimentary substrate at each point controls, and is controlled by, the flux of wave energy into the bottom. The resulting surface is concave upward, steeper toward the shoreface. Grain size decreased with depth and distance seaward, and as a direct function of the diminishing input of wave energy into the sea floor. The model derived its sediment from coastal erosion rather than from river input. Despite its qualitative expression and limited applicability, the model was in advance of its time in its dynamical, systems analysis approach.

Oregon Continental Margin Structure and Stratigraphy: A Test of the Imbricate Thrust Model

Abstract: The Cenozoic structural and stratigraphic framework of the Oregon continental margin records several intervals of significant tectonism (uplift) with subsequent erosion and truncation of older structures. During late Cenozoic time this framework displayed many of the characteristics of a fore-arc structure defined by Karig (1970). It is also characterized by a substantial amount of continental accretion, interpreted to be the result of underthrusting of the oceanic plate. The compressional thrust model (Seely, et al., this volume; Burk, 1968) offers the best explanation for the late Cenozoic evolution of the Oregon margin.

Plate Tectonics Model for the Evolution of the Arctic

Abstract: Following a search of critical literature on the geology and geophysics of the Arctic, we have constructed a model for the post-Permian evolution of the Arctic Ocean that follows the tenets of plate tectonics We consider the history of the Arctic as the study of two separate basins, the Cenozoic Eurasian Basin and the Mesozoic-Cenozoic Amerasian Basin, and we have utilized the detailed pattern of opening of the North Atlantic worked out by Pitman and Talwani (1972) to determine the relative positions of Eurasia and North America during the past 81 my We propose that the Amerasian Basin as we now know it opened by sea-floor spreading during the Jurassic magnetic quiet period, 180 to 150 my ago We reject the interpretation of the Alpha-Mendeleyev Ridge complex as an early Cenozoic spreading center and show that this feature is better interpreted as a fossil subduction zone-incipient island arc The Eurasian Basin is an extension of the North Atlantic, which has opened by sea-floor spreading during the past 63 my Prior to 63 my, the Lomonosov Ridge formed the seaward edge of the Eurasian continental margin

Holocene Carbonate Sediments of Continental Shelves

Abstract: Carbonate sediments occur on most modern continental shelves. Because the various skeletal and nonskeletal grains are not moved far from their environments of formation, their distribution is a reliable tool on continental shelves for interpreting the history of Holocene deposition.

Evolution of the Continental Margins Bounding a Former Southern Tethys

Abstract: Now that the characteristics of modern active and inactive continental margins are being studied, it is appropriate to see whether or not they can be identified in supposed ancient continental margins that have been involved in later crogeny. If so, then the nature of these margins can be determined and the results applied to the study of the pre-orogenic evolution of the region. The present paper is an attempt to apply this procedure to the southern part of the Tethyan orogenic belt, from the Mediterranean Sea eastward to Indonesia.

Deep-Sea Sedimentation

Abstract: In a volume on the geology of continental margins, a section on deep-sea sediments would seem in need of explanation. First, deep-sea sedimentation includes both the eupelagic and the hemipelagic facies domain, the latter being greatly influenced by continental margin effects. Second, all oceanic sedimentation is part of a global geochemical system, so processes in one realm have profound effects on sedimentation in the other. Third, the plate tectonics paradigm provides for long-term interaction between continental and oceanic crust at the continental margins. Thus a record can be preserved, however jumbled, of the workings of the biogeochemical fractionation apparatus that is the world ocean—be it obscure, as in the rocks and ore bodies of Andean mountains, or in ophiolite suites and ancient pelagic sediments of the Alpine chains.

Circulation on the New England Continental Shelf: Response to strong winter storms

Abstract: Current and sea level observations made on the New England continental shelf during several winter storms show that these short intense wind events dominate the circulation over the shelf, and account for most of the observed net flow. The observations show that large westward mass transports along the shelf were produced by strong easterly winds, while westerly winds produced little along-shore flow. Significant cross-shelf and along-shelf surface pressure gradients occur during the storms. A simple conceptual model is proposed to explain the observations.

The Balkanids as an Instance of Back-Arc Thrust Belt: Possible Relation with the Hellenids

Abstract: The Late Cretaceous calc-alkaline and Late Cretaceous–early Eocene alkaline (shoshonitic) magmatism of Srednogora in the Balkan chain of Bulgaria are interpreted as associated with an ancient active continental margin of Cordilleran (Andean) type. A tectonic evolution of back-arc thrust-belt type agrees, for the Balkanids, with the lack of large overthrust, ophiolite and radio-larite rock-type, eumiogeosyncline in this area. The distribution of magmatism from the Mesozoic to the Recent and the tectonic evolution of the Balkanic-Hellenic area suggest a discontinuous southward Tertiary migration of an arc-trench system away from the southwestern edge of the Rhodope massic to its present location in the eastern Mediterranean.

Atlantic Continental Margin of North America

Abstract: The continental margin bordering eastern North America includes all the features common to Atlantic-type margins. Morphologically, the margin is characterized by a broad, flat continental shelf, a relatively gentle continental slope, and an even gentler-sloping continental rise. An exception to this rule is found east of Florida and the Bahama Islands.

Evidence for continental crust beneath the Faeroe Islands

Abstract: Recent crustal seismic refraction experiments in the North Atlantic indicate that the Faeroe Islands are underlain by continental crust. This suggests that the whole Rockall-Faeroe Plateau may be a microcontinent which formed during the early evolution of the North Atlantic.

Continental Breakup and the Development of Post-Paleozoic Sedimentary Basins around Southern Africa

Abstract: Since Paleozoic time, the development of sedimentary basins on the continental margin of southern Africa has been controlled by the structures formed during the breakup of Gondwanaland. In Mozambique, the earliest rift (∼180 m.y. B.P.), between East and West Gondwana, produced a north-south–trending series of large horsts and grabens which were buried beneath detritus from the Limpopo and Zambezi river systems. Oceanward sediment dispersion was controlled by the Mozambique Ridge. This stage of continental breakup coincided with the establishment of marine conditions in the older, epicontinental basins which lay over the present-day Agulhas Bank and off the Transkei and Natal coasts (Outeniqua, St. Johns, and Durban basins). When West Gondwana broke up (125 to 130 m.y. B.P.), a large sediment wedge (Orange Basin) was initiated on the west coast of southern Africa by discharge from the Orange River and associated rivers onto a downfaulted, tensional-formed margin. At the same time, a large transform fault (Agulhas fracture zone) truncated the Outeniqua to Durban Basins as the Falkland Plateau separated from south and east Africa. These movements resulted in the formation of new ocean basins and the enlargement of older adjacent ones. Subsequent major sea-level movements are attributable to epeirogenic/eustatic events which are possibly related to variations in world-wide ocean-ridge spreading rates. Most variations in sediment accumulation rates are related to the distribution of marginal traps rather than differences in detrital discharge rates from the major river systems.

Continental Collisions in the Appalachian-Caledonian Orogenic Belt: Variations Related to Complete and Incomplete Suturing

Abstract: Certain oceans, whose obliteration during Paleozoic time led to climactic deformation of the Appalachian-Caledonian orogenic belt, closed at different times along the belt. Irregularity of the continental margins that collided during Late Silurian, Early Devonian, and Middle Devonian times caused great variations in structural style and igneous and metamorphic associations. The impingement of projections caused intense deformation leading to cryptic sutures and, in places, basement reactivation. Wider zones of less intense deformation are believed to represent embayments in the colliding margins. Late granite and associated high-grade metamorphic aureole rocks, are more common in the narrower, more intensely deformed regions; we suggest that they represent partial melts from lower parts of a thickened continental crust in these areas. “Granite” in embayments, floored by deformed oceanic basement, is suggested to represent the partial melting of that basement and (or) the covering sediment. In both cases, the magmatism reflects depression of crustal material to a depth below a particular critical isotherm and does not necessitate any abnormal heat input from below the crust.

Holocene History of the Laptev Sea Continental Shelf

Abstract: The 400-km-wide, low gradient Laptev Sea continental shelf consists of flat terrace-like features at regular depth intervals from 10 to 40 m below present sea level. The five large submarine valleys traversing the shelf do not continuously grade seaward, but contain elongated, closed basins. These terraces and closed basins plus deltaic sediments associated with the submarine valleys quite possibly mark sea level Stillstands, and enable reconstruction of the paleogeography of the Laptev Sea shore line at five periods during post-Wisconsin (Holocene) time.

The Ancient Continental Margin of Eastern North America

Abstract: It has long been recognized in North America that orogens represent deformed ancient continental margins, although the basic concepts have changed greatly from the early views of Hall, Dana, and Schuchert to later ones of Kay, Drake, and Dietz. This North American view stemmed from the fact that on this continent all the Phanerozoic orogens are at or near the present edge of the continent. The doctrine that geosynclines are open toward the ocean (one-sided) and that continents grow by the outward addition of younger orogenic belts was popular in North America during the 1950s and early 1960s, and it was enhanced by the interpretation of Paleozoic volcanic belts as island-arc complexes (Kay, 1951). The doctrine was further supported by early oceanographic studies at the present Atlantic margin, e.g., Drake et al. (1959), and by the models of Dietz (1963) and Dietz and Holden (1966) based upon actual comparisons between ancient and modern continental margins.

Plate Tectonics Origin of the Caribbean Mountain System of Northern South America: Discussion and Proposal

Abstract: A brief summary of the petrotectonic assemblages involved in the Cretaceous to early Tertiary orogeny of the Caribbean Mountain System is presented in order to delineate those geological constraints pertinent to a plate tectonic interpretation of the area. In particular, it is concluded that the Mesozoic sediments caught up in the orogeny were originally deposited along a quiescent Atlantic-type continental margin or “tensile open series geosyncline” (Wang, 1972) and not in association with an active subduction zone. Moreover, paired metamorphic belts are not found, and all metamorphic rocks, whose grade increases from south to north across the orogenic belt, are of medium- to high-pressure type. Thus a collision-type model appears to be more appropriate as the major mountain-building mechanism than the cordilleran-type mechanism proposed for the Caribbean Mountain System by various authors. An alternate tectonic model which incorporates the following main stages in tectonic development is suggested: (1) formation of an Atlantic-type continental margin along northern South America by continental rifting in middle Mesozoic time or earlier; (2) movement of an oceanic plate with a leading edge characterized by a subduction-zone–island-arc complex toward this continental margin; (3) collision in Late Cretaceous time; (4) overriding of the island arc over the continental margin, resulting in widespread gravity sliding and thrusting, and high p/T metamorphism of the shelf and rise sediments; and (5) isostatic readjustment due to reorganized plate movement in the early Tertiary. Ephemeral subduction zones active during this sequence of events explain the singular pattern of sporadic volcanism observed during this time span. The direct applicability of this model to areas outside of the region of interest is limited. Necessary restrictions on timing and plate movement in the Caribbean area, however, suggest that a probable feature of the most likely model for the tectonic evolution of this area will be the insertion of an oceanic plate from the Pacific between North and South America in late Mesozoic time.

Continental Margins of Galicia-Portugal and Bay of Biscay

Abstract: The continental margins in the Bay of Biscay and off Galicia-Portugal (Fig. 1) appear now to be stable margins, as are most Atlantic margins. But they have a singularity that greatly complicates their interpretation: the present structure is due not only to Atlantic history during the Mesozoic and Cenozoic but also to the Tethys (Mediterranean) history. They are the result of well-known distension movements on the margins formed by rifting as well as of compression movements responsible for building the Pyrenees.

Continental Slope Construction and Destruction, West Africa

Abstract: According to the generally accepted concept, a continental margin of the Atlantic (passive) type develops by outbuilding and upbuilding of sediments. The sediment material is thought to be supplied mainly from land. A shallow-water terrace is always present between the coast and the continental slope. The shelf sediments may build up to a height of several tens of meters below sea level, determined by water movement (see Rona, 1973a, for numerous references). If more sediment material is supplied, it migrates onto the continental slope. Therefore, shallow-water sediments several kilometers in thickness, often encountered, require subsidence of the basement.

Cretaceous Stratigraphy and Structure, Western Andes of Peru Between Latitudes 10 degrees -10 degrees 30'

Abstract: Cretaceous tectonics, sedimentation, and volcanism were dominated by oscillatory vertical movements of strips of basement bounded by major shear belts. Two main episodes of movement occurred: a Valanginian to Senonian episode of general subsidence, and a Senonian to Tertiary episode of uplift. In the West Peruvian trough, the products of the first episode comprise a western shale-graywacke-volcanic facies and an eastern sandstone-limestone-shale facies. The facies meet along the Tapacocha axis, a major steep basement shear zone, which allowed the western side to subside faster than the eastern side. Fracturing of the western block in its descent provided channels for volcanic eruptions, whereas, on the eastern intact block, shelf deposits were laid down. During uplift, the sediments on the eastern block were folded as a single unit by decollement on underlying shale; those on the western block were strongly deformed in narrow belts above steep basement fractures. The belt of strongest deformation is along the Tapacocha axis and was accompanied by metamorphism of greenschist facies. The response of the Andes to lateral movement of oceanic crust beneath was to break into ribbonlike strips, parallel with the continental margin, which oscillated in vertical planes.

Recent Sediments and Environment of Southern Brazil, Uruguay, Buenos Aires, and Rio Negro Continental Shelf

Abstract: The southern Brazil continental shelf is a narrow and gently sloping sediment-covered submerged coastal plain, with a width of 62 n.m. Southward it increases in width to 170 n.m. and steepens. The shelf break ranges from 35 fm in the north to 50 fm in the south. In the middle area the shelf-slope transition is steep, related to overlapping relict deltas.

Transfer of Nazca Ridge Pelagic Sediments to the Peru Continental Margin

Abstract: A complex set of lithologies, including calcareous oozes, hemipelagites, and turbidites, was recovered from the landward wall of the Peru Trench at its intersection with the Nazca Ridge. The sequence occurs at a water depth of 4,900 m and overlies an acoustic basement in the lowermost continental slope. Early Pliocene calcareous ooze overlies Pliocene to Quaternary ooze; both of these deposits are sandwiched between late Pleistocene (⩽ 400,000 yr), organic-rich turbidites and hemipelagic deposits typical of the Peru Trench and margin. Planktonic and benthic foraminiferal assemblages indicate that the early Pliocene ooze originally was deposited on the Nazca Ridge above the calcium carbonate compensation depth (4,000 m) and to the west of the cool Peru-Chile Current. The Pliocene-Pleistocene ooze contains a temperate fauna associated with the Peru-Chile Current. Block faulting at the terminus of the Nazca Ridge displaced the calcareous ooze 1,900 m from the top of the ridge to the trench below. Apparently these lithologies were then folded against or thrust beneath the lower continental slope within the past 400,000 yr. The stratigraphic sequence and the physiographic setting of the Nazca Ridge–Peru Trench intersection indicate convergence of the Nazca Ridge with the South American block. A minimum convergence rate of 0.8 cm/yr is calculated for the Pleistocene based upon the past and present geographic positions of the calcareous ooze. The best estimate of the rate is 2.8 cm/yr.