Showing papers on "Continental margin published in 1984"

Biological communities at the Florida escarpment resemble hydrothermal vent taxa.

Abstract: Dense biological communities of large epifaunal taxa similar to those found along ridge crest vents at the East Pacific Rise were discovered in the abyssal Gulf of Mexico. These assemblages occur on a passive continental margin at the base of the Florida Escarpment, the interface between the relatively impermeable hemipelagic clays of the distal Mississippi Fan and the jointed Cretaceous limestone of the Florida Platform. The fauna apparently is nourished by sulfide rich hypersaline waters seeping out at near ambient temperatures onto the sea floor.

Phanerozoic addition rates to the continental crust and crustal growth

Abstract: Phanerozoic addition rates to the continental crust are calculated by using seismic profiles through magmatic arcs to measure the crustal volumes added during the active lifespans of the arcs. Data for 17 arcs give addition rates per kilometer of arc in the range 20 to 40 km³ km−1 Ma−1. From these data we deduce a world-wide addition rate of 1.65 km³ a−1 after adding other contributions to the formation of the continental crust, e.g., from hot spot volcanism. We infer a subtraction rate, mainly by subducting sediments, of 0.6 km³ a−1 and arrive at a net crustal growth rate of about 1 km³ a−1. Growth of the continental crust is necessary to maintain approximately constant freeboard, because the secular decline in the heat production of the mantle causes the ocean basins to deepen. An equation for the growth of the continents as a function of the decline in terrestrial heat flow yields approximately constant growth rate since the Archean of 0.9 km³ a−1, in good agreement with the above estimate. On the average, Archean growth rates must have been 3 to 4 times the present rate. Island arc growth rates are inadequate to explain the formation of the Arabian-Nubian Shield and the Archean granite-greenstone terrain of the Superior Province, and a captured island chain in Oregon. We confirm the oceanic island origin of the Oregon terrain on the basis of the large growth rates of hotspot islands.

Thermogenic Gas Hydrates in the Gulf of Mexico

Abstract: Thermogenic gas hydrates were recovered from the upper few meters of bottom sediments in the northwestern Gulf of Mexico. The hydrates were associated with oil-stained cores at a water depth of 530 meters. The hydrates apparently occur sporadically in seismic "wipeout" zones of sediments in a region of the Gulf continental slope at least several hundred square kilometers in area.

A model for the evolution of the Grampian tract in the early Caledonides and Appalachians

Abstract: Analogy with models developed for Newfoundland suggests that the ophiolites of the Grampian tract were emplaced as hot, young oceanic crust. The conversion of a mid-ocean ridge-fracture zone system into a subduction zone resulted in the northward emplacement of a giant ophiolite nappe onto the continental margin in the early Ordovician. This obduction occurred long before the collision between the Laurentian and Baltic Shields. The general features of this model may be characteristic of orogens resulting from oceanic closure.

Permian-early Mesozoic tectonism and continental margin formation in Israel and its implications for the history of the Eastern Mediterranean

Abstract: Summary The Early Mesozoic tectonic reorganization of the Tethys region also affected the Levant area. Here two processes were superimposed: (a) The pattern of long-wavelength vertical motions of the Arabo-Nubian platform changed in the Late Permian when Israel and nearby areas began to subside. (b) Rifting occurred in this subsiding area, probably in several phases: Late Anisian (and Ladinian?), Carnian-Norian, and Liassic. Differential movements reached 2–3 km and magmatism occurred in the Liassic and perhaps also in the Triassic. The tectonism was strongly felt up to 50 km landward of the present coast. The crust was thinned and modified under the present continental margin and in the main rifts. Passive margin conditions were established in Late Liassic times over the previously faulted area. Later in the Jurassic a carbonate shelf was constructed along this subsiding margin; its basinward edge was 1–1.5 km high, which shows that the adjacent SE Mediterranean was already a deep sea. Hence this basin and its passive margin are considered to have been shaped by the Early Mesozoic rifting which is recognized in Israel. The rifting process probably signifies oblique separation of the Tauride block from the Levant part of Gondwana.

Preferential rifting of continents: A source of displaced terranes

Abstract: Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, the total strength of different lithospheres is compared by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future rifting episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.

From passive margin to continental collision; a tectonic scenario for the Western Alps

Abstract: General transverse section just south of the Pelvoux Massif, European margin of Tethys An ancient atlantic-type margin, it was subject to extension from Triassic to the beginning of Late Cretaceous times and to collision thereafter, as the result of convergence of the Eurasian and African continents Paleogeographic evolution is outlined; a new Alpine tectogenetic reconstruction is proposed An attempt is made to establish how the previous extensional structures influenced the later tectonic evolution--Modified journal abstract

Cenozoic volcanic and tectonic evolution of baja california sur, mexico

Abstract: Tertiary to Recent volcanic and sedimentary rocks in Baja California Sur record the geologic events caused by the interactions of lithpspheric plates, notably the transition from subduction to strike-slip motion between the North American and adjacent oceanic plates. As the Farallon Plate was subducted beneath North America during the early Tertiary, Baja California was a stable marine continental shelf receiving volcanic detritus from the active Sierra Madre Occidental calc-alkaline volcanic arc in western Mexico. Volcanic activity migrate d westward and arrived along what is now eastern Baja California about 24 m.y. ago accompanied by uplift of the Baja platform. Calc-alkaline volcanism dominated the geologic events in Baja California Sur from 24 m.y. to about 12 m.y. The focus for this volcani c activity lay along the eastern margin of the present peninsula of Baja California. The volcanic and volcaniclastic materials that constitute the Isidro and Comondu Formations were shed to the west from the arc, first onto a shallow marine shelf, then onto a network of volcanic and detrital nonmarine fans built across Baja California. These arcrelated volcanic rocks range from andesitic flows and lahars to rhyolitic ash-flow tuffs. The abundance of rhyolitic volcanics generally decreases t o the north from the Bahi'a de La Paz region. Subduction ceased off Baja California by 12 m.y. ago, closely followed by the extinction of the Comondu volcanic arc. Consumption of the Farallon Plate resulted in the juxtaposition of the Pacific Plate with Baja California along the Tosco-Abreojos fault. Lack of depositional unit s above the Comondu volcanics indicates a separation of the peninsula from the easterly volcanic and sedimentary sources probably due to graben formation along the locus of a proto-Gulf of California. Coeval with the development of the proto-Gulf of California, a series of basalts yielding dates from 12.4 to at least 0.47 m.y. erupted from widely distributed local sources in Baja California Sur. These largely alkalic basalts are associated with the extensional structural regime and further document the transition from the earlier period of calc-alkaline volcanism that accompanied the mid-Tertiary subduction. Structurally, the southern Baja California Peninsula was a generally stable continental margin during the Miocene arc volcanism. One major discontinuity, the La Paz Fault, was active prior to the Miocene and evidence shows that normal, east-side down, and left-lateral strikeslip motion has occurred along this structure from the late Miocene to possibly Recent times.

Paleoceanographic model of neogene phosphorite deposition, u.s. Atlantic continental margin.

Abstract: The Neogene stratigraphic section of the southeastern U.S. continental shelf-coastal plain system is characterized by (i) a series of major regional phosphogenic episodes; (ii) a strong spatial relationship between the structural or topographic framework and phosphate deposition; and (iii) distinct cyclical and regional patterns of deposition of the terrigenous, carbonate, and phosphate lithofacies. The complex depositional patterns are explained by a paleoceanographic model based upon the interaction of glacial eustatic sea-level fluctuations, associated changes in climate, and the dynamics of the Gulf Stream in response to the bathymetric configurations of the continental margin during the past 20 million years.

Short-lived 1.9 Ga continental margin and its destruction, Wopmay orogen, northwest Canada

Abstract: A 1.9 Ga continental margin in the northwest of the Canadian Shield evolved in less than 15 m.y. from a zone of crustal stretching, through a phase of passive-margin subsidence, to attempted subduction beneath an exotic(?) microcontinent. Reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m.y. following microcontinental accretion. Terminal collision occurred 90 m.y., at most, after initial rifting. Certain differences in detail between this collided margin and those of Phanerozoic age may be related to subduction of newly rifted lithosphere. Rapid evolution of the orogen, the contracted passive-margin phase in particular, is consistent with (but does not prove) more rapid recycling of oceanic plates at that time.

Geology and paleobiology of islands in the Ordovician Iapetus Ocean: Review and implications

Abstract: A history of islands in the Iapetus Ocean and their destruction is recorded by widespread occurrences of Lower to Upper Ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the Appalachian-Caledonide orogen in North America and northwestern Europe. Assemblages of Arenigian and Llanvirnian benthic shelly fossils, chiefly brachiopods, from these rocks constitute the Celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces, some that are younger than elsewhere, and others that have not hitherto been found together. Evidence from these fossils and from the geology of the rocks pertaining to them suggests that North American terranes that included islands of Arenigian-Llanvirnian age lay at middle to high latitudes, both in mid-ocean and within the Armorican continental margin, whereas coeval rocks in west-central Ireland and Norway that have Toquima–Table Head shelly fossils were originally part of the Laurentian continental margin. Eradication of the Celtic assemblage and the reduction of Ordovician faunal provinciality in the Appalachian-Caledonide region coincide with Llanvirnian obduction-subduction of hundreds of kilometres of Iapetus Ocean crust. Late Llanvirnian and younger Ordovician rocks in the remnant basin, including some of peri-insular origin, have brachiopods of Scoto-Appalachian affinities. No discrete islands of Ashgillian age have been recognized, although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places. Shelly fossils in these rocks, and those in the siltstones and limestones from the Gaspe Peninsula, are like those of Ashgillian age elsewhere.

Structure and development of the mid-Norway Continental Margin

Abstract: The area reviewed covers the Mid-Norway Continental Margin between 62°N and 68°N. The main structural elements of the Mid-Norway Continental Shelf and slope basins, as defined at the base Cretaceous level, are the Trondelag Platform, which underlies the inner shelf, the More and Voring Basins, which are located beneath the outer shelf and slope, and the More Platform and the Outer Voring Plateau, which form a base of slope trend of highs.

A Thermal-Tectonic Model for High-Pressure Metamorphism and Deformation in the Sesia Zone, Western Alps

Abstract: The Sesia Zone, Western Alps, is a slice of Austroalpine continental crust that was once metamorphosed at T = 500°-560°C and P > 14-16 kbar at ~130-100 Ma B.P. Pervasive blueschist/eclogite facies assemblages record this event, and a later greenschist facies overprint is only well developed in the NW part of the zone. On the basis of thermal calculations, it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift/erosion. An alternative model is proposed in which the Sesia Zone was emplaced by underthrusting of the Austroalpine continental margin prior to subduction of oceanic lithosphere of the Piemonte basin. Subduction of relatively cool oceanic lithosphere below the underthrust Sesia Zone then continued for a period of time (e.g., 20 Ma), thus suppressing thermal recovery of this latter unit. Exhumation of Sesia Zone rocks began during this subduction episode by uplift on the Insubric Line, erosion, and by underplating of oceanic ...

Continental margins and the extent and number of the continents

Abstract: A new map of the extent of continental crust shows that the continents are more extensive and less dispersed than they are conventionally thought to be. Their total area is 210.4 × 106 km², or 41% of the earth's surface, and there are 14 of them. Continent-ocean contacts are mapped on the basis of a literature survey, and continent-continent contacts are drawn along plate boundaries; thus although Eurasia is still the largest continent, it does not include the separate continents India and Arabia and surrenders northeastern Siberia to North America. Much of the Arctic Ocean is underlain by North American continental crust, and North America is the second-largest continent. Central America and New Zealand are recognized as continents, the latter being nine-tenths submerged, and there are four microcontinents: Rockall, Seychelles, Agulhas, and Jan Mayen. Several oceanic windows, enclaves within the continents, must be recognized if the extent of the continents is to be accurately delineated. The average thickness of continental crust is 36 km; small continents seem to be thinner than large ones, and height and thickness are strongly interdependent. The total volume of continental crust is 7.2 × 109 km³, somewhat less than the 8.0 × 109 km³ estimate of Ronov and Yaroshevskiy (1977). Mean heights are significantly lower than the 875 m commonly quoted from Kossinna (1933), because submerged parts of the continents (30.6% of their total area) are correctly accounted for; the new estimate of mean height is ∼120 m. The relationship between mean height and area for individual continents, however, is still observed. Modal heights are not correlated with area and are ∼ 250 m for most of the large continents. Certain continents have anomalous mean or modal heights, suggesting that the mean height-area relationship arises from greater relative lengths of orogenic belts around and internal tectonism within larger continents; anomalous modal heights suggest anomalous epeirogenic activity. These results may have implications for the hypsometry of ancient continents: in particular, Pangea need not have had a modal height much different from that observed today.

Cenozoic polyphase landscape and tectonic evolution of the Cordillera Occidental, southernmost Peru

Abstract: Landforms on the Pacific slope of the Cordillera Occidental (Western Cordillera) in southernmost Peru reflect the episodic epeirogenic uplift of the Central Andes since the earliest Miocene. In the geomorphic history, three major stages are distinguished. Initial uplift followed a mid-Tertiary period of tectonic and magmatic quiescence in which a low-lying, subdued landscape formed through dissection of an Eocene subaerial volcano-plutonic terrain. Uplift and resulting erosional beveling of this landscape, coinciding in their later stages with the explosive reactivation of the magmatic arc, gave rise to the regionally extensive, lower Miocene Altos de Camilaca surface (I), which was mantled soon after its formation by ignimbrites of the Huaylillas Formation. Major relief was achieved in the middle and early-Late Miocene, when a succession of uplift episodes generated a sequence of four discrete pediments, constituting the Multiple Pediment Stage (II). Uplift was accompanied by widespread eruption of ignimbrites of the Chuntacala Formation. Subsequently, in the Pliocene, pedimentation was succeeded as the major erosional process by canyon incision across the lower slopes of the cordillera. During this Valley and Terrace Stage (III), the immediate littoral margin experienced the development of two major terraces, contemporaneous with a transition from ignimbrite eruption to predominantly andesitic volcanism in the High Cordillera in the early Pleistocene. During this time, it is probable that epeirogenic uplift, rather than glacio-eustatic sea-level changes, exerted the dominant control on landform development. In the absence of evidence for large-scale crustal compression in this immediate region since the Cretaceous, it is inferred that Neogene uplift resulted from episodic thickening of the continental crust through “underplating” by basic and/or intermediate magmas. The Neogene chronology of landform development herein defined is remarkably similar to that outlined in other transects of the Central Andes, implying that the continental margin has, on a broad scale, responded uniformly to plate subduction since the early Miocene. However, the low relief evident in the mid-Tertiary in the study area, and not, for instance, in northern Chile (27°S), suggests that crustal thickening through the Neogene was more rapid, and more extensive, in the vicinity of the Arica deflection than in transects to the north and south. The estimated Neogene uplift rate for the transect was in the range 0.06 to 0.19 mm/yr.