Showing papers on "Continental margin published in 2000"

Fluvial responses to climate and sea‐level change: a review and look forward

Abstract: Summary Fluvial landforms and deposits provide one of the most readily studied Quaternary continental records, and alluvial strata represent an important component in most ancient continental interior and continental margin successions. Moreover, studies of the long-term dynamics of fluvial systems and their responses to external or ‘allogenic' controls, can play important roles in research concerning both global change and sequence-stratigraphy, as well as in studies of the dynamic interactions between tectonic activity and surface processes. These themes were energized in the final decades of the twentieth century, and may become increasingly important in the first decades of this millennium. This review paper provides a historical perspective on the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geology, and then summarizes key processes that operate to produce alluvial stratigraphic records over time-scales of 103−106 years. Of particular interest are changes in discharge regimes, sediment supply and sediment storage en route from source terrains to sedimentary basins, as well as changes in sea-level and the concept of accommodation. Late Quaternary stratigraphic records from the Loire (France), Mississippi (USA), Colorado (Texas, USA) and Rhine–Meuse (The Netherlands) Rivers are used to illustrate the influences of climate change on continental interior rivers, as well as the influence of interacting climate and sea-level change on continental margin systems. The paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea-level change. At present, empirical field-based research on fluvial response to climate and sea-level change lags behind: (a) the global change community's understanding of the magnitude and frequency of climate and sea-level change; (b) the sequence-stratigraphic community's desire to interpret climate and, especially, sea-level change as forcing mechanisms; and (c) the modelling community's ability to generate numerical and physical models of surface processes and their stratigraphic results. A major challenge for the future is to catch up, which will require the development of more detailed and sophisticated Quaternary stratigraphic, sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings. There is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter (102−103 years) and longer (104−106 years) time-scales than has commonly been the case to date, as well as in larger river systems, from source to sink. Studies of Quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre-Quaternary rock record.

Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography

Abstract: We present results from a combined multichannel seismic reflection (MCS) and wideangle onshore/offshore seismic experiment conducted in 1996 across the southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure. We employ a hybrid ray-tracing scheme based on the graph method and the local ray-bending refinement to efficiently obtain an accurate forward solution, and we employ smoothing and optional damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model for the 350-km-long transect, and a long-wavelength structure with strong lateral heterogeneity is recovered, including (1) ∼30-km-thick, undeformed continental crust with a velocity of 6.0 to 7.0 km/s near the landward end, (2) 30- to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone, and (3) 15- to 9-km-thick oceanic crust toward the seaward end. The thickness of the igneous upper crust characterized by a high-velocity gradient also varies from 6 km within the transition zone to ∼3 km seaward. The bottom half of the lower crust generally has a velocity higher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A nonlinear Monte Carlo uncertainty analysis is performed to estimate the a posteriori model variance, showing that most velocity and depth nodes are well determined with one standard deviation of 0.05–0.10 km/s and 0.25–1.5 km, respectively. Despite significant variation in crustal thickness, the mean velocity of the igneous crust, which serves as a proxy for the bulk crustal composition, is surprisingly constant (∼7.0 km/s) along the transect. On the basis of a mantle melting model incorporating the effect of active mantle upwelling, this velocity-thickness relationship is used to constrain the mantle melting process during the breakup of Greenland and Europe. Our result is consistent with a nearly constant mantle potential temperature of 1270–1340°C throughout the rifting but with a rapid transition in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.

From continents to island arcs: a geochemical index of tectonic setting for arc-related and within-plate felsic to intermediate volcanic rocks

Abstract: Three distinct tectonic regimes were identified for felsic and intermediate volcanic rocks using published datasets from twenty-six different geographical locations around the world. The three well-defined tectonic regimes include oceanic arcs, active continental margins and within-plate volcanic zones. This subdivision is based on concentrations and ratios of the incompatible trace elements Ta, Th and Yb as geochemical tectonic discriminants. The separation of tectonic regimes is demonstrated on two discriminant diagrams, where the three zones are separated by ca. 45° diagonal lines on one, and by horizontal lines on the other. The ca. 45° trends of the boundaries between tectonic provinces on a Ta/Yb versus Th/Yb diagram are due to the similar incompatibility of Th and Ta relative to the somewhat lower incompatibility of Yb. On a Th/Ta versus Yb diagram, the three tectonic zones are separated by horizontal lines; datasets within individual zones have characteristic Th/Ta values, ca. 1–6 for within-plate volcanic zones, >6–20 for active continental margins, and >20–90 for oceanic arcs. These discriminant diagrams can be successfully used to identify the tectonic environments of intermediate and felsic volcanic rocks, and to evaluate the tectonic history of a region.

Subtropical shelf front off eastern south america

Abstract: Historical hydrographic data from the continental shelf off eastern South America are used to examine the thermohaline properties of the water masses in the region between 20°S and 40°S. The continental shelf water masses are originated by dilution of open ocean waters of the western boundary currents of the South Atlantic Ocean. On the basis of temperature-salinity relation, two distinct water masses are identified, namely, the Subantarctic Shelf Water and the Subtropical Shelf Water. Subantarctic Shelf Water originates by dilution of Subantarctic Water, primarily in the southeast Pacific, due to excess precipitation and continental runoff and enters the continental shelf near 55°S. The Subtropical Shelf Water is modified South Atlantic Central Water diluted by continental runoff from the coast of Brazil. In addition, substantial dilution of the upper shelf waters takes place at the mouth of Rio de la Plata (approximately located at 36°S) and, in a lesser extent, at the Patos-Mirim Lagoon (at 32°S). The Rio de la Plata and the Patos outflows form a low-salinity tongue that caps the shelf water leading to a salinity decrease to values <30. The low-salinity tongue extends northward over the shelf penetrating farther north in winter than in summer. The extent of the low-salinity water has a strong impact on the vertical stratification and acts to limit winter convection to the layer above the halocline. There is little or no indication of mixing between Subantarctic Shelf Water and Subtropical Shelf Water. An intense temperature, salinity, and nutrient front separates these water masses. The front is oriented along the north-south direction, located on average near the 50 m isobath at 32°S and extends southward toward the shelf break near 36°S. Between 32° and 34°S the Subtropical Shelf Front follows the 100 to 200 m isobaths and separates Subantarctic Shelf Water from the oceanic South Atlantic Central Water. On the basis of the temperature and salinity distributions, beneath the low-salinity surface layer, the Subtropical Shelf Front appears as an extension of the Brazil-Malvinas Confluence over the continental shelf of South America. Thus the location of the Subtropical Shelf Front may be linked to the migrations of the separation point of the Brazil-Malvinas Confluence from the continental slope.

The tectonic evolution of the Norwegian Sea Continental Margin with emphasis on the Vøring and Møre Basins

Abstract: Abstract The Norwegian Sea continental margin is dominated by two major basins with a very thick Cretaceous basin fill: the Vøring and Møre Basins. The basins are flanked by the uplifted mainland and the Cretaceous Trøndelag Platform to the east and by the Møre and Vøring Marginal Highs capped by Eocene lavas to the west. The tectonic development of the area is controlled by two structural trends: NE-SW and NW-SE. The area has been tectonically active from Carboniferous to Late Pliocene time with the main tectonic phases in Late Palaeozoic, late Mid-Jurassic-Early Cretaceous and Late Cretaceous-Early Tertiary time. The general tectonic development comprised a long period of extension and rifting that ended in Early Eocene time by continental separation, major volcanism and subsequent sea-floor spreading in the Norwegian-Greenland Sea. In Carboniferous to Early Cretaceous time the extensional tectonics were related to within-plate continental rifting. The tectonics of the Late Cretaceous and the Tertiary periods were controlled by the relative movements along plate boundaries. The overall NE-SW structural grain is constituted by faults and basin axes that probably originated in Late Palaeozoic time and were active during all subsequent tectonic phases. The transverse NW-SE trend is expressed as major lineaments that probably reflect the old, Precambrian grain of the basement. These lineaments, two of which are the continuation into the continental crust of major oceanic fracture zones, controlled the tectonic activity throughout Cretaceous and Tertiary time and constitute the boundaries between the major structural provinces of the area. The differentiation into the Cretaceous basins and the bounding platforms and marginal highs started by the late Mid-Jurassic-Early Cretaceous extensional phase. The subsequent Cretaceous subsidence history, where the basin flanks formed by flexuring rather than faulting, resulted in an exceptionally thick basin fill. In the Vøring Basin the Cretaceous development comprised an early thermal subsidence phase and a post-Cenomanian phase of tectonically driven subsidence involving intermittent phases of normal faulting and compression and folding. The Vøring Basin was tectonically active also during Tertiary time with the main phases of strike-slip-compression coinciding with the Alpine orogenies in Late Eocene and Mid-Miocene time. Within the Vøring Basin there is evidence of the formation of a fossil opal A-opal-CT transition and extensive regional marine erosion in Mid-Miocene and Late Pliocene times. In contrast, the Møre Basin was generally tectonically quiet throughout the Cretaceous and Tertiary periods, experiencing mainly continuous subsidence.

Enormous Ganges-Brahmaputra sediment discharge during strengthened early Holocene monsoon

Abstract: Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development. However, little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing climatic regimes. In this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca. 11000 yr B.P., when rising sea level flooded the Bengal basin, thereby trapping most of the river's discharge on the inner margin. Chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of the river's past sediment load. Results reveal that ∼5 × 1012 m3 of sediment was stored in the Bengal basin from ca. 11000 to 7000 yr B.P., which corresponds to a mean load of 2.3 × 109 t/yr. In comparison, modern sediment load of the Ganges-Brahmaputra is ∼1 × 109 t/yr, ranking it first among the world's rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoon-related sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespread fluviosedimentary response, the potential magnitude of which is showcased by the Ganges-Brahmaputra system.

A re‐assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle?

Abstract: SUMMARY We investigate the depth distribution of earthquakes within the continental lithosphere of southern Iran, the Tien Shan and northern India by using synthetic seismograms to analyse P and SH body waveforms. In the Zagros mountains of southern Iran, earthquakes are apparently restricted to the upper crust (depths of <20 km), whereas in the Tien Shan and northern India they occur throughout the thickness of the continental crust, to depths of ∼40–45 km. We find no convincing evidence for earthquakes in the continental mantle of these regions, in spite of previous suggestions to the contrary, and question whether seismicity in the continental mantle is important in any part of the world. In some regions, such as Iran, the Aegean, Tibet and California, seismicity is virtually restricted to the upper continental crust, whereas in others, including parts of East Africa, the Tien Shan and northern India, the lower crust is also seismically active, although usually less so than the upper crust. Such variations cannot reliably be demonstrated from published catalogue or bulletin locations, even from ones in which depth resolution is generally improved. In contrast to the oceanic mantle lithosphere, in which earthquakes certainly occur, the continental mantle lithosphere is, we suggest, virtually aseismic and may not be significantly stronger than the lower continental crust. These variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness, suggesting that the strength of the continental lithosphere resides in the crust, and require some modification to prevalent views of lithosphere rheology.

Cenozoic depositional history of the Gulf of Mexico basin

Abstract: A Geographic Information System (GIS) database incorporating information from 241 publications, theses, and dissertations; well logs and paleontologic reports; and interpreted University of Texas Institute for Geophysics (UTIG) deep-basin seismic lines was used to map and interpret 18 basinwide genetic stratigraphic sequences that form the Gulf of Mexico basin Cenozoic fill. Eight principal extrabasinal fluvial axes provided the bulk of the sediment infill in the basin. First-order temporal and spatial use of these axes reflects four continent-scale phases of crustal uplift. Abundant sediment supply has prograded the northern and northwestern basin margin 150 to 180 mi (240 to 290 km) from its inherited Cretaceous position. Margin outbuilding has been locally and briefly interrupted by hypersubsidence due to salt withdrawal and mass wasting. Three depositional systems tracts characterize Cenozoic genetic sequences: (1) fluvial --> delta --> delta-fed apron, (2) coastal plain --> shore zone --> shelf --> shelf-fed apron, and (3) delta flank --> submarine fan. One or more examples of the fluvial --> delta --> delta-fed apron systems tract occur in each of the major genetic sequences. Immense volumes of sand have bypassed the shelf margin to be deposited in slope and base-of-slope systems, primarily within fluvial --> delta --> delta-fed apron system tracts, during all major Paleogene and Neogene depositional episodes. Deposition and preservation of volumetrically significant coastal plain --> shore zone --> shelf --> shelf-fed apron tracts is typical of Paleogene through Miocene depositional episodes only. Fan system origin was commonly associated with major continental margin failures, but large submarine canyons occur mainly in Pleistocene sequences. Thick, potential reservoir sand bodies occur in offlapping delta-fed slope and subjacent basin floor aprons, in autochthonous slope aprons and related infills of slide scars and canyon cuts, and in submarine fans.

Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles : The IAM-9 transect at 40^° 20^' N

Abstract: We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320-km-long wide-angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170-km-wide ocean-continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault-bounded blocks of continental crust. The profile lies ∼40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2–4 km thick with velocities of between 4.5 and 7.0 km s−1 and generally a high-velocity-gradient (1 s−1), and a lower layer up to 4 km thick with a velocity of ∼7.6 km s−1 and a low-velocity-gradient. A weak Moho reflection in this zone was seen only on wide-angle profiles at an offset of ∼30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault-bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin-parallel features. Finally, ODP boreholes, ∼40 km north of our profile and within the interpreted transition zone, have recovered up to 140-m-thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.

NE Atlantic continental rifting and volcanic margin formation

Abstract: The NE Atlantic rift system, which has been under extensional deformation since the Caledonian orogeny, experienced a series of rift episodes in Late Paleozoic to Early Cenozoic times (Fig. 1). Late Paleozoic rifting is poorly constrained, particularly with respect to timing. However, rifted basin geometries, inferred to be of this age, are observed at depth in seismic data from the flanks of the younger rift structures.

Deep structure of the Namibia continental margin as derived from integrated geophysical studies

Abstract: During the Geophysical Measurements Across the Continental Margin of Namibia (MAMBA) experiments, offshore and onshore refraction and reflection seismic as well as magnetic data were collected. Together with the existing free-air gravity data, these were used to derive two crustal sections across the ocean-continent transition. The results show that the Early Cretaceous continental breakup and the separation of South Africa and South America were accompanied by excessive igneous activity offshore. Off Namibia we found a 150–200 km wide zone of igneous crust up to 25 km thick. The upper part of this zone consists of an extrusive section comprising three units of basaltic composition: two distinct wedges of seaward dipping reflectors (SDRs) separated by flat-lying volcanic flows. The inner wedge of SDRs can be modeled as the source of a long-wavelength magnetic anomaly that borders long parts of both South Atlantic margins (anomaly G). The crust underneath these extrusives is characterized by high-velocity and high-density material (average values 7 km s−1, 3×103 kg m−3). Free-air gravity anomalies along both sides of the high-density crust are interpreted as edge effects resulting from juxtaposition with normal oceanic and continental crust on either side. We define the abrupt landward termination of this zone as the continent-ocean boundary, and consequently, the crust seaward is interpreted as exclusively igneous material and not intruded continental crust. Extrapolation of the interpreted geophysical features along the southwest African margin suggests a fast prograding narrow rift zone and sharp lithospheric rupture leading to the formation of a margin-parallel magmatic belt south of the Walvis Ridge. The influence of the Tristan da Cunha mantle plume may explain the widening of this thick igneous crust near the Walvis Ridge.

Geomorphology and global tectonics

Abstract: Geomorphology and Global Tectonics: Introduction (M. Summerfield). GENERAL MODELS AND EMPIRICAL APPROACHES. Application of Digital Elevation Models to Macroscale Tectonic Geomorphology (L. Mayer). Coupled Tectonic-Surface Process Models with Applications to Rifted Margins and Collisional Orogens (C. Beaumont, et al.). Fission-Track Thermochronology and the Long-Term Denudational Response to Tectonics (A. Gleadow & R. Brown). Macroscale Process Systems of Mountain Belt Erosion (N. Hovius). MORPHOTECTONIC EVOLUTION IN INTERPLATE SETTINGS. Geodynamic Processes in the Southern Alps, New Zealand (J. Tippett & N. Hovius). Morphotectonic Evolution of Taiwan (J.-C. Lin). Morphotectonic Evolution of Japan (H. Ohmori). Large-Scale Geomorphology of the Andes: Interrelationships of Tectonics, Magmatism and Climate (L. Kennan). Morphotectonic Evolution of the Himalayas and Tibetan Plateau (E. Fielding). MORPHOTECTONIC EVOLUTION IN INTRAPLATE SETTINGS. Geomorphological Evolution of the East Australian Continental Margin (P. Bishop & G. Goldrick). Morphotectonic Evolution of the South Atlantic Margins of Africa and South America (R. Brown, et al.). Late Cenozoic Landscape Evolution of the US Atlantic Passive Margin: Insights Into a North American Great Escarpment (F. Pazzaglia & T. Gardner). Linking Tectonics and Landscape Development in a Passive Margin Setting: The Transantarctic Mountains (A. Kerr, et al.). Morphotectonic Evolution of the Western Ghats, India (Y. Gunnell & L. Fleitout). The Growth and Decay of Oceanic Islands (A. Watts). Index.

On the circulation and water masses over the Antarctic continental slope and rise between 80 and 150°E

Abstract: The circulation and water masses in the region between 80 and 150°E and from the Antarctic continental shelf to the Southern Boundary of the Antarctic Circumpolar Current (ACC) (∼62°S) are described from hydrographic and surface drifter data taken as part of the multi-disciplinary experiment, Baseline Research Oceanography Krill and the Environment (BROKE). Two types of bottom water are identified, Adelie Land Bottom Water, formed locally between 140 and 150°E, and Ross Sea Bottom Water. Ross Sea Bottom Water is found only at 150°E, whereas Adelie Land Bottom Water is found throughout the survey region. The bottom water mass properties become progressively warmer and saltier to the west, suggesting a westward flow. All of the eight meridional CTD sections show an Antarctic Slope Front of varying strength and position with respect to the shelf break. In the water formation areas (between 140 and 150°E) and 104°E, the Antarctic Slope Front is more “V” shaped, while elsewhere it is one-sided. The shape of the slope front, and the presence or absence of water formation there, are consistent with other meridional sections in the Weddel Sea and simple theories of bottom-water formation (Gill, 1973. Deep-Sea Research 20, 111–140; Whitworth et al., 1998. In: Jacobs and Weiss (Eds.), Ocean, Ice and Atmosphere: Interactions at the Antartic Continental Margin, Antarctic Research Series. American Geophysical Union, Washington, pp. (1−27). ADCP surface velocities and buoy drift tracks show a strong westward flow over the shelf and slope regions. In the region 90–100°E there is a strong eastward flow of the waters just south of the Southern Boundary of the ACC, suggesting a recirculation of the westward slope current and the presence of a weak cyclonic gyre. Using the ADCP velocities as a reference for the CTD data, the average westward transport in this region is 29.4±14.7 Sv.

Continental margin sedimentation, with special reference to the north‐east Atlantic margin

Abstract: The north-east Atlantic continental margin displays a wide range of sediment transport systems with both along-slope and down-slope processes. Off most of the north-west African margin, south of 26°N, upwelling produces elevated accumulation rates, although there is little fluvial input. This area is subject to infrequent but large-scale mass movements, giving rise to debris flows and turbidity currents. The turbidity currents traverse the slope and deposit thick layers on the abyssal plains, while debris flows deposit on the continental slope and rise. From the Atlas Mountains northwards to 56°N, the margin is less prone to mass movements, but is cut by a large number of canyons, which also funnel turbidity currents to the abyssal plains. The presence of a lithospheric plate boundary off SW Iberia is believed to have led to high rates of sediment transport to the deep sea. Even larger quantities of coarse sediments have fed the canyons and abyssal plains in the Bay of Biscay as a result of drainage from melting icecaps. Bottom currents have built sediment waves off the African and Iberian margins, and created erosional furrows south of the Canaries. The Mediterranean outflow is a particularly strong bottom current near the Straits of Gibraltar, depositing sand waves and mud waves in the Gulf of Cadiz. North of 56°N, the margin is heavily influenced by glacial and glaciomarine processes active during glacial times, which built glacial trough-mouth fans, such as the North Sea Fan, and left iceberg scour marks on the upper slope and shelf. Over a long period, especially during interglacials, this part of the margin has been greatly affected by along-slope currents, with less effect by turbidity currents than on the lower latitude margins. Large-scale mass movements are again a prominent feature, particularly off Norway and the Faeroes. Some of these mass movements have occurred during the Holocene, although high glacial sedimentation rates may have contributed to the instability.

Sediment erosion thresholds and characteristics of resuspended aggregates on the western European continental margin

Abstract: Sediment erosion thresholds and characteristics of resuspended aggregates were experimentally determined on cores from the western European continental margin with a ship-borne erosion chamber augmented by image analysis. Bottom sediments (212–4940 m water depth) had a thin surface layer that was resuspended as aggregates (median diameter 125–2403 μm) under critical shear velocities ( u ∗c ) of 0.4–1.2 cm s −1 . For the underlying sediments, eroded as primary particles, u ∗c increased with water depth from 0.7 cm s −1 (sandy shelf sediments) to 2.1 cm s −1 (lower slope sediments). A two-layer concept of the sediment interface is discussed which distinguishes between an underlying sediment layer, bound by both physico-chemical and biological adhesion, and a more easily resuspendable surface aggregate layer. The surface layer consists mainly of aggregates in the 140–450 μm size range and is resuspended at mean thresholds u ∗c of 0.8–0.9 cm s −1 . These aggregates can subsequently be transported in tide-related resuspension–deposition loops over long distances.

Large Igneous Provinces and Plate Tectonics

Abstract: © 2000 by the American Geophysical Union. Large Igenous Provinces (LIPs) constitute broad areas, > 10 5 km 2 , of mafic volcanic and plutonic rocks erupted over ~106 yr. Compared with plate dimensions, LIPs are smaller, localized features; they form both in plate interiors and at plate boundaries. LIPs originate in the mantle, via mass and energy transfer which acts both independently o￡ and in conjunction with, the wide-ranging upwellling systems producing new oceanic crust by sea floor spreading along the mid-oceanic ridge system. Plate tectonic theory does not readily explain the massive magmatism, which is most commonly attributed to mantle plumes. Most LIPs form in extensional oceanic or continental plate tectonic settings, suggesting a relationship with thinned lithosphere. Deformed LIP complexes in intraplate continental settings suggest formation throughout most of Earth history. The post-150 Ma LIP record reveals both many events and large melt volumes from 135-85 Ma, and a distinct decline since 50 Ma. These trends may reflect variations in mantle circulation and have links to global environmental change. Following formation, oceanic LIPs may be carried laterally by the plates to regions of plate convergence. Subsequent accretion of major LIPs into continental crust contributes episodically to continental growth.

Dating of pore waters with (129)I: relevance for the origin of marine gas hydrates

Abstract: Pore waters associated with gas hydrates at Blake Ridge in the Atlantic Ocean were dated by measuring their iodine-129/iodine ratios. Samples collected from sediments with ages between 1.8 and 6 million years ago consistently yield ages around 55 million years ago. These ages, together with the strong iodine enrichment observed in the pore waters, suggest that the origin of iodine is related to organic material of early Tertiary age, which probably is also the source of the methane in the gas hydrates at this location.

The role of the oceanic oxygen minima in generating biodiversity in the deep sea

Abstract: Many studies on the deep-sea benthic biota have shown that the most species-rich areas lie on the continental margins between 500 and 2500 m, which coincides with the present oxygen-minimum in the world's oceans. Some species have adapted to hypoxic conditions in oxygen-minimum zones, and some can even fulfil all their energy requirements through anaerobic metabolism for at least short periods of time. It is, however, apparent that the geographic and vertical distribution of many species is restricted by the presence of oxygen-minimum zones. Historically, cycles of global warming and cooling have led to periods of expansion and contraction of oxygen-minimum layers throughout the world's oceans. Such shifts in the global distribution of oxygen-minimum zones have presented many opportunities for allopatric speciation in organisms inhabiting slope habitats associated with continental margins, oceanic islands and seamounts. On a smaller scale, oxygen-minimum zones can be seen today as providing a barrier to gene-flow between allopatric populations. Recent studies of the Arabian Sea and in other regions of upwelling also have shown that the presence of an oxygen-minimum layer creates a strong vertical gradient in physical and biological parameters. The reduced utilisation of the downward flux of organic material in the oxygen-minimum zone results in an abundant supply of food for organisms immediately below it. The occupation of this area by species exploiting abundant food supplies may lead to strong vertical gradients in selective pressures for optimal rates of growth, modes of reproduction and development and in other aspects of species biology. The presence of such strong selective gradients may have led to an increase in habitat specialisation in the lower reaches of oxygen-minimum zones and an increased rate of speciation.

Was the Late Triassic orogeny in Turkey caused by the collision of an oceanic plateau

Abstract: A belt of Late Triassic deformation and metamorphism (Cimmeride Orogeny) extends east-west for 1100 km in northern Turkey. It is proposed that this was caused by the collision and partial accretion of an Early-Middle Triassic oceanic plateau with the southern continental margin of Laurasia. The upper part of this oceanic plateau is recognized as a thick Lower-Middle Triassic metabasite-marble-phyllite complex, named the Niltifer Unit, which covers an area of 120 000 km 2 with an estimated volume of mafic rocks of 2 x 105 km 3. The mafic sequence, which has thin stratigraphic intercalations of hemipelagic limestone and shale, shows consistent within-plate geochemical signatures. The Niliifer Unit has undergone a high-pressure greenschist facies metamorphism, but also includes tectonic slices of eclogite and blueschist with latest Triassic isotopic ages, produced during the attempted subduction of the plateau. The short period for the orogeny (< 15 Ma; Norian-Hettangian) is further evidence for the oceanic plateau origin of the Cimmeride Orogeny. The accretion of the Niltifer Plateau produced strong uplift and compressional deformation in the hanging wall. A large and thick clastic wedge, fed from the granitic basement of the Laurasia, represented by a thick Upper Triassic arkosic sandstone sequence in northwest Turkey, engulfed the subduction zone and the Niltifer Plateau. An east-west trending belt of latest Triassic deformation and regional metamorphism extends for over 1100 km in northern Turkey. The Early Mesozoic deformation (but not the regional metamorphism) was known previously ($eng6r 1979; Bergougnan & Fourquin 1982) and was referred to as the Cimmeride deformation ($eng6r et al. 1984). The Cimmeride deformation was ascribed to the closure of the Palaeotethys ocean following the collision of a Cimmerian continental sliver with the southern margin of Laurasia ($eng6r 1979; Seng6r et al. 1984). Here, an alternative explanation, involving the collision and partial accretion of an oceanic plateau to the southern margin of Laurasia, is proposed for the origin of the latest Triassic deformation and metamorphism in northern Turkey. A tectonic map of Turkey and the surrounding region is shown in Fig. 1. During the Palaeozoic and Mesozoic, the various continental blocks that make up present-day Turkey were situated on the continental margins of the Tethys Ocean. The Pontides, which comprise the Strandja, istanbul and Sakarya Zones, show Laurasian stratigraphic affinities, while the Anatolide-Tauride Block and the Klr~ehir Massif are tectonically and stratigraphically related to Gondwana ($eng6r & Yllmaz 1981; Okay et aL 1996; Okay & Ttiystiz 2000). The istanbul Zone is a continental fragment, which was translated south from the Odessa Shelf with the Cretaceous opening of the oceanic West Black Sea Basin (Fig. 1; Okay et al. 1994). Its stratigraphy is similar to that of the Scythian and Moesian platforms, with a fully developed Palaeozoic sedimentary sequence unconformably overlain by Triassic and younger sedimentary rocks (Haas 1968; Dean et al. 1997; G6rtir et al. 1997). In the Istanbul Zone, a weak latest Triassic deformation is marked by an unconformity between the Norian siliciclastic turbidites and the overlying Upper Cretaceous carbonates. The Strandja Zone consists of a Late Hercynian metamorphic and granitic basement unconformably overlain by Lower Triassic-Middle Jurassic sedimentary rocks (Chatalov 1988; Okay et al. 1996). The Anatol ide-Tauride Block and the Klr~ehir Massif are also devoid of Triassic metamorphism, and of any significant Triassic deformation. Several well studied Lower Mesozoic stratigraphic sections in the Taurides, including those in the Bornova Flysch Zone (Erdo~an et al. 1990.). and in the central Taurides (Gutnic et al. 1979; Ozgti11997), show a continuous transition between Triassic and Jurassic with no evidence of an intervening deformation phase. The pre-Jurassic thrusting, described by Monod & Akay (1984) from a small locality in the central Taurides, is as yet of unknown significance. Late Triassic deformation and regional metamorphism in Turkey are predominantly found in the Sakarya Zone, which will form the main subject of this paper. From: BOZKURT, E., WINCHESTER, J. A. & PIPER, J. D. A. (eds) Tectonics and Magmatism in Turkey and the Surrounding Area. Geological Society, London, Special Publications, 173, 25-41.1-86239-064-9/00/$15.00 (C) The Geological Society of London 2000.

Proto-Avalonia: A 1.2–1.0 Ga tectonothermal event and constraints for the evolution of Rodinia

Abstract: The Neoproterozoic evolution of Avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of Rodinia. Similar Sm-Nd isotopic signatures for different periods of arc activity suggest that Avalonian basement, or proto-Avalonia, was generated in a series of primitive oceanic island arcs between 1.2 and 1.0 Ga. Because this interval coincides with the amalgamation of Rodinia, proto-Avalonia is inferred to have been located in a Panthalassa-like peri-Rodinian ocean. An early (760–660 Ma) phase of Avalonian arc activity is attributed to renewed subduction in the peri-Rodinian ocean following the breakup of Rodinia, which caused the accretion of Avalonian terranes to the Gondwanan margin by ca. 650 Ma. Further subduction along the margin occurred outboard of these terranes and resulted in the onset of main-phase Avalonian volcanism at 630 Ma. The diachronous cessation of arc magmatism is attributed to ridge-trench collision and the generation of a continental transform. The geodynamic link between Avalonia and Rodinia is analogous to that between the Mesozoic dispersal of Pangea and the tectonothermal evolution of western North America. This event also resulted in the accretion of outboard terranes and in arc-related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the San Andreas transform. The model implies that the Neoproterozoic evolution of Avalonia and other peri-Gondwanan terranes provide important constraints on the tectonic history of a large portion of the Rodinian continental margin.

Late quaternary geological history of rio grange do sul coastal plain, southern brazil

Abstract: The sedimentary deposits and geomorphic features preserved in the Rio Grande do Sul coastal plain, southern Brazil, represent a significant record of late Quaternary climatic changes with its associated glacio-eustatic sea-level fluctuations. The sediments of the coastal plain belong to two major depositional systems - an alluvial fan system developed along the inner part of the coastal plain and a barrier-lagoon complex with four distinctive transgressive-regressive cycles seaward. The alluvial fan sediments were derived from igneous and metamorphic rocks of the Precambrian shield and from sedimentary and volcanic rocks of the Parana Basin. Subsequently, they were reworked by four barrier- lagoon systems each representing a transgressive-regressive cycle. Each barrier probably originated at the landward limit of a transgression and was preserved due to regression of the shoreline forced by a glacio-eustatic sea-level fall. The four barrier-lagoon systems are believed to have formed during the last 400 ka assuming a correlation with the highstands represented by the last major peaks of the oxygen isotopic record. INTRODUCTION The Rio Grande do Sul coastal plain is an elongate (620 km) and wide (up to 100 km) physiographic province underlain by the Pelotas Basin. This basin, the southernmost of the Brazilian continental margin, has accumulated more than 10,000 m of mainly terrigenous sediments since its formation in the Early Cretaceous, when the South Atlantic began opening. The younger section of this sedimentary record is now exposed on the Rio Grande do Sul coastal plain and contains one of the most complete records of Brazilian coastal Quaternary sedimentation (Fig. 1). This paper summarizes knowledge about Rio Grande do Sul coastal plain sediments and suggests an evolutionary model for them during the Quaternary. PHYSICAL AND GEOLOGICAL SETTING OF THE STUDY AREA The coastal plain of Rio Grande do Sul has a nearly straight shoreline between about 29° S and 34° S latitude. Covering an area of about 33,000 km 2 , this large lowland embraces a great number of coastal water bodies, some of them of large dimensions, such as the Patos Lagoon with an area of 10,000 km 2 and Mirim Lagoon with an area of 3,770 km 2 . At the northern end of the coastal plain the adjacent highlands consist of Paleozoic and Mesozoic sedimentary and volcanic rocks of the Parana Basin that locally reach 1,000 m. At the southern section, igneous and metamorphic rocks of the Precambrian shield form lower highlands. At present, all sandy sediments eroded from these highlands and transported by rivers to the coast are trapped in the coastal lagoons and other backbarrier environments and none reaches the oceanic shoreline. The climate of the region is temperate, humid, with an even distribution of rain throughout the year, averaging around 1300 mm. The coastal plain is dominated by a bimodal high-energy wind re- gime. The dominant wind comes from NE and is more active in spring and summer months. The secondary W-SW wind becomes more important in the autumn-winter months. The coast of Rio Grande do Sul is a wave-dominated microtidal coast with semidiurnal tides with a mean range of about 0.5 m. The region is affected by swell waves approaching from SE that produce a net northerly alongshore transport of sediment. Besides the swell action, sea waves from E and NE and episodic storm waves from E and SE control erosional and depositional processes along the seashore. The adjacent continental shelf has an average width of 150 km and the shelf break is situated at a depth of about 170 m. Bottom sediments on the shelf are predominantly terrigenous elastics with some biodetrital concentrations that seems to be mostly relic. COASTAL PLAIN DEPOSITIONAL SYSTEMS Alluvial fan system This system includes fades formed through sediment gravity flows and alluvial processes along the inner part of the coastal plain. The sediments consist of mass flow deposits in proximal regions (mainly massive debris flow and colluvial slide deposits) grading seaward into waterlaid fades associated to braided channels with cross-stratified sandy and gravelly deposits. The composition and texture of the alluvial fan fades mainly reflects the nature (relief and composition) of their primary source area. To the north, the fades