Showing papers on "Continental margin published in 2012"

Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone

Abstract: Turbidite systems along the continental margin of Cascadia Basin from Vancouver Island, Canada, to Cape Mendocino, California, United States, have been investigated with swath bathymetry; newly collected and archive piston, gravity, kasten, and box cores; and accelerator mass spectrometry radiocarbon dates. The purpose of this study is to test the applicability of the Holocene turbidite record as a paleoseismic record for the Cascadia subduction zone. The Cascadia Basin is an ideal place to develop a turbidite paleoseismologic method and to record paleoearthquakes because (1) a single subduction-zone fault underlies the Cascadia submarine-canyon systems; (2) multiple tributary canyons and a variety of turbidite systems and sedimentary sources exist to use in tests of synchronous turbidite triggering; (3) the Cascadia trench is completely sediment filled, allowing channel systems to trend seaward across the abyssal plain, rather than merging in the trench; (4) the continental shelf is wide, favoring disconnection of Holocene river systems from their largely Pleistocene canyons; and (5) excellent stratigraphic datums, including the Mazama ash and distinguishable sedimentological and faunal changes near the PleistoceneHolocene boundary, are present for correlating events and anchoring the temporal framework. Multiple tributaries to Cascadia Channel with 50to 150km spacing, and a wide variety of other turbidite systems with different sedimentary sources contain 13 post-Mazama-ash and 19 Holocene turbidites. Likely correlative sequences are found in Cascadia Channel, Juan de Fuca Channel off Washington, and Hydrate Ridge slope basin and Astoria Fan off northern and central Oregon. A probable correlative sequence of turbidites is also found in cores on Rogue Apron off southern Oregon. The Hydrate Ridge and Rogue Apron cores also include 12–22 interspersed thinner turbidite beds respectively. We use 14C dates, relative-dating tests at channel confluences, and stratigraphic correlation of turbidites to determine whether turbidites deposited in separate channel systems are correlative—triggered by a common event. In most cases, these tests can separate earthquake-triggered turbidity currents from other possible sources. The 10,000-year turbidite record along the Cascadia margin passes several tests for synchronous triggering and correlates well with the shorter onshore paleoseismic record. The synchroneity of a 10,000-year turbidite-event record for 500 km along the northern half of the Cascadia subduction zone is best explained by paleoseismic triggering by great earthquakes. Similarly, we find a likely synchronous record in southern Cascadia, including correlated additional events along the southern margin. We examine the applicability of other regional triggers, such as storm waves, storm surges, hyperpycnal flows, and teletsunami, specifically for the Cascadia margin. The average age of the oldest turbidite emplacement event in the 10–0-ka series is 9,800±~210 cal yr B.P. and the youngest is 270±~120 cal yr B.P., indistinguishable from the A.D. 1700 (250 cal yr B.P.) Cascadia earthquake. The northern events define a great earthquake recurrence of ~500–530 years. The recurrence times and averages are supported by the thickness of hemipelagic sediment deposited between turbidite beds. The southern Oregon and northern California margins represent at least three segments that include all of the northern ruptures, as well as ~22 thinner turbidites of restricted latitude range that are correlated between multiple sites. At least two northern California sites, Trinidad and Eel Canyon/pools, record additional turbidites, which may be a mix of earthquake and sedimentologically or storm-triggered events, particularly during the early Holocene when a close connection existed between these canyons and associated river systems. The combined stratigraphic correlations, hemipelagic analysis, and 14C framework suggest that the Cascadia margin has three rupture modes: (1) 19–20 full-length or nearly 1 Oregon State University, Corvallis, OR 2 Instituto Andaluz de Ciencias del la Tierra, Universidad de Granada, Granada, Spain 3 Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia; Chevron Energy Technology Company, Houston, TX 4 Centre Mediterrani d’Investigacions Marines i Amvientals Unitat de Tecnologia Marina, Barcelona, Spain 5 Department of Earth and Environmental Sciences, California State University, East Bay, Hayward, CA 6 Geological Survey of Canada—Pacific, Sydney, B.C., Canada 7 Geological Survey of Canada—Pacific, Sydney, B.C., Canada; Royal Roads University, Victoria, B.C., Canada 8 U.S. Geological Survey, Menlo Park, CA 2 Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone full length ruptures; (2) three or four ruptures comprising the southern 50–70 percent of the margin; and (3) 18–20 smaller southernmargin ruptures during the past 10 k.y., with the possibility of additional southern-margin events that are presently uncorrelated. The shorter rupture extents and thinner turbidites of the southern margin correspond well with spatial extents interpreted from the limited onshore paleoseismic record, supporting margin segmentation of southern Cascadia. The sequence of 41 events defines an average recurrence period for the southern Cascadia margin of ~240 years during the past 10 k.y. Time-independent probabilities for segmented ruptures range from 7–12 percent in 50 years for full or nearly full margin ruptures to ~21 percent in 50 years for a southern-margin rupture. Time-dependent probabilities are similar for northern margin events at ~7–12 percent and 37–42 percent in 50 years for the southern margin. Failure analysis suggests that by the year 2060, Cascadia will have exceeded ~27 percent of Holocene recurrence intervals for the northern margin and 85 percent of recurrence intervals for the southern margin. The long earthquake record established in Cascadia allows tests of recurrence models rarely possible elsewhere. Turbidite mass per event along the Cascadia margin reveals a consistent record for many of the Cascadia turbidites. We infer that larger turbidites likely represent larger earthquakes. Mass per event and magnitude estimates also correlate modestly with following time intervals for each event, suggesting that Cascadia full or nearly full margin ruptures weakly support a time-predictable model of recurrence. The long paleoseismic record also suggests a pattern of clustered earthquakes that includes four or five cycles of two to five earthquakes during the past 10 k.y., separated by unusually long intervals. We suggest that the pattern of long time intervals and longer ruptures for the northern and central margins may be a function of high sediment supply on the incoming plate, smoothing asperities, and potential barriers. The smaller southern Cascadia segments correspond to thinner incoming sediment sections and potentially greater interaction between lowerplate and upper-plate heterogeneities. 42° 40° 44° 46° 48° 130° 128° 126° 124°

An analysis of principal features of tectonic evolution in South China Block

Abstract: Studies suggest that at least four stages of regional-scale tectonic and magmatic events have taken place in the South China block,namely,geodynamic processes of Neoproterozoic and Late Mesozoic active continental margins,Early Paleozoic and Early Mesozoic intracontinental orogenies.The Cathaysia block was a pre-Nanhua basement consisting mainly of Neoproterozoic rocks instead of a stable old land.It experienced a complex evolution from assembly through break-up to re-assembly.The intracontinental shortening during Silurian led to the stabilization of the united South China continent.The entire South China Block was under a shore-shallow sea-slope setting,with no translithospheric fault,no regional-scale volcanism and mantle-derived magmatism in the period from Sinian to Jurassic,during which polyphase tectonic and magmatic events occurred in the united South China lithosphere.It evolved into a part of the Late Mesozoic Western Pacific active continental margin after the Early-Middle Jurassic transformation from Tethysian to Pacific tectonic regimes.The South China lithosphere experienced polyphase continental growth due to the dominant lateral accretion of block assembly accompanied by vertical growth of magma up-swarming.During the Cretaceous,the South China basin and range tectonics occurred in the western shore of the Pacific Ocean due to strong intracontinental extension caused by the northwestward subduction of the Pacific Ocean.Long-term intraplate tectonism and polystage granitic magmatism provided South China with a favorable condition of metallization,forming various large-size ore deposits and resources.Neoproterozoic Nanhua Period and Early Cretaceous were two dominant stages of metallization,with various types of ore deposits being chiefly formed in the Early Cretaceous period.

Understanding continental margin biodiversity: a new imperative.

Abstract: Until recently, the deep continental margins (200–4,000 m) were perceived as monotonous mud slopes of limited ecological or environmental concern. Progress in seafloor mapping and direct observation now reveals unexpected heterogeneity, with a mosaic of habitats and ecosystems linked to geomorphological, geochemical, and hydrographic features that influence biotic diversity. Interactions among water masses, terrestrial inputs, sediment diagenesis, and tectonic activity create a multitude of ecological settings supporting distinct communities that populate canyons and seamounts, high-stress oxygen minimum zones, and methane seeps, as well as vast reefs of cold corals and sponges. This high regional biodiversity is fundamental to the production of valuable fisheries, energy, and mineral resources, and performs critical ecological services (nutrient cycling, carbon sequestration, nursery and habitat support). It is under significant threat from climate change and human resource extraction activities. Serious...

Distribution of major elements in Atlantic surface sediments (36°N–49°S): Imprint of terrigenous input and continental weathering

Abstract: [1] Numerous studies use major element concentrations measured on continental margin sediments to reconstruct terrestrial climate variations. The choice and interpretation of climate proxies however differ from site to site. Here we map the concentrations of major elements (Ca, Fe, Al, Si, Ti, K) in Atlantic surface sediments (36°N–49°S) to assess the factors influencing the geochemistry of Atlantic hemipelagic sediments and the potential of elemental ratios to reconstruct different terrestrial climate regimes. High concentrations of terrigenous elements and low Ca concentrations along the African and South American margins reflect the dominance of terrigenous input in these regions. Single element concentrations and elemental ratios including Ca (e.g., Fe/Ca) are too sensitive to dilution effects (enhanced biological productivity, carbonate dissolution) to allow reliable reconstructions of terrestrial climate. Other elemental ratios reflect the composition of terrigenous material and mirror the climatic conditions within the continental catchment areas. The Atlantic distribution of Ti/Al supports its use as a proxy for eolian versus fluvial input in regions of dust deposition that are not affected by the input of mafic rock material. The spatial distributions of Al/Si and Fe/K reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas. High biogenic opal input however influences the Al/Si ratio. Fe/K is sensitive to the input of mafic material and the topography of Andean river drainage basins. Both ratios are suitable to reconstruct African and South American climatic zones characterized by different intensities of chemical weathering in well-understood environmental settings.

Overview of the Palaeozoic–Neogene evolution of Neotethys in the Eastern Mediterranean region (southern Turkey, Cyprus, Syria)

Abstract: Valid palaeotectonic and palaeogeographical reconstructions of the easternmost Mediterranean and adjacent region involve a long-lived Tethys (Rheic, Palaeotethyan and Neotethyan oceans), northward subduction beneath Eurasia and rifting of continental fragments from Gondwana. Rifted microcontinents bordering Gondwana were separated (from south to north) by the Southern Neotethyan ocean, the Berit ocean (new name), the Inner Tauride ocean and the Izmir–Arkara–Erzincan ocean. Mid-Permian to Mid-Triassic pulsed rifting culminated in Late Triassic–Early Jurassic spreading of the Southern Neotethyan oceans (the main focus here). After Early–Mid-Jurassic passive subsidence, the Late Jurassic–Early Cretaceous was characterized by localized alkaline, within-plate magmatism related to plume activity or renewed rifting. Late Cretaceous ophiolites formed above subduction zones in several oceanic basins. Ophiolites were emplaced southwards onto the Tauride and Arabian platforms during the latest Cretaceous. The Southern Neotethys sutured with the Arabian margin during the Early–Middle Miocene, while oceanic crust remained in the Eastern Mediterranean further west. The leading edge of the North African continental margin, the Eratosthenes Seamount, collided with a subduction trench south of Cyprus during the Late Pliocene–Pleistocene, triggering rapid uplift. Coeval Plio-Quaternary uplift of the Taurides may relate to break-off or delamination of a remnant oceanic slab.

Alpine inversion of the North African margin and delamination of its continental lithosphere

Abstract: [1] This paper aims at summarizing the current extent and architecture of the former Mesozoic passive margin of North Africa from North Algeria in the west up to the Ionian-Calabrian arc and adjacent Mediterranean Ridge in the east. Despite that most paleogeographic models consider that the Eastern Mediterranean Basin as a whole is still underlain by remnants of the Permo-Triassic or a younger Cretaceous Tethyan-Mesogean ocean, the strong similarities documented here in structural styles and timing of inversion between the Saharan Atlas, Sicilian Channel and the Ionian abyssal plain evidence that this portion of the Eastern Mediterranean Basin still belongs to the distal portion of the North African continental margin. A rim of Tethyan ophiolitic units can be also traced more or less continuously from Turkey and Cyprus in the east, in onshore Crete, in the Pindos in Greece and Mirdita in Albania, as well as in the Western Alps, Corsica and the Southern Apennines in the west, supporting the hypothesis that both the Apulia/Adriatic domain and the Eastern Mediterranean Basin still belong to the former southern continental margin of the Tethys. Because there is no clear evidence of crustal-scale fault offsetting the Moho, but more likely a continuous yet folded Moho extending between the foreland and the hinterland beneath the Mediterranean arcs, we propose here a new model of delamination of the continental lithosphere for the Apennines and the Aegean arcs. In this model, only the mantle lithosphere of Apulia and the Eastern Mediterranean is still locally subducted and recycled in the asthenosphere, most if not all the northern portion of the African crust and coeval Moho being currently decoupled from its former, currently delaminated and subducted mantle lithosphere.

Elevated, passive continental margins: Not rift shoulders, but expressions of episodic, post-rift burial and exhumation

Abstract: Many studies of elevated, passive continental margins (EPCMs) assume that their characteristic, large-scale morphology with high-level plateaux and deeply incised valleys has persisted since rifting and crustal separation, and that the absence of post-rift sediments is evidence of non-deposition. The high mountains in West Greenland, however, expose evidence of km-scale, post-rift subsidence, and recent studies showed that typical EPCM morphology with elevated plateaux formed c. 50 Myr after breakup through a process of uplift and dissection of a regional, post-rift erosion surface. Since the West Greenland margin shares all the morphological characteristics of EPCMs, the results from West Greenland lead us to question the common assumption that EPCMs have remained high since the onset of continental separation. We present published evidence of post-rift burial followed by uplift and exhumation from a number of EPCMs and their adjacent basins to support the notion that EPCMs are not permanent highs and that their morphology is unrelated to rifting and continental breakup. Geodynamic models that explain EPCMs as permanent highs since the time of rifting require either no lithospheric mantle extension below extending crust or effective elastic thicknesses > 100 km. Such models are, however, not consistent with the subsidence history inferred from actual rifts and their margins. Geodynamic models using low elastic thicknesses and a much more uniform distribution of strain within the lithosphere are more consistent with observations of early post-rift behaviour, but some additional process is needed to uplift the margins later. We suggest that EPCMs represent anticlinal, lithospheric folds formed under compression where an abrupt change in crustal or lithospheric thickness occurs between cratons and rift basins. We propose that EPCMs are expressions of episodes of post-rift burial followed by compression-induced uplift and exhumation; one episode of uplift results in erosion of the region to produce a low-relief surface near the level of the adjacent, opening ocean, and a second (or more) episode(s) raises the plateau to its present elevation, after which the plateau is dissected by fluvial and possibly glacial erosion.

From rifting to oceanic spreading in the Gulf of Aden: a synthesis

Abstract: We present here a synthesis of the evolution of rifted continental margin systems in the Gulf of Aden. These margins are volcanic to the west of the Gulf of Aden, where they are influenced by the Afar hotspot, and non-volcanic east of longitude 46° E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic, stratigraphic and sedimentological) and oil well data allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo-Triassic to the Oligocene, the Arabian-African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene-Early Oligocene, 34-33 Ma ago, rifting started to localise along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localised on the northern and southern borders of the inherited grabens, in the direction of the Afar hotspot. In the areas with non-volcanic margins (in the east), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the west of the Gulf, up to the Guban graben in the southeast and as far as the southern boundary of the Bahlaf graben in the northeast. Seaward dipping reflectors can be observed on many oil industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterised by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localisation during the final rifting stage, just before formation of the Ocean-Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, at the time of the continental break-up, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hotspot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. Uplift of the northern proximal margin was still active (e.g. stepped beach rocks exposed at 60 m of 2 Ma; 30 m of 35,200 years; 10 and 2 m) and active volcanoes can be inferred at depths of between 70 and 200 km beneath the margin (at 5-10 km distance from the coast). On the distal margin, heat flow measurements show a high value that is associated with post-rift volcanic activity and the development of a volcano (with flows and sills) shortly after the formation of the OCT. The Afar hotspot is therefore important for several reasons. It allows the localisation of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up; its influence also seems to persist during the post-rift period.

Quantification and causes of the terrigeneous sediment budget at the scale of a continental margin: a new method applied to the Namibia–South Africa margin

Abstract: The terrigeneous sediment budget of passive margin basins records variations in continental relief triggered by either deformation or climate. Consequently, it becomes a major challenge to determine sediment accumulation histories in a large number of basins found in various geodynamic contexts. In this study, we developed a GIS‐based method to determine the sediment budget at the scale of a whole basin (from the upstream continental onlap to the most distal deepest marine deposits) and the associated uncertainties. The volume of sediments preserved in the basin for each time interval was estimated by interpolation between cross‐sections and then corrected from in situ production and porosity to obtain terrigeneous solid volumes. This approach was validated by applying it to Namibia–South African passive margin basins for which independent data are available. We determined by a statistical approach the variances associated with each parameter of the method: the geometrical extrapolation of the section (8–43%), the uncertainties on seismic velocities for the depth conversion (2–10%), on the absolute ages of stratigraphic horizons (0.2–12%), on the carbonate content (0.2–46%) and on remaining porosities estimation (3–5%). Our estimates of the accumulated volumes were validated by comparison with previous estimates at a lower temporal resolution in the same area. We discussed variations in accumulation rates observed in terms of relief variations triggered by climate and/or deformation. The high accumulation rates determined for the Lower Cretaceous, progressively decreasing to a minimum in the Mid‐Cretaceous, are consistent with the progressive relaxation of a rift‐related relief. The following increase to an Upper Cretaceous maximum is consistent with a major relief reorganization driven either by an uplift and/or a change to more humid climate conditions. The lower accumulation rate in the Cenozoic suggests a relief reorganization of lesser amplitude over that period.

Late Cretaceous‐Palaeogene stratigraphic and basin evolution in the Zhepure Mountain of southern Tibet: implications for the timing of India‐Asia initial collision

Abstract: This article presents combined stratigraphic, sedimentological, subsidence and provenance data for the Cretaceous–Palaeogene succession from the Zhepure Mountain of southern Tibet. This region records the northernmost sedimentation of the Tethyan passive margin of India, and this time interval represents the transition into continental collision with Asia. The uppermost Cretaceous Zhepure Shanpo and Jidula formations record the transition from pelagic into upper slope to delta-plain environments. The Palaeocene–lower Eocene Zongpu Formation records a carbonate ramp that is overlain by the deep-water Enba Formation (lower Eocene). The upper part of the Enba Formation records shallowing into a storm-influenced, outer shelf environment. Detrital zircon U–Pb and Hf isotopic data indicate that the terrigenous strata of the Enba Formation were sourced from the Lhasa terrane. Unconformably overlying the Enba Formation is the Zhaguo Formation comprising fluvial deposits with evidence of recycling from the underlying successions. Backstripped subsidence analysis indicates shallowing during latest Cretaceous-earliest Palaeocene time (Zhepure Shanpo and Jidula formations) driven by basement uplift, followed by stability (Zongpu Formation) until early Eocene time (Enba Formation) when accelerated subsidence occurred. The provenance, subsidence and stratigraphy suggest that the Enba and Zhaguo formations record foredeep and wedge-top sedimentation respectively within the early Himalayan foreland basin. The underlying Zongpu Formation is interpreted to record the accumulation of a carbonate ramp at the margin of a submarine forebulge. The precursor tectonic uplift during latest Cretaceous time could either record surface uplift over a mantle plume related to the Re´union hotspot, or an early signal of lithospheric flexure related to oceanic subduction, continental collision or ophiolite obduction. The results indicate that the collision of India with Asia occurred before late Danian (ca. 62 Ma) time.

How does the continental crust thin in a hyperextended rifted margin? Insights from the Iberia margin

Abstract: The discovery of hyperthinned continental crust and exhumed mantle on present-day deep-water rifted margins leads to two fundamental questions: (1) in detail, how does the crust thin in extension, and (2) what controls extreme crustal thinning and mantle exhumation? Reflection and refraction seismic lines across the Iberian margin show decoupling levels in the crust cut by structures that eventually transfer deformation to mantle levels. The region of decoupled extension appears to be more broadly distributed on the northern Iberian margin and more localized on the southern margin. Based on drill hole data, the transition from decoupled to coupled deformation occurred during Tithonian time (ca. 145 Ma). This evolution from decoupled to coupled deformation may help explain the crustal architecture of the Iberian margin. An apparent delay of subsidence across the hyperextended coupled zone of the Iberian margin may indicate that crustal thinning had to occur simultaneously with lithospheric necking and the advection of heat related to lithospheric mantle thinning.

Opening and evolution of the South China Sea constrained by studies on volcanic rocks: Preliminary results and a research design

Abstract: The South China Sea (SCS) is characterized by abundant seamounts, which provide important information about the evolution of the SCS and related deep processes. Cenozoic volcanism in the SCS and its surroundings comprises three stages relative to the spreading of the SCS: pre-spreading (>32 Ma), syn-spreading (32-16 Ma), and post-spreading (<16 Ma). The pre-spreading magmatism predominantly occurs on the northern margin of the SCS and in South China coastal areas and shows a bi-modal affinity. The syn-spreading magmatic activity was very limited on the periphery of the SCS, but may be concentrated in the SCS. However, seafloor samples of this stage are not available yet because of overlying thick sedimentary deposits. Post-spreading magmatism is widespread in the central and southwest sub-basins of the SCS, Hainan Island, Leizhou Peninsula, Thailand, and Vietnam. These are mainly alkali basalts with subordinate tholeiites, and display OIB-type geochemical characteristics. The Dupal isotope anomaly and presence of high-magnesian olivine phenocrysts suggests their possible derivation from the Hainan mantle plume. The temporal and spatial distribution of Cenozoic volcanism in the SCS and its surroundings may be accounted for either by plate stress re-organization before and after SCS spreading, or by ridge suction of plume flow during opening of the SCS. If the latter is the case, the volcanic rocks within the SCS basin may not be typical mid-ocean ridge basalts (MORB). It remains puzzling, however, that the transition between the South China continental margin and the SCS basin does not have features typical of a volcanic rifted margin. Clearly, the relationship between mantle plume and SCS opening needs further evaluation. A better understanding of the link between deep processes and opening of the SCS not only requires enhanced studies on igneous petrogenesis, but also is heavily dependent on systematic sampling of seafloor rocks.

Episodic burial and exhumation in NE Brazil after opening of the South Atlantic

Abstract: It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has undergone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceous breakup, the margin under went burial beneath a thick sedimentary cover; (2) uplift episodes in the Campanian and Eocene led to almost complete removal of these deposits; (3) the resulting large-scale, low-relief erosion surface (peneplain) was deeply weathered and finally reburied at the Oligocene-Miocene transition; and (4) Miocene uplift and erosion produced a new, lower-level peneplain by incision of the uplifted and re-exposed Paleogene peneplain. Previous studies have identified aspects of this interpretation, but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed Early Cretaceous rifting and Eocene–Oligocene peneplanation. We suggest that a late sedimentary cover protected Paleogene weathering profiles until the present day. The uplift phases in Brazil are synchronous with uplift phases in Africa and the Andes. The Andean phases coincided with rapid convergence on the western margin of South America, and the Campanian uplift coincided with a decline in spreading rate at the Mid-Atlantic Ridge. Consequently, we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause, which is lateral resistance to plate motion.

Growth of Archean continental crust in oceanic island arcs

Abstract: Understanding the origin of the continental crust is one of the key objectives of earth sciences because as a land species we owe our existence to continents. In addition, change in the volume of the continental crust and distribution of continents on Earth's surface have profound effects on major

Late Palaeozoic–Cenozoic tectonic development of Greece and Albania in the context of alternative reconstructions of Tethys in the Eastern Mediterranean region

Abstract: The objective of this article is to use the geology and tectonics of a critical part of the Tethyan orogen, represented by Greece and Albania, to shed light on the tectonic development of Tethys on a regional to global scale. A review of existing Tethyan reconstructions reveals little consensus concerning key aspects, such as the timing and direction of subduction, arc magmatism, ophiolite genesis, and continental collision. The regional to local-scale geology of individual regions, therefore, has to be considered in detail to test existing models and to develop a viable tectonic reconstruction. For Carboniferous time, much evidence suggests that the Korabi–Pelagonian crustal unit as exposed in Albania and Greece formed above a northward-dipping subduction zone along the Eurasian continental margin, with Palaeotethys to the south. However, there is also evidence of southward subduction beneath Gondwana, especially from southern Greece and central-southern Turkey. Palaeotethys is inferred to have closed in...

European Margin Sediment Dynamics: Side-Scan Sonar and Seismic Images

Abstract: Summary of Geophysical Techniques.- The Norwegian Margin.- GLORIA Lang-Range Sidescan Sonar Mosaic of the North Norwegian Margin (67-75 DegreesN).- Large-Scale Slides on the North Norwegian Margin Imaged by GLORIA.- The Northern Storegga Slide Escarpment - Morphology and Features.- A Weak Layer Feature on the Northern Storegga Slide Escarpment.- Morphology of a Non-glacigenic Debris Flow Lobe in the Helland Hansen Area Investigated with 3D Seismic Data.- The Traenadjupet Slide: Sediment Disintegration and Flow.- Morphology and Acoustic Character of the Middle and Lower North Sea Fan.- Debris Flow Activity on the Bear Island and Trough Mouth Fan from GLORIA and 3.5 kHz Records..- Submarine Glacigenic Debris Flows on the Bear Island Trough Mouth Fan, Western Barents Sea: Aspects of Flow Behaviour.- Morphology of Glacigenic Debris Flows on the Upper North Sea Fan.- Canyon and Channel Systems in the Lofoten Basin, Norwegian Margin.- Morphology of the Lofoten Basin Channel.- The Lofoten Contourite Drift: High-Resolution Seismic Stratigraphy.- Gas Hydrates at Storegga Slide..- Seabed and Subsurface Features on the Southern Voring Plateau and Northern Storegga Slide Escarpment.- The Hakon Mosby Mud Volcano.- The Faeroe-Shetland Margin.- Leng-Range Side-Scan Sonar Imagery of the North Faeroes Margin.- A Major Channel System on the North Faeroes Margin from Side-Scan Sonar Records.- A Record of Mid-Cenozoic Strong Deep-Water Erosion on the Faeroe-Shetland Channel.- Morphology of an Ice-Sheet Limit and Constructional Glacially-Fed Slope Front, Faeroe-Shetland Channel.- Sandy Contourites and Pathways of the Norwegian Sea Overflow Water, West of the Faeroe Bank Channel.- The Rockall-Porcupine Margin.- Images of Sliding and Slumping Along the Porcupine and SW Rockall Trough Margins.- Holocene Shelf-Margin Submarine Landslides, Donegal Fan, Eastern Rockall Trough.- Canyon Heads and Channel Architecture of the Gollum Channel, Porcupine Seabight.- The Continental Rise West of Porcupine Seabight, Northeast Atlantic.- Growth and Partial Destruction of a Neogene Sediment Drift, Western Rockall Trough.- Development of a Sediment Drift: Feni Drift, NE Atlantic Margin.- Giant Carbonate Mounds and Current-Swept Seafloors on the Slopes of the Southern Rockall Trough.- Giant Carbonate Mounds Along the Porcupine and SW Rockall Trough Margins.- Mounds and Sediment Drift in the Porcupine Basin, West of Ireland.- The Celtic and Armorican Margins - a New View.- Morphology and Seismic Stratigraphy of the Manche Paleoriver System, Western Approaches.- The Celtic Deep-Sea Fan: Seismic Facies, Architecture and Stratigraphy.- Morphology and Depositional Processes of the Celtic Fan, Bay of Biscay.- Physiography of the Armorican Turbidite System (Bay of Biscay).- The Iberian and Canaries Margin including NW Africa.- Sediment Dynamic Features on the Eastern Gulf of Cadiz (SW Spain).- Reassessment of Sedimentary Evidence for Deep Contour Currents West of Iberia.- Debris Avalanche Deposits on the Flanks of the Canary Islands: Contrasts Between El Hierro and Tenerife.- Flow Processes in the Saharan Debris Plow.- Turbidity Current Processes and Deposits in the Moroccan Turbidite System.- Oligocene to Recent Processes on the Agadir Continental Margin and Basin (Central Eastern Atlantic).- Canyon Switching in the Moroccan Turbidite System, Northwest African Margin.- Large-Scale Slides on the Flanks of the Canary Islands.- Giant Landslides off the Islands of La Palma.- El Hierro: Shaping of an Oceanic Island by Mass Wasting.

Seismic investigation of the transition from continental to oceanic subduction along the western Hellenic Subduction Zone

Abstract: [1] The western Hellenic subduction zone (WHSZ) exhibits well-documented along-strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab density are believed to drive deformation rates along the WHSZ; however, this hypothesis has been difficult to test given the limited seismic constraints on the structure of the WHSZ, particularly beneath northern Greece. Here, we present high-resolution seismic images across northern and southern Greece to constrain the slab composition and mantle wedge geometry along the WHSZ. Data from two temporary arrays deployed across Greece in a northern line (NL) and southern line (SL) are processed using a 2D teleseismic migration algorithm based on the Generalized Radon Transform. Images of P- and S-wave velocity perturbations reveal N60E dipping low-velocity layers beneath both NL and SL. The ∼8 km thick layer beneath SL is interpreted as subducted oceanic crust while the ∼20 km thick layer beneath NL is interpreted as subducted continental crust. The thickness of subducted continental crust inferred within the upper mantle suggests that ∼10 km of continental crust has accreted to the overriding plate. The relative position of the two subducted crusts implies ∼70–85 km of additional slab retreat in the south relative to the north. Overall, our seismic images are consistent with the hypothesis that faster sinking of the denser, oceanic portion of the slab relative to the continental portion can explain the different rates of slab retreat and deformation in the overriding plate along the WHSZ.

Late Cenozoic erosion of the high-latitude southwestern Barents Sea shelf revisited

Abstract: The southwestern Barents Sea has experienced profound erosion during the last ∼2.7 m.y. that has resulted in the development of a characteristic glacial morphology of the continental shelf and deposition of a several-kilometer-thick sediment fan along the western margin prograding into the deep sea. In the period from ca. 2.7 to 1.5 Ma, proglacial processes, including fluvial and glaciofluvial erosion, dominated. For this period, the total average erosion of the shelf was 170–230 m, the average erosion rate was 0.15–0.2 mm/yr, and the average sedimentation rates on the fan were 16–22 cm/k.y. Subglacial erosion affected an area of ∼575,000 km 2 during the period from ca. 1.5 to 0.7 Ma. Total average erosion is estimated at 330–420 m for this interval, and the average erosion rate was 0.4–0.5 mm/yr. Average sedimentation rates were 50–64 cm/k.y. During the last ∼0.7 m.y., glacial erosion mainly has occurred beneath fast-flowing paleo-ice streams topographically confined to troughs (∼200,000 km 2 ). The total average erosion is estimated at 440–530 m, average erosion rate is 0.6–0.8 mm/yr, and average sedimentation rate on the continental slope is 18–22 cm/k.y. The amount of erosion was mainly determined by the duration of the glaciations and the location, velocity, and basal properties of the ice streams. In total, glacial erosion of the troughs has been relatively high throughout the last ∼2.7 m.y. at ∼1000–1100 m. For the banks, erosion is inferred to have increased from ca. 2.7 Ma to a peak between 1.5 and 0.7 Ma. Subsequently, little erosion occurred in these areas, which implies a total of 500–650 m of erosion. Compared with other high-latitude areas, our rates are among the highest so far reported. This comparison also demonstrates that there have been large variations in the rate of sediment delivery to the glaciated continental margins.

Geochemical Constraints on the Provenance and Depositional Setting of the Devonian Liuling Group, East Qinling Mountains, Central China: Implications for the Tectonic Evolution of the Qinling Orogenic Belt

Abstract: The Qinling orogenic belt of Central China is one of the largest collision orogens in eastern Asia and is considered to mark the site where the eastern part of the Paleotethyan ocean, which separated the North and South China plates, was consumed. Many researchers have described the major units and tectonic framework of the Qinling orogenic belt, but there is little agreement over its Paleozoic history, particularly the tectonic setting of the Devonian assemblages. In this paper geochemistry is used to constrain the provenance and depositional setting of the Middle–Upper Devonian Liuling Group clastic sedimentary rocks from the East Qinling Mountains. Chondrite-normalized REE patterns are uniform with light REE enrichment (LaN/YbN c. 9.6), negative Eu anomalies (Eu/Eu* c. 0.62), and flat heavy REE patterns (GdN/YbN c. 1.6), indicating an upper-continental-crustal source and/or juvenile differentiated arc material. Trace-element discriminant diagrams also suggest a continental arc setting, albeit major-element chemistry can show some characters of passive-margin sandstones. Chemical index of alteration and A-CN-K relations, Nb-Ta negative anomalies (PAAS-normalized data), and high Cr-Ni-V-Ti anomalies all suggest that the source area was dominated by non-steady-state weathering regimes indicative of active uplift along an active continental convergent plate boundary, and not a passive margin. This indicates that the Liuling Group was probably related to subduction accretion with rapid uplift of its source areas. The minor recycling of older sedimentary components and oceanic-crust-related mafic inputs could be derived from the basement of the North Qinling arc and ophiolites or an accretionary complex, respectively. We conclude that the Middle to Upper Devonian Liuling Group was deposited at an active continental margin at the southern edge of the North China plate, and hence closure of eastern Paleotethys was post-Devonian.

Holocene subsurface temperature variability in the eastern Antarctic continental margin

Abstract: [1] We reconstructed subsurface (∼45–200 m water depth) temperature variability in the eastern Antarctic continental margin during the late Holocene, using an archaeal lipid-based temperature proxy (TEX86L). Our results reveal that subsurface temperature changes were probably positively coupled to the variability of warmer, nutrient-rich Modified Circumpolar Deep Water (MCDW, deep water of the Antarctic circumpolar current) intrusion onto the continental shelf. The TEX86Lrecord, in combination with previously published climatic records, indicates that this coupling was probably related to the thermohaline circulation, seasonal variability in sea ice extent, sea temperature, and wind associated with high frequency climate dynamics at low-latitudes such as internal El Nino Southern Oscillation (ENSO). This in turn suggests a linkage between centennial ENSO-like variability at low-latitudes and intrusion variability of MCDW into the eastern Antarctic continental shelf, which might have further impact on ice sheet evolution.