The Ocean-Continent Transition at Rifted Continental Margins: What Is It, How Is It Formed, and How Do We Locate It? I
Presiding: G Karner, Lamont-Doherty Earth Observatory; N Kusznir, Department of Earth and Ocean Sciences,
T41A-01 08:30h
Attempts to reconcile plate reconstructions with interpreted positions of the continent-ocean boundary in the South Atlantic.
South Atlantic continental margins are separated into two major domains, volcanic margins south of the Walvis and Rio Grande Ridges and non-volcanic margins north of the ridges. In addition, variable age of the continent-ocean boundary (COB) in the South Atlantic reflects diachronous separation between Africa and South America. South of the Walvis Ridge, along the volcanic margin, the COB is progressively younger to the north, as evidenced by oceanic spreading magnetic anomalies adjacent to the COB that are about 5 million years older in the far south than near the Walvis Ridge. This age progression has generally been interpreted to continue to the north as far as the Niger Delta, with first oceanic crust at the delta younger still than at the Walvis Ridge. Age of this more northerly crust cannot be dated by oceanic spreading anomalies as these margins formed during the Cretaceous normal-polarity interval. Along the non-volcanic margins north of the ridges there are extensive salt basins developed on highly extended continental crust. Seismic and potential field evidence suggests that the COBs in these basins approximately (i.e. within 20-30 km) coincide with the outer edge of salt. It is generally assumed that the edge of salt is an isochron, i.e. salt deposition stopped at the same time along the entire 2,000 km strike length of the salt basins. Plate reconstructions that attempt to match the COBs from South America and Africa along the entire South Atlantic demonstrate, however, that it is not possible to match the COBs along the length of the salt basin margins. This means that the edges of salt and thus the COBs are not isochrons. Possible explanations for this observation include 1) that the edge of salt is not the COB or 2) that the COB and end of salt deposition are diachronous. We tend to favor the diachronous model as, even if the edge of salt is not the COB, it would mean that salt would have had to move out onto oceanic crust by as much as 200 km to account for the mismatch. This talk discusses our tectonic observations and implications of the diachronous model for our understanding of salt deposition in evolving rift basins and the transition from continental to oceanic crust.
T41A-02 09:00h
Steep Satellite Altimetry Gradients as a Proxy to the Edge of the Continental Crust
Tight-fit plate reconstructions are produced using a global database constrained by marine magnetic anomalies tied to a consistent timescale, paleomagnetic poles, seafloor age dates based on drilling results, and fracture zone and transform fault lineations picked from ship-track and satellite altimetry data. Where a prominent steep gradient in the satellite altimetry data is present near the continental-ocean transition, it is used as a proxy to the continental shelf break [CSB]. Continental block outlines are based on digitization of the steep gradient. In some places, notably off Namibia, there is a very close correlation between that gradient and the ocean-continent boundary deduced from seismic refraction and reflection data. In other regions, there may be some stretched continental crust oceanward of the steep gradient but for reconstruction purposes we assume the crust to be predominantly continental landward of the boundary and oceanic, seaward of the line. Good matchs for conjugate CSBs are found in many places world-wide along passive margins and these will be highlighted. Particularly good matches are observed between the cratonic edges of East Antarctica as determined by sub-ice topographic highs seen along the margins of East Antarctica with respect to Madagascar, Sri Lanka, the southern half of the eastern margin of India, and the region of Australia between 124° E and 133° E along the Great Australian Bight (GAB). There are overlaps of the reconstructed conjugate CSBs, with one overlap between India and East Antarctica (70° E to 85° E) and one between East Antarctica and the western section of the GAB (105° E to 120° E). These two overlaps are coincident with the outer margins of the Lambert Graben - Prydz Bay Basin and the Aurora Subglacial Basin, respectively. It is known that there are substantial glacially-derived sediments prograded off the continental margin onto oceanic crust at Prydz Bay where there may be as much as 200 km in width of Eocene? and younger sediments outboard of the original CSB in the Prydz Bay region. When there is not a good match between conjugate CSBs then post-break up features can frequently be invoked to explain the mismatch. Abroulhos Bank off Brazil is an example of a post-break up [~40 m.y. later] magmatic construction that alters where the steep gradient in the satellite altimetry data is picked. In general, the CSBs are a good proxy to the original continental edge.
T41A-03 09:15h
The Ocean-Continent Transition at the North Atlantic Volcanic Margins
The continental margins of the northern North Atlantic are the best studied volcanic margins in the world. There is a wealth of integrated wide-angle and deep seismic profiles across the continent-ocean transition and the adjacent oceanic and continental crust, several of which form conjugate margin studies. We show new results from the integrated Seismic Imaging and Modelling of Margins (iSIMM) profiles across the Faroes continental margin which image both the extruded volcanics which generate seaward dipping reflector sequences and the underlying lower-crustal intrusions from which the extruded basalts are fed. This enables estimation of the degree of continental stretching and the total volume of melt generated from the mantle at the time of continental breakup. The new results are set in the context of profiles along the entire northern North Atlantic margins. The pattern of melt generation during continental breakup and the initiation of seafloor spreading allows us to map the pattern of enhanced sub-lithospheric mantle temperatures caused by initiation of the Iceland mantle plume over this period. The initial mantle plume thermal anomalies have the shape of rising hot sheets of mantle up to 2000 km in length, which focus into a more axisymmetric shape under the present location of Iceland. These spatial and temporal variations in the mantle temperature exert important controls on the history of uplift and subsidence and thermal maturation of the sediments near the continental margin and its hinterland. The iSIMM Scientific Team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, R Spitzer, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms, J Eccles and D Healy. The iSIMM project is supported by Liverpool and Cambridge Universities, Schlumberger Cambridge Research, Badley Technology Limited, WesternGeco, Amerada Hess, Anadarko, BP, ConocoPhillips, ENI-UK, Statoil, Shell, the NERC and DTI. We thank WesternGeco for provision of Q-streamer data.
T41A-04 09:30h
Predicting Structure and Location of the Ocean-Continent Transition at Rifted Margins Using a New Model of Continental Breakup and Rifted Margin Formation
The structure and width of the ocean-continent transition (OCT) at rifted continental margins shows great diversity for both non-volcanic and volcanic margins. Some non-volcanic margins show broad regions of exhumed mantle within the OCT while others show broad regions of very thin upper continental crust above mantle. While many volcanic margins show a sharper OCT, others show broad regions of continental crustal thinning. Both non-volcanic and volcanic rifted continental margins, including conjugate margin pairs, show depth-dependent lithosphere stretching in which stretching of the lower continental crust and lithosphere greatly exceeds that of the upper crust. Determining the structure and location of the OCT at rifted continental margins and the processes responsible for its formation represent key goals of rifted continental margin research and challenges to deep-water hydrocarbon exploration at rifted margins. Depth-uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup; it cannot explain observed depth-dependent lithosphere stretching. A new model of continental lithosphere thinning leading to breakup and sea-floor spreading initiation has been developed which assumes that thinning occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere. Model formulation uses corner-flow predicted by kinematic iso-viscous stream-function or finite element solutions to advect continental lithosphere and asthenosphere material and their temperature fields. Flow is defined by Vx, the divergence half-velocity, and Vz, the vertical upwelling velocity. The new model predicts depth-dependent lithosphere stretching prior to sea-floor spreading initiation for both non-volcanic and volcanic margins, and mantle exhumation at non-volcanic margins. Thinning of continental margin crust and lithosphere temperature evolution are sensitive to the velocity ratio Vz/Vx. For Vz/Vx of the order of 1, corresponding to plate boundary driven divergence, the model predicts exhumation of continental lithospheric mantle and a broad OCT with lateral dimension up to 100 km as observed at non-volcanic margins. For large initial values of Vz/Vx, corresponding to buoyancy assisted flow at volcanic margins, the predicted OCT is sharper with little or no exhumation of continental lithospheric mantle. The rate of upward propagation of the upwelling divergent flow field through continental lithosphere during breakup deformation initiation has a strong control on margin structure, OCT width, and the extent of mantle exhumation. Crustal thinning and lithosphere temperature predicted by the new margin formation model are used to determine rifted margin bathymetry and gravity anomalies. Observed bathymetry and gravity anomalies have been used to invert for breakup deformation kinematic parameters which may be used to predict margin structure, OCT location, and subsidence and heat-flow history. The new margin formation model has been successfully applied to the Goban Spur, Grand Banks, and Iberian non-volcanic margins, and the Lofoten and Voering volcanic margins using observed bathymetry, gravity and sediment thickness data to invert for the kinematic parameters describing lithosphere breakup deformation. The preferred models of these margins require only small components of pre-breakup depth-uniform lithosphere stretching. This work forms part of the NERC Margins iSIMM project.