A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of approximately 3-4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of approximately 25 mm x yr(-1), but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north-south mantle flow.

[1] Reliable piercing points on both sides of the Sea of Marmara enabled us to obtain an estimate of the slip-rate over time scales of 10-15 ka on different fault strands of the North Anatolian Fault (NAF) system. We analyzed geomorphic features in the gulfs of Izmit, Gemlik (Sea of Marmara) and Saros (NE Aegean Sea), which were passively displaced by the NAF strands after their abandonment related to the post-glacial sea level rise. Results for the main northern strand, consistently similar on both sides of the Marmara pull-apart, are in the order of 10 mm/yr, about one half of that expected from geodetic measurements and accepted plate-tectonic models. In the southern branch of the NAF, the estimated rate of ∼4 mm/yr is only slightly higher than that given by geodetic models. Our findings have implications for both neo-tectonic reconstructions of the submerged portion of the NAF system, and fault interactions and seismic hazard estimates in the Marmara region. They suggest that, either the total Anatolia/Eurasia plate motion is more diffuse than previously reported, or geodetic data are not representative of the geological time-scale deformations. Moreover, they suggest that a significant amount of stress is accommodated along the southern strand of the NAF system, on which the last large (M ≈ 7) earthquakes dates back

The central Red Sea, an oceanic basin floored by Miocene evaporites reaching kilometres in thickness in places, is at an early stage of development, where seafloor spreading has geologically only recently replaced continental rifting. Surveying with a high-resolution multibeam echo-sounder around Thetis Deep, a new spreading centre, has revealed a remarkable series of structures resembling viscous gravity flows, which are 2 here interpreted as originating from flowage of the evaporites laterally unloaded by axial rifting and other processes developing the relief of the deep. The flow margins are marked by stream-wise lineaments and some apparently rotated markers. Their fronts in the floor of the deep are rounded in plan view and profile. Their surfaces contain small closely spaced features resembling extensional faults. In one area below declining gradients, the surface contains along-slope ridges and valleys typical of compression folds (ogives). Flow-parallel lineaments and extensional faults lie, respectively, parallel and orthogonal to the direction of maximum seabed gradient. Movement is apparently heterogeneous, at least in part by varied blocking by relief of underlying basement observed protruding between flows. Flowage is currently transporting materials into the floor of the deep where it has the potential to become incorporated into the young oceanic crust by repeated eruption of axial lavas over them. In the light of these new data, we reexamine the possibility and implications of flowage in the South Atlantic marginal evaporites, in particular whether flowage contaminated early oceanic crust in such areas.

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