Variations in amount and direction of seafloor spreading along the northeast Atlantic Ocean and resulting deformation of the continental margin of northwest Europe

International audienceThe NE Atlantic Ocean opened progressively between Greenland and NW Europe during the Cenozoic. Seafloor spreading occurred along three ridge systems: the Reykjanes Ridge south of Iceland, the Mohns Ridge north of the Jan Mayen Fracture Zone (JMFZ), and the Aegir and Kolbeinsey Ridges between Iceland and the JMFZ. At the same time, compressional structures developed along the continental margin of NW Europe. We investigate how these compressional structures may have resulted from variations in the amount and direction of seafloor spreading along the ridge system. Assuming that Greenland is rigid and stationary, we have used a least squares method of palinspastic restoration to calculate differences in direction and rate of spreading along the Reykjanes, Kolbeinsey/Aegir and Mohns Ridges. The restoration generates relative rotations and displacements between the oceanic segments and predicts two main periods of left-lateral strike slip along the main oceanic fracture zones: (1) early Eocene to late Oligocene, along the Faeroe Fracture Zone and (2) late Eocene to early Oligocene and during the Miocene, along the JMFZ. Such left-lateral motion and relative rotation between the oceanic segments are compatible with the development of inversion structures on the Faeroe-Rockall Plateau and Norwegian Margin at those times and probably with the initiation of the Fugløy Ridge in the Faeroe-Shetland Basin during the Eocene and Oligocene. The Iceland Mantle Plume appears to have been in a position to generate differential seafloor spreading along the NE Atlantic and resulting deformation of the European margin

Sr and Nd isotope data for arc-related (meta) volcanics (SW Iberia)

In the southern sector of the Ossa-Morena Zone (Iberian Variscan Chain), along its boundaries with the Beja-Acebuches Ophiolite and the South-Portuguese Zone, Upper Palaeozoic igneous mafic and intermediate rocks, both intrusive and extrusive, are widely represented. The so-called Odivelas Unit (Andrade,1983), include (meta-) basalts and (meta-) andesites, which, according with previous studies, display low-K tholeiitic to calc-alkaline signatures and, therefore, are interpreted as remnants of an active margin volcanic arc. Santos et al. (1990) subdivided those volcanics into two groups: in Alfundão-Peroguarda, the tholeiitic nature is dominant; in Odivelas-Penique, the calc-alkaline signature becomes more pronounced. Intercalation of limestone layers provided some age constraints, showing that the subduction-related volcanic activity in the studied area began in the Lower Devonian and continued, at least, through the Middle Devonian (Conde & Andrade, 1974; Machado et al., 2010). In this work, samples previously studied by Santos et al. (1990) and Silva et al. (2011) were analysed for Sm-Nd and Rb-Sr isotopes. Considering that the volcanics were systematically affected by hydrothermal metamorphism, it is expected that the Sr signatures show significant disturbance. In contrast, Nd isotope ratios probably reflect the primary features. Alfundão-Peroguarda samples show a very limited range of positive initial εNd, from +5.1 to +4.3 (assuming 400 Ma), showing no evidence for significant crustal assimilation and, therefore, allowing the attribution of negative Nb and Ta anomalies to arc-related processes On the other hand, 87Sr/86Sr varies from 0.7044 to 0.7060 (for 400Ma). These samples rocks define a horizontal trend on the initial εNd vs. initial 87Sr/86Sr plot, typical of co-genetic rocks that underwent interaction with seawater. On the other hand, Odivelas-Penique volcanics show wide spectra for both initial 87Sr/86Sr (from 0.7038 to 0.7066) and εNd (from +4.6 to -4.1). Significantly, the highest εNd values for this group are within the narrow range defined by Alfundão-Peroguarda tholeiitic basalts, suggesting a common mantle source (or very similar sources) for the most mafic magmas of both sectors. The whole set of Nd isotope ratios supports the distinction previously proposed between the two groups of volcanics. In addition, the variation from positive to negative initial εNd values in the Odivelas-Penique suite shows that its geochemical features were likely influenced by assimilation of continental crustal material

Thrust wedges and fluid overpressures: Sandbox models involving pore fluids

International audienceThe well-known model for the critical taper of an accretionary wedge includes overpressure as a first-order parameter. Fluid overpressures reduce frictional resistance at the base of a wedge but they also act as body forces on all material particles of the wedge, in addition to that of gravity. By means of sandbox modeling, many workers have tried to verify the predictions of the critical taper model, but few of them have so far incorporated true fluid pressures. We have used scaled experiments, in which compressed air flows through sand packs, so as to model the deformation of overpressured wedges. A new apparatus provides for a horizontally varying fluid pressure, for example, a linear variation, as in the critical taper model. We have done three series of experiments, involving horizontal shortening of homogeneous or multilayered sand models for various gradients of fluid pressure. As predicted by the critical taper model, the apical angle of the resulting wedge depends on the overpressure gradient. In homogeneous sand at a high overpressure gradient, deformation becomes diffuse and looks ductile. In multilayered models, detachments form beneath layers of low permeability, so that thrusts propagate rapidly toward the undeformed foreland. The efficiency of a detachment and its ability to propagate depend not only on the fluid pressure but also on the permeability ratios between the various layers

Horizontal compression and stress concentration at passive margins: causes, consequences, and episodicity

1 p.Late uplift and exhumation at regional scale are common to many a passive margin. By consensus, amongst the likely mechanisms are thermal relaxation after rifting, deep-seated mantle flow, offshore sedimentation, onshore exhumation, and tectonic stress. Probably no one mechanism can account for all of the uplift and exhumation of a margin. Here I will make the case for horizontal compression, as an important mechanism in some instances. The World Stress Map project provides useful insights into current stress distributions. Horizontal compression dominates in many plates, including some (but not all) continental margins. Continental collision, ridge push, and slab pull can account for horizontal compression. However, radial patterns of trajectories provide evidence for stress concentrations. A well-known example is Central Asia, where the cause is indentation by a colliding India. In the northern hemisphere, there is good evidence that the Iceland plume, whatever its origin, is another source of stress concentration. The pattern is best seen on azimuthal projections, which centre on Iceland itself. It would appear to be responsible for thrust mechanisms of recent earthquakes in Scandinavia. In the geological record, there is also evidence for anomalies in seafloor spreading and basin inversion around Iceland, at least since the Neogene. Excess seafloor spreading appears to have transmitted, via reactivated fracture zones, to the continental margin of Norway. Thus the Iceland plume and the North Atlantic ridge, acting together, may have provided enough compression to account for basin inversion on North Atlantic margins. They may even account, at least in part, for Neogene to recent mountain building in adjacent onshore areas of Scandinavia, Scotland, Svalbard, Greenland, Baffin Island, Ellesmere Island and Labrador. More generally, for such a mechanism to be viable, a ridge-centred plume should close to a continental margin. I discuss the likelihood of such a mechanism having operated in the South Atlantic during the Late Cretaceous and Palaeogene, and having contributed to inversion in SE Brazil. Even more generally, uplift and exhumation on passive margins would seem to be episodic in nature. Some of the known episodes correlate with phases of mountain building and rapid plate convergence worldwide. This in itself is an argument for horizontal compression being a significant mechanism in the uplift and exhumation of some passive margin

Mechanical modelling in structural geology and tectonics - past, present and future

In structural geology and tectonics, most fundamental advances have come through observation. However, mechanical and kinematical modelling has provided an understanding of physical processes, as well as ideas for further investigation. As in other sciences, the main techniques in tectonics have been three: analytical, physical and numerical. For past and present times, I will compare and contrast these techniques, concentrating on their advantages, disadvantages, and dangers. For the future, I will attempt to predict their development, even though this exercise may be subjective and risky. Analytical modelling has had a long but erratic history, punctuated by occasional breakthroughs. Outstanding examples have been the theories of folding and flexure. The main advantages of the method are mathematical precision and elegance. The main disadvantages are (1) high requirements of mathematical ability, and (2) difficulties in incorporating nonlinear behaviour or complex boundary conditions. Physical modelling had a long history, but a slow start, until Hubbert (1937) formulated the rules for proper scaling. The method has developed very rapidly in the last few decades. Of great importance have been technical breakthroughs, especially (1) the use of weak materials, for which a centrifuge is not necessary, (2) the addition of surface processes, and (3) the development of visualization in 2D and 3D. The main advantages of the method are that (1) the results are real, visual, and instructive, (2) the models have good resolution, (3) sedimentation and erosion are simple to apply, (4) the experimenter has little need of experience in mathematics. The main disadvantages are that (1) the results are not easily reproducible, (2) the work may be time-consuming, even physically demanding, (3) boundary conditions may be difficult to control, (4) the range of suitable model materials is small, and (5) the exercise may appear childish to newcomers. Numerical modelling has the shortest history, but its development has been almost exponential in the last few decades, in pace with computers. The main advantages of the method are (1) reproducibility, (2) speed, (3) ability to incorporate non-linear behaviour, (4) ease of investigation of different parameters, (4) ease of presentation of results, (5) an appearance of sophistication. The main disadvantages are (1) high cost or unavailability of some software, (2) possible programming mistakes, (3) possible numerical errors. Ironically perhaps, the main danger of the method may be its ease of application. There is a current tendency for numerical modelling to substitute for good observations. Although physical modelling may remain useful for testing new ideas, the future probably lies in numerical modelling. I foresee many new developments in the numerical modelling of nonlinear processes, especially those involving feedback between mechanical, thermal and chemical aspects, as well as fluid flow, in two or three dimensions, and on all scales up to that of planet Earth. On the other hand, I would urge prudence in accepting some of the result

Hydrocarbon generation, a mechanism of detachment in thin -skinned thrusts belts

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Sandbox experiments on gravitational spreading and gliding in the presence of fluid overpressures

International audienceWhereas in previous analogue experiments on gravitational spreading and gliding, detachment occurred on a ductile layer, we have used a relatively new technique of injecting compressed air into sand packs so as to simulate the effects of fluid overpressures in sedimentary strata and to trigger slope instabilities. In our experiments, the governing equations yield scales for dimensions, stresses and fluid pressure. However, the more transitory phenomena of production and decrease of overpressure cannot be suitably scaled. By using layers of differing permeability, we are able to produce sharp detachments in models made of sand alone. The experiments involve gravity spreading or gravity gliding. In gravity spreading, propagation of the detachment and of extensional deformation depends on the fluid pressure. For medium values of fluid overpressure, normal faults are closely spaced, numerous and bound rotated blocks. They propagate progressively toward the back of the model. For the highest pressures, the deformation propagates very fast and faults bound non-rotated blocks, which slide on an efficient basal detachment. Fault dips are also controlled by fluid pressure and by frictional resistance at the base. To model gravitational gliding required an apparatus with a more complex system of air injection. We did a series of experiments using injection windows of various lengths and compared the results with predictions from a quasi-3D analytical model of sliding. In contrast with predictions for an infinite slope, sliding depends on (1) the fluid overpressure on the detachment, (2) the fluid overpressure in the body of the sliding sheet, and (3) the shape of the detachment surface. In particular, we show that frictional resistance at the lower edge is a primary control on the dynamics of gliding