Fluid pressure evolution in overpressured limestone reservoir at basin-scale: example of the Bighorn Basin (Wyoming, USA) and lessons from comparison with other reservoirs

Nicolas Beaudoin, Olivier Lacombe, Nicolas Bellahsen, Khalid Amrouch, and Jean-Marc Danie

Tertiary sequence of deformation in a thin-skinned/thick-skinned collision belt: The Zagros Folded Belt (Fars, Iran)

International audienceWe describe how thin-skinned/thick-skinned deformation in the Zagros Folded Belt interacted in time and space. Homogeneous fold wavelengths (15.8 ± 5.3 km), tectono-sedimentary evidence for simultaneous fold growth in the past 5.5 ± 2.5 Ma, drainage network organization, and homogeneous peak differential stresses (40 ± 15 MPa) together point to buckling as the dominant process responsible for cover folding. Basin analysis reveals that basement inversion occurred ∼20 Ma ago as the Arabia/Eurasian plate convergence reduced and accumulation of Neogene siliciclastics in foreland basin started. By 10 Ma, ongoing contraction occurred by underplating of Arabian crustal units beneath the Iranian plate. This process represents 75% of the total shortening. It is not before 5 Ma that the Zagros foreland was incorporated into the southward propagating basement thrust wedge. Folds rejuvenated by 3–2 Ma because of uplift driven by basement shortening and erosion. Since then, folds grew at 0.3—0.6 mm/yr and forced the rivers to flow axially. A total shortening of 65–78 km (16–19%) is estimated across the Zagros. This corresponds to shortening rates of 6.5–8 km/Ma consistent with current geodetic surveys. We point out that although thin-skinned deformation in the sedimentary cover may be important, basement-involved shortening should not be neglected as it requires far less shortening. Moreover, for such foreland folded belts involving basement shortening, underplating may be an efficient process accommodating a significant part of the plate convergence

Subduction And Plate Motion In Laboratory. European Geosciences Union.

We have performed 3-D laboratory experiments to test the influence of factors controlling plate and trench motion of a subducting plate. The systematic study has been carried out widely changing geometrical and rheological parameters of the system and testing the relative influence on the kinematics of subduction. A total number of 60 different experiments have been performed using variable combinations of thickness, viscosities, densities of the plate and mantle. We have scaled down our experiments to natural gravity field using silicone putty and honey to simulate the viscous behavior of slab and mantle, respectively. Our models can be divided in two classes typified by a different style of subduction: a "retreating" style of subduction characterized by the backward motion of the trench and the slow advancing motion of the plate, and an "advancing" style, where trench is stationary or advance towards the upper plate and the plate move in faster way. The latter condition is found to be more difficult to be obtained. In general terms, the two behaviors depend upon the distribution of the forces active into the system. If driving forces are much more elevated than resisting ones the system is more prone to retreat. The result we obtained in terms of velocity of subduction, in addition, is original with respect to previous works. We found a velocity of subduction which is always much greater than the one calculated by using simple equations. Hence we pointed out that a precise evaluation of subduction velocity could not be done assuming a constant radius of curvature

Dynamics of Subduction and Plate Motion in Laboratory Experiments.

3-D laboratory experiments have been designed to investigate the way slab-bearing plates move during subduction inside the mantle. The boundary conditions are as simple as possible: a viscous plate rests in the center of a large tank filled up by honey and subducts under its negative buoyancy once a small instability at the plate edge is created. Varying thickness, width, viscosity, density of the plate and mantle, three characteristic modes of subduction are observed: a retreating trench mode (Mode I), a retreating trench mode following a transient period of advancing trench (Mode II), and an advancing trench mode (Mode III). These modes are characterized by different partitioning of the amount of subduction into plate and trench motion. Our experiments show that the velocity of subduction can be roughly modeled by the dynamic interaction between acting and resisting forces and that some parameters such as the slab viscosity or thickness have the opposite influence than the one usually suggested in the literature. This result is interpreted as the consequence of the dependence (measured in the experiments) of the slab radius of curvature on the plate viscosity and thickness. However, it is still far from being simple to predict how the trench and plate move. Our results suggest that the complexity of the style of subduction could also be controlled by simple geometrical rules of a plate bending inside a stratified mantle: our planet system is in the crucial range where the length of the slab pulling down the plate is about the double of its radius of curvature

Distribution, style, amount of collisional shortening, and their link to Barrovian metamorphism in the European Alps

International audienceWe review estimates of collisional shortening along the Alpine Chain and reassess its amounts, showing that it increases from south to north in the Western Alps, attaining a maximum in the Central Alps, and decreasing in the Eastern Alps. We suggest that previous calculations overestimated shortening in the Western Alps, but underestimated it in the Central Alps. Based on these new determinations, we conclude that the convergence direction during Alpine collision was more likely oriented NW instead of WNW. A new map compilation of peak metamorphic temperatures related to syn-collisional, Barrovian metamorphism and of cooling ages for the Western Alps form the base for a discussion and interpretation of the spatial distribution of Barrow-type metamorphism throughout the Alpine Chain. We show that a simple correlation exists between the inferred temperature of areas where Barrow-type metamorphism is exposed at the surface and the amount of collisional shortening, which is mainly localized in the External Zone in the Western Alps, but in the Internal Zone in the Central and Eastern Alps. We conclude with a conceptual model, suggesting that major differences in the spatial distribution of shortening and exhumation of Barrovian metamorphic units across the Alpine Chain depend on the convergence direction, but also on the presence and size of the Briançonnais continental nappe stack at the onset of collision

Relating orogen width to shortening, erosion, and exhumation during Alpine collision

We investigate along-strike width changes of the thickened, accreted lower plate (TALP) in the Central and in the Eastern Alps. We set the width of the TALP in relation to the inferred amount of collisional shortening and exhumation along six orogen-scale cross sections. Taking the present-day, along-strike gradients in the amount of collisional shortening to represent the temporal evolution of the collisional wedge, it may be concluded that the cross-sectional area of the TALP diminishes during ongoing shortening, indicating that the erosional flux outpaced the accretionary flux. Higher amounts of collisional shortening systematically coincide with smaller widths of the TALP and dramatic increases of the reconstructed eroded rock column. Higher amounts of shortening also coincide with larger amplitudes of orogen-scale, upright folds, with higher exhumation and with higher exhumation rates. Hence, erosion did play a major role in reducing by >30 km the vertical crustal thickness in order to accommodate and allow shortening by folding. Long-term climate differences cannot explain alternating changes of width by a factor of almost 2 along straight segments of the orogen on length scales less than 200 km, as observed from the western Central Alps to the easternmost Eastern Alps. Sedimentary or paleontological evidences supporting such paleo-climatic differences are lacking, suggesting that erosional processes did not directly control the width of the orogen

Fault reactivation and rift localization: Northeastern Gulf of Aden margin

[1] The Gulf of Aden and the Sheba spreading ridge ( Gulf of Aden) forms the southern boundary of the Arabian Plate. Its orientation ( 075 degrees E) and its kinematics ( about 030 degrees E divergence) are interpreted as the result of an oblique rifting. In this contribution, a field study in the northeastern Gulf of Aden allows us to confirm the Oligo-Miocene synrift directions of extension and to precise the normal fault network geometry. The synrift extensions are 020 degrees E and 160 degrees E ( possibly in this chronological order); the normal faults strike 070 degrees E, 090 degrees E, and 110 degrees E. The results show that some characteristics are consistent with oblique rifting analogue models, while some others are not. Especially, fault reactivation of Mesozoic structures is shown to have occurred significantly at the beginning and during rifting. These data are therefore compared to analogue models of oblique reactivation, and this comparison demonstrates that fault reactivation played a key role during the early stage of the Gulf of Aden rifting. Finally, scenarios of the lithospheric evolution during the eastern Gulf of Aden opening ( preexisting weaknesses in the lithosphere or not) are discussed to better constrain the deformation history of the northern margin. Especially, we show that rift localization processes may imply stress rotations through time

The contribution of Anisotropy of Magnetic Susceptibility (AMS), Anisotropy of P-wave Velocity (APV) and Petrophysic analysis in the strain characterisation at Sheep Mountain Anticline (Wyoming, USA)

Robion, P., Amrouch, K., Callot, J., Lacombe, O., Daniel, J., Bellahsen, N