Nicolas Bellahsen

Why did Arabia separate from Africa? Insights from 3-D laboratory experiments

Abstract We have performed 3-D scaled lithospheric experiments to investigate the role of the gravitational force exerted by a subducting slab on the deformation of the subducting plate itself. Experiments have been constructed using a dense silicone putty plate, to simulate a thin viscous lithosphere, floating in the middle of a large box filled with glucose syrup, simulating the upper mantle. We examine three different plate configurations: (i) subduction of a uniform oceanic plate, (ii) subduction of an oceanic–continental plate system and, (iii) subduction of a more complex oceanic–continental system simulating the asymmetric Africa–Eurasia system. Each model has been performed with and without the presence of a circular weak zone inside the subducting plate to test the near-surface weakening effect of a plume activity. Our results show that a subducting plate can deform in its interior only if the force distribution varies laterally along the subduction zone, i.e. by the asymmetrical entrance of continental material along the trench. In particular, extensional deformation of the plate occurs when a portion of the subduction zone is locked by the collisional process. The results of this study can be used to analyze the formation of the Arabian plate. We found that intraplate stresses, similar to those that generated the Africa–Arabia break-up, can be related to the Neogene evolution of the northern convergent margin of the African plate, where a lateral change from collision (Mediterranean and Bitlis) to active subduction (Makran) has been described. Second, intraplate stress and strain localization are favored by the presence of a weakness zone, such as the one generated by the Afar plume, producing a pattern of extensional deformation belts resembling the Red Sea–Gulf of Aden rift system.

Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide-angle, multichannel seismic, and heat flow survey

Continental rifts and passive continental margins show fundamental along-axis segmentation patterns that have been attributed to one or a number of different processes: extensional fault geometry, variable stretching along strike, preexisting lithospheric compositional and structural heterogeneities, oblique rifting, and the presence or absence of eruptive volcanic centers. The length and width scales of the rift stage fault-bounded basin systems change during the late evolution of the new plate boundary, and the role of magmatism may increase as rifting progresses to continental rupture. Along obliquely spreading ridges, first-order mid-ocean ridge geometries originate during the synrift stage, indicating an intimate relationship between magma production and transform fault spacing and location. The Gulf of Aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore. This obliquely spreading rift is considered magma-poor and has several large-offset transforms that originated during late stage rifting and control the first-order axial segmentation of the spreading ridge. Widely spaced geophysical transects of passive margins that produce only isolated 2-D images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models. Using closely spaced new geophysical and geological observations from the Gulf of Aden we show that rift sectors between transforms have a large internal variability over short distances (∼10 km): the ocean-continent transition (OCT) evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed. We suggest that this small-scale variability may be explained (1) by the distribution of volcanism and (2) by the along-strike differences in time-averaged extension rate of the oblique rift system. The volcanism may be associated with (1) the long-offset Alula-Fartak Fracture Zone, which may enhance magma production on its younger side, or (2) channeled flow from the Afar plume material along the newly formed OCT and the spreading ridge. Oblique extension and/or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma-poor margins.

Fracture patterns in the Zagros Simply Folded Belt (Fars, Iran): constraints on early collisional tectonic history and role of basement faults

Pre-/early folding fracture patterns were recognized in several anticlines from three structural domains in the Simply Folded Belt of the Fars arc. These fracture sets were characterized in terms of opening mode and relative chronology and used to reconstruct the main compressional trends related to the early Zagros collisional history. The palaeostress reconstructions based on these fracture sets were further refined by combination with newly collected or already available fault-slip and calcite twin data. As an alternative to previous models of rigid block rotations or regional stress rotation, we propose that the complex pattern of pre-folding fractures and the contrasting palaeostress orientations through time in the different domains investigated are related to the presence of basement faults with N–S and WNW trends, above which basement and cover were variably coupled during stress build-up and early deformation of the Arabian margin. Beyond regional implications, this study draws attention to the need to carefully consider pre-existing fractures, possibly unrelated to folding, to build more realistic conceptual fold–fracture models.

Analog models of oblique rifting in a cold lithosphere

New lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution, basin segmentation, and mantle exhumation occurring during rift localization. We present two models: one with a preexisting oblique lithospheric weakness (model B) and another with no weakness zone (model A). Both oblique rifts have an obliquity of about 40°. The main results are as follows. (1) The fault populations, especially during the early stages of deformation, are composed of faults that in strike are largely intermediate between rift-parallel and perpendicular to displacement. This fault population is characteristic of oblique rifts observed in previous studies. (2) In later stages, faults parallel to the rift become numerous in both models. Buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift-parallel faults. (3) During the final stages of extension, in model B the crust is deformed by rift-parallel faults, while in the basins the small-scale deformation pattern is composed of displacement-normal faults. However, in model A, displacement-normal faults tend to accommodate most of the extension, controlling its final stages. They probably also control the formation of the ocean-continent transition, any possible mantle exhumation, as well as the geometry of oceanic accretion centers. These results provide an insight into the possible evolution of the Gulf of Aden conjugate margins, which developed in an oblique context and most probably without any preexisting rift-parallel localizing heterogeneity in the lithosphere.

Collision kinematics in the western external Alps

The kinematics of the collision in Western Alps are investigated through five balanced cross sections of the whole external domain from the Oisans to the Mont Blanc massif. These cross sections were built using published data for the Jura and subalpine fold-and-thrust belts and new structural and field analysis for the External Crystalline Massifs. Five units are defined: the sedimentary nappes from innermost parts of the external zone (e.g., ultra-Dauphinois/Helvetic), the crystalline units with their dysharmonically folded cover (e.g., Morcles nappe), sedimentary nappes over the frontal parts of the crystalline massifs (the Aravis-Granier unit), the subalpine belts (e.g., Vercors, Chartreuse, Bauges, and Bornes), and the Jura. Except for the ultra-Dauphinois nappes, the shortening, including the cover shortening, always corresponds to basement shortening. The total amount of shortening increases from south (28 km, 20%) to north (66 km, 27%). Moreover, the shortening is slightly older in the south than in the north; deepwater turbidites (flysch) and shallow marine to freshwater clastics (molasse) basins are more developed in the north; pressure and temperature conditions are higher in the north; the average uplift rates are about 3 times higher in the north and more localized in space. We propose that these differences are due to along-strike variations in the structure of the European continental margin inherited from Mesozoic times. We then build five palinspastic maps: one at Cretaceous times showing the inherited European Mesozoic margin structure and four from Priabonian to upper Miocene times showing the collision kinematics and the related rotation of Adria.

Fracture‐controlled paleohydrogeology in a basement‐cored, fault‐related fold: Sheep Mountain Anticline, Wyoming, United States

New geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of Sheep Mountain Anticline, Wyoming, United States. Oxygen, carbon, and strontium isotope studies were performed on prefolding and fold-related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results. We show that most of the cements precipitated from Paleogene meteoric fluid. Stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth (T > 110°C). This implies that fluids migrated along the basement thrust underlying the fold and/or at the base of the cover. Above the fault tip, the fluids circulated rapidly in the diffuse synfolding (and early folding) fracture network. The zone of preferential migration of the warm fluids is currently located in the backlimb of the fold, which supports some of the previously published structural interpretation of the subsurface. This study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement-involved shortening in Laramide-style forelands.

Paleostress magnitudes in folded sedimentary rocks

Using Sheep Mountain Anticline (Wyoming, USA) as a case study, we propose a new approach to quantify effective paleo-principal stress magnitudes in the uppermost crust. The proposed mechanical scenario relies on a well-documented kinematic and chronological sequence of development of faults, fractures and microstructures in the folded strata. Paleostress orientations and regimes as well as differential stress magnitudes based on calcite twinning paleopiezometry are combined with rock mechanics data in a Mohr construction to derive principal stress magnitudes related to the successive steps of layer-parallel shortening and to late stage fold tightening. Such quantification also provides original insights into the evolution of the fluid (over)pressure and amount of syn-folding erosion.

Curvature and fracturing based on global positioning system data collected at Sheep Mountain anticline, Wyoming

We investigate the curvature-fracture relationship at Sheep Mountain anticline, Wyoming, by coupling fracture mapping with structural mapping using high-precision global positioning system data. Carrier-phase post-processing techniques of spatial data collected across patches of bedding surfaces result in a high-resolution data set. Differential geometry tools form the basis for curvature analysis, allowing for a quantitative understanding of the shapes of these surfaces. Comparison of principal curvature magnitudes with fracture measurements indicates that greater curvature correlates with greater spherical variance of fracture set orientations. Fracture intensities, however, correlate only loosely with curvature, because fracturing mechanisms other than curvature of bedding must be taken into account.

Influence of viscous layers on the growth of normal faults: insights from experimental and numerical models

Abstract The influence in space and time of viscous layers on the deformation pattern of brittle layers is investigated using wet clay/silicone putty analogue models in extension. Brittle and brittle–viscous experiments at various extension velocities are compared. Numerical models are also performed to confirm the results and to control the boundary conditions. Our results show that: (i) the presence of a basal viscous layer localizes the deformation by creating faults with very large throw. This kind of deformation distribution constrains the location of small faults, with scattered orientations, in the vicinity of the larger, in particular in relay zones. (ii) A lower strength of the viscous layer (i.e. a low extension velocity) enhances this localization of the deformation. (iii) The displacement–length relationship and the spatial distribution of small-scale faults are strongly influenced by both the rheology of the model and the amount of extension. This study shows that they are important parameters, especially when characterizing the whole fault network evolution and the relationship between large and small faults.

Mechanical constraints on the development of the Zagros Folded Belt (Fars)

We synthesize available structural, seismotectonics and microtectonics studies, mechanical modelling of the topography as well as stratigraphic constraints on the timing of Plio-Pleistocene folding and Zagros basin evolution in order to examine which mechanical behaviour would explain the development of the Zagros Folded Belt at both local and regional scale.