Mountain belts are often the result of former inverted rifts or rifted margins. Up to date, relics of magma-poor rifted margins have been found in orogens allowing the investigation of how these margins reactivate and control the formation of mountain belts. In contrast, magma-rich rifted margins have barely been recognized within orogenic systems, and consequently how these margins reactivate and control subsequent orogenesis is poorly understood. We use a thermo-mechanical model to investigate the mechanical behaviour of reactivated magma-rich versus magma-poor rifted margins during early orogenesis (i.e. margin inversion). Input data for our modelling experiments are obtained from two natural laboratories. One is the Demerara Plateau characterized by a mildly shortened magma-rich rifted margin. Its décollement level is observed at the top of the syn-kinematic volcanics, which propagates downwards into a frozen incipient subduction. The other study-case is the Basque-Cantabrian Belt consisting of a reactivated magma-poor hyper-extended rift system with the décollement level at the bottom of the syn-rift sediments. Our modelling results, for inverted magma-rich rifted margin, show that the syn-kinematic volcanics undergo subduction as they are mechanically coupled with the underlying rifted lithosphere. Hence, only post-rift sediments are accreted within the early orogenic accretionary wedge. In contrast, both the syn- and post-rift sediments from a reactivated magma-poor rifted margin are expected to be preserved within the accretionary wedge. Therefore, we conclude that the presence or absence of syn-rift sediments within an accretionary prism may represent a robust indicator to determine the nature of reactivated rifted margins within orogens. We believe that our results may strongly contribute to recognize, the so far poorly identified, magma-rich rifted margins within present-day orogenic systems.
To investigate the effect of crustal heterogeneities inherited from previous tectonic phases on magma-poor rifting processes, we performed numerical experiments of lithospheric extension with initial conditions that included strength variations from inherited crustal fabrics. Crustal fabrics were introduced in the model by using an element-wise bimineralic composition in which mineral phases were distributed in a way that was compatible with the orientation and distribution of kilometric-scale heterogeneities observed in seismic reflection data. Our numerical models show that strength variations from inherited crustal fabrics strongly influence the mechanisms of deformation in the stretching and thinning phases of rifting. The strength variations also generate alternative models for the evolution of faulting during distributed stretching and localized thinning phases that are usually associated with detachment or sequential faulting models. During the stretching phase, inherited strength variations control the distribution and the processes of deformation. Vertical fabrics favor the formation of horst-and-graben structures. Horizontal and dipping fabrics favor the formation of detachment faults and core complexes. During the thinning phase, processes differ depending on the orientation of the crustal fabrics and involve either a combination of detachment faults and sequential normal faults or an alternative model in which deformation remains decoupled between the upper crust and lithospheric mantle, with the formation of high-angle faults in the upper crust and a low-angle detachment fault in the upper mantle. As a consequence, strength variations inherited from crustal fabrics also control the resulting geometry of the margin and the width of the necking and hyperextended domains. Finally, our models demonstrate that inherited crustal fabrics do not control breakup and mantle exhumation. These processes are ubiquitously associated with the development of new detachment faults exhuming mantle to the seafloor.
By demonstrating that extensional inheritance plays a decisive role in the formation of orogens, recent studies have questioned the ability of a unique, complete Wilson cycle model to explain the diversity of collisional orogens. For 5 years, the OROGEN Research Project had therefore the ambition to challenge this classical Wilson cycle model. By focusing on the diffuse Africa-Europe plate boundary in the Biscay-Pyrenean-Western Mediterranean system, the project questioned the preconceived “Orogen singularity” assumption and investigated the role of divergent and convergent maturities in orogenic and post-orogenic processes. This work led us to rethink the development of collisional orogens in a genetic (or process-driven) way and to propose an updated version of the ” classical Wilson cycle”, the Wilson Cycle 2.0, and the ORO-Genic ID concept presented in this paper. The particularity of the Wilson Cycle 2.0 is to take into account the divergence and convergence maturity reached during extensional and orogenic processes in proposing different tectonic tracks associated with different ORO-Genic ID numbers. The ORO-Genic ID is composed of a letter (or track), corresponding to the maturity of divergence reached and a number corresponding to the maturity of convergence reached during the formation of the orogen. This new concept relies on the observed pre- and syn- convergent tectono- stratigraphic and magmatic record and deformation history and can be identified in using diagnostic criteria presented in this paper. It represents therefore a powerful tool that can be used to characterize the evolution and the architectural type of an orogenic system. Moreover, as a mappable concept, it can be easily used worldwide and can help us to explain differences in the style of deformation at crustal scale between orogens.
Abstract Orogenesis in the Variscan belt of Western Europe was followed by a major magmatic event during the Permian that formed a mafic lower crust by crystallizing pyroxenite and gabbros from mantle‐derived melts at the base of the continental crust. Partial melting of the asthenosphere left a significantly depleted mantle that was progressively incorporated into the subcontinental lithospheric mantle as the orogenic domain cooled. The potential impact of such large‐scale thermal and lithologic layering has never been taken into account in the study of the Alpine Tethys and North Atlantic rift systems that developed subsequently in Western Europe. Here we investigate via numerical modeling how a mafic heterogeneity within the lowermost part of a quartzo‐feldspathic continental crust and/or a zone of depleted mantle within the lithospheric mantle beneath a former orogenic domain could have influenced subsequent rifting. Our numerical modeling results indicate that, in a thermally equilibrated lithosphere, a mafic body within the lower continental crust or a zone of shallow depleted mantle prevents rifting of overlying weaknesses (e.g., faults and suture zones). We propose that the regional mafic lower crust beneath the Variscan orogenic domain may explain why the Tethyan and southern North‐Atlantic rift systems did not localize at former suture zones of the Western European orogenic lithosphere.
Abstract Many mountain belts exhibit significant along‐strike variation in structural style with changes in the width of the orogen, the geometry and kinematics of the crustal‐scale thrust system, and the degree of partitioning between pro‐ and retro‐wedge deformations. Although the main factors controlling first‐order structural style are understood, the cause of these lateral variations remains to be resolved. Here we focus on the Pyrenees, characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part. Similarly, the prior Mesozoic rifting event was characterized by significant lateral variation in structure. We integrate available geological and geophysical data with forward lithospheric scale numerical models. We show that lateral variation in crustal strength attributed to inherited Variscan crustal composition accentuated during Mesozoic rifting explains the variation in structural style observed during Pyrenean mountain building.
Recent observations and models of continental rifting in magma‐poor environments have led to the concept of multiphase stages of lithospheric extension. In these concepts it is shown that extreme crustal thinning of the crust predates exhumation of lower crustal and subcontinental mantle rocks during final rifting. The Bay of Biscay is a V‐shaped ocean basin that opened in Aptian‐Albian time. In front of this propagating ocean, several rift basins formed that show evidence for extreme crustal thinning and locally also mantle exhumation (the Parentis, Arzacq‐Mauleon, and Cantabrian basins). In this paper we propose, based on geological and geophysical observations and using numerical modeling, a model that can explain the extreme crustal thinning observed in the Arzacq‐Mauléon and Parentis basins. Our results show that rifting in the Bay of Biscay was initiated by distributed oblique stretching (latest Jurassic to Early Aptian) before it underwent an more orthogonal asymmetric thinning and exhumation phase from Late Aptian to Albian time. These last two stages of deformation are similar to those observed in orthogonal rift systems. We show that thinning is accomplished by the formation of a semibrittle shear zone that allows for the transfer of middle to lower crustal material from the side of the rift collocated with the hanging wall to the side of the rift collocated with the footwall of the detachment system. The main difference with an orthogonal rift system appears to be generated by the formation of flower structures during the distributed oblique phase and the capacity of localizing the deformation in the subsequent stages. These oblique slip faults form very steep normal faults that induce the development of strongly localized, compartmentalized, and asymmetric rift basins. In the case of the Parentis and Arzacq‐Mauleon basins, these strike‐slip faults separate upper plate sag basins to the north from lower plate sag basins to the south. While the northern sag basins do not show any evidence for exhumation, the southern ones are more complex and floored by detachment faults, as indicated by the occurrence of syntectonic and posttectonic sediments onlapping directly onto exhumed lower crustal and mantle rocks.
Abstract We investigate how lithospheric scale compositional heterogeneities affect kilometric scale deformation processes. To this end, we perform numerical experiments of lithospheric extension in which we vary the Moho temperature and the mineralic composition of the mantle and the crust. In both the crust and the mantle, we use an explicit bimineralic composition by randomly distributing two mineral phases in the materials. Comparison of our models to simulations using an implicit bimineralic composite (one average viscous flow laws for a two‐phase aggregate) crust and mantle demonstrates that an explicit bimineralic composition assimilated to heterogeneities succeeds in explaining observations related to the formation of rifted margins such a: (1) the absence of a sharp deformation zone at the brittle ductile transition (BDT), (2) the initiation of the rifting process as a wide delocalized rift system with multiple normal faults dipping in both directions; (3) the development of anastomosing shear zones in the middle/lower crust and the upper lithospheric mantle similar to the crustal scale anastomosing patterns observed in the field or in seismic data; (4) the preservation of undeformed lenses of material leading to lithospheric scale boudinage structure and resulting in the formation of continental ribbons as observed along the Iberian‐Newfoundland margin.
