Suzon Jammes

Extreme crustal thinning in the Bay of Biscay and the Western Pyrenees: From observations to modeling

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-Mauleon 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.

Interaction between prerift salt and detachment faulting in hyperextended rift systems: The example of the Parentis and Mauleon basins (Bay of Biscay and western Pyrenees)

Prerift salt layers associated with extensional detachment faults exhuming mantle and deeper crustal rocks at the sea floor are observed in the Parentis and Arzacq-Mauleon basins located at the eastern termination of the Bay of Biscay. How detachment faults interact with salt in hyperextended rift systems is yet little understood. Based on field observations and drill-hole and seismic data, we propose a new model to explain the interaction between salt tectonics and extensional detachment systems. We demonstrate that the presence of a thick prerift salt layer in an area undergoing extreme crustal thinning can control the geometry and evolution of rift systems and obscure the rift-related structures in the underlying basement. During an initial stage of rifting, prerift salt layers act as a decoupling horizon between sub- and suprasalt units and hinder crustal detachment faults to cut through the salt layers and form breakaways at the sea floor. As a consequence, they sole out along the ductile salt layer and no subsalt material can be exhumed to the sea floor. Thus, sub- and suprasalt layers deform by different deformation modes, which makes that detachment fault difficult to identify on seismic images. In a later stage, when salt has migrated and thinned out, sub- and suprasalt layers can locally couple and detachment faults can be exposed at the surface, resulting in windows of exhumed basement surrounded by extensional allochthons formed by suprasalt sedimentary units.

Localization and delocalization of deformation in a bimineralic material

We investigate how localization and delocalization of deformation occurs in a bimineralic material composed of a strong plagioclase and a weaker quartz phase. We perform numerical, meter-scale shear experiments in which we vary the temperature and the ratio of the two mineral phases. Three micromechanical deformation fields are identified according to the mechanical behavior of the minerals at play (brittle or ductile when both phases are in the brittle or ductile regime, respectively, and semibrittle when one phase is in the brittle and the other in the ductile regime). Besides these micromechanical deformation fields, we identify three deformation types characterizing the degree of localization (type I: localized shear zone, type II: localized anastomosing shear zone, and type III: delocalized shear zone). Type I is expected in the brittle deformation field. In the semibrittle field, all deformation types can be observed depending on the amount of weak phase present. In the ductile field, deformation is dependent on the strength ratio between the two phases. For a low strength ratio, deformation of type III is always observed. For high-strength ratios, deformation of type II can be observed for a moderate amount of weak phase. A small amount of weak phase (<10%) reverses the mechanical behavior of the strong phase and leads to the formation of a narrow anastomosing shear zone (type II) where fully ductile (type III) behavior is expected. This highlights the importance of a bimineralic material for the deformation localization and overall large-scale deformation processes.

The effect of bimineralic composition on extensional processes at lithospheric scale

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.