The effect of bimineralic composition on extensional processes at lithospheric scale

Abstract:

In a bimineralic material the deformation localization is strongly dependent on the mechanical behavior of the two phases, the proportion, and the strength contrast between them. If these parameters vary at the scale of a rock sample, they also vary at lithospheric scale when units with different lithologies are juxtaposed leading to the strongly heterogeneous crust and mantle. In the lithosphere, moreover, the dependence with depth of the pressure-temperature imposes that the strength ratio between the different phases is also dependent on the depth and geotherm. Based on our previous result, we expect therefore that the compositional heterogeneities strongly influence the localization and that the resulting complex mechanisms of deformation are dependent of the geotherm and proportion of different materials constituting the lithosphere.

To test these ideas, we perform lithospheric scale models with a bimineralic composition in the crust and the mantle to take into account the compositional heterogeneities. We deform the model in an extensional setting and systematically change the Moho temperature and the composition of the crust and mantle.. Comparison of our models to simulations using a monomineralic crust and mantle demonstrates that a bimineralic composition assimilated to heterogeneities succeed in explaining observations related to the formation of rifted margins such as : 1) the absence of a sharp brittle ductile transition,2) the initiation of the rifting process as a wide delocalized rift system where multiple normal faults dipping in both directions; 3) the development of an anastomosing shear zone in the middle/lower crust and the upper lithospheric mantle similar to the crustal scale anastomosing pattern observed on 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.