From Bedding To Cleavage: The Evolution Of Clay Fabric Near A Thrust.

Abstract:

In foothills, the propagation of faults within incompetent bed is potentially accompanied by the development of oblique cleavage. This is particularly demonstrated in the Southern Pyrenees where the out-of-sequence propagation of a flat thrust imposed the development of oblique cleavage within the flat-lying Pamplona marls. Over hundreds of meters, it is possible to trace step by step the cleavage development. The horizontal bedding is gradually superimposed by oblique cleavage (dip ~60°N). At the end of the studied outcrop, a pervasive ~mm spaced cleavage is observed. The anisotropy of magnetic susceptibility (AMS) is a classic tool for study the deformation in shales. In the Pamplona marls, AMS is essentially controlled by clays fabric. AMS shows that the degree of anisotropy Pj is not the best parameter to highlight the degree of deformation of marls. This parameter is positively correlated to the bulk magnetic susceptibility Km (~10^-4 SI). On the contrary, the shape parameter T is more consistent with the degree of deformation: higher is the degree of deformation observed in outcrop (occurrence of pervasive cleavage), lower is the T parameter. As m-spaced cleavage starts to develop, the shape parameter T decreases linearly from ~0.8 to ~0.2, reflecting a gradual disorganization of clay particles. Despite the development of mm-spaced cleavage, the magnetic fabric remains oblate and is still dominated by the sedimentary fabric. This means that the bulk fabric of clay particles remains parallel to the bedding plane. Our study demonstrates that AMS is a powerful tool to trace the deformation of clay rocks and that the study of the shape parameter T is a robust and fast gauge of clays fabric. .