Finding strength where it lies in the lithospheric mantle: An exposed extensional plate boundary shear zone, Turon de Técouère, France

Julie Newman, Texas A&M Univeristy
Basil Tikoff, University of Wisconsin-Madison
Vasilis Chatzaras, University of Sydney

Abstract

The Turon de Técouère massif of the French Pyrenees preserves a Cretaceous, magma-poor hyperextended plate margin within the mantle lithosphere. The massif records a: 1) A 40 m-wide mylonite zone at deformation conditions of 1000° C, 1 GPa, and strain rates of 10-11 1/sec; 2) An inferred ~40 m-wide ultramylonite zone for conditions of 850° C, 7.5 GPa, and strain rates of 10-11 1/sec; and 3) A ~200 m-wide mylonite zone for conditions of 700° C, 5 GPa, and strain rates of 4 x 10-12 1/sec. We utilize the unique relationship, with a zone of simple shearing, between displacement rate, strain rate, and shear zone thickness. Using the displacement rates, determined from tectonic analyses, and the strain rate estimates determined from microstructural analysis, we correctly determine the width of the shear zone at all three different lithospheric levels. This result establishes the Turon de Técouère shear zone as the exposed Cretaceous plate boundary. These width observations are not consistent with shear zone widths that depend on far-field strength. Significantly, the inferred ~40 m-wide ultramylonite zone – deforming at ~850° – records the highest differential stress values (~200 MPa) and thus corresponds to the strongest part of the lithospheric plate boundary. The study indicates that constant displacement rate boundary conditions is the best approach to consider plate-boundary shear zones, as both the stresses and strain rates vary during deformation.

Session

Deformation of mafic or ultramafic rocks