STRAIN LOCALIZATION IN THE UPPER MANTLE AT A LOW-ANGLE EXTENSIONAL PLATE BOUNDARY: MIGRATING DEFORMATION WITHIN A ZONE OF CONSTANT STRAIN RATE

The Turon de Técouère massif of the French Pyrenees records a shear zone developed at mantle levels. The deformation is interpreted to have occurred at a magma-poor extensional margin; the region preserves Early Cretaceous sedimentary rocks directly overlying highly sheared lower crustal and ultramafic rocks. The massif is composed of spinel- and plagioclase-bearing lherzolite, and the shear zone formed as a result of grain size reduction associated with reaction. Olivine-based paleopiezometry indicates that differential stresses were variable both spatially across the zone and temporally during exhumation. The strain rate (10⁻¹² to 10⁻¹¹·s⁻¹) remained constant over changing temperature conditions, despite changes in stress. From a tectonics perspective, the high strain rate supports the interpretation of a plate boundary setting. A constant deformation rate at any particular plate boundary is consistent with deformation driven by plate motions. The stresses within the shear zone vary to accommodate this deformation at different lithospheric levels.

It is generally agreed that the plate motion rates are ≤ 1 cm/year. If displacement occurred uniformly across the Turon de Técouère shear zone (600 m) at the strain rates recorded by the microstructures, the resulting displacement would be unrealistic for this geologic setting. Consequently, the zone of active deformation in the shear zone must vary over space and time. Assuming simple shear kinematics, we quantify how much of the shear zone can be active at any time, given variable dip of the plate interface, rate of plate divergence, and percentage of slip accommodated during high-strain events.