Shear in the lower crust during extension: not so pure and simple

As lower crustal reflectivity appears to be related to extension, it is sensible to consider the role of the lower crust in the extension of the lithosphere and, in particular, the way the lower crust deforms. The occurrence of transparent zones wrapped around by reflective bands can be interpreted as low-strain regions enclosed within anastomosing, shear zones, suggesting that deformation in the lower crust is by the relative movements of low-strain lozenges along acoustically reflective, high-strain zones. Although this deformation can easily accommodate bulk pure shear, it can also develop during bulk simple shear. A component of subhorizontal simple shear within the lower crust is necessary if deformation in the mantle is localised into shear zones with the geometry and distribution imaged on BIRPS profiles. The lower crust may be the critical link in the lithosphere, transferring deformation between the relatively strong upper crust and upper mantle and being strongly sheared in the process. The form of this shearing is a direct consequence of the way the entire lithosphere extends.