HOW DOES THE EVOLVING FABRIC OF ROCKS IN SHEAR ZONES AFFECTS THE STRENGTH OF CONTINENTAL OR OCEANIC LITHOSPHERE?

The geology of our planet is characterized by plate tectonic, a global deformation regime according to which deformation in the lithosphere localizes unto relatively narrow deformations zones or plate boundaries. Geodetic, geodynamic, and structural data show that plate boundaries are more intensely localized in the oceanic lithosphere than the continental lithosphere. However, in both cases, localization occurs at a variety of scales from plate boundaries proper to outcrop-scale or even microscopic shear zones. I will address how the localization processes identified in ductile shear zones influence the large-scale strength of the lithosphere in both oceanic and continental domains.

Shear zones rocks typically feature a different fabric than the surrounding rocks. Grain size reduction and the development of layers (foliation) are hallmarks of shear zones. I previously described how these changes decrease the strength of rocks and concluded that the most efficient weakening (and therefore localization) processes are 1) layer development in presence of phyllosilicates in the mid-crust and 2) grain size reduction in the uppermost mantle, especially if dislocation-accommodated grain boundary is an active deformation mechanism. Here, I discuss how these changes influence the strength profile and integrated strength of the lithosphere.

Geologically, weakening is best understood as an increase in strain rate rather than a reduction of stress. When applied to the entire lithosphere, the total energy dissipated across the deformation zone is conserved. Two end-member cases can be imagined: 1) the width of the deformation zone decreases at constant velocity and stress (a geodynamic view) and 2) velocity increases while the stress decreases and deformation zone thickness remains constant (a structural view). The range of strain rate observed in the oceanic lithosphere favors the earlier scenario. Localization may be less intense in the continental lithosphere because the uppermost mantle is generally at higher temperature than in the oceans and localization by layer development is restricted to the middle crust.