Partitioning of strain rates and stresses between a thin shear zone and its walls under transpression or transtension

Abstract

Within a mid-crustal domain deforming in transpression or transtension, thin zones may exhibit higher strain than their immediately adjacent walls. The lower transpressive strain of the host may sometimes not be recognized and these thin bands might thus be interpreted as simple shear zones. However, their kinematic indicators should reflect the transpressional or transtensional strain imposed by the less deformed host. In order to explore the contrasting strain between such a zone and its walls, we analyse a simple model of a narrow low-viscosity oblique transcurrent shear zone held coherently within a broader, higher-viscosity host rock. The model uses strain rate as a proxy for strain.

We evaluate the contrasts in strain rate intensities and in shape parameters between such a shear zone and its walls as functions of viscosity contrast and of convergence angles. We find important differences between transpressional or transtensional shear zones and the more familiar simple shear zones.

Strain fabrics observed in some crustal scale shear zones and their walls, such as the Archean Larder Lake-Cadillac Deformation Zone (LCDZ) in Ontario and Québec, Canada, which combine transcurrent shear sense indicators and steeply plunging lineations, can indeed be explained by transpression.

We also examine the contrast in stresses between the shear zone and its walls. We find that, in transpression, buoyant fluids that might be generated in the deep crust, near the base of a deformation zone such as the LCDZ, should in fact move through the wall rock rather than within that high-deformation zone. This has implications for the location of mineral deposits traditionally associated with such deformation zones.