On the Relative Importance of Foliation Development and Syntectonic Metamorphism on Shear Zone Formation

A theoretical description of the formation of ductile shear zones remains an elusive goal of geodynamics modeling. Standard approaches such as shear heating and grain size evolution seem unable to produce spontaneously shear zones out of an essentially homogeneous, slowly deforming, rock. Progress in this field hinges on the inclusion of geological information to guide further modeling effort. Interconnected layers of weak minerals, especially phyllosilicates, typically mark ductile shear zones in the continental middle to lower crust. Shear zone material is often richer in phyllosilicates than the original host rock. These characteristics need to be included into the next generation of shear zone development models. The authors of this abstract have proposed in independent publications mathematical descriptions of the rate at which phyllosilicate enrichment and layer development proceed. In this contribution, we determine to what extend these processes might contribute to the formation of localized shear zones. To achieve this goal, we apply a variety of simple theoretical analysis to these phenomena. First, we calculate how much faster a shear zone rock is expected to shear compared to the host rock (localization potential). Then, we quantify the rate at which weakening is occurring through the calculation of the effective stress exponent of each of these processes. Finally, we integrate numerically the ODEs that describe these processes to follow the strength evolution of rock undergoing layer development, phyllosilicate enrichment, or both. These analyses are repeated for a range of ambient temperature and starting material to explore under which circumstance one or the other of these phenomena is expected to dominate.