MAGNITUDE OF WEAKENING DURING CRUSTAL-SCALE SHEAR ZONE DEVELOPMENT

The viscosity structure of the crust controls strain distribution, but quantifying the pattern of relative strength and weakness in both space and time has proved elusive. We describe a field-based method for calculating the bulk strength of a heterogeneous material and apply that method to a crustal-scale shear zone in the Grenville Province of southeastern Ontario.

We calculate bulk strength as the ratio between the surface traction needed to deform a square block in simple shear and the velocity gradient across that block. In order to replicate natural shear zone geometries for use in the model, we map in the field by relative strength in addition to lithology. For these purposes, we define strength as effective viscosity. We assign relative viscosities within the model based primarily on geometrical criteria, with most viscosity contrasts being between one and ten.

The c.1160 Ma Parry Sound domain is a granulite facies nappe whose core remained rigid during subsequent Grenvillian events. The margin of the domain, however, developed into the domain-bounding Twelve Mile Bay shear zone, which accommodated several tens of km of transport. Fracturing and fluid infiltration drove development of an amphibolite facies meter-scale shear zone network that evolved into the Twelve Mile Bay structure. We analyzed three sites across the ~5km-wide strain gradient from near the granulitic domain core to the large scale shear zone. The rocks at the margins of the shear zone weakened by approximately 30%, and those in the shear zone by at least 90%. Thus, across several km, the crust was at least 30% weaker than the protolith. Results such as these inform quantitative and qualitative geodynamic predictions related to crustal-scale strain events as disparate as orogenesis and postglacial rebound by providing a ground-truthed basis for strength patterns and strain weakening algorithms.