35 Rheological Information from Structures and Microstructures

Tuesday, 19 March 2013
8:45 AM - 12:00 PM
Jefferson Room

HOW IMPORTANT ARE SHEAR ZONES IN DEFORMATION OF DEEP OROGENIC CRUST?

Paper number: 35-2
Program: Theme Sessions
Time: 8:55 AM – 9:10 AM

Authors

Abstract

One of the unanswered questions about the rheology of the deep crust is the extent to which shear zones sensu stricto (zones of deformation with distinguishable walls) dominate the strain budget. In the Central Gneiss Belt of the Grenville Province, distributed high strain zones are areally more significant (>80%) than shear zones (ss).

In the Central Gneiss Belt ductile thrust sheets that record thickening and nappe flow are composed of a combination of monocyclic (Grenville deformation only) and polycyclic (Grenville + pre-Grenville deformation) materials. Four nappe types are recognized: (a) monocyclic migmatitic; (b) monocyclic granulitic; (c) monocyclic migmatitic + polycyclic; and (d) polycyclic granulitic. Initial Grenvillian deformation of monocyclic material in any nappe type is expressed as a uniform gneissosity (amphibolite facies migmatitic or granulitic) i.e. a zone of distributed, high strain with preservation of few, small-scale strain gradients (m-scale or less). Polycyclic components may be either homogeneously deformed sheets of straight gneiss with pre-Grenville metamorphism identified isotopically or heterogeneously deformed containing m-scale shear zones (ss) with walls preserving pre-Grenville fabrics.

M-scale shear zones (ss) are most common where the early high strain gneissosity in monocyclic nappes was transposed during nappe flow. Transposition related shear zones may be along zones of hydration accompanying pegmatite emplacement into granulite (with extensive preservation of early gneissosity in the wall rock). Alternately, where early gneissosity is amphibolite facies, wall rock may be thoroughly buckled prior to shearing. Systems of such m-scale shear zones lie within the strain gradients bounding several regional-scale shear zones.

Taken together, the observations from the Central Gneiss Belt of the Grenville Province suggest that shear zones certainly play a role in the strain budget, but may not be the dominant factor.