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Length-Scales of Permeability in Deep Crustal Rocks During Progressive Shearing

Jeffrey Marsh
Department of Earth Sciences,University of Maine, Orono, ME, USA; [email protected]

Ductile shear zones are of primary importance in transporting fluids within the Earth’s crust. This study investigates outcrop-scale shear zones that rework the perimeter of Parry Sound domain granulites, Grenville Province, Ontario, Canada. In the interior of the domain incipient shear zones appear to have nucleated on spaced, mineralized fractures that display roughly symmetric, centimeter-scale alteration halos. Toward the margin well-defined mineralogical and microstructural transitions related to fluid infiltration, metamorphic reactions, and localized shear deformation are observed from the undeformed host rocks into the zones. These outcrop-scale shear zones have been chosen for investigation due to their well-preserved shearing related textures and mineralogy, and constrained fluid-rock interaction histories. The evolving microstructure apparently resulted in an enhanced permeability and bulk rheological weakening that developed progressively through positive feedback with deformation and reaction progress.

High-fluid fluxes within shear zones is commonly observed; however the grain-scale mechanisms for fluid transport within the ductile regime is largely unresolved. Observations and preliminary modeling indicate that fluids may be pumped through interconnected pore spaces by heterogeneously deforming mineral grains during transient frictional (brittle) and viscous deformation processes. Thus, conceptual models for focusing of fluid flow through ductile shear zones must incorporate anisotropic fabric development and transient grain-scale stresses during progressive deformation. The goal of this study is to develop a numerical model which explores the effect of progressive microstructural evolution on the permeability of a deforming volume. Model boundary conditions, geometries, and mechanical properties are based on observations and data collected from the studied shear zones.

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid