EVOLUTION OF FRACTURE POROSITY AND PERMEABILITY DURING FOLDING BY CATACLASTIC FLOW: IMPLICATIONS FOR SYNTECTONIC DILATIONAL PUMPING

Fold tightening in the Canyon Range (CR) thrust sheet, central Utah, occurred during the later part of the Sevier orogeny under shallow overburden (<5 km). Deformation occurred by cataclastic flow, a process that involves blocks of various sizes sliding past each other along a stable network of fractures. The CR thrust sheet has uniform lithologies, variable fold geometry along strike, well constrained timing relationships of progressive fold growth, and superbly exposed fracture networks. This unique combination of geologic features provides an ideal setting to address the geometric and kinematic relationships between folding and fracture-network development. Fracture populations form a deformation fabric that is penetrative at the outcrop scale. The CR syncline exposes four orthoquartzite units which all behave slightly differently due to minor variations in bedding thickness, grain size, matrix composition, initial porosity, etc. However, their deformation is interlinked, with fracturing in any unit affecting fracture development in adjoining units. Two representative transects across the CR syncline, with interlimb angles of 60 and 124 degrees respectively, are used to evaluate how fracture density varies with fold geometry and with lithologic variations. Microstructural characteristics also control the outcrop scale fracture development.

The fracture networks dominate the porosity/permeability in the CR syncline and would have controlled the migration of fluids and hydrocarbons during folding. These migration/reservoir patterns reveal how fracture networks redistribute fluids and hydrocarbons through space and time and how porous/permeable zones assist in syntectonic dilational pumping. The fracture network patterns also help to elucidate the role lithology plays in fluid/hydrocarbon localization. Finally, these patterns help us in beginning to understand if fracture network geometry and fluid localization is a self generating process, i.e. do the over-pressured fluids and/or hydrocarbons localize fracture development and vice versa (Sibson 1996).