HR: 1340h
AN: T53C-1438    [Abstracts]
TI: Fault Damage Zone Characteristics and the Evolution of Fluid Flow Properties
AU: * Mitchell, T M
EM: t.mitchell@liv.ac.uk
AF: University of Liverpool, Rock Deformation Laboratory, Department of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: Faulkner, D R
EM: faulkner@liv.ac.uk
AF: University of Liverpool, Rock Deformation Laboratory, Department of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AB: Faults are a major control on fluid flow in the crust. Fault damage zones are represented by both microfracturing of the rock matrix and by macroscopic fracture networks. The empirical characterization of fracture patterns at various scales, as well as understanding the geometry of the fault zone fracture population help to predict both fault growth processes and bulk hydraulic properties of a fault zone. Here, we characterize the damage around a large strike slip fault zone in order to understand the fluid flow properties and their temporal and spatial variation. We studied faults of various displacements that cut crystalline rock within the excellently exposed and exhumed Atacama Fault Zone, northern Chile. The core zone of the largest displacement fault (~2km offset) displays discrete high strain ultracataclasite layers that bound highly fractured zones of protocataclastic granodiorite. Bulk fluid flow appears to have been largely restricted to this multiple-core zone, as the damage zone displays a lack of hydrothermal activity although fluid inclusion planes indicate early percolation through microcracks. Micro- and macroscale fracture densities within the damage zones of faults with displacements ranging over 3 orders of magnitude (0.2m - 2000m) have been characterized. These show a log-linear decrease in density with perpendicular distance from the fault plane. Macrofractures are a combination of open and shear mode fractures, and are orientated at a high angle to the main fault trace. Microfractures are represented by fluid inclusion planes in a predominantly mode I orientation. Macro and micro-fracture densities fall to background levels at ~50mm for the smallest displacement fault (0.2-2m offset) and ~100m for the two largest faults (200m and 2000m offset). The maximum fracture density of all faults appears to scale with log fault displacement. This differs from other studies where a there is a constant maximum microfracture density that is independent of fault length. Our observations imply that fluid flow in fault damage zones will be controlled by both macroscopic fracture networks and microscopic damage. A paucity in cross-cutting relationships for both macro- and microscopic fracture sets in the damage zone indicates a lack of cyclic damage and implies fluid flow may have been confined to the core zone.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: Fall Meeting 2005