2005 Salt Lake City Annual Meeting (October 16–19, 2005)
Paper No. 226-6
Presentation Time: 9:15 AM-9:30 AM

FAULT ROCK GENERATION, FRICTIONAL PROPERTIES, AND PERMEABILITY IN FAULT ROCKS OF THE MOAB FAULT, UTAH

DAVATZES, Nicholas C.1, SOLUM, John1, LOCKNER, David A.2, and STANCHITS, Sergei3, (1) U.S. Geological Survey, 345 Middlefield Road, MS977, Menlo Park, CA 64025, ndavatzes@usgs.gov, (2) Earthquake Hazards Team, U. S. Geol Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, CA 94025, (3) GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg, 14473, Potsdam, Germany

The mechanical and hydrologic evolution of fault rocks is strongly dependent on rock type and deformation mechanism. In the Moab fault, Utah, fault rocks were derived from sandstone and shale. Fault rocks in sandstone are associated with cataclasis of quartz and feldspar in either deformation bands or due to abrasion of joint surfaces reactivated in shear. Fault rocks in shale are associated with: 1) Shale smear which consists of pervasive ductile deformation that attenuated and entrained shale into the fault zone but largely preserved sedimentary layering and 2) Clay gouge generated by abrasion of shale, typically against slip surfaces in sandstone, which also contains with authigenic clays. The frictional strength (μ) and permeability (k) of samples of these fault and host rocks were measured at effective confining pressures of 10 - 40 MPa consistent with the burial depth during faulting at ~60 Ma. In the Jurassic aeolian sandstone, μ ranges from 0.71 to 0.91. k of experimentally deformed sandstone varies between units from ~9E-18 to ~120E-18 m2. In clay-rich fault rocks, μ is ~0.39 in the shale smear and ~0.37 in the clay gouge. k in the clay gouge is ~3E-21 m2 at 40 MPa confining pressure. The fault rocks derived from sandstone have remained distinct from those derived from shale. These clay-rich fault rocks are present in all exposures of the fault juxtaposed or offset past shale. This partitioning and apparent continuity suggest that once developed, strain is concentrated in clay-rich fault rocks as expected from their relatively low μ. Authigenesis of clays in the gouge represents continued development beyond mechanical disruption and grain size reduction that promotes clay gouge continuity and changing μ and k. Precipitation of minerals such as calcite that contribute to the regeneration of fault zone strength (by increasing cohesion and μ) and reduce permeability (by filling pores) are largely restricted to relays and intersections between segments of the Moab fault subjected to increased dilatancy evidenced by joints and breccia. Thus, in the damage zone μ and k vary strongly due to the influence of distinct deformation mechanisms in both rock types. In the fault core, it appears likely that μ and k evolved to a steady state dominated by clays that maintained seal integrity and fault weakness over geologic time spans.

 

2005 Salt Lake City Annual Meeting (October 16–19, 2005)
General Information for this Meeting
Session No. 226
Fracturing and Faulting of the Clastic Rocks of the Colorado Plateau
Salt Palace Convention Center: 150 DEF
8:00 AM-10:00 AM, Wednesday, 19 October 2005

Geological Society of America Abstracts with Programs, Vol. 37, No. 7, p. 498

© Copyright 2005 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.