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_Journal of Structural Geology 32 (2010) 1576e1589_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Internal structure, fault rocks, and inferences regarding deformation, fluid flow, and mineralization in the seismogenic Stillwater normal fault, Dixie Valley, Nevada_ _Jonathan Saul Caine a,*, Ronald L. Bruhn b, Craig B. Forsterb_ _a U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, CO 80225, USA_ _b Department of Geology and Geophysics, University of Utah, 115 South 1460 East, Salt Lake City, UT 84112, USA _article info_ _Article history: Received 15 January 2009 Received in revised form 28 January 2010 Accepted 9 March 2010 Available online 17 March 2010_ _This work is dedicated to the memory of Craig B. Forster who died in a tragic accident on December 28, 2008. It is a reflection of his exceptional enthusiasm and dedication to bringing students and colleagues together in the pursuit of collaborative scientific research._ _Keywords: Fault zone Seismicity Fluid flow Hydrothermal Breccia textures Silicification_ _abstract_ _Outcrop mapping and fault-rock characterization of the Stillwater normal fault zone in Dixie Valley, Nevada are used to document and interpret ancient hydrothermal fluid flow and its possible relationship to seismic deformation. The fault zone is composed of distinct structural and hydrogeological components. Previous work on the fault rocks is extended to the map scale where a distinctive fault core shows a spectrum of different fault-related breccias. These include predominantly clast-supported breccias with angular clasts that are cut by zones containing breccias with rounded clasts that are also clast supported. These are further cut by breccias that are predominantly matrix supported with angular and rounded clasts. The fault-core breccias are surrounded by a heterogeneously fractured damage zone. Breccias are bounded between major, silicified slip surfaces, forming large pod-like structures, systematically oriented with long axes parallel to slip. Matrix-supported breccias have multiply brecciated, angular and rounded clasts revealing episodic deformation and fluid flow. These breccias have a quartz-rich matrix with microcrystalline anhedral, equant, and pervasively conformable mosaic texture. The breccia pods are interpreted to have formed by decompression boiling and rapid precipitation of hydrothermal fluids whose flow was induced by coseismic, hybrid dilatant-shear deformation and hydraulic connection to a geothermal reservoir. The addition of hydrothermal silica cement localized in the core at the map scale causes fault-zone widening, local sealing, and mechanical heterogeneities that impact the evolution of the fault zone throughout the seismic cycle._ _Published by Elsevier Ltd._ _1. Introduction_ _The presence and flow of fluids in the upper crust has a major impact on the mechanics of faulting (Hubbert and Rubey, 1959; Nur and Booker, 1972; Sibson, 1977, 1981, 1990, 1996; Power and Tullis, 1989; Bruhn et al., 1990, 1994; Parry and Bruhn, 1990; Scholz, 2002; Chester et al., 1993; Rice, 1992; Byerlee, 1993; Keller and Loaiciga, 1993; Evans and Chester, 1995; Caine et al., 1996; Miller et al., 1996; Seront et al., 1998; Tanaka et al., 2001; Wibberley, 2002; Faulkner et al., 2006; Lockner et al., 2009). Fluid flow and its interactions with heterogeneous permeability structures in a fault zone can control the magnitude of local principal stresses (Nemcok et al., 2002). This, in turn, affects local fluid-pressure gradients, mechanical failure, propagation of pressure transients._ _∗ Corresponding author. E-mail address: jscaine@usgs.gov (J.S. Caine)._ _0191-8141 $ e see front matter Published by Elsevier Ltd. doi:10.1016/j.jsg.2010.03.004_ _Fluid infiltration into and out of a fault zone via fault-valve mechanisms (e.g., Sibson, 1992), and fault-zone sealing and healing (e.g., Faulkner et al., 2008). Fluid flow in fault zones can control the location, emplacement, and evolution of economic mineral deposits and geothermal systems (e.g., Newhouse, 1942; Cox et al., 2001; Sibson, 2001; Micklethwaite, 2009), and may also impact the locations and magnitudes of foreshock, earthquake and aftershock distributions (Miller et al., 2004). Yet fault zones are heterogeneous geological and hydrological structures that commonly are not well exposed. Even in well-exposed fault zones direct links between internal structure, fault rocks, and mineral assemblages that uniquely indicate a seismogenic origin are uncommon (cf. Sibson, 1986b; Cowan, 1999; Ujiie et al., 2007; Woodcock et al., 2007; Smith et al., 2008). Thus, the study of exposed, seismogenic fault zones that may record fluid flow-related processes associated with earthquakes remains important for understanding the mechanics of faulting._ _J.S. Caine et al. Journal of Structural Geology 32 (2010) 1576e1589_ _1577_ _Fault zones are commonly composed of distinct, three-dimensional, mappable components that include a fault core and damage zone within relatively undeformed protolith (Chester and Logan, 1986; Smith et al., 1990; Forster et al., 1991; Caine et al., 1996). Most of the strain is accommodated in a fault core indicated by rocks such as fault-related breccias and clay-rich gouge. Fault zones can also have multiple core zones interspersed with pods of heterogeneously deformed host rock (cf. Faulkner et al., 2006). A damage zone is the mappable network of subsidiary structures that surrounds a fault core or fault-core zone and is related to the nucleation, evolution, and growth of the fault zone (Chester and Logan, 1986; Scholz, 2002; Caine et al., 1996; Knipe et al., 1998). Damage-zone fracture networks commonly have orientations mechanically related to the master fault and are of higher intensity than found in the protolith (e.g., Caine and Forster, 1999). The fault core and damage zone are surrounded by the protolith where fault-related structures are generally absent._ _The bulk permeability structure and strength of a fault zone are controlled by preexisting and newly developed structures, the regional and local stress state, fault-zone geometry, and changes in lithology resulting from the coupling of mechanical, thermal, fluid flow, and reactive geochemical processes. For example, the creation of new hydraulically contrasting lithologies and structures, such as clay-rich cataclasites and complex fracture networks, has been documented to result from as well as impact fluid flow in diverse brittle fault-zone settings (Sibson, 1986a; Chester and Logan, 1986; Scholz, 2002; Bruhn et al., 1994; Antonellini and Aydin, 1994; Goddard and Evans, 1995; Caine et al., 1996; Faulkner et al., 2006). These fault-related physical attributes in the upper crust create hydraulic and mechanical heterogeneity and anisotropy that have a significant impact on rupture and the arrest of failure (Parry et al., 1991; Byerlee, 1993; Miller et al., 1996; Seront et al., 1998) as well as growth and widening of a fault zone._ _Previous theoretical research in earthquake mechanics has focused on the role of fluid circulation and hydrothermal alteration associated with faulting processes (Sibson, 1981; Bruhn et al., 1994; Parry et al., 1991; Rice, 1992; Byerlee, 1993; Scholz, 2002; Unsworth et al., 1997). Although there have been studies of exhumed and well-exposed seismogenic fault zones (e.g., Hancock and Barka, 1987; Ghisetti et al., 2001), details regarding the physical pathways along which fluid flow occurs, and the characteristics of structures and rock types that result from coupled deformation and fluid flow remain sparsely documented._ _This paper describes field observations from the Mirrors locality of the Stillwater Fault Zone (SFZ) in Dixie Valley, Nevada (Fig. 1). This is an area of geological interest due to exposures of exhumed portions of the footwall of this normal fault with a record of historic earthquakes and surface ruptures associated with the fault. There are also epithermal gold deposits, and a productive geothermal reservoir hydraulically connected to the fault zone. Outcrop mapping, hand-sample and thin-section fault-rock studies are used to extend previous work and (1) document the internal structure and geometry of part of the fault zone, (2) infer the paleo-permeability structure, (3) document the textural attributes, composition, and spatial and temporal distribution of fault rocks, and (4) infer deformation-related fluid flow processes associated with seismicity and growth of the fault zone._ _2. Geologic setting and previous work_ _The SFZ is historically active and capable of generating magnitude (M) > 6 earthquakes. Ground surface rupture associated with the 1954 M ? 6.8 earthquake was 30e40 km long (Caskey et al., 1996). The SFZ, also called the Dixie Valley Fault, is the eastern range-bounding fault between the Stillwater Mountains and Dixie Valley graben (Fig. 1). Fault segments that range from several kilometers to a few tens of kilometers in length form the SFZ. The SFZ is one segment in a 300 km long belt of normal and normaloblique slip faults (Wallace and Whitney, 1984; Caskey et al., 1996). The Stillwater Range is composed of Mesozoic metasedimentary, plutonic, and volcanic rocks. Igneous rocks include the Jurassic gabbroic Humboldt igneous complex, Cretaceous granites, a multiphase Oligocene graniteegranodioriteequartz monzonite complex, and various extrusive rocks of Oligocene and Miocene age (Page, 1965; Speed and Armstrong, 1971; Wilden and Speed, 1974; Speed, 1976)._ _The SFZ has been seismically active since Oligocene to early Miocene times (Parry et al., 1991; Bruhn et al., 1994; Caskey et al., 1996; Seront et al., 1998). The Holocene fault scarps that cut the basin fill along the eastern base of the Stillwater Range are ground surface ruptures probably formed during major earthquakes within_ _Stillwater Range, 40° N Latitude, Nevada, 118° W Longitude, Km 0 10 20 30 40, N, Stillwater Range Dixie Valley Box Canyons Oxbow Geothermal_ Ключевые слова: figs, angular, fault rock, faulting, range, mirrors locality, journal structural, hydrothermal, internal structure, matrix, forster, university, fault zone, fault-valve behaviour, wibberley, mirrors, slip surface, network, minor kaolinite, matrix-supported texture, matrix supported, fracture permeability, rock type, society, carbonate, mineral assemblage, generally, clast, fluid, eds, fault core, structural, economic, mineral, power tullis, parry, fault gouge, matrix-supported breccia, sfz, ?uids, localized area, data, j?brak, shear fracture, earth, deformation event, aydin, dixie, normal fault, silicied clast, thrust fault, intensity, rounded, angular clast, open space, breccia texture, local, pod, sillitoe, tullis, breccia pod, journal structural geology, cut, reservoir, deformation, ?eld, mechanical, structural geology, fault-zone component, wall rock, heterogeneity, report, seismic cycle, mechanical property, rounded clast, average orientation, type, damage-zone, varying degree, sample, in-situ stress, nature, hand sample, representative sample, letters, caine forster, supported breccia, amplitude ratio, miller, ?ow, approximate location, ?uid ?ow, rock, space, cowan, hydrothermal uids, power, supported, byerlee, damage zone, ore, normal, speed, cf, pressure, whitney caskey, map scale, mechanical heterogeneity, red arrow, conceptual model, cross fracture, fluid flow, surface, fracture, uid, geology, polished face, nevada, fault, bruhn, clast-supported breccia, wa, permeability heterogeneity, gradational contact, quartz, damage, sibson, geothermal, orientation, formed, polished, geophysical, silici?ed, valley, texture, rapid precipitation, component, uid pressure, sagy, slip, critically stressed, shear, lutz, caine, journal, hybrid dilatant, mechanical strength, smith, slip direction, permeability, vein, locality, key observation, breccia body, woodcock, earthquake, chester, london, lineation direction, clastsupported breccia, breccia type, contact, clast-supported, stillwater range, wall, micklethwaite, caskey, evans, hydrothermal alteration, structure, breccia, protolith suggesting, dilatational strain, seront, stillwater, matrix-supported, seismological society, bulletin, fault-core breccia, tectonic stress, scale, alteration, fault-zone strength, matrixsupported breccia, hanging wall, core, outcrop scale, geothermal reservoir, fault-related breccia, stress, composed, local stress, map, evidence, outcrop, common extinction, hydraulic connection, permeability enhancement, jigsaw puzzle, fault-zone, sfz parry, protolith, dixie valley, goodwin, journal geophysical, ?uid, fracture network, faulkner, zone