Elsevier

Journal of Structural Geology

Volume 32, Issue 11, November 2010, Pages 1576-1589
Journal of Structural Geology

Internal structure, fault rocks, and inferences regarding deformation, fluid flow, and mineralization in the seismogenic Stillwater normal fault, Dixie Valley, Nevada

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.
https://doi.org/10.1016/j.jsg.2010.03.004Get rights and content

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.

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, Sibson, 1981, Sibson, 1990, Sibson, 1996, Power and Tullis, 1989, Bruhn et al., 1990, Bruhn et al., 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, WibberleyFaulkner 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, 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.

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.

Section snippets

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 30–40 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

The Mirrors map area exposure, overview, and methods

Each fault-zone component is well exposed in the bedrock at the Mirrors locality (Fig. 2, Fig. 3, Fig. 4). This footwall remnant of the SFZ extends approximately 250 m vertically upward from the base of the Stillwater Range. Although the hanging wall is composed of Quaternary basin fill in depositional contact with the crystalline footwall and no recent ground surface rupturing fault scarps were observed directly against it, we have mapped this as a fault contact to portray the crystalline

Fault zone orientation, component contact relationships, and mineralogy

The SFZ strikes northeast to east-northeast and dips from 32° to 70° southeast (Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5). The broad range in orientation of major slip surfaces shown in Fig. 5 reflects significant map to outcrop-scale variation in the topography of multiple slip surfaces (cf. Power and Tullis, 1989, Sagy et al., 2007). Slickenlines indicate that the fault is dominantly normal dip–slip with minor components of left and right lateral strike–slip (Fig. 5).

Fig. 2, Fig. 3 show that

Discussion

The SFZ at the Mirrors locality comprises a complex set of structures and rock types that include multiple slip surfaces, distinct fault rocks and breccia bodies, fault and non-fault-related fracture networks. We infer that several processes acted together during the formation of the fault zone and that some of the rocks may record coseismic deformation and associated fluid flow. In the following discussion we present key observations and a conceptual model that links deformation and fluid flow

Conclusions

Field mapping along the Stillwater Fault Zone provides a detailed view of the internal structure of the footwall of a seismogenic normal fault zone. Distinct mechanical and hydraulic components include a fault core, damage zone, and protolith (cf. Seront et al., 1998). The different components are distinguished by distinctive variations in lithology, amount and type of hydrothermal alteration, fracture intensity, matrix-scale hydraulic properties, and structure. The fault core shows evidence

Acknowledgements

Funding for this work was provided by a U.S. Geological Survey, National Earthquake Hazards Reduction Program Grant # 1434-93-G-2280 to Forster and Bruhn who supported Caine with a graduate research assistantship at the University of Utah, Department of Geology and Geophysics from 1992 through 1995. We thank Bill Parry, Laurel Goodwin, Darrel Cowan, George Breit, Don Sweetkind, and Albert Hofstra, for helpful comments on an early version of this manuscript. Lyndsay Ball, Dan Faulkner, David

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