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Hot and fast versus cold and slow:

Transition element thermometry of seismic slip

Technical Summary

We have developed a new fault surface paleothermometer that exploits the temperature-sensitive oxidation state changes in transition metals that are a result of shear heating during seismic slip (Evans et al., 2014). We use transition metal surface chemistry to document iron and Mn oxidation state changes and determine the peak temperatures attained during slip on faults coated by Fe-oxides. We (Ault et al., 2014; submitted) have also developed methods to directly date the slip on faults using U-Th/He geochronology. With these methods, we are dating several faults of the San Andreas Fault, exhumed in the Mecca Hills, and determining the likely peak temperatures of faults of the West Salton Detachment Fault, faults of the Elsinore fault, and the San Andreas Fault. Currently we have examined with optical and scanning electron microscopy the texture of the faults, and isolated 10 samples for U-Th/He dating. We are waiting for time at the University of Utah X-Ray Photoelectric Spectrographic lab, and the Stanford Linear Accelerator lab XANES lab to determine peak temperatures.

Our work to date under the current funding focuses on fieldwork in the Mecca Hills, where we have examined the five largest faults east of the San Andreas Fault, with 12 study sites where detailed fault-related data were collected. We have collected data on subsidiary fault orientations, made observations regarding fault slip timing, collected samples for microscopy, whole-rock geochemistry, U-Th/He dating, and transition element thermometry. We document the presence of hematite-coated faults and veins that appear to have formed relatively late in the histories of the faults, and also document the mineralization along faults formed in the Quaternary Palm Spring and Mecca Formations. These faults are coated with a curious silica-rich material and exhibit strike-slip, oblique slip, and dip slip striations, and striations that bend around curves on the faults. Some of the faults exhibit iridescent coatings akin to other faults where we have documented high temperatures associated with slip, and thus the Mecca Hills faults are candidates for U-Th/He dating and paleothermometry. We will also conduct detailed microscopy to determine the nature of mineralization and slip, seeking evidence for silica gel formation and slip. Faults in the area are simple planar structures in the overlying sedimentary rocks, but fault surface exhibit a range of slip surfaces. In the crystalline rocks the faults are complex structures with slip surfaces, veins, and mineralization that suggest a complex, and perhaps pre-SAF, history.

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Hot and fast versus cold and slow: Transition element thermometry of seismic slip

By Amy Moser, Kelly Bradbury, James P. Evans

Technical report The Mecca Hills, Southern California, is an area of localized uplift and contractional

deformation related to transpression along the adjacent San Andreas Fault (Fig. 1)(Dibblee, 1954; Sylvester and Smith, 1976). It is the location for seminal studies on transpressional deformation (Dibblee, 1954; Sylvester and Smith, 1976), deformation along the San Andreas (Sylvester and Smith, 1976), and San Andreas Fault geophysics (Lindsey et al., 2014, Fattaruso et al., 2014). While an iconic area for studying transpressional deformation, manifested in Quaternary sedimentary rocks, and deformation along the San Andreas Fault zone, the history and timing of faulting in the Mecca Hills remains unclear. We aim to determine the relationship between the subsidiary faults that parallel the San Andreas in the Mecca Hills and the adjacent San Andreas Fault. We pose the question: what can

the timing and history of faulting in the Mecca Hills tell us about the development of the San Andreas Fault in the area? We focus on faults in the crystalline

basement: Precambrian gneisses and the Late

Cretaceous/Early Tertiary Orocopia Schist (Dibblee, 1954; Crowell, 1975;

Sylvester and Smith, 1976; Jacobson et al., 2007; Fattaruso et al., 2014), The

Orocopia Schist (Graham and England, 1976; Malin et al., 1995; Jacobson et al.,

1996; Wood and Saleeby, 1997; Jacobson et al., 2002 was exhumed via a series of normal faults during the Late Tertiary and Early Cretaceous, while transpression is responsible for very recent exhumation (Goodman and Malin, 1992; Matti and Morton, 1993; Jacobson et al. 1996).

The Mecca Hills (Figure 1) lie at the northern end of a north-plunging anticlinorium and consist of several high-angle faults accommodating transpression along the sub-parallel San Andreas Fault zone (Dibblee, 1954; Sylvester and Smith, 1976; Sheridan et al., 1994 a and b). These faults include the Skeleton Canyon Fault (SCF), Painted Canyon Fault (PCF), Platform Fault (PF), Eagle Canyon Fault (ECF) and Hidden Springs Fault (HSF)(Fig. 2). Sylvester and Smith (1976) propose these faults bound three distinct structural domains, or blocks, in the Mecca Hills: a downfaulted crustal block west of the San Andreas Fault, a 1.5 km wide zone of intensely deformed basement and sedimentary rocks between the San Andreas Fault and the Painted Canyon Fault, and a relatively undeformed crustal block east of the Painted Canyon Fault (Figure from Sylvester and Smith, 1976). Their observations revealed the crystalline basement rocks between the San Andreas Fault and the Painted Canyon Fault (exposed in Painted Canyon) are highly sheared and fractured, providing evidence of cataclastic deformation in response to transpressional forces from the adjacent San Andreas Fault. While it appears horizontal shear strain dominated during cataclastic flow of the basement rocks, the basement was uplifted due to compressional stresses from the lateral and convergent slip of the

Figure 1. Geologic map of the Mecca Hills area (Fattaruso et al., 2014). Area shown is highlighted in Figure 1. Fault abbreviations are the same as in the text.  

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crustal blocks east and west of the Mecca Hills (Sylvester and Smith, 1976). It is clear from their work that the crystalline basement rocks of the Mecca Hills record a rich history of deformation related to the adjacent San Andreas Fault.

We are in the midst of a detailed structural analysis of subsidiary faults that parallel the main trace of the San Andreas Fault and cut the crystalline rocks in the Mecca Hills (PCF, PF, ECF, HSF), investigating the question of what the relationship is between the subsidiary structures and the main trace of the San Andreas Fault. The major subsidiary faults in the Mecca Hills (SCF, PCF, PF, ECF, HSF) that parallel the San Andreas Fault, and which cut the crystalline rocks exposed in blocks between these faults as well as the Quaternary sedimentary rocks. We have examined 5 major faults of the area, at 12 study sites, and this detailed structural mapping at a large scale and characterizing the fault structure (i.e. determining if fault core and damage zones are present, mapping their widths, sampling along transects) of the faults in the Mecca Hills. We have also examined the microstructures and geochemistry of the major strike-slip faults in the Mecca Hills. Mineralization patterns from major fault zones, in fractures, and in veins provided intriguing observations from field work, including hematite-filled fractures, epidote-coated slip surfaces, interesting oxidation patterns, polished silica-rich surfaces in the sedimentary rocks, and other evidence for fluid-rock interaction (Figures 2 and 3). These observations imply that the geochemical and microstructural elements of the

Figure 2. Field images from winter 2015 field season. (A) Metallic slip surface in Precambrian/Mesozoic crystalline rock. Outcrop is located ~90 m from the main trace of the Platform Fault. (B) Epidote-mineralized fracture. Epidote is fairly pervasive throughout the Mecca Hills. (C) Exposure of the main trace of the Platform Fault showing drag folding of Upper Palm Spring Formation and gouging of Precambrian/Mesozoic crystalline rock. (D) Delicate clay slip surfaces exposed in clay gouge of the Platform Fault. Multiple orientations of slip vectors were observed on this surface. (E) Brecciated mixed zone observed both at the Eagle Canyon Fault and the Hidden Springs Fault. The brecciated mixed zone is composed of very angular clasts of both Orocopia Schist and Upper Palm Spring Formation and displays no evidence of bedding. (F) Clay gouge exposed in the Painted Canyon fault zone. Note the acicular gypsum crystals in the outcrop. (G) Outcrop near the Hidden Springs Fault exposing a number of white silica-rich slip surfaces. (H) White silica-rich surface from the same outcrop. At least three different directions of slip vectors can be observed on this surface.  

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fault system in the Mecca Hills have played a large role in the development of fault structure, fault processes, and deformational mechanisms. Microstructural and geochemical analyses will therefore add an important element to the deformation history story of the area. The trace-element chemistry of the Precambrian and Mesozoic crystalline rocks lends the faults to examination for temperature measurements suggested by Evans et al. (2014). We have collected over 300 samples for geochemical and microstructural analyses, and we have at least 10 samples selected for further processing for (U-Th)/He dating of the formation of iron oxides along fault surfaces in the Mecca Hills. Our fieldwork in the Painted Canyon area, where the Precambrian gneisses and Late Cretaceous/Early Tertiary Orocopia Schist are cut by of a range of small and moderate displacement faults, and reveals iron oxide coated slip surfaces and iron oxide veins in the crystalline basement rocks. We have at least 20 candidate samples for the transition metal thermometry experiments, and While our interest lies in the crystalline basement rocks (Precambrian/Mesozoic gneisses and Late Cretaceous/Early Tertiary Orocopia Schist), we use the faults that cut the Middle Tertiary and younger sedimentary rocks (Figure 3) to focus on faults that have formed, or at least have some recent geologic slip history, and thus will place important constraints on the broader structural context of transpression in the Mecca Hills, related to the development of the San Andreas Fault in the area.

These studies help constrain the nature of fault development, the nature of fluid-rock interactions along the faults, and the usefulness of metallic slip surfaces in dating fault slip and determining temperature of fault slip. Focusing on the structural analysis, microstructures, geochemistry, and hematite thermochronology will serve to answer the main questions addressed in this study: (1) What are the kinematics of the major faults in the Mecca Hills which sub-parallel the San Andreas fault; (2) What are the ages of these faults; (3) What is the relationship between these faults and the San Andreas Fault, as well as their likely connection to the ECSZ; and (4) What is the fault zone structure of these faults? We have completed a structural analysis in the field concentrated on faults that cut the crystalline bedrock and the overlying sedimentary rocks. This will include mapping at the meso (~1:100 outcrop scale) and microscopic scale, as well as gathering orientation data of fault planes, slip surfaces, veins, and fractures. We will also gather structural data from the

metamorphic rocks, mapping foliation and lineation in the Orocopia Schist and Precambrian/Cretaceous gneisses. The fault zones of the major faults which cut the crystalline rocks (PCF, PF, ECF, HSF) will be analyzed and described, gathering data about the fault core and damage zone while completing transects across the faults. Fault transect samples (slip surfaces, fault gouge, deformation zone, host rock) will be collected for geochemical and textural laboratory analysis and microstructural analysis via oriented thin sections. Iron-oxide slip surfaces will be collected for (U-

Figure 3. Field photos and preliminary analyses of iron-oxide coated slip surfaces of the Mecca Hills, Southern California. (A) Field photo of iron-oxide slip surface in Precambrian/Mesozoic metamorphic basement rock. (B) Scanning electron microscope (SEM) image of a similar iron-oxide slip surface from the Mecca Hills showing slickenline lineations. (C) Same SEM image showing qualitative elemental chemistry of the slip surface using energy-dispersive X-ray spectroscopy (EDS). Elemental iron was the second most abundant after oxygen.

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Th)/He dating, geochemical and textural analysis, and microstructural analysis via oriented thin sections. Future work includes geochemical and microstructural analyses of fault rocks and corresponding undeformed host rock in the Mecca Hills. These analyses include X-ray diffraction (XRD) to determine mineralogical variations across and along the faults, X-ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to examine the whole-rock geochemical compositions, Scanning Electron Microscopy (SEM; see Fig 3) to examine deformation textures, and optical thin section analysis to examine microstructures. XRD analyses will be completed in the lab at Utah State University using

an X’Pert PRO diffractometer. The XRD analyses will be completed to determine minerals present in fault rocks, iron oxide slip surfaces, and in undeformed host rock. Fault rocks, slip surfaces, and host rock of interest will be sent off for whole-rock geochemical analyses using XRF (or ICP-MS). SEM analyses will be completed in the lab at Utah State University and used to image slip surface textures as

well as for energy-dispersive x-ray spectroscopy (EDS), creating elemental chemistry maps of sample surfaces. Oriented thin sections will be made from fault rocks, undeformed host rock, and crystalline bedrock containing iron oxide slip surfaces. Thin sections will be used to study microstructures as well as mineralogy of fault rocks, undeformed host rock, crystalline bedrock containing iron oxide slip surfaces, and silica and/or clay-rich slip surfaces in the Quaternary sedimentary rocks. These methods have been developed in a range of fault zone studies of the San Andreas Fault (Schulz and Evans, 1998; Bradbury et al., 2011) as well as in other fault zones (Wibberly and Shimamoto, 2003).

In the initial field work for this project, we have collected ~300 samples of fault-related rocks, iron oxide coated slip surfaces, and undeformed crystalline host rock, as well as 100s of measurements of slip surfaces, slickenlines, foliation, and fractures (Fig. 4). The purest, thickest, and most reflective metallic slip surfaces are in Precambrian gneisses and metavolcanics near the main traces of the Painted Canyon Platform faults (Fig. 2A). Epidote and hematite mineralization is common throughout fractures in the crystalline bedrock throughout the Mecca Hills (Fig. 2B).

The Platform fault, Eagle Canyon fault, and Hidden Springs fault each contain a clay gouge zone, or multiple clay gouge zones along the main fault trace (Fig. 3C). Gouged crystalline rock is often a vibrant green color. Numerous delicate slip surfaces are found within the clay gouge, several with nearly perfect strike-slip lineations. However, delicate clay gouge slip surfaces with multiple orientations of slip vectors have been identified (Fig. 2D). The delicate nature of these slip surfaces may indicate very recent slip (several hundred years) on the subsidiary faults in the Mecca Hills, as it is not expected that these delicate surfaces would be preserved for very long. A brecciated mixed zone has been observed at the Eagle Canyon fault and at the Hidden Springs fault (Fig. 2E). The brecciated mixed zone is interpreted as fault-related due to the angular nature of the clasts, the clasts composed of Upper Palm Spring Formation and Orocopia Schist, as well as the complete lack of bedding in the unit. Gouge in the Painted Canyon fault zone appears to approach the spectrum of ultra-cataclasite, and preserves multiple orientations of slip vectors. Gypsum has also been observed within Painted Canyon fault zone gouge (Fig. 2F).

White, silica-rich slip surfaces have been identified at the Eagle Canyon fault and at the Hidden Springs fault. These surfaces have only been observed in the sedimentary units (largely Upper Palm Spring Formation). The surfaces are highly light reflective, preserve multiple orientations of slip vectors on the same surface, and can be observed at multiple orientations in the same outcrop, including low angle (Fig. 3G and 3H). Preliminary XRD analyses reveal these surfaces are largely composed of quartz, while preliminary SEM analyses reveal a Ca and S – rich phase (likely gypsum)

Figure  4.    Poles  to    hematite  coated  fault  surfaces  at  oen  site  along  the  Platform  Fault,  mecca  hills,  indicating  a  NE  strike  to  the  faults.    

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on these surfaces in addition to the silica phase. To our knowledge, this work will represent the first systematic structural analyses of faults in the Mecca Hills. The data presented in this work will build on the detailed mapping and other work completed by Dibblee (1954), Sylvester and Smith (1996), Rymer et al. (1994) Sheridan et al. (1994 a and b), Dibblee and Minch (2008a and b), and others, providing a better understanding of fault zone structure, fault zone processes, and timing of faulting in the Mecca Hills. As discussed previously, this work is important in the broader context of the San Andreas Fault zone, particularly in an area that lacks a history of large historic earthquakes, and is in a complicated tectonic setting, including both San Andreas Fault and East California Shear Zone deformation. Broader impacts This work supports one MSc student, Amy Moser, and travel and field expenses for Postdoctoral researcher Kelly Bradbury. Communication with the general public occurred through the fieldwork season, where the general public, hiking in the popular canyons of the Mecca Hills, asked a range of questions. Moser, Bradbury, Evans, and field assistant Simona Clausnitzer answered a range of questions and gave impromptu talks ranging from 5 to 20 minutes in length on topics such as the tectonic, rock, and seismic cycles; the San Andreas fault and seismic hazards; southern California geology; rock mechanics; seismology; and the nature of the SCEC program. We estimate having substantive discussions with ~ 50 people in January and February. References Ault, A.K et al., 2013; Geological Society of America Abstracts with Programs, v. 45, no. 7, p. 223. Bilham, R. and Williams, P., 1985, Geophysical Research Letters, v. 12, no. 9, p. 557-560. Bird, P. and Rosenstock, R.W., Geological Society of America Bulletin, v. 95, no. 8, p. 945-957. 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