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Examination of Exhumed Faults in the Western San Bernardino Mountains, California: Implications for Fault Growth and Earthquake Rupture
Joseph R. Jacobs
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EXAMINATION OF EXHUMED FAULTS IN THE WESTERN SAN BERNARDINO
MOUNTAINS, CALIFORNIA: IMPLICATIONS FOR FAULT GROWTH
AND EARTHQUAKE RUPTURE
by
Joseph R. Jacobs
A thesis submitted in partial fulfillment of the requirements for the degree
of
MASTER OF SCIENCE
in
Geology
Approved:
James P. Evans Susanne U. Janecke Major Professor Committee Member
Peter T. Kolesar Laurens H. Smith, Jr. Committee Member Interim Dean of Graduate Studies
UTAH STATE UNIVERSITY Logan, Utah
2005
ii ABSTRACT
Examination of Exhumed Faults in the Western San Bernardino Mountains, California:
Implications for Fault Growth and Earthquake Rupture
by
Joseph R. Jacobs, Master of Science
Utah State University, 2005
Major Professor: Dr. James P. Evans Department: Geology
The late Miocene Cedar Springs fault system is a high-angle transpressional system in the Silverwood Lake area, western San Bernardino Mountains, southern
California. This thesis presents the study of oblique-slip faults with modest amounts of slip, which represent the early stages of fault development by using slip as a proxy for maturity. A structural and geochemical characterization is provided for six fault zones ranging from 39 m of slip to 3.5 km of offset in order to develop a model of fault zone geometry and composition. Basic geometric and kinematic results are provided for an additional 29 small-displacement (cm- to m-scale) faults. The main faults of this study can be divided into the fault core composed of sheared clay gouge and microbreccia, the primary damage zone made up of chemically altered rock with microstructural damage and grain-size reduction, and the secondary damage zone, which is characterized by an increased fracture density relative to the host rock. Although there appears to be a general increase in fault core thickness with increasing slip, the correlation is iii insignificant when analyzing all faults. Both the primary and secondary damage zones appear to thicken with increased slip on the main fault.
Overall, the structure and composition of the faults studied here are similar to those of larger strike-slip and reverse faults. This indicates that the fault core develops early in a fault’s history. Subsequent slip appears to be focused along these narrow zones, with some deformation accumulating in the damage zone. Whole-rock geochemical analyses typically show a reduction in the abundance of Na, Al, K, and Ca in the fault core and primary damage zone relative to the host rock. This indicates enhanced fluid-rock interactions in these zones. Calculations of the energy consumed to produce the chemical alteration in the fault core indicate that a considerable amount of the total earthquake energy may be lost to alteration. This thesis concludes that fault processes are similar throughout the different stages of development, and the study of relatively small-displacement faults can therefore be used to understand fault evolution through time and the processes of larger faults in the brittle crust.
(226 pages)
iv ACKNOWLEDGMENTS
First and foremost, I would like to thank my advisor, Jim Evans, for the valuable support on this project. Jim has been an incredible resource of information and this thesis would not have possible without him. This work was funded through the Southern
California Earthquake Center (SCEC) Cooperative Agreement No. 02HQAG0008 issued by the US Geological Survey under CFDA No. 15.807 and I greatly appreciate their financial assistance and interest in the study of exhumed fault zones.
I would like to thank Pete Kolesar for assistance with x-ray diffraction methods and interpretations. Pete also helped me immensely with calculating the energy associated with chemical alterations. Susanne Janecke has also been very helpful in improving previous drafts of this thesis and I appreciate her insightful reviews.
Discussions with Judy Chester and Ron Biegel helped to gain understanding into fault zone processes. Additional gratitude is given to Tony Williams and Jason Kneedy for a weekend of field assistance.
Last, but not least, I would like to acknowledge the many friendships (too many to mention here) that have made my stay here very enjoyable. Stefan Kirby and I started our work here at the same time and it was good to have a friend going through the same experiences as me. Alex Steely, Scott Friedman, and Ben Kessel have also been good friends that all helped to make getting this degree a little easier. Thanks to everyone.
Joseph Jacobs
v CONTENTS
Page
ABSTRACT ...... ii
ACKNOWLEDGMENTS ...... iv
LIST OF TABLES ...... vii
LIST OF FIGURES ...... viii
CHAPTER
1. INTRODUCTION ...... 1
1-1. Introduction ...... 1 1-2. Methodology ...... 4 1-3. Seismicity and Los Angeles Basin Analog ...... 7 1-4. Summary of Work ...... 8 References ...... 10
2. STRUCTURAL AND GEOCHEMICAL CHARACTERIZATION OF MULTIPLE REVERSE FAULTS IN THE WESTERN SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA ...... 16
Abstract ...... 16 2-1. Introduction ...... 17 2-2. Mesoscopic Analysis ...... 27 2-3. Microstructure ...... 36 2-4. Mineralogy and Geochemistry ...... 40 2-5. Earthquake Energy Budget ...... 47 2-6. Discussion ...... 51 2-7. Conclusions ...... 59 References ...... 62
3. THE CEDAR SPRINGS FAULT SYSTEM: ANALYSIS OF MICRO- SEISMICITY AND COMPARISON TO THE PUENTE HILLS BLIND- THRUST SYSTEM ...... 118
Abstract ...... 118 3-1. Introduction ...... 119 3-2. Microseismicity ...... 120 3-3. Eastwood Fault and Puente Hills Blind-Thrust Analog ...... 122 vi 3-4. Conclusions ...... 126 References ...... 128
4. CONCLUSIONS ...... 139
References ...... 144
APPENDICES ...... 147
APPENDIX A: Slip calculations for constrained faults ...... 148 APPENDIX B: Thin section descriptions ...... 167 APPENDIX C: X-ray diffraction patterns ...... 170 APPENDIX D: Whole-rock geochemistry raw data ...... 202 APPENDIX E: Principal component analysis ...... 204
vii LIST OF TABLES
Table Page
2-1 List of all faults where true slip is constrained ...... 71
2-2 Summary of XRD results for all samples ...... 72
2-3 Summary of geochemical trends of all oxides in the Grass Valley fault zone ...... 74
2-4 Summary of geochemical trends of trace elements and LOI in the Grass Valley fault zone ...... 74
2-5 Summary of geochemical trends of oxides in the Eastwood fault zone ...... 75
2-6 Summary of geochemical trends of trace elements and LOI in the Eastwood fault zone ...... 75
2-7 Summary of geochemical trends of oxides in the Cleghorn fault zone ...... 76
2-8 Summary of geochemical trends of trace elements and LOI in the Cleghorn fault zone ...... 76
2-9 Summary of energy calculations for the Eastwood fault ...... 76
viii LIST OF FIGURES
Figure Page
1-1 Simplified geologic map of the Silverwood Lake area ...... 14
1-2 Diagram showing transect methodology ...... 15
2-1 Fault core and damage zone model for the North Branch San Gabriel strike-slip fault zone from Chester et al. (1993) ...... 77
2-2 Shaded relief map of southern California showing major roads (red), rivers (blue), and faults (yellow) ...... 78
2-3 Shaded relief map of the western San Bernardino Mountains ...... 79
2-4 Geologic map of the Silverwood Lake area ...... 80
2-5 Simplified cross-section trending northeast through the Silverwood Lake area and Cedar Springs fault system to Deep Creek ...... 81
2-6 Cross-sections showing estimates of minimum slip constraints based on lack of the Crowder Formation at higher topographic elevations ...... 82
2-7 Histogram of all 31 slip-constrained faults studied ranging from 2 cm to 183 m of slip ...... 83
2-8 Outcrop photographs showing elements used for slip calculations ...... 84
2-9 Photomosaic and simplified sketch of the Grass Valley fault zone with view to the south ...... 85
2-10 Transect data from the Grass Valley reverse right-lateral fault ...... 86
2-11 View to the southeast of a subsidiary fault in the Grass Valley fault zone ...... 87
2-12 Photomosaic and simplified sketch of the Eastwood reverse fault with view to the east ...... 88
2-13 Transect data from the Eastwood reverse fault hanging wall ...... 89
2-14 Photographs of the Cleghorn fault (reverse left-lateral slip) juxtaposing crystalline basement on top of the Miocene Crowder Formation ...... 90
ix 2-15 Transect data from the Cleghorn reverse left-lateral fault hanging wall ...... 91
2-16 Transect data from the Powell Canyon right-lateral fault footwall ...... 92
2-17 Photomosaic and simplified sketch of two faults in the Lakeview reverse left- lateral fault zone ...... 93
2-18 Photomosaic and simplified sketch of the Westwood reverse left-lateral fault (Station 54) ...... 94
2-19 Plots of fault core thickness with respect to slip ...... 95
2-20 Microstructure of the Grass Valley fault ...... 96
2-21 Microstructure of the Grass Valley fault zone hanging wall ...... 97
2-22 Microstructure of the Eastwood fault ...... 98
2-23 Microstructure of the Eastwood fault zone hanging wall ...... 99
2-24 Microstructure of the Cleghorn fault 30 cm into the hanging wall from the fault core ...... 100
2-25 Microstructure of the Cleghorn fault zone hanging wall ...... 101
2-26 Microstructure of the Lakeview fault ...... 102
2-27 Microstructure of the Westwood fault ...... 103
2-28 Microstructure of the Westwood fault footwall damage zone ...... 104
2-29 Diffraction patterns of fault cores showing original sample (blue) and glycolated sample (red) ...... 105
2-30 Whole-rock geochemistry of the Grass Valley fault zone ...... 106
2-31 Whole-rock geochemistry of the Eastwood fault zone ...... 107
2-32 Whole-rock geochemistry of the Cleghorn fault zone ...... 108
2-33 Whole-rock geochemistry of the Powell Canyon fault zone ...... 109
2-34 Spider diagrams at different positions throughout the Grass Valley fault zone ..110
2-35 Spider diagrams at different positions throughout the Eastwood fault zone ...... 111 x 2-36 Spider diagrams at different positions throughout the Cleghorn fault zone ...... 112
2-37 Spider diagrams at different positions throughout the Powell Canyon fault zone ...... 113
2-38 Variations in shear stress (τ) and displacement (u) over time for seismic and aseismic crustal fault zones ...... 114
2-39 Diagram showing the concept of activation energy ...... 115
2-40 Plots showing seismic relationships used to calculate total moment energy for the Eastwood fault ...... 116
2-41 Diagram showing the extent of the primary and secondary damage zones for five major fault zones studied ...... 117
3-1 Simplified geologic map of the Silverwood Lake area showing orientation of maximum horizontal stress (SHmax) from Townend and Zoback (2001) ...... 130
3-2 Microseismicity plots of 33 events from 1983-2001 for the Silverwood Lake area ...... 131
3-3 Oblique along-strike view to the northwest of the San Andreas fault ...... 132
3-4 Geologic map of the Eastwood fault area combining work from Weldon (1986) with field mapping performed as part of this research ...... 133
3-5 Cross-section C-C’ to accompany Figure 3-4 ...... 134
3-6 Trishear model of the Eastwood fault using FaultFold software of Allmendinger ...... 135
3-7 Comparison of trishear model with present geology ...... 136
3-8 Photographs of the Eastwood fault ...... 137
3-9 Seismic reflection profiles of the Santa Fe Springs segment of the Puente Hills thrust ...... 138
CHAPTER 1
INTRODUCTION
1-1. Introduction
Fault growth and evolution is of much interest to the seismological community because it is related to topics of earthquake mechanics, such as the width of the fault surface that slips during an earthquake (Kanamori and Heaton, 2000), fault zone structure
(Rubin et al., 1999), and rupture propagation processes (Heaton, 1990; Andrews and Ben-
Zion, 1997; Harris and Day, 1999). Characterization of fault zone structure and composition provides insight into these processes of fault nucleation, propagation, and termination.
There has been extensive fault and rock mechanics oriented research on large- displacement strike-slip faults in southern California, such as the San Gabriel and
Punchbowl faults (Chester and Logan, 1986, 1987; Chester et al., 1993; Evans and
Chester, 1995; Schulz and Evans, 2000; Wilson et al., 2003). The work presented here is a continuation of this structural and compositional approach to understanding fault growth processes. However, the faults studied here differ from those cited above in that they are predominantly high-angle oblique reverse faults that range in scale of slip from cm to hundreds of m. Thus, it may be more appropriate to compare these faults with other reverse faults in crystalline rocks, such as the “thick-skinned” Laramide-style faults in western Wyoming (Mitra et al., 1988; Evans, 1993; Mitra, 1993; Yonkee and Mitra,
1993). Previous studies in southern California and western Wyoming typically examined one or two faults in detail (e.g., Chester and Logan, 1987; Evans, 1993; Mitra, 1993;
Yonkee and Mitra, 1993; Schulz and Evans, 2000; Wilson et al., 2003), and slowly build 2 our “fault catalog” by examining only one stage of fault development. Other studies that have used a multi-fault data set typically focused on the scaling relationship between fault length and displacement (Dawers et al., 1993; Dawers and Anders, 1995; Clark and
Cox, 1996; Nicol et al., 1996; Schlische et al., 1996; Watterson et al., 1996; Yielding et al., 1996) and focused more on normal faults. Our work is unique in that it provides a structural and geochemical characterization for six fault zones and basic geometric and kinematic observations from 29 other faults. Combined with previous work of larger- displacement reverse faults, a model of fault evolution can be developed to further our understanding of earthquake processes in the brittle crust.
Studying faults with modest amounts of slip (m to hundreds of m of slip) allows for a characterization at early stages of fault growth by using slip as a proxy for time. It is assumed that smaller-displacement faults are younger and larger-displacement faults are more mature. Previous work on relatively small faults primarily focused on fault initiation, linkage, and termination processes (Lim, 1998; Robeson, 1998; Pachell and
Evans, 2002; Katz et al., 2003), or damage zone geometries and structures (Kim et al.,
2004, and references therein), without addressing chemical changes and fluid-rock interactions. This thesis continues the structural analysis of relatively small-displacement faults (< 200 m) in order to study the early stages of fault development, but also addresses geochemical topics, such as formation of clays, fluid-rock interactions, and energy associated with chemical alteration. One drawback to studying relatively small- displacement faults is the limited opportunity to make observations of along-strike changes due to the short trace length of the fault.
3 Whether or not scaling laws exist for fault populations is an important question that has received attention by some workers. In order to assess this question, workers have either analyzed thickness-displacement relationships (Robertson, 1983, 1988; Hull,
1988, 1989; Blenkinsop, 1989; Evans, 1990) or fault length-displacement relationships
(Dawers et al., 1993; Dawers and Anders, 1995; Clark and Cox, 1996; Nicol et al., 1996;
Schlische et al., 1996; Watterson et al., 1996; Yielding et al., 1996). Both types of approaches typically used combined data sets, often from faults in different rock types and structural settings. The work presented here comes from a small range of fault types with varying amounts of slip. The faults share a similar rock type and tectonic setting.
With these variables held relatively constant, we can better analyze scaling relationships.
This research only tests for correlation between displacement and thickness, not fault length, because lengths are difficult to quantify in the area due to widely varied exposure.
Although lithology is mostly constant, granite pegmatite dikes intruded into finer- grained granodiorite provide some lateral lithologic and mechanical contrast. Field observations were made to describe the effects of this lithologic contrast. Data were also collected in order to characterize fault zone geometry and kinematics and to define the nature and extent of the damage zone. Other questions addressed in this work are: what is the rheological significance of clay gouge on slip surfaces and what are the effects of fluid-rock interactions at seismogenic depths? Together, these data give a mesoscopic
(outcrop-scale), microscopic, and geochemical characterization of several fault zones.
Fault zones can be divided into two separate domains: the damage zone and the fault core (Chester and Logan, 1986; Chester et al., 1993, 2004; Evans and Chester, 1995;
Wilson et al., 2003). The damage zone is characterized by an increase in subsidiary
4 faulting, fracturing, and mineral alteration as compared to the undeformed host rock
(Chester and Logan, 1987; Chester et al., 1993). The fault core is defined as the zone that contains increased foliation, clay gouge, and/or fine-grained fault rocks such as ultracataclasite or microbreccia. It has been proposed by these workers that nearly all of the slip occurs within this relatively thin fault core, a zone of high shear strain, extreme comminution, and enhanced fluid-rock interactions. The damage zones referred to in this thesis correspond to the “wall damage zones” in the classification scheme of Kim et al.
(2004).
1-2. Methodology
1-2.1 Field Methods
The field area lies within two 7.5-minute USGS topographic quadrangles: Cajon and Silverwood Lake. The main focus of this study was in the Silverwood Lake area due to the quality of exposure and the accessibility to the area. Field mapping of the western
San Bernardino Mountains by Meisling and Weldon (1989) has documented the complex structural geology of the area (Figure 1-1). Faults were located in the field using geologic maps from Weldon (1986) and from reconnaissance mapping. Offset markers and slip vectors provided fault kinematics. True slip was calculated using fault orientation and rake, separation, and orientation of offset marker (Appendix A). Detailed outcrop sketches were made to record fault geometries, offsets, and cross-cutting relationships of faults, fractures, and veins. Photomosaics were also made with digital photos in order to aid in the small-scale mapping of structures. Densities and orientations of damage elements (subsidiary faults, fractures, and veins) were determined in the
5 damage zone by using a linear point intercept method combined with 70 cm vertical and horizontal linear transects that record everything under line (Figure 1-2) (e.g., Schulz and
Evans, 2000, and references therein). An outcrop-specific, systematic (i.e., non-random) sampling plan was developed to representatively sample the host rock, damage zone, and fault core at the various fault zones studied. A total of 81 hand samples were collected during field work.
1-2.2 Laboratory Methods
1-2.2.1 Thin Sections
In order to understand the microscopic processes occurring throughout fault zones, thin sections were prepared from the host rock, damage zone, and fault core. A total of 68 samples were prepared at Utah State University and then sent to Burnham
Petrographics for professional slide mounting and polishing. The steps for billet preparation were:
1. Dry samples at 120° C for several hours;
2. Coat samples with two-part Raka epoxy (resin 127 and hardener 606);
3. Vacuum samples and recoat until fully impregnated;
4. Cure epoxied samples at 120° C;
5. Cut samples wet with rock saw and then dry at 135° C for at least one hour; and
6. Coat billet with Petropoxy 154 and cure at 135° C.
Optical microscopy analysis of the completed thin sections, including acquisition of digital photomicrographs, was done at Utah State University.
6 1-2.2.2 X-Ray Diffraction
Samples to be analyzed with X-ray diffraction (XRD) were crushed to pea size gravel in a Sepor shatterbox and then put in a Rocklabs grinding mill in a Syalon
(lightweight ceramic) head for several minutes, depending on the hardness of the sample.
This process reduced the samples to a fine powder. The powder was placed in aluminum holders for XRD analysis. The majority of whole-rock samples were analyzed using a
Philips X-Pert PRO PANalytical machine with the accompanying X-Pert Highscore software for result interpretation. Some samples were analyzed with a 1970’s Philips
XRD machine with a Jade software package. A total of 80 bulk samples were analyzed.
Oriented clay samples were also analyzed with XRD to further identify the mineralogy of fault cores. One gram of loose material from the original sample was measured and added to 10 mL of distilled water. This mixture was then agitated with a sonic dismembrator for 10-15 seconds in order to loosen the clay from the sample. The mixture was allowed to settle for several minutes leaving the clay minerals on top. One mL was then drawn from the very top and put on a slide to dry, thus creating an oriented clay sample. In order to distinguish smectite from illite, an ethylene glycol treatment was applied to the clay slides. The slides were put into a closed chamber with ethylene glycol and baked for at least one hour at 65° C. A heat treatment (550° C for > 1 hour) was also performed on selected samples in order to more accurately identify the presence of kaolinite. The X-Pert PRO PANalytical machine was used for analysis of 11 oriented clay samples.
7 1-2.2.3 Whole-Rock Geochemistry
Whole-rock geochemistry was performed at SGS Mineral Services in Toronto,
Ontario, Canada using an Inductively Coupled Plasma (ICP) method to determine the major, minor, and trace elements of 39 samples. All major and minor elements and loss on ignition (LOI) had a detection limit of 0.01 %, while trace elements had a detection limit of 10 ppm. Four duplicate samples were analyzed for quality assurance.
Principal Component Analysis (PCA) was performed on the oxides and loss on ignition (LOI) data in order to determine the most significant variables. Multi-Variate
Statistical Package 3.1 was used for this analysis and was performed at Utah State
University. The whole-rock geochemical data are percentages of a total, which comprise a closed data set. This means that changing one variable (i.e., Si) causes the others to change, even though that is not necessarily the case. Therefore, a log 10 transformation was performed on the data in order to convert it to an open data set. The data were also standardized and centered prior to PCA.
1-3. Seismicity and Los Angeles Basin Analog
Historical seismic data obtained from the Southern California Earthquake Center
(SCEC) reveal a pronounced lack of microseismicity beneath the field area (Aguilar et al., 2003). From August 1, 1983 to August 2, 2001, a total of 33 events were recorded.
These events ranged from M 0 to M 2.93 and from 2.8 to 11 km in depth. Townend and
Zoback (2001) show that the maximum horizontal stress in the field area is oriented
NNE-SSW. The NW-SE strike of the Cedar Springs fault system is almost optimally
8 oriented for failure, and yet there is a lack of microseismicity. Potential reasons for this paradox are explored in Chapter 3.
The Eastwood fault zone is a thrust sheet that places crystalline bedrock on top of the Miocene Crowder Formation. The frontal thrust of this fault zone propagated from the crystalline bedrock into softer sedimentary rocks, similar to blind thrust faults beneath the Los Angeles basin, such as the Puente Hills system (e.g., Shaw et al., 2002). Fault bend and fault-propagation folding have been documented above the Puente Hills blind thrust system (Allmendinger and Shaw, 2002; Shaw et al., 2002). The Eastwood reverse also appears to represent fault-propagation folding of the crystalline basement and can be modeled successfully using trishear (e.g., Erslev, 1991; Hardy and Ford, 1997;
Allmendinger, 1998). Although folding is more difficult to assess in the crystalline rock, the topography of the Eastwood fault hanging wall may be structurally controlled, thus reflecting the folded basement. Therefore, the Eastwood fault zone may be a good exposed analog for the blind thrust faults beneath the Los Angeles basin. Direct observations of the Eastwood fault are available due to exhumation from 1-2 km depth
(Spotila and Sieh, 2000). The Eastwood frontal thrust cuts this crystalline-sedimentary contact and is defined by a 2-5 cm thick layer of clay gouge. The Eastwood-Puente Hills analog is further discussed in Chapter 3.
1-4. Summary of Work
Chapter 2 of this thesis is a manuscript that characterizes the mesostructure, microstructure, and geochemistry of six fault zones and a detailed study of 29 additional faults. We use a rock mechanics and structural petrologic approach to study relatively
9 small-displacement exhumed reverse faults with varying degrees of slip in order to gain understanding of fault processes at seismogenic depths and their implications for earthquake evolution. Estimates of the energy associated with chemical alteration in the fault core are made in this chapter to help solve the discrepancy between total earthquake energy and lesser radiated energy. Thickness-displacement relationships are tested for all faults cataloged in this study in order to evaluate whether or not there is a correlation between these fault variables. The results of this work are compared with similar studies of the Punchbowl and San Gabriel strike-slip faults (Chester et al., 1993; Schulz and
Evans, 2000; Chester et al., 2004) and Laramide-style reverse faults in the Wind River
Mountains (Mitra et al., 1988; Evans, 1993; Mitra, 1993; Yonkee and Mitra, 1993).
In Chapter 3, we step back from the detailed analysis to address more regional topics. The Eastwood thrust sheet is used as an analog for the Puente Hills blind thrust system beneath the Los Angeles basin in order to better understand the properties of thrust faults at depth. Additionally, there is a pronounced lack of microseismicity in the
Silverwood Lake area despite the favorable orientation of faults for failure. Possible reasons for this paradox are explored in this chapter 3.
Chapter 4 concludes the thesis with a summary of all chapters and final remarks.
The results of this work indicate that the narrow clay-rich cores form early in the development of a fault. Subsequent slip appears to be focused along these narrow zones, with some deformation accumulating in the damage zone. The lack of observed pseudotachylyte and the increased LOI values (indicating trapped fluids) in the fault core implies that fluid pressurization is an important mechanism for reducing shear resistance during fault rupture. Fault processes are similar throughout the different stages of
10 development, and large amounts of slip can occur on very narrow slip surfaces.
Therefore, the study of relatively small-displacement faults can be used to understand fault evolution through time and the processes of larger faults in the brittle crust.
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Nicol, A., J. J. Walsh, J. Watterson, and P. A. Gillespie (1996), Fault size distributions – are they really power-law?, J. Struct. Geol., 18, 191-197.
Pachell, M. A., and J. P. Evans (2002), Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: The Gemini fault zone, Sierra Nevada, California, J. Struct. Geol., 24, 1903-1924.
Robertson, E. C. (1983), Relationship of fault displacement to gouge and breccia thickness, Min. Eng., 35, 1426-1432.
Robertson, E. C. (1988), Shearing of rocks in faults, Trans. Am. Geophys. Un., 69, 1448, 1988.
Robeson, K. R. (1998), Three-dimensional structure of small strike-slip fault zones in granitic rock: implications for fault-growth models, M.S. thesis, 324 pp., Utah State University, Logan.
Rubin, A. M. (1999), Streaks of microearthquakes along creeping faults, Nature, 400, 635-641.
Schlische, R. W., S. S. Young, R. V. Ackermann, and A. Gupta (1996), Geometry and scaling relations of a population of very small rift-related normal faults, Geology, 24, 683-686.
13 Schulz, S. E., and J. P. Evans (2000), Mesoscopic structure of the Punchbowl Fault, Southern California and the geologic and geophysical structure of active strike- slip faults, J. Struct. Geol., 22, 913-930.
Shaw, J. H., A. Plesch, J. F. Dolan, T. L. Pratt, and P. Fiore (2002), Puente Hills Blind- Thrust System, Los Angeles, California, B. Seismol. Soc. Am., 92, 2946-2960.
Spotila, J. A., and K. Sieh (2000), Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface, Tectonics, 19, 589-615.
Townend, J., and M. D. Zoback (2001), Implications of earthquake focal mechanisms for the frictional strength of the San Andreas fault system, in The Nature and Tectonic Significance of Fault Zone Weakening, edited by R. E. Holdsworth, R. A. Strachan, J. F. Magloughlin, and R. J. Knipe, pp. 13-21, Geol. Soc. London, Spec. Pub., 186.
Watterson, J., J. J. Walsh, P. A. Gillespie, and S. Easton (1996), Scaling systematics of fault sizes on a large-scale range fault map, J. Struct. Geol., 18, 199-214.
Weldon, R. J. (1986), The late Cenozoic geology of Cajon Pass; implications for tectonics and sedimentation along the San Andreas fault, Ph.D. thesis, 481 pp., Calif. Inst. of Technol., Pasadena.
Wilson, J. E., J. S. Chester, and F. M. Chester (2003), Microfracture analysis of fault growth and wear processes, Punchbowl fault, San Andreas system, California, J. Struct. Geol., 25, 1855-1873.
Yielding, G., T. Needham, and H. Jones (1996), Sampling of fault populations using sub- surface data: a review, J. Struct. Geol., 18, 135-146.
Yonkee, W. A., and G. Mitra (1993), Comparison of basement deformation styles in parts of the Rocky Mountain foreland, Wyoming, and the Sevier orogenic belt, northern Utah, in Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, edited by C. J. Schmidt, R. B. Chase, and E. A. Erslev, pp. 197-228, Geol. Soc. Am. Spec. Pap. 280, Boulder, Colo.
14
34o 20' 40'' 34 o 20' 40''
Grass Valley fault
Eastwood fault Lakeview fault
Powell Canyon fault
o 34 o 15' 34 15' o 117 o 14' 08'' 117 26'15'' 60 Younger Alluvium Pliocene Phelan Peak Deposits _J__ D D BEDDING ATTITUD E Older Alluvium Miocene Crowder Formation ~-6...:.:'.. Pleistocene D D FOLIATION ATTITUDE Shoemaker Gravel ..... ·---? Mesozoic crystalline basement complex CONT ACT; D D CONC EALED; INFERRED Harold Formation 1 5 km 45 :t u ~ • ...... 1 ... D D ~ FAULT; DIP; SLIP; THRUST CONCEALED; INFERRED Figure 1-1. Simplified geologic map of the Silverwood Lake area. SAF: San -+.- ·· ? - Andreas fault; WSBA: western San Bernardino arch. Adapted from Meisling ~ .. ? ANT ICLINE ; SYNCLINE and Weldon (1989). CONC EALED; INFER RED 25
70 cm vertical and horizontal linear transects (i.e., scan lines) I I I I I I I I I I I I I I I___ I___ I___ I___ I___ I___ I___ • • • • • • • Linear point intercept with 1 meter spacing
Figure 1-2. Diagram showing transect methodology. At set distances away from the main fault, two scan lines were performed; one vertical and one horizontal. The spacing throughout fault damage zones was not always one meter. 16 CHAPTER 2
STRUCTURAL AND GEOCHEMICAL CHARACTERIZATION OF MULTIPLE
REVERSE FAULTS IN THE WESTERN SAN BERNARDINO MOUNTAINS,
SOUTHERN CALIFORNIA1
Abstract
The topic of fault growth and evolution is of much interest to the seismological community because it relates to numerous subjects of earthquake mechanics, such as the width of the fault surface that slips during an earthquake, fault zone structure, and rupture propagation processes. The work presented here employs a fault and rock mechanics approach to examine 35 predominantly reverse faults with centimeters to hundreds of meters of slip in the western San Bernardino Mountains, southern California. Using slip as a proxy for time, these small-displacement faults represent relatively young faults in contrast to much of the previous work done on the mature San Gabriel and Punchbowl strike-slip faults in southern California and the Laramide-style (basement-involved) reverse faults in northwest Wyoming.
We provide a detailed structural and geochemical characterization of six exhumed fault zones with varying amounts of slip along with basic geometric and kinematic results from 29 small-displacement (i.e., cm to m scale) faults. Results from the six main fault zones are used to develop a model of fault zone geometry and composition. The majority of these faults can be divided into a fault core, primary damage zone (intense microstuctural damage and chemical alteration), and secondary damage zone (negligible
1 Coauthored by J. R. Jacobs, J. P. Evans, and P. T. Kolesar. 17 alteration, but greater fracture density than the host rock). We observed well- developed fault cores and damage zones on the order of tens of meters in half-thickness perpendicular to the fault, despite the modest amounts of slip on these faults. The fault cores and primary damage zones of most faults analyzed typically exhibit a reduction in the abundance of Na, Al, K, and Ca relative to the host rock; a sign of enhanced fluid- rock interactions in these zones. Fault cores studied are a mixture of comminuted protolith and alteration products such as smectite, palygorskite, and laumontite. Our calculations suggest that syntectonic alteration in the fault core can consume a considerable amount of the total earthquake energy. Thickness-displacement relations for 26 faults show no correlation between fault core thickness and displacement. Both the primary and secondary damage zones thicken with increased slip.
The results of this work indicate that the narrow fault cores form early in the development of a fault, when there is as little as several cm of slip. Subsequent slip appears to be focused along these narrow zones, with some continued deformation accumulating in the damage zone. We suggest that fault processes are similar throughout the different stages of development and that large amounts of slip can occur on very narrow slip surfaces.
2-1. Introduction
Although there has been fairly extensive fault and rock mechanics-oriented research on large-displacement faults of the San Andreas system in order to define the structure and composition of fault zones and relate these to fault processes (Chester et al.,
1993; Evans and Chester, 1995; Schulz and Evans, 2000; Wilson et al., 2003), much less 18 has been done on smaller-displacement faults in southern California or elsewhere.
Research on small-displacement faults is critical in order to understand the scaling relationships between faults of various sizes. If the mechanical and chemical processes of small faults can be used as analogs for larger faults, then a much larger data set is available to us to study fault growth and earthquake processes. The study of small faults is important in order to understand how faults grow because small-displacement faults represent relatively young faults, as opposed to mature faults that have accumulated much slip. Using the amount of slip as a proxy for time, we are able to examine faults at different stages of development.
The relationship between thickness and displacement of fault zones is important for its application to fault-zone growth models (Robertson, 1983, 1988; Hull, 1988, 1989;
Blenkinsop, 1989; Evans, 1990). However, much of this work combined data sets from a wide range of rock types, structural settings, and fault processes (Evans, 1990). We test for thickness-displacement correlation from 26 predominantly reverse faults, with slip that ranges from 2 cm to 183 m, in a similar rock type and structural regime.
The topic of fault growth and evolution is of much interest to the seismological community because it relates to numerous subjects of earthquake mechanics, such as the width of the fault surface that slips during an earthquake (Kanamori and Heaton, 2000) and fault zone structure (Rubin et al., 1999). The direct study of faults at depth poses both logistical and economic challenges, although it certainly may be accomplished (i.e., the San Andreas Fault Observatory at Depth – SAFOD). The study of exhumed faults has been previously shown by such workers as Chester and Logan (1986, 1987), Chester et al. (1993), Lim (1998), Robeson (1998), Schulz and Evans (2000), Pachell and Evans 19 (2002), and Wilson et al. (2003) as a means to investigate active and inactive faults at the earth’s surface that are representative of faults at depth. The work presented here is a mesoscopic (outcrop scale), microscopic, and geochemical characterization of relatively small-displacement faults exhumed from 1-2 km depth in the San Bernardino Mountains, southern California. Study of this relatively shallow depth is important in understanding seismic processes because of a “shallow trapping structure” that has been interpreted for many fault zones (Ben-Zion et al., 2003; Peng et al., 2003, and references therein).
2-1.1 Objectives
A primary motivation of this work is to provide the structural geologic and seismological communities with constraints on fault structure and composition. This includes determining the mesoscopic fault zone geometry, such as the nature and extent of the fault core and damage zone (Figure 2-1), and fault kinematics. The damage zones referred to in this paper correspond to the “wall damage zone” in the classification scheme of Kim et al. (2004). Characterization of fault zone structure and composition provides insight into the processes of fault nucleation, propagation, and termination and has applications to numerous subjects of earthquake mechanics, such as the width of the fault surface that slips during an earthquake (Kanamori and Heaton, 2000) and rupture propagation processes (Heaton, 1990; Andrews and Ben-Zion, 1997; Harris and Day,
1999).
The other primary objective of this work is to further our knowledge of fault and rock mechanics in the brittle crust. We address questions such as, how does lithology affect the development and nature of faults? What is the rheological significance of clay 20 gouge on slip surfaces? Fault gouge may significantly affect fault strength, which may in turn determine whether a fault moves by catastrophic displacement (seismic) or steady- state creep (Vrolijk and van der Pluijm, 1999). What are the effects of fluid-rock interactions at seismogenic depths? Pore-fluid pressure can have a drastic effect on the stress state due to fluid migration through the fault (Rice, 1992; Chester et al., 1993) or to fluids trapped in fault core gouge zones (Byerlee, 1990). Fluid-assisted mechanisms recorded by chemical alteration and veining (Evans and Chester, 1995) may reduce the frictional strength in the fault core and result in rate strengthening behavior (Logan and
Rauenzahn, 1987; Chester and Higgs, 1992).
2-1.2 Geologic Setting
2-1.2.1 Tectonics
The San Bernardino Mountains are part of the Transverse Ranges in southern
California. The Transverse Ranges have an anomalous east-west trend through the predominant northwest-southeast tectonic grain of California and the western United
States. This obliquity is due to clockwise rotation of the western and easternmost
Transverse Ranges in association with dextral shear along the Pacific-North American plate boundary (Garfunkel, 1974; Kamerling and Luyendyk, 1979, 1985; Luyendyk et al.,
1980, 1985; Dickenson, 1996). The San Bernardino Mountains have not been rotated
(Dickenson, 1996). The San Andreas fault (SAF) dissects the Transverse Ranges, dividing the San Gabriel Mountains to the west from the San Bernardino Mountains to the east (Figure 2-2). 21 The San Bernardino Mountains reflect a complex history of rapid uplift in the
Late Cenozoic in association with transpressive deformation (Dibblee, 1975; Meisling and Weldon, 1982; Spotila and Sieh, 2000; Blythe et al., 2000) that generated both the highest elevations and most extensive upland plateau within the Transverse Ranges
(Sadler, 1982). This broad upland plateau is called the Big Bear plateau, and has been uplifted by two east-west striking thrust faults with opposing dips: the North Frontal thrust system and the Santa Ana thrust (Spotila and Sieh, 2000). Spotila and Sieh (2000) have used a deeply weathered erosion surface in the crystalline bedrock as a structural datum to constrain the amounts of displacement along these thrust fault systems to 1-2 km. Thus, the fault exposures analyzed in this work formed at depths of 1-2 km and have subsequently been exhumed due to uplift on the Santa Ana and North Frontal thrusts.
The field area is in the far western portion of the Big Bear plateau in the Silverwood Lake area (Figure 2-3).
2-1.2.2 Lithologies
The western San Bernardino Mountains consist of a Cretaceous crystalline basement complex that is primarily composed of granodiorite, diorite, and quartz- monzonite (Meisling and Weldon, 1982). These crystalline rocks commonly contain
Precambrian to Paleozoic roof pendants and inclusions of marble, quartzite, and garnet- mica schist of Cordilleran miogeoclinal affinity (Dibblee, 1967; Stewart and Poole, 1975;
Miller and Morton, 1980; Cameron, 1981). All of these lithologies are cut by abundant pegmatitic and aplitic dikes of unknown age (Meisling and Weldon, 1982). Fresh bedrock is commonly overlain by up to 30 m of deeply weathered rock consisting of 22 residual boulders in a matrix of grus (Oberlander, 1974; Meisling and Weldon, 1982).
This weathered surface serves as a structural datum to constrain vertical deformation in the San Bernardino Mountains (Spotila and Sieh, 2000). Two main phases of cooling, caused by pluton uplift, are documented in the San Bernardino Mountains using apatite fission-track and U-Th/He dating: late Cretaceous and late Tertiary (Blythe et al., 2000).
The most recent phase of cooling and bedrock uplift probably began after 3 Ma due to contractional deformation (Blythe et al., 2000) associated with modern strike-slip faulting.
Abundant metamorphic rocks in the area include amphibolite, gneiss, and schist that exhibit strong foliation, mineral segregation, and shearing textures (Meisling and
Weldon, 1982). The age of these metamorphic complexes is debated. According to
Meisling and Weldon (1982), the metamorphism was Cretaceous because the degree of metamorphism increases with depth, and there are gradual transitions of the Cretaceous intrusive rocks to their respective gneissic equivalents. Conversely, some believe this metamorphic complex to be Proterozoic in age. If so, the gneissic rocks are inclusions in the Mesozoic granite (Rogers, 1967; Dibblee, 1975; Ehlig, 1975, 1981; Barth et al.,
1995).
The Miocene Crowder Formation overlies the deeply weathered erosion surface of the San Bernardino Mountains and is composed of up to 980 meters of non-marine arkosic sandstone and conglomerate in Crowder Canyon (Meisling and Weldon, 1989) that are poorly to moderately well indurated (Foster, 1982). The Crowder Formation is believed to have been deposited between 17 and 9.5 Ma (Reynolds, 1985) and exposure is limited to the western portion of the San Bernardino Mountains (Meisling and Weldon, 23 1989). Offset of this unit provides minimum throw constraints on several faults in the field area.
The Pliocene Phelan Peak deposits consist of as much as 500 m of interbedded siltstone, swelling claystone, siliceous ash, sandstone, and conglomerate (Weldon, 1984;
Meisling and Weldon, 1989). These deposits unconformably overlie the Crowder
Formation (Weldon, 1986) and are important to this study because they help to constrain the timing of the Cedar Springs fault system (Meisling and Weldon, 1989).
Coarse fanglomerates were shed as an alluvial apron from the northern escarpment of the San Bernardino Mountains during rapid Quaternary uplift of the north- central plateau and they conformably and unconformably overlie the Pliocene units along the northwestern margin of the San Bernardino Mountains (Meisling and Weldon, 1989).
These fanglomerates are generally called the Victorville fan deposits and are divided into several units: the Harold Formation, the Shoemaker Gravel, the Ord River deposits
(Meisling, 1984), and older alluvium (Noble, 1954). These units are not discussed in detail due to the lack of fault exposure throughout them.
2-1.2.3 Structure
Field mapping of the western San Bernardino Mountains by Meisling and Weldon
(1989) has documented the complex structural geology of the area (Figures 2-4 and 2-5).
Faults exposed in the field area are predominantly late Miocene (9.5-4.1 Ma) high-angle reverse faults with displacements ranging from centimeters to hundreds of meters. Their formation is attributed to the obliquity between the SAF system in southern California
(i.e., the “Big Bend”) and the plate motion vectors, thus causing oblique convergence, or 24 transpression (Spotila and Sieh, 2000). The timing of these faults is bracketed by the fact that they deform the upper Crowder Formation and they are truncated by the unconformity at the base of the Phelan Peak deposits (Meisling and Weldon, 1989).
These south-southwest vergent contractional structures are collectively called the Cedar
Springs fault system, which is composed of northwest to east-west striking arcuate faults.
East-west striking fault segments typically exhibit a pure north-side-up dip-slip motion whereas northwest striking segments often have a component of strike-slip, thus making them right-oblique slip faults (Meisling and Weldon, 1989). Within the Cedar Springs fault system are a set of north-plunging anticlines and synclines, which locally deform the
Crowder Formation and basement rocks. The high-angle reverse faults cut the folds and therefore post-date them (Meisling and Weldon, 1989). Meisling and Weldon (1989) note that when bedding of the Crowder Formation is restored by unfolding the
Pleistocene Western San Bernardino arch, the north-dipping faults decrease in dip by
20°- 30°, making many of these faults oblique-slip thrusts. Tilting from the Western San
Bernardino arch was removed during the construction of cross sections for this paper
(Figure 2-6).
Contemporaneous with the late Miocene Cedar Springs fault system is the east- west striking, north-dipping Santa Ana thrust, which is responsible for the initial uplift of the Big Bear Plateau east of the field area (Spotila and Sieh, 2000). Continued uplift of this central plateau in the early-middle Pleistocene occurred via the North Frontal thrust system, composed of primarily south-dipping faults that strike east-west along the northern boundary of the San Bernardino Mountains (Spotila and Sieh, 2000). Although these two major thrust systems die out east of the field area, they are responsible for 25 much uplift of the San Bernardino Mountains. Spotila and Sieh (2000) have used a deeply weathered erosion surface in the crystalline bedrock as a structural datum to constrain throw along these thrust fault systems to 1-2 km. Thus, the fault exposures analyzed in this work formed at depths of 1-2 km and have subsequently been exhumed due to uplift on the Santa Ana and North Frontal thrusts.
The Cleghorn fault, the largest fault studied for this work, strikes east-west through the Silverwood Lake area (Figure 2-3). This steeply-dipping (85°N-vertical) fault was formed with the Cedar Springs Fault System in late Miocene as a south-block- down reverse fault accumulating approximately 300 m of vertical separation (Meisling and Weldon, 1989). The Cleghorn fault was then reactivated as a left-lateral strike-slip fault and has accumulated 3.5-4.0 km of offset in the Quaternary based on offset of older faults and folds and disturbed alluvial fans (Meisling and Weldon, 1989). This fault will serve as the large-displacement end member of faults studied in this work.
The Eastwood reverse fault places crystalline basement on top of the Crowder
Formation and has a minimum slip constraint of 183 m. This reverse fault strikes ESE through the eastern portion of the field area and dips approximately 55° to the north.
When tilt from the western San Bernardino arch is removed, the Eastwood fault dips 34° to the north (Figure 2-6). Slip vector measurements on the fault plane indicate pure dip- slip motion.
In addition to the Cleghorn and Eastwood faults, four other main faults are studied: the Westwood, Lakeview, Grass Valley, and Powell Canyon faults (Figure 2-4).
Except for the Powell Canyon fault, these are oblique reverse faults most likely related to the Cedar Springs fault system. The Powell Canyon fault is a right-lateral strike-slip fault 26 that appears to be related to SAF dextral deformation, although it may have initiated with the Cedar Springs fault system. These four faults strike E-W to SE-NW and have dips to both the north and south.
2-1.3 Methods
2-1.3.1 Field Methods
Faults were located in the field using geologic maps from Weldon (1986) and from reconnaissance mapping. Slip on small displacement faults (< 50 m) was determined, when possible, using fault orientation and rake, separation, and orientation of offset markers, such as pegmatite dikes. Vertical offset of the Crowder Formation allowed for minimum slip calculations for the Eastwood and Lakeview faults (Figure 2-
6). These calculations assume that the measured slip vector represents the overall slip vector for the fault. The dip-slip component of the Cleghorn fault was also determined from vertical offset of the Crowder Formation.
Detailed outcrop sketches were made and digital photos were taken to record fault geometries, offsets, and cross-cutting relationships of damage elements (faults, fractures, and veins). Densities and orientations of damage elements were determined in the fault zones by using a linear point intercept method combined with 70 cm vertical and horizontal linear transects that record everything under the line (e.g., Schulz and Evans,
2000, and references therein). An outcrop-specific, systematic (i.e., non-random) sampling plan was developed to representatively sample the host rock, damage zone, and fault core at the various fault zones studied. A total of 81 hand samples were collected for thin section and geochemical analysis. 27 2-1.3.2 Laboratory Methods
In order to understand the micro-scale processes occurring in the fault zones, a total of 68 thin sections were prepared from the host rock, damage zone, and fault core.
Optical microscopy analysis of the completed thin sections, including digital photomicrographs, was done at Utah State University to determine microstructures and infer deformation mechanisms in the faults.
X-ray diffraction (XRD) was done at Utah State University to determine whole- rock mineralogy of 80 samples. Eleven oriented clay samples were also analyzed with
XRD to further identify the mineralogy of fault cores. Whole-rock geochemistry was done at SGS Mineral Services in Ontario, Canada using an Inductively Coupled Plasma
(ICP) method to determine the major, minor, and trace elements of 39 samples. All major and minor elements and loss on ignition (LOI) had a detection limit of 0.01 %, while trace elements had a detection limit of 10 ppm.
2-2. Mesoscopic Analysis
2-2.1 Fault Inventory
A generalized geologic map of the field area, along with station locations and cross section lines, is shown in Figure 2-4. Station locations show specific fault zones studied, or some other significant area, such as lithologic contacts or host rock outcrops.
Note the general WNW-ESE trend of the Cedar Springs fault system. Net slip was constrained for a total of 31 faults, with slip ranging from 2 cm to 183 m (5 orders of magnitude). The mean slip for these faults is 14.7 m while the median is only 28 cm. 28 The low medians show the skewed nature of the data set toward the small displacement end. Eighteen of the faults studied have less than a meter of slip (Figure 2-7).
Net slip was calculated for faults that provided a discernable slip vector (i.e., rake on the fault plane) and offset marker (Figure 2-8). The orientations of the fault plane and offset marker were measured. Using a stereonet program, the “trace angle” was measured. This is the angle from horizontal to the intersection of the fault plane and offset marker plane, measured along the plane of the fault. Net slip was resolved with a
2-D calculation using the separation, slip vector, and trace angle. This was done for all constrained faults (Table 2-1 and Appendix A). The Eastwood and Lakeview faults have minimum slip constraints only based on offset of the Crowder Formation (Figure 2-6). It is important to note that the certainty of the slip calculations has an unquantifiable error.
However, a qualitative assessment of certainty was performed taking into account the quality of the slip vector and confidence of the measured offset. A rating of poor, fair, or good for the confidence of calculated slip was assigned to each constrained fault.
2-2.2 Fault Zone Descriptions
We present detailed outcrop descriptions along with the data collected from the transect work in order to constrain the geometries of the fault zone compartments (i.e., fault core and primary and secondary damage zones). This helps to further characterize fault zones and can be used in future modeling. The extent of the secondary damage zone was determined by comparing to “background” levels of fracture density. Data from scanlines were collected in areas where major faults are absent in order to capture this background density. A density of 8 ± 3 fractures per 70 cm scanlines (vertical and 29 horizontal) was determined as the background density in the study area. Observations, such as fault asymmetry and lithologic control on fault style, are described to characterize the various fault zones. Six main fault zones are discussed separately, while the rest of the faults are presented together as “Other Faults.”
2-2.2.1 Grass Valley Fault
The Grass Valley fault has an orientation of 105/45° S and is located in the northeast portion of the field area (Station 32 on Figure 2-4). This fault has 39 m of reverse right-lateral slip along a 55° slip vector from the east and a fault core thickness ranging from 2-12 cm (Table 2-1). There are multiple sub-parallel faults in the hanging wall and less damage, but more variability of fault orientation in the footwall (Figure 2-
9). The damage zone is characterized by many fractures with some subsidiary faults. No veins were observed at this fault zone (Figure 2-10a). There are approximately 50 damage elements per 70 cm scans near the main fault. This density drops off abruptly within a few meters (Figure 2-10b). Fracture orientations are bimodal, one sub-parallel to the main fault and the other at a high angle to the main fault. These orientations are especially clear on the 2, 4, and 8 m stereonet plots in the hanging wall (Figure 2-9).
Two types of lithologies exist within this fault zone: a weathered, grus-forming granodiorite host rock cut by competent granite pegmatite dikes, which serve as excellent offset markers. Faults through the more competent dikes are manifested as a zone of dense fractures and faults instead of the well-defined clay gouge fault core characteristic of the softer granodiorite (Figure 2-11). This shows the control lithology has on fault geometry. 30 2-2.2.2 Eastwood Fault
The Eastwood fault exposure is located near the northeast corner of Silverwood Lake and has an orientation of 316/55° NE (Figure 2-4). This is a pure dip-slip reverse fault with a minimum slip constraint of 183 m (Figure 2-6) and a fault core thickness of 2-5 cm
(Table 2-1). The Eastwood fault places crystalline basement on top of the Miocene
Crowder Formation (Figure 2-12). A foliated, maroon clay gouge layer was observed along the Crowder Formation fault wall and may record a slip preference to the footwall.
Data were collected in the hanging wall only because of the lack of observable damage in the Crowder Formation. Damage elements are mostly fractures, with some subsidiary faults and minor veins, indicating the presence of fluids (Figure 2-13). The hanging wall is characterized by an initial 5 m of fault breccia extending away from the fault core that has a light pink color and chemically altered appearance. This defines the primary damage zone. Outside of the primary damage zone is the secondary damage zone that lacks the pink color, brecciation, and chemically altered appearance. Rather, it is characterized by a decreasing density of damage elements away from the main fault.
Damage element densities reach background levels at approximately 12 m from the fault core. Orientations of damage elements are more variable in this fault zone than in the
Grass Valley fault zone.
2-2.2.3 Cleghorn Fault
The Cleghorn fault strikes E-W through the field area and dips steeply (~ 85°) to the north (Figure 2-4). Meisling and Weldon (1989) estimate 300 m of throw when this fault formed with the rest of the Cedar Springs fault system. Minimum constraints of the 31 dip-slip component using the Crowder Formation as a marker are > 150 m of throw and > 200 m of reverse slip, assuming pure dip-slip movement (Figure 2-6), which are consistent with Meisling and Weldon’s (1989) estimate. The Cleghorn fault was later reactivated as a left-lateral strike-slip fault accumulating 3.5-4.0 km of offset in the
Quaternary (Meisling and Weldon, 1989), thus making it the largest fault studied for this paper. This amount of left-lateral motion is based on offset of older folds and faults and offset of Quaternary terrace deposits and stream courses (Meisling and Weldon, 1982,
1989).
Like the Eastwood fault, the Cleghorn fault juxtaposes deeper granitic rocks on the Crowder Formation and exhibits an asymmetry of the fault core. A 2 cm thick foliated, maroon clay layer is localized next to the Crowder Formation, or footwall
(Figure 2-14). Total fault core thickness is 32 cm. The Cleghorn fault has many subsidiary faults in its damage zone (Figure 2-15a), unlike the other fault zones, which are dominated by Mode I fractures. No veins were observed in this fault zone. The first
9 meters of the hanging wall are very damaged and can be considered the primary damage zone (Figure 2-15b). Due to lack of exposure, the width of the secondary damage zone is uncertain, but must be less than 65 m in half-thickness because an outcrop 65 m into the hanging wall exhibits a fracture density near background levels.
2-2.2.4 Powell Canyon Fault
The Powell Canyon fault is a NW-SE striking right-lateral strike-slip fault with a steep SW dip (~ 80°) and approximately 1.2 km of apparent displacement in map view based on offset of a granite-gneiss contact. This fault is located in the southwest portion 32 of the field area (Figure 2-4) and likely formed along with the Cedar Springs fault system, but may also be related to SAF deformation due to its dextral slip sense and parallel strike to the SAF. The fault core was not exposed. However, there is very good exposure of the footwall damage zone, which may be a good analog for rocks at depth.
The damage zone has many fractures, some subsidiary faults, and no veins (Figure 2-
16a). The host rock for this fault zone is a competent gneiss that is different than rocks from most of the other fault zones, which are developed in grus-forming granodiorite and granite. The secondary damage zone extends approximately 75 m into the footwall before reaching background levels. However, exposure is missing for the first 25 m, obscuring the primary damage zone, if it exists (Figure 2-16b).
2-2.2.5 Lakeview Fault
The Lakeview fault is a reverse left-lateral fault that rakes 47° W and has an orientation of 067/42° S. This fault is located east of Silverwood Lake (Figure 2-4) and
has an estimated slip of -> 180 m (Figure 2-6). The Lakeview fault is an intensely damaged zone comprised of two main faults (Figure 2-17). The north fault zone is 1.3 m thick and stands out in relief as a hardened, nearly brecciated zone with numerous green- coated slickensides on the fault surface. This fault appears to be a subsidiary (i.e., less slip) to the fault to the south, although it may be a separate strand of the same fault. The south fault has a fault core 90 cm thick that is comprised of four distinct lithologic zones, or compartments (Figure 2-17). Steps and mineral fibers along the fault indicate oblique slip, with reverse and left-lateral strike-slip components. The primary damage zone is characterized by pink and green, highly fractured and faulted granite. Outcrop of the 33 hanging wall primary damage zone is covered by vegetation to the south. The thickness of the footwall primary damage zone to the north is approximately 5 m. The secondary damage zone extends for an undetermined distance.
2-2.2.6 Westwood Fault
The Westwood fault is a reverse left-lateral fault that strikes NW-SE through
Silverwood Lake and dips steeply (~ 80°) to the SW (Station 54 on Figure 2-4).
Although slip on the Westwood fault is not constrained, it is a useful fault zone because it provides many small subsidiary faults and good exposure of the footwall damage zone
(Figure 2-18). Based on current fault geometry, the SW side of the fault appears to be the hanging wall, but it is actually the footwall once tilt on the western San Bernardino arch is removed (Figure 2-6), and will be referred to as such throughout this paper. Similar to other fault zones, the Westwood fault can be divided into a fault core and primary and secondary damage zones. The fault core and primary damage zone are 50 cm thick and 6 m in half-thickness, respectively. The secondary damage zone is of undetermined thickness and forms a sharp contact with the primary damage zone, which is characterized by pink-brown-white breccia that is foliated nearest to the fault core (zone
B in Figure 2-18) and coarsens away from the main fault. This primary damage zone is absent in the gneissic hanging wall block, which creates an asymmetry within the fault zone similar to the fault core asymmetry observed at the Eastwood and Cleghorn faults.
However, the Westwood fault asymmetry differs in that it is not within the fault core, but within the primary damage zone. The primary damage zone in the footwall progressively 34 decreases for approximately 6 m away from the fault core, while the transition from fault core to hanging wall host rock (or secondary damage zone) is abrupt.
2-2.2.7 Other Faults
Basic geometric and kinematic data were collected from 29 other faults. The majority of the very small-displacement faults (< 1 m) have mm-cm thick fault cores.
Several other larger fault zones were examined in detail, but are not discussed in this paper due to lack of slip constraints. The damage zones of these other faults, as well as the six fault zones described above, provide many small faults that are used for the thickness-displacement correlation testing.
There are two other faults worth mentioning because of the different nature of the fault core compared to the rest of the faults studied. At the Silverwood Lake fault zone
(Station 37 on Figure 2-4), there is a 2 cm thick fault core standing out in relief as a
“hardened fin” with slickensides and the fault has 55 cm of slip. Similarly, the Pilot
Rock fault (Station 11 on Figure 2-4) has these same hardened fins along each side of the fault core, which varies from 15 to 37 cm in thickness. These anomalously resistant fault cores may represent healing due to increased fluids and are discussed in more detail later.
2-2.3 Fault Core Thickness-Displacement Relationship
Does fault thickness correlate with the amount of slip? This question is important for its application to fault zone growth models (Robertson, 1983, 1988; Hull, 1988, 1989;
Blenkinsop, 1989). However, it is not always clear how “fault thickness” is defined
(Evans, 1990), as there are separate compartments within a fault zone. Chester and
Logan (1986), Chester et al. (1993), and Evans and Chester (1995) have determined that 35 many, or a class of fault zones can be divided into two separate domains: the damage zone and the fault core (Figure 2-1). The damage zone is characterized by an increase in subsidiary faulting and fracturing as compared to the undeformed host rock. The fault core is defined as the zone that contains increased foliation, clay gouge, and/or fine- grained fault rocks such as ultracataclasite. It has been proposed by these workers that nearly all of the fault slip occurs within this relatively thin fault core, as small as several cm thick, which reflects high shear strain, extreme comminution, and enhanced fluid-rock interactions. Principal slip surfaces may be even as thin as millimeters (Chester et al.,
1993; Kanamori and Heaton, 2000; Heermance et al., 2003; Sibson, 2003).
We use the fault core, instead of damage zone, as the measure of fault thickness for correlation testing for several reasons.
1. The fault core is quickly and easily measured on most faults in the field and was
defined by the central gouge and/or microbreccia layer (zone 4 from Figure 2-1).
2. Faults are prevalent throughout the entire field area, creating much damage and
some ambiguity on individual fault damage zone thicknesses.
3. Hanging wall and footwall blocks are not always exposed. Measuring damage
zone thickness on just one side of the fault may not be representative of total
damage zone thickness, since there could be asymmetry in the damage zone
causing the fault core to not be in the center of the fault zone (Schulz, 1997;
Heermance et al., 2003).
4. Very small faults (i.e., cm-scale slip) have a definable fault core, but do not create
much of a damage zone. 36 5. Most importantly, the fault core is where the majority of strain is
accommodated and thus, may be a more appropriate measure of the fault for
thickness-displacement correlation testing.
The slip-constrained faults shown in Table 2-1 were used to plot fault core thickness with respect to slip. However, the faults with a certainty rating of “poor” for the slip constraint were disregarded for these plots, leaving a total of 26 faults with 2 cm to 183 m of slip. Fault core thicknesses range from 1 mm to 3.5 cm. Although there appears to be a general increase in fault core thickness with increased slip, there is no significant correlation (R2 = 0.1514) for all faults studied here (Figure 2-19). This indicates that faults do not necessarily thicken with increased slip. Rather, faults may localize into narrow zones of deformation.
2-3. Microstructure
This section presents the results of optical microscopy analysis (the complete descriptions are in Appendix B) in order to characterize the microstructure of rocks in the fault zones studied, which is important for several reasons. The documentation of the intensity of microstructural damage helps to further define the structure of the fault zone.
The study of the fabric of microfractures (i.e., open or filled) can be informative with respect to the timing of deformation and to the amount of fluid-rock interactions (Chester et al., 2004). Thirdly, the study of microfractures assists in understanding the processes of fault growth and wear (Wilson et al., 2003). Optical microscopy is also used for evaluating changes in mineralogy across a fault zone, which may be indicative of fluids in the system. 37 2-3.1 Grass Valley Fault (39 m of oblique slip)
Except for the main fault, there is relatively little microstructural damage in the
Grass Valley fault zone. Fractures are mostly open, but some are partially filled with iron oxide and chlorite. Minor alteration of feldspar grains is common. Thin- section analysis from the main fault reveals intense comminution and clay development. A primary slip surface with surrounding shear fabric is present in the fault core of the main fault (Figure
2-20). This intense microstructural damage is typically absent 1 m into the hanging wall
(Figure 2-21), showing the concentration of shear in the fault core.
2-3.2 Eastwood Fault (>- 183 m of reverse slip)
Compartmentalization of the fault zone into fault core, primary damage zone, and secondary damage zone is seen at the microscopic level in the Eastwood fault. The primary damage zone is characterized by many fractures (open and clay, microbreccia, and/or iron oxide filled), grain comminution, and feldspar alteration (Figure 2-22). This damage decreases towards the secondary damage zone, which only has minor grain cracking with few through-going fractures and little grain comminution and alteration
(Figure 2-23).
2-3.3 Cleghorn Fault (3.5 km of left-lateral offset)
Due to its unconsolidated nature, thin sections were not prepared from the
Cleghorn fault core. Microstructures at 30 and 55 cm into the hanging wall are characterized by shattered grains in a clay-rich microbreccia matrix and by many fractures, some with calcite and microbreccia fill (Figure 2-24, Figure 2-25a and b). The calcite veins indicate the presence of fluids at some time during the fault’s history. There 38 is twinned and untwinned calcite around the Cleghorn fault. The twinned calcite presumably formed before or during deformation while the untwinned calcite formed after deformation (Spang, 1974). This requires at least two different episodes of vein filling and indicates the presence of fluids at multiple times throughout the Cleghorn fault’s history. Although the microstructural damage decreases away from the Cleghorn fault, it persists for a greater distance than microstructural damage at the other fault zones studied. This can be attributed to the greater amount of slip on the Cleghorn fault and its resultant wider damage zone. At 9.5 m into the hanging wall from the fault core, the rock is characterized by open and microbreccia-filled fractures and feldspar alteration (Figure
2-25c and d), which persists at a low level up to the farthest sample at 16 m.
2-3.4 Powell Canyon Fault (1.2 km of right-lateral offset)
The fault core was not observed along the Powell Canyon fault and the damage zone is characterized by little microstructural damage. Feldspar alteration and minor grain cracking with few through-going fractures typifies the damage. Samples closest to the fault (within 10 m of the fault proper) in the footwall show slightly elevated damage, but not as much as would be expected from a fault of this magnitude. This may be because samples are from the footwall, which often has less damage than the hanging wall (Heermance et al., 2003).
2-3.5 Lakeview Fault (>- 180 m of oblique slip)
One sample from the hanging wall edge of the fault core in a breccia sub- compartment (Figure 2-17) exhibits high microstructural damage characterized by microbreccia and fractures (Figure 2-26). The fractures are both open and iron oxide 39 filled or lined. There is a very fine grained matrix surrounding the microbreccia that appears to have a high clay content.
2-3.6 Westwood Fault (slip unknown)
Although a formal transect was not performed at this station (because slip on the main fault is not constrained), several samples were collected in the damage zone at various distances from the fault core. Samples from the secondary damage zone are from subsidiary faults and document the microstructure associated with cm-scale slip. The primary damage zone of the Westwood fault is characterized by intense comminution, alteration, and fractures (Figure 2-27). The fractures are often through-going with some microbreccia lining and calcite vein fill. Subsidiary fault 57 (Figure 2-18) has a defined fault core with surrounding fractures and veins (Figure 2-28).
2-3.7 Other faults
The majority of the small-displacement faults cataloged for thickness- displacement correlation testing show relatively little damage outside of the fault core.
Where microstructural damage does exist in other fault zones, it is characterized by grain- size reduction, micro-fractures, and mineral alteration, as seen in the fault zones discussed above.
2-3.8 Microstructure Summary
Microstructural damage of faults ranging from cm to tens of m in slip is mostly confined to the fault core, as seen in the Grass Valley fault and numerous other faults.
Faults that have accumulated slip of approximately 100 m or more have primary damage 40 zones that are characterized by an elevated density of micro-fractures, grain-size reduction, and increased alteration (particularly of feldspar grains) as compared to the host rock. This microstructural damage typically extends for approximately 5-10 m from the edge of the fault core. Although lining of micro-fractures with microbreccia and iron oxide is common, the majority do not have vein fill. This suggests limited to moderate fluid-rock interactions in this system. However, this relatively shallow setting (1-2 km depth) is not conducive to antitaxial vein growth, which requires high fluid pressures.
This may help to explain the scarcity of veins, even with moderate quantities of fluids in the system.
2-4. Mineralogy and Geochemistry
The geochemistry of several fault zones was analyzed in order to gain insight into alteration processes and fluid-rock interactions associated with deformation. X-ray diffraction (XRD) was performed on 80 samples to identify the mineralogy (Appendix
C). This allows for analysis of mineralogic alteration, which helps to characterize the fault zones. Clay mineralogy is particularly useful in understanding the properties and mechanical effects of the fault core (e.g., Anderson et al., 1980, 1983; Evans, 1988;
Vrolijk and van der Pluijm, 1999; Choo and Chang, 2000; Solum et al., 2003). Whole- rock geochemistry can be used to evaluate elemental changes across a fault zone
(Anderson et al., 1983; Evans and Chester, 1995; Goddard and Evans, 1995; Schulz,
1997). Whole-rock geochemical analyses were performed across the Grass Valley,
Eastwood, Cleghorn, and Powell Canyon fault zones (Appendix D). Previous work has 41 shown the depletion of many mobile elements around the fault core, indicating
syntectonic fluid flow (Goddard and Evans, 1995).
2-4.1 X-Ray Diffraction
Results of XRD typically show granodiorite mineralogies for the host rocks that
are consistent with thin section analysis (Table 2-2 and Appendix C). Quartz is present in
nearly every sample. Albite (the sodium-feldspar end member of the plagioclase series)
is the primary plagioclase in most of the samples. Anorthite and andesine are also
present in some samples. Potassium feldspars are typically orthoclase and microcline.
Micas are present in many samples and include biotite, phlogopite, and muscovite.
Hornblende is the dominant mafic mineral in the host rocks.
Altered rocks from the primary damage zone of each of the six main fault zones,
except for the Powell Canyon fault, commonly contain epidote and/or laumontite in
addition to host rock minerals. The core of most faults is clay rich, typically consisting of
smectite, an expanding clay. This was the case for each of the six main fault zones analyzed, except for the Lakeview fault, which contained vermiculite. The Powell
Canyon fault core was not analyzed due to lack of exposure. The presence of smectite was determined based on the peak shift after the sample was glycolated (Figure 2-29).
Palygorskite, a fibrous clay mineral, is also present in the Grass Valley, Eastwood, and
Cleghorn fault zones. Although uncommon, palygorskite has been documented in other
fault gouges (Soong and Perrin, 1983). Calcite is present around the Cleghorn fault core.
The “hardened fin” fault cores of the Silverwood Lake fault 3 (Station 37) and the Pilot 42 Rock fault (Station 11) are composed of quartz, consistent with thin section identification.
In general, alteration products in the fault cores studied are smectite clays
(nontronite, montmorillonite, volkonskoite, and saponite), zeolites (laumontite and gismondine), and palygorskite. Chemical reactions leading to the breakdown and alteration to these minerals (using mineralogy from the Eastwood fault zone) include:
4 CaAl2Si2O8 + 9 H4SiO4 ↔ 2 Ca2Al4Si4O16 · 9 H2O + 9 SiO2 (2-1) (anorthite) (dissolved silica) (gismondine) (amorphous)
+ + 2 NaAlSi3O8 + 9 H2O + 2 H ↔ Al2Si2O5(OH)4 + 2 Na + 4 H4SiO4 (2-2) (albite) (kaolinite) (dissolved silica)
2+ + + 2 Al2Si2O5(OH)4 + 2 Fe + 0.3 Na + 6 H ↔ Fe2Al4Si4O10(OH)2Na.3 · 6 H2O (2-3) (kaolinite) (nontronite)
2+ Fe2Al4Si4O10(OH)2Na.3 · 6 H2O + 4 SiO2 + 2 Mg (2-4) (nontronite) (amorphous) 2+ + + ↔ 2 (Mg,Al)2Si4O10(OH) · 4 H2O + 2 Fe + 0.3 Na + 4 H (palygorskite)
Although kaolinite is present only in the Cleghorn fault core, we propose this intermediate reaction for the other fault zones in order to form nontronite, which subsequently alters to palygorskite. Gismondine is a direct alteration product of anorthite.
Although some alteration products, such as laumontite, are present in many of the primary damage zones, clay minerals are almost entirely confined to the fault cores. This observation suggests that clay growth is directly related to faulting. Earthquake rupture may help overcome the kinetic barrier in the reactions shown above, as suggested for the
Lewis thrust in Canada (Vrolijk and van der Pluijm, 1999). Consequently, the newly 43 formed clays would reduce the coefficient of friction (µ) along the fault, thus reducing the tectonic load required for failure (Scholz, 2002). The fact that smectite is the dominant clay mineral further argues for fault weakening because of its very low µ value.
Montmorillonite, a type of smectite, is the weakest clay, with a µ of about 0.2 (Morrow et al., 1992). However, it is important to note that the fault cores are a mixture of clay and comminuted host rock minerals, so the overall µ would be higher than 0.2.
2-4.2 Whole-Rock Geochemistry
Whole-rock geochemical analysis was performed on 39 samples across four different fault zones. The first step in analyzing the whole-rock data was to perform principal component analysis (PCA), an eigenvector method used to reduce data, on all oxides and loss on ignition (LOI) in order to identify the significant variables that have the most weight in defining the geochemistry of the sample (Appendix E). More than
80% of the variation for each of the four fault zones can be represented on two axes of a
PCA plot and illustrate the effectiveness of this method. The length of the eigenvector indicates the importance of the variable (e.g., oxide) to the sample. Once the PCA was performed, plots of the oxides and LOI with respect to distance from the main fault were constructed, keeping the significant variables in mind. This shows the geochemical changes across the fault zone. Figures 2-30 through 2-33 show the results of PCA along with the plots of selected oxides and LOI across each of the four fault zones. Finally, spider diagrams were made of oxides, LOI, and trace elements. These help to visualize the geochemical changes in the fault zones and are discussed in the following sections.
44 2-4.2.1 Grass Valley Fault
The PCA results indicate that Na2O, Al2O3, and LOI are the most significant variables in defining the Grass Valley fault zone whole-rock geochemistry (Figure 2-
30a). Comparison of the weight percent of oxides in the fault core and at various positions in the damage zone reveals that both Na2O and Al2O3 are reduced in the fault
core relative to the hanging wall and footwall, which is typical of many of the other
oxides as well (Figure 2-34a, Table 2-3). The reduction of Na2O, CaO, and K2O is
consistent with the alteration of feldspars to clays as shown by XRD analysis. However,
MgO, MnO, and Fe2O3 show an increase in the fault core relative to the hanging wall, or
protolith. This is also consistent with the mineralogic changes in the fault core.
Nontronite and palygorskite contain Fe and Mg, respectively, which would increase the
relative abundance of these elements in the fault core. The three hanging wall plots track
each other very well and indicate little difference in geochemistry at various distances in
the hanging wall. There are also differences in geochemistry between the hanging wall
and footwall. The footwall is enriched relative to the hanging wall for all oxides except
SiO2 and K2O.
Analysis of the trace elements gives additional insight into the geochemical changes across the fault zone and implications for fluid-rock interactions (Figure 2-34b,
Table 2-4). In general, the fault core is depleted in trace elements compared to the hanging wall and footwall. However, Zr is enriched in the fault core relative to the hanging wall. This indicates that a volume loss has occurred. The fault core also has a much higher LOI value than the hanging wall and footwall. This shows that the fault core has excess fluids in it, possibly trapped in the gouge or in the clay mineral structure 45 itself. Similar to the oxide results, the foootwall is enriched relative to the hanging
wall in all trace elements except Sc, which does not show any change.
2-4.2.2 Eastwood Fault
Results of PCA of the Eastwood fault indicate the most significant variables to be
Na2O, Al2O3, TiO2, and LOI (Figure 2-31a). All three of the most significant oxides are depleted in the fault core relative to the damage zone (Figure 2-35a, Table 2-5). The secondary damage zone has the highest percent of all variables except for SiO2 and K2O.
Thus, the primary damage zone and fault core are depleted in oxides relative to the host rock. This is consistent with field and optical observations that show an increase in geochemical alteration nearest to the fault. The reduction of Na2O and CaO in the fault core is also consistent with the alteration of feldspars to clay. The secondary damage zone is characterized by mesoscopic damage without alteration, and should geochemically represent the host rock. The primary damage zone is actually more depleted than the fault core in MgO, MnO, and Fe2O3 relative to the secondary damage
zone, or host rock. This shows the transition between the host rock and fault core. The
host rock contains pargasite (a hornblende), which has Fe and Mg, and the fault core
contains nontronite and palygorskite, which has Fe and Mg, respectively. Therefore, the
primary damage zone is the most depleted in the relative abundance of these elements
because it does not contain Fe and Mg bearing minerals.
Where changes do occur, there is typically a decrease in trace elements in the
fault core relative to the secondary damage zone (Figure 2-35b, Table 2-6). There is a
slight increase in Zr in the fault core relative to the secondary damage zone. This 46 suggests that some volume loss has occurred. The fault core has an increase in LOI
relative to the primary damage zone. The primary damage zone has mixed trends of trace
element changes when compared to the secondary damage zone.
2-4.2.3 Cleghorn Fault
Results of PCA for the Cleghorn fault indicate that the most significant variables
are Al2O3, TiO2, and CaO (Figure 2-32a). There is little change in the concentration of
Al2O3 across the fault zone. The concentration of CaO is enhanced in the fault core relative to the secondary damage zone (Figure 2-36a, Table 2-7). This is probably due to the presence of calcite in and near the fault core, as observed in thin section and XRD analysis. The concentration of TiO2 is increased in the fault core relative to both the primary and secondary damage zones. The decrease in Na2O and K2O in the fault core
relative to the secondary damage zone is consistent with the dissolution and alteration of
feldspars. The marked increase in the concentration of MgO in the fault core can be
attributed to the formation of palygorskite, a Mg bearing mineral. There is not as much
difference between the primary and secondary damage zones as there is at the Eastwood
fault zone.
The majority of trace elements are reduced in the fault core relative to the
secondary damage zone, or protolith (Figure 2-36b, Table 2-8). Where changes do occur,
the primary damage zone is reduced relative to the secondary damage zone. The
concentration of LOI has a consistent increase when approaching the fault core indicating
more CO2 and/or H2O trapped in the gouge or in the clay mineral structure.
47 2-4.2.4 Powell Canyon Fault
Results of PCA for the Powell Canyon fault show P2O5 and Na2O as the most
significant variables (Figure 2-33a). There is not as much variation of the oxides and
LOI in this fault zone as the others. However, a sample from the fault core was not
collected due to lack of exposure and may show a different geochemistry. There does not
appear to be a clear trend of oxide or trace element abundance with respect to distance
from the main fault (Figure 2-37). This suggests that samples were collected outside of
the primary damage zone where there has been little geochemical change.
2-5. Earthquake Energy Budget
Lachenbruch and McGarr (1990) developed a model of the energy balance for
earthquakes. Equation 2 from their manuscript shows that the total earthquake energy (E)
must equal the radiated seismic energy (Ea), plus the energy lost to heat dissipation (Er),
plus unknown sinks of energy (?):
E = Ea + Er + ? (2-5)
Lachenbruch and McGarr (1990) put aside the (?) term because they felt it was unconstrained. More recent energy budgets have been proposed that include frictional energy loss and fracture energy associated with fault propagation (e.g., Kanamori and
Heaton, 2000), but it is still clear that there are unknown or unconstrained sinks of energy. This is due to the discrepancy of expected fault stress between laboratory measurements and dynamic measurements of seismic energy and frictional heat production (Lay and Wallace, 1995, pp. 394-395). The average shear stress on a fault should exceed 50 MPa, but this value is five times that allowed by heat-flow 48 measurements (Lay and Wallace, 1995, pp. 394-395). Several mechanisms have been proposed to reconcile this discrepancy. Lachenbruch and McGarr (1990) state that the creation of new fractures would account for some energy loss. A significant amount of energy may also be lost converting Mode I fractures to subsidiary faults, as observed in the damage zone of the Cleghorn fault. The creation of new surface area of small grains due to comminution is proposed as a sink of earthquake energy (Reches, pers. comm.,
2004). Chemical alteration may be another sink of energy along the fault surface (Evans and Chester, 1995; Lay and Wallace, 1995, pp. 394-395; Schulz and Evans, 1998).
We use the Eastwood fault as a “model” fault to determine how much energy may be consumed by chemical alteration over the history of the fault. The Eastwood fault core consists of gismondine (a zeolite mineral), nontronite (a smectite mineral), and palygorskite (a fibrous clay mineral), and is representative of the other fault cores analyzed. The reactions (equations 2-1 through 2-4) for mineral alteration in the
Eastwood fault are used to calculate the energy required to form these clay and zeolite minerals. It is important to note that the energy calculated here is for all clay and zeolite formation, some of which may have formed after the active life of the Eastwood fault.
However, most clay formation is syntectonic because: (1) Clay is exclusively found in the fault core, (2) The clay gouge exhibits a sheared texture in both outcrop and thin section, and (3) Clay occurs in very small faults (< 1 m of slip), which indicates early clay formation by using slip as a proxy for fault maturity. We propose that the external energy source for these alteration reactions is the earthquakes that created the faults. At the instant of slip, large amounts of energy are released, with typically less than a third of the near-fault energy (McGarr and Fletcher, 2002) consumed as far-field radiated energy. 49 The non-radiated energy is “deposited in a relatively small fault zone over a time scale
of less than a minute” (Kanamori and Heaton, 2000). This energy would therefore be
available in the hypocentral region as heat that would dissipate and be consumed by
alteration reactions. These reactions may occur largely during Sibson’s (1986, 1989)
low-strain rate phases of the seismic cycle (Chester et al., 1993) (Figure 2-38) or shortly
after seismic events. Thus, even though clay growth probably does not occur co-
seismically, it still may affect the earthquake energy budget by consuming energy
between seismic events.
2-5.1 Energy Calculations for the Eastwood Fault
Although it is well documented that mineral alterations do occur in fault zones,
particularly in the fault core (Vrolijk and van der Pluijm, 1999; Choo and Chang, 2000;
Solum et al., 2003), estimates of how much energy may go into the alteration process
were not found in the literature. The energy required for these reactions can be calculated
using the experimentally determined Gibb’s Standard free energies (ΔG°f) for each
mineral and element (Woods and Garrels, 1987, and references therein) in the reaction.
The net change in free energy can be calculated by:
ΔG°f = ∑ ΔG°f(products) - ∑ ΔG°f(reactants) (2-6)
If this value is positive, then the reaction requires energy to be added from an external source. If it is negative, then the reaction would release energy. In addition to ΔG°f calculations, we consider the activation energy (Ea) required for these reactions to occur
(Figure 2-39). Even if ΔG°f is negative, energy must be added to the system to get over
the Ea (Table 2-9). 50 In order to determine the amount of energy consumed by these alterations, we
need to know the number of moles of each mineral in the fault core. The molar volume
was calculated by dividing the mineral density by the molecular weight (Table 2-9).
Fault core volume was calculated using known values of trace length (7 km) and
thickness (3.5 cm). The total depth of the fault was assumed to be 5 km. The distance
along the fault at a restored dip of 34° (removing tilt from the western San Bernardino
arch) is 8.9 km. This yields a total fault core volume of 2.2 x 1012 cm3. The exact quantities of clay and zeolite minerals in the fault core are not known. Estimating 10% gismondine, kaolinite, and nontronite, and 7% palygorskite, the total energy associated with alteration is 4.6 x 1011 kJ (Table 2-9). A total energy release of 2 x 1012 kJ (Figure
2-40) is estimated for the history of the Eastwood fault based on the Gutenburg-Richter law (Main et al., 1999) and the fact that there is a linear relationship between magnitude and source dimension (Abercrombie, 1995). A maximum magnitude of 5.0 was assumed for the 7 km long Eastwood fault. Our calculations suggest that syntectonic alteration in the fault core may consume approximately 20% of the total earthquake energy.
This is a very crude calculation with much uncertainty associated with some variables. The fault core volume is an approximation based on an assumed depth. The amount of each alteration mineral in the fault core is also estimated based on thin section analysis and bulk XRD. It is important to note that the final energy calculation is very sensitive to the amount of each alteration mineral in the fault core. Future work of this nature should tightly constrain the abundance of clay minerals and the volume of affected rock. The ΔG°f used are also for 25° C and 1 atm. Reactions occurring at depth, with 51 elevated pressure and temperature may require less energy than what is calculated here
(Kolesar, pers. comm., 2004).
2-6. Discussion
We show that, in the Silverwood Lake study area of the northwest San Bernardino
Mountains, modest slip reverse faults exhibit a primary and secondary damage zone
(Figure 2-41), based on field observations, microstructural evidence, and geochemistry.
The primary damage zone can be defined as the area immediately surrounding the fault core that is characterized by microstructural damage, grain-size reduction, and chemical alteration. This zone is typically several meters in half-thickness and is recognized with optical microscopy and XRD analysis, as well as outcrop observation. The primary damage zone exhibits greater chemical alteration based on XRD analysis, and thus is interpreted as having had greater fluid-rock interactions than the secondary damage zone.
The increased number of microfractures would increase the permeability, allowing more fluids into this fault zone compartment. The secondary damage zone lies in between the primary damage zone and the host rock and has a similar microstructure and mineralogy as the host rock, but has an increased density of mesoscopic fractures and subsidiary faults. This is a thicker, more gradational zone that is on the order of tens of meters in half-thickness. We find that the boundary between primary and secondary damage zones is typically sharp for the faults studied here. The primary damage zone may roughly equate to the “foliated zone” that surrounds the “central ultracataclasite layer” in the
Chester et al. (1993) model (Figure 2-1). 52 Caine et al. (1996) developed a conceptual model for four types of permeability
structures in fault zones: (1) localized conduit, (2) distributed conduit, (3) localized
barrier, and (4) combined conduit-barrier. We find that the six fault zones studied in
detail for this work fit the combined conduit-barrier model best, where both the fault core
and damage zone are well developed. This architectural style is composed of a localized
cataclastic zone (fault core) and a distributed zone of subsidiary structures (damage
zone). The fault core, behaving as an aquitard, is sandwiched between two aquifers, or
damage zones, thus making the maximum permeability parallel to the plane of the fault
zone.
There are some general trends in damage element orientation in many of the fault
zones studied. Often times, as in the Cleghorn, Grass Valley, and Powell Canyon fault
zones, damage elements have an orientation at either a low or high angle to the main
fault. Townend and Zoback (2001) determined that the current maximum horizontal
stress (SHmax) in the field area is NNE-SSW. Although the stress regime in the late
Miocene is not definitively known, the motion and orientation of the Cedar Springs fault system is consistent with the current stress state (south-block-down reverse faults striking
NW-SE to E-W). This stress regime accounts for the formation of subsidiary faults that are sub-parallel to the main fault. Subsidiary faults at a high angle to a main fault are sub-parallel to the SHmax, which is the preferred orientation for Mode I fracture
development. These high angle subsidiary faults may initiate as Mode I fractures and be
converted to faults over time, even though they do not have the optimum orientation for
failure. Evidence of this Mode I fracture to fault conversion is seen when faults with
varying degrees of slip are compared. For example, the Cleghorn fault has much more 53 slip than any of the other faults examined here and the damage zone is characterized by a greater number of subsidiary faults. Damage zones of the smaller-displacement faults are dominated by Mode I fractures. This observation implies that fractures develop first in a damage zone and are converted to subsidiary faults over time, consistent with observations from Kim et al. (2004).
Faults juxtaposing crystalline basement rocks over the Crowder Formation exhibit much damage in the uplifted crystalline fault block and little damage in the Crowder fault block. This can be attributed to several potential controls. Firstly, the Crowder
Formation is a sedimentary unit that is much softer than the crystalline rocks. The soft nature may minimize the brittle deformation that can occur and localize the fault.
Additionally, this lithologic contrast between fault blocks may create a “wrinklelike pulse” that causes asymmetric motion on different sides of the fault, thus causing asymmetric damage (Andrews and Ben-Zion, 1997; Ben-Zion and Andrews, 1998; Ben-
Zion and Huang, 2002). Secondly, the faults have not been in contact with the Crowder
Formation for as long as they have been in contact with the crystalline rocks. The faults likely initiated in the crystalline rocks and did not cut the Crowder Formation until much later. This is an important point because it implies that the crystalline rocks record all cycles of deformation associated with faulting. Thirdly, the Crowder Formation is in the footwall whereas the crystalline rocks are in the hanging wall. Footwall fault blocks of reverse faults often have less damage associated with faulting than hanging wall fault blocks (Heermance et al., 2003). This study provides additional evidence for the concentration of damage in the hanging wall of reverse faults. 54 Nearly all of the faults examined in this work have a well defined fault core consisting of foliated clay gouge and/or microbreccia. Fault cores range from mm to tens of cm in thickness for the faults studied, with fault size ranging from 2 cm of slip to 3.5 km of offset. Based on microscopic and geochemical analysis, the fault cores are characterized by a mixture of comminuted protolith and alteration products. Results of
XRD analyses indicate that the majority of fault cores contain smectite, laumontite, and palygorskite. Most of these alteration minerals formed during fault development because: (1) Clay is exclusively found in the fault core, (2) The clay gouge exhibits a sheared texture in both outcrop and thin section, and (3) Clay occurs in very small faults
(< 1 m of slip), which indicates early clay formation by using slip as a proxy for fault maturity. The presence of expanding clays in the fault cores explains the typical increase in LOI relative to the damage zone, due to the trapped fluids in the impermeable clay-rich gouge. Fluids may be allowed into this relatively impermeable fault core during rupture events, when there is some opening along the fault plane. Frictional heating during rupture may produce thermal pressurization of these trapped fluids causing fault weakening as proposed by several workers (Byerlee, 1990; Rice, 1992; Chester et al.,
1993; Sibson, 2003). This enrichment of the fault core with smectite is consistent with previous work on the Punchbowl fault (Chester and Logan, 1986; Solum et al., 2003).
The low coefficient of friction of smectite, particularly montmorillonite, may help to further weaken the fault. The presence of smectite without illite also helps to constrain the depth of clay formation. Smectite will convert to illite at depths of 2 to 3.7 km
(Hower et al., 1976). The lack of illite constrains clay formation to less than 2 km depth, which is consistent with an exhumation of 1-2 km (e.g., Spotila and Sieh, 2000). The 55 association of laumontite with active and ancient fault zones is also well documented
(Evans and Chester, 1995; Solum et al., 2003, and references therein) and is observed in
many of the faults analyzed here. Laumontite and other zeolite minerals are interpreted
as being alteration products from calcic plagioclase, such as anorthite. We interpret the
palygorskite to be an alteration product from smectite (equation 2-4).
As proposed by previous workers (Chester et al., 1993; Evans and Chester, 1995;
Heermance et al., 2003), the data presented here show that the fault core appears to
accommodate the majority of the slip within the fault zone, as documented by transect
work and microstructural analysis. The thickness of the co-seismic slip zone has
significant effects on the physics of earthquakes (Kanamori and Heaton, 2000; Sibson,
2003). For example, reduction of shear resistance via friction-melting and fluid
pressurization requires slip to be restricted to a few centimeters or tens of centimeters,
respectively (Sibson, 2003). Due to the lack of pseudotachylyte in the faults of this
study, we propose that fluid pressurization is the dominant process in reducing shear
resistance. Fluids are most likely allowed into the fault core during some ruptures, when
there is opening along the fault plane. The fluids are then trapped there due to the
impermeable nature of the clay-rich gouge and become pressurized during future slip
events, thus reducing shear resistance. The thickness of the co-seismic slip zone also
affects the characteristic displacement, Dc, (Biegel and Sammis, in press, 2005) which is defined as the displacement over which the frictional strength of a fault drops from a static value to a lower dynamic value (Dieterich, 1978a, b; Dieterich, 1979a, b), thus causing earthquake nucleation as a stick-slip friction instability (Dieterich, 1972, 1974;
Rice, 1980). The thicker the co-seismic slip zone (i.e., involves much of the fault core), 56 the larger Dc will be (Biegel and Sammis, in press, 2005). Although there appears to be a general increase in fault core thickness with slip, there is not a significant correlation when analyzing all faults in this study (Figure 2-19). This localization of deformation into narrow zones reflects strain-softening processes (Yonkee and Mitra, 1993).
Without the presence of pseudotachylyte, we cannot absolutely differentiate between seismic and aseismic faults in the field (Cowan, 1999). Some workers, however, believe that slickensides owe their origin to seismic slip (Power and Tullis, 1989; Doblas et al., 1997) and that the majority of brittle deformation, particularly localized brecciation and transgranular cracking, occurs primarily during seismic slip (Chester et al., 1993), or the high strain-rate γ phase of the seismic cycle (e.g., Sibson, 1986, 1989) (Figure 2-38).
The faults studied in this work are interpreted as having been produced by seismic deformation, based on the presence of slickensides and extensive brittle deformation around the fault. Fracture sealing and mineralogic alteration may largely occur during
Sibson’s (1986, 1989) low strain-rate phases (α, β, and δ) of the seismic cycle (Chester et al., 1993). These slower processes contribute to the healing of the fault zone, particularly the fault core, between the high strain rate γ events. The result is a fault core that stands out in relief as a “hardened fin,” such as fault 3 of the Silverwood Lake fault zone and the
Pilot Rock fault. Results of XRD and petrographic analysis indicate that the mineralogy of these fins is entirely quartz. Considering that all faults in the Cedar Springs fault system are inactive except for the Cleghorn fault, it is unclear why these two faults are completely healed while most faults are composed of non-cohesive gouge and microbreccia. 57 The soft, non-cohesive fault cores from the Silverwood Lake area are different than what has been observed from previous work on the North Branch San Gabriel and
Punchbowl faults (Chester et al., 1993), and Laramide-style reverse faults in the Wind
River Range, Wyoming (Mitra, 1993; Yonkee and Mitra, 1993). These strike-slip and reverse faults are characterized by a relatively narrow and localized cataclasite and ultracataclasite fault core (Chester and Logan, 1986; Chester et al., 1993; Mitra, 1993;
Yonkee and Mitra, 1993). The ultracataclasites are different than the gouge- and breccia- dominated fault cores of this study in that they have primary cohesion (e.g., Sibson
1977). The non-cohesive fault cores observed in this study can be attributed to the shallow depth of fault formation (1-2 km), whereas the North Branch San Gabriel and
Punchbowl faults were formed between 2-5 km and 2-4 km, respectively (Chester and
Logan, 1986; Evans and Chester, 1995) and the Laramide reverse faults were formed at approximately 5 km depth (Mitra, 1993; Yonkee and Mitra, 1993). Chester et al. (2004) explain that the progression from the non-cohesive breccia-gouge series to the cohesive cataclasites and mylonites corresponds to increasing depth of formation, causing different deformation and recovery mechanisms (e.g., Christie, 1960; Reed, 1964). Thus, the non- cohesive breccia-gouge fault cores observed in this study are likely a function of shallow depth of formation.
An asymmetry in the fault core was observed at some faults studied in this work.
Particularly, the Cleghorn and Eastwood faults show that the principle slip surface is offset to the footwall side of the fault core. This slip preference is manifested as a strongly foliated, maroon clay gouge layer up against the fault wall that is typically more sheared than the rest of the fault core. Since both of these faults juxtapose crystalline 58 basement against the Crowder Formation, it is interpreted that the asymmetry is caused
by contrasting lithologies, with a slip preference to the softer sedimentary rocks.
Alternatively, Ben-Zion and Andrews (1998) believe that “… rupture may tend to
migrate from within the fault zone to an interface between the fault zone and the
surrounding rock,” causing a change in rupture character from a cracklike mode to a
wrinkelike pulse. This compartmentalization within the fault core further increases the
localization of slip, confining it predominantly against the footwall fault block of these
two faults.
Both the Grass Valley and Eastwood fault zones have a decrease in the most
significant oxides (based on PCA) in the fault core relative to the damage zone. Where a
cross-fault suite of data was examined in the Grass Valley fault, the footwall showed an
increase in all oxides except Si and K, and all trace elements present. This suggests
movement of these mobile elements into the footwall, either from the host rock, or
possibly from the fault core, since it is depleted in these elements. The reduction of many
elements, such as Na, Al, K, and Ca, in the fault core of both fault zones indicates
enhanced fluid flow through the cores that carried away these mobile elements. The
increased LOI concentrations in the fault core indicate greater amounts of trapped CO2
and/or H2O, and thus document fluids in the faults. Thus, even though fluids may be dissolving out some mobile elements, the fault core retains some of the excess fluids.
Since these are relatively small faults, using slip as a proxy for time, these results indicate that fluid-rock interactions are important early in the fault development process. The abundance of Mg, Mn, and Fe is increased in the fault core relative to the primary damage zone and hanging wall of the Eastwood and Grass Valley faults, respectively. 59 This concentration could possibly be attributed to the early development of alteration products rich in these oxides, such as nontronite and palygorskite.
The Cleghorn fault zone exhibits different geochemical trends than the Grass
Valley and Eastwood faults, perhaps because it has a different history. This is not surprising considering that the Cleghorn fault has been reactivated since initial formation in the Miocene Cedar Springs fault system (Meisling and Weldon, 1989). Many of the oxides show an increase in the fault core relative to the damage zone. In fact, the fault core shows an increase in all oxides relative to either the primary or secondary damage zone except for Na, Si, and K. This trend may be caused by the trapping of mobile elements in the fault core. In this model, the primary damage zone behaves as a zone of enhanced fluid flow, which increases the occurrence of chemical reactions and removes mobile elements, depositing them in the clay-rich fault core.
2-7. Conclusions
We provide a structural and geochemical characterization of six exhumed fault zones with varying magnitudes of slip (39 m to 3.5 km) along with basic geometric and kinematic results from 29 small-displacement (i.e., cm to m scale) faults in the
Silverwood Lake area. Detailed analysis of the six fault zones provides a model of fault zone geometry and composition as a function of slip. By examining faults with different amounts of slip in the same rocks and in the same structural setting, we can use the amount of displacement as a proxy for maturity of the faults. The majority of the main faults studied can be divided into the fault core, primary damage zone, and secondary damage zone. The fault core is defined by the presence of clay gouge and microbreccia 60 and averages 36 cm in thickness for the six faults studied in detail. Fault cores of the
29 small-displacement (cm to m) faults have an average thickness of 3.6 cm. Results of
XRD and thin section analysis indicate that the fault cores studied are a mixture of comminuted protolith and alteration products such as smectite, palygorskite, and laumontite. The primary damage zone is characterized by intense microstructural damage, grain-size reduction, and chemical alteration and has a typical half-thickness
(i.e., the thickness on one side of the fault) of several meters. The secondary damage zone is the transitional zone between the primary damage zone and the host rock. It is characterized by an elevated density of mesoscopic damage compared to the host rock and has a half-thickness on the order of tens of meters for the faults studied here. Despite the modest amounts of slip on these faults, we observed well-developed wall damage zones. This indicates that a considerable amount of the damage is the result of fracture propagation, or crack tip interactions, rather than damage associated with bends or other geometric complexities because these faults are relatively immature and have not underwent major linking. Further damage may accumulate in the wall rock as slip continues on the main fault (Kim et al., 2004).
Using slip as a proxy for time, these small-displacement faults represent relatively young faults. Thus, field observations and geochemical analyses show that fault core development and fluid-rock interactions occur early in a fault’s history. Whole-rock geochemical analysis typically shows a reduction of elements in the fault core and primary damage zone relative to the host rock, indicating enhanced fluid-rock interactions in these zones. Thickness-displacement correlation testing was performed on a total of 26 faults from the study area and shows little to no correlation between fault 61 core thickness and displacement of all faults. Both the primary and secondary damage zones appear to thicken with increased slip on the main fault.
We use the Eastwood fault as a “model” fault to determine how much energy may be consumed by chemical alteration over the history of the fault. The reactions for mineral alteration in the Eastwood fault are used to calculate the energy required to form the clay and zeolite minerals present. We propose that the external energy source for these alteration reactions is the earthquakes that created the faults. Our calculations suggest that syntectonic alteration in the fault core could consume a considerable amount of the total earthquake energy. The abundance of alteration minerals and the affected volume of rock are very important to the calculation and should be tightly constrained in future work.
The results of this work indicate that the narrow cores form early in the development of a fault. Subsequent slip appears to be focused along these narrow zones, with some deformation accumulating in the damage zone. The lack of pseudotachylyte and the increased LOI values (indicating trapped fluids) in the fault core implies that thermal pressurization helps to reduce shear resistance during fault rupture. We suggest that fault processes are similar throughout the different stages of development, and that large amounts of slip can occur on very narrow slip surfaces. Therefore, the study of relatively small-displacement faults can be used to understand fault evolution through time and the processes of larger faults in the brittle crust.
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69 Schulz, S. E. (1997), Geochemical, petrologic, and structural characterization at multiple scales of deformation associated with the Punchbowl fault, southern California, M.S. thesis, 150 pp., Utah State University, Logan.
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71 Table 2-1. List of all faults where net slip is constrained.
Fault Zone UTM Coordinates1 Fault Fault Type2 Orientation; Rake3 Slip; Certainty4 Fault Core Width Water Tank (St. 15) 3794083 N, 471272 E FW 5 normal,llss 278/65 N; 30 R W 10.5 m; poor 10 cm HW 5 llss 108/90; 08 R W 28.5 cm; fair 1 mm
Eastwood (St. 17) 3795358 N, 472471 E main reverse 316/55 NE; 90 R > 183 m; fair 2-5 cm 6 reverse, rlss 266/88 N; 45 R N 1.5 m; good 1-3 cm
Grass Valley (St. 32) 3796779 N, 475200 E main fault 1 reverse, rlss 105/45 S; 55 R SE 39 m; good 2-12 cm HW 2 reverse, rlss 105/46 S; 20 R E 1.9-2.7 m; good 1-2 cm 3 reverse, rlss 090/46 S; 30 R E 1.5 m; good 1-2 cm 4 reverse, rlss 105/53 S; 30 R E 2.3-3.3 m; good < 1 cm 6 reverse, rlss 095/52 S; 30 R E 36.8 m; good 8 cm 11 reverse, rlss 117/79 S; 35 R E 58 cm; good < 1 cm FW 4 reverse, rlss 091/55 S; 50 R E 5.7 cm; fair 1-2 cm 6 normal, rlss 107/61 S; 50 R W 4 m; fair 3 cm 16 normal, rlss 160/69 W; 40 R N 13 cm; fair 2 cm
Clipper Reverse (St.9) 3792380 N, 463015 E 9 reverse 282/65 N; 90 R 3 m; good 2 mm-3 cm
Seeley Jr (St. 40) 3791904 N, 472147 E main reverse, rlss 022/78 E; 40 R N 40 cm; good 1 cm
Silverwood Lake (St. 37) 3795989 N, 471407 E 3 reverse, rlss 163/76 W; 63 R S 75 cm; good 2 cm 5 normal, rlss 112/81 S; 43 R W 28 cm; poor 3-15 cm 4 reverse, llss 081/81 S; 57 R W 3.3 cm; good 2 mm
Lakeview (St. 47) 3794737 N, 471858 E 20 normal, llss 302/30 NE; 70 R W 2.4 cm; poor 2 mm 35 reverse, llss 067/42 S; 47 R W > 180 m; poor 90 cm
Westwood (St. 54) 3794380 N, 468320 E 2 reverse, rlss 158/18 SW; 45 R S 9.4 cm; good 1-2 mm 21 reverse, rlss 160/23 W; 55 R S 6.7-14.5 cm; good 1 mm 56 normal 289/68 N; 90 R 11 cm; fair 1-2 mm 57 reverse, llss 240/70 NW; 50 R E 4 cm; good 1-3 cm 58 normal 275/61 N; 90 R 50 cm; fair 2-4 cm
St. 56 3795016 N, 467307 E 9 normal 112/77 S; 85 R W 35 cm; good 1-2 cm 10 normal 112/62 S; 85 R E 17 cm; good 2-3 cm 11 reverse, llss 273/60 N; 70 R E 2 m; poor 30-45 cm 8 reverse 295/71 N; 90 R 6 cm; fair 1mm
St. 58 3795091 N, 467212 E 1 reverse 010/78 E; 70 R S 25 cm; fair 3-4 cm 3794826 N, 467234 E 5 normal 030/66 SE; 80 R S 14 cm; fair 2 cm 1: UTM coordinates in meters; Zone = 11 North; Datum = WGS 1984; Geoid = DMA 10 x 10 (Global) 2: llss=left-lateral strike-slip; rlss=right-lateral strike-slip (most faults have a component of strike-slip and dip-slip) 3: Read 30 R W as "30 degree rake from the west" 4: Certainty of slip calculation = poor, fair, or good (poor calculations were discarded for fault core vs slip plots) HW: hanging wall; FW: footwall
72 Table 2-2. Summary of XRD results for all samples.
distance from structura l fault/slip (m) sa mple fault (m) position feldspars clays other Eastwood 17-1 0.05 HW albite, orthoclase epidote, annite 17-2 0.3 HW albite, microcline clinozoisite nontronite*, > 183 17-3 fault core fault core albite gismondine palygorskite 17-4 2.5 HW albite, microcline epidote, clinozoisite 17-5 3 HW albite, microcline laumontite 17-6 5 HW albite, sanidine magnesiohornblende 17-7 5.5 HW albite, anorthite, microcline magnesiohornblende 17-8 7.5 HW albite, anorthite, microcline magnesiohornblende 17-9 8 HW albite, anorthite, microcline muscovite 17-10 9 HW albite, anorthite, microcline ferropargasite 17-11 9.5 HW albite, anorthite, microcline muscovite 1.5 17-12 "fault 6" HW albite, anorthite, microcline manganacolumbite unknown 31-1 fault 12 core HW laumontite, annite unknown 31-2 fault 12 core HW orthoclase laumontite unknown 31-3 fault 12 core HW laumontite unknown 31-5 fault 13 core HW laumontite 31-6 HW albite, orthoclase Grass Valley nontronite*, 39 32-1 fault core fault core orthoclase, albite palygorskite 32-2A 0.5 HW microcline, albite volk ons k oit e* laumontite 32-3 7 HW microcline, albite 32-4 1 HW albite, sanidine 32-5 2 HW microcline, albite 32-6 3 HW microcline, albite cuprite 32-7 4 HW microcline, albite, anorthite 32-8 6 HW albite, anorthoclase 32-9 8 HW microcline, albite 32-10 0.25 FW microcline, albite 32-11 2 FW albite 32-12 4 FW albite, anorthite phlogopite 32-13 5 FW albite minimal 32-14 5 ("fault 12") FW albite, sanidine 37 32-15 0.05 from "fault 6" HW microcline, albite Cleghorn fault kaolinite, (3500) 3-1 fault core fault core palygorskite, calcite montmorillonite* 3-2 0.3 HW albite, orthoclase epidote, calcite 3-3 0.55 HW albite, orthoclase epidote, calcite 2-1 9.5 HW albite, orthoclase 2-2 16 HW albite, microcline Powell Canyon (1200) 43-1 <10? FW albite biotite biotite, 43-2 <10? FW albite, anorthoclase, microcline magnesiohornblende 43-3 <10? FW albite, microcline phlogopite PC-1 25 FW albite, sanidine muscovite PC-2 40 FW albite, anorthoclase biotite PC-3 60 FW albite, microcline muscovite biotite, PC-4 115 FW albite, andesine potassicpargasite biotite, PC-5 150 FW albite, anorthoclase potassicpargasite biotite, muscovite, PC-6 230 FW albite, orthoclase diopside PC-7 305 FW albite phlogopite unknown 7-1 0.05 HW albite, orthoclase phlogopite, greenalite 0.05 from Clipper 3 9-1 HW albite, microcline reverse
73
Pilot Rock (220) 11-1 fault core fault core laumontite 11-2 fault core fault core orthoclase epidote 11-3 hardened fin fault core quartz Water Tank .05 from "fault 5" 0.29 15-1 HW albite, orthoclase area 2 unknown 15-2 "fault 4b" area 2 fault core albite, microcline 0.05 from "fault 5" 10.5 15-3 FW albite, orthoclase area 1 "fault 5" area 1 10.5 15-4 fault core albite laumontite fault core Silverwood Lake 0.75 37-1 "fault 3" hard fin fault core quartz 0.28 37-2 .05 from "fault 5" FW albite, orthoclase epidote 0.28 37-3 "fault 5" core fault core microcline muscovite 0.03 37-4 "fault 4" core fault core albite, orthoclase epidote Seeley Junior 0.4 40-1 0.05 FW albite, anorthoclase Lakeview > 180 47-1 fault core fault core albite, orthoclase vermic ulit e epidote Westwood 0.09 54-1 0.05 from "fault 2" HW albite, anorthoclase (0.5) 54-2 0.05 from "fault 10" FW albite, sanidine saponite* laumontite 0.1 54-3 0.05 from "fault 21" FW albite potassicpargasite, (1.4) 54-4 0.05 from "fault 29" HW albite smectite laumontite (1.2) 54-5 0.05 from "fault 30" HW albite, orthoclase 0.04 54-6 0.05 from "fault 57" HW albite, orthoclase laumontite unknown 54-7 1.2 HW albite epidote unknown 54-8 fault core fault core albite, microcline montmorillonite* epidote Other Faults magnesiohornblende, 0.75 49-1 0.05 from "fault 1" HW albite, microcline ferropargasite 0.06 49-2 0.05 from "fault 2" HW albite, microcline, anorthite unknown 51-1 fault core fault core montmorillonite* laumontite magnesiohornblende, unknown 52-1 0.05 from "fault 1" FW albite, microcline diopside 0.7 55-1 0.05 FW albite, microcline 0.35 56-1 0.05 from "fault 9" FW albite, orthoclase, microcline 0.17 56-2 0.05 from "fault 10" HW albite, microcline 0.25 58-1 0.05 from "fault 1" HW orthoclase, anorthite muscovite (1.2) 58-2 0.05 from "fault 4" HW albite, microcline augite 0.14 58-3 0.05 from "fault 5" HW albite, sanidine ferropargasite * Smectite mineral Note: All samples have quartz except for 7-1, 31-3, and 31-5 HW: hanging wall; FW: footwall fault "slip" in parentheses is actually offset
74 Table 2-3. Summary of geochemical trends of all oxides in the Grass Valley fault zone. FCHW: fault core relative to hanging wall; FCFW: fault core relative to footwall; FWHW: footwall relative to hanging wall. Values show percent change between various zones within the fault. For example, the fault core has 17% the amount of Na2O as the hanging wall. Hanging wall and footwall distances used for comparisons are 4 m and -4 m, respectively.
Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3
FCHW 17% 3300% 88% 88% 100% 19% 80% 820% 222% 542%
FCFW 14% 133% 69% 99% 6% 64% 22% 82% 33% 98%
FWHW 116% 2480% 128% 89% 1667% 30% 369% 1000% 667% 555%
Table 2-4. Summary of geochemical trends of trace elements and LOI in the Grass Valley fault zone. FCHW: fault core relative to hanging wall; FCFW: fault core relative to footwall; FWHW: footwall relative to hanging wall. Values show percent change between various zones within the fault. Hanging wall and footwall distances used for comparisons are 4 m and -4 m, respectively.
Sr Y Zr Nb Ba Sc LOI
FCHW 72% 100% 317% 144% 75% 100% 2580%
FCFW 26% 60% 68% 100% 39% 100% 12900%
FWHW 281% 167% 469% 144% 193% 100% 20%
75 Table 2-5. Summary of geochemical trends of oxides in the Eastwood fault zone. FCPDZ: fault core relative to primary damage zone; FCSDZ: fault core relative to secondary damage zone; PDZSDZ: primary damage zone relative to secondary damage zone. Values show percent change between various zones within the fault.
Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3
FCPDZ 28% 209% 83% 99% 40% 92% 67% 88% 175% 169%
FCSDZ 24% 75% 74% 116% 27% 129% 42% 45% 58% 67%
PDZSDZ 88% 36% 90% 117% 67% 141% 62% 51% 33% 40%
Table 2-6. Summary of geochemical trends of trace elements and LOI in the Eastwood fault zone. FCPDZ: fault core relative to primary damage zone; FCSDZ: fault core relative to secondary damage zone; PDZSDZ: primary damage zone relative to secondary damage zone. Values show percent change between various zones within the fault.
Sr Y Zr Nb Ba Sc LOI
FCPDZ 72% 36% 80% 56% 59% 100% 148%
FCSDZ 50% 44% 113% 100% 100% 64% 95%
PDZSDZ 70% 122% 142% 178% 168% 64% 64%
76 Table 2-7. Summary of geochemical trends of oxides in the Cleghorn fault zone. FCPDZ: fault core relative to primary damage zone; FCSDZ: fault core relative to secondary damage zone; PDZSDZ: primary damage zone relative to secondary damage zone. Values show percent change between various zones within the fault.
Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3
FCPDZ 9% 521% 110% 82% 138% 60% 89% 200% 175% 211%
FCSDZ 9% 692% 95% 80% 180% 38% 248% 171% 233% 205%
PDZSDZ 99% 133% 86% 97% 130% 64% 279% 86% 133% 97%
Table 2-8. Summary of geochemical trends of trace elements and LOI in the Cleghorn fault zone. FCPDZ: fault core relative to primary damage zone; FCSDZ: fault core relative to secondary damage zone; PDZSDZ: primary damage zone relative to secondary damage zone. Values show percent change between various zones within the fault.
Sr Y Zr Nb Ba Sc LOI
FCPDZ 64% 118% 132% 100% 52% 122% 343%
FCSDZ 64% 68% 67% 100% 40% 122% 1440%
PDZSDZ 101% 58% 51% 100% 77% 100% 420%
Table 2-9. Summary of energy calculations for the Eastwood fault.
Molar % of Ea1 ΔG° 2 Volume Reaction f volume fault Moles kJ (kJ/mol) (kJ/mol) (cm3) (mol/cm3) core 2-1 35 6808.2 3.2 x 10-3 10 2.2 x 1011 7.0 x 108 4.80 x 1012 2-2 65 2.1 1.0 x 10-2 10 2.2 x 1011 2.2 x 109 1.49 x 1011 2-3 31 2489.1 4.3 x 10-3 10 2.2 x 1011 9.4 x 108 2.36 x 1012 2-4 82 -9081.5 5.0 x 10-3 7 1.5 x 1011 7.6 x 108 -6.85 x 1012 1: Data from Lasaga (1984), Chen and Brantley (1997), Cama et al. (1999), and Eberl and Hower (1976), respectively 2: Data from Woods and Garrels (1987)
77
( I ) (2 ) ( 3 ) (4) (3) (2) ( I )
1} Undeformed Host Rock
2) Damaged Host Rock Fault Zone 3) Foliated Zone } { 4) Central ultracataclasite layer Fault Core
Figure 2-1. Fault core and damage zone model for the North Branch San Gabriel strike-slip fault zone from Chester et al. (1993). Zone 3 of this model may correspond to the primary damage zone discussed in this paper. Garlock faultMojave
Lenwood fault Helendale fault Field Area
NFT Transverse Ranges
San Andreas fault LA Basin
Peninsular Salton Ranges Trough 33"0'0"N W+EN s
,1e ~O'O"'W 117"f11TW
Figure 2-2. Shaded relief map of southern California showing major roads (red), rivers (blue), and faults (yellow). Note the anomalous east- west trend of the Transverse Ranges. The San Bernardino Mountains make up the eastern part of this range. The San Andreas fault defines the southern portion of the San Bernardino Mountains. NFT: North Frontal thrust. Eastwood fault
Powell Canyon fault
San Bernardin9 ~CA
N 0 1.25 2.5 5 Kilometers Figure 2-3. Shaded relief map of the western San Bernardino I I I Mountains. Geographic locations are shown in blue and faults t in red. Green box shows approximate extent of field area. 80 34o 20' 40'' 34 o 20' 40''
A'
B' A
B
o 34 o 15' 34 15' o o 60 117 14' 08'' 117 26'15'' Younger Alluvium __L_ BEDDINGA TTITUDE 0 0.5 1 2 3 4 D Kilometers Older Alluvium Figure 2-4. Geologic map of the Silverwood Lake N D Pleistocene ~ FOLIATION ATIITUDE area. See Figure 2-3 for location. Note the -- Shoemaker Gravel D ...... ?-· CONTACT; arcuate shape of the Cedar Springs fault system. D Harold Formation CONCEALED; INFERRED The Cleghorn fault trends roughly east-west Pliocene Phelan Peak Deposits 45 f ~~A ... --? through the field area. Red circles represent D FAULT; DIP; SLIP; THRUST CONCEALED: INFERRED stations from field work. Station numbers are + D Miocene Crowder Formation -+.- - ? keyed to Table 2-1. Geology from Meisling and Mesozoic crystalline basement ~ .. ? D ANTICLINE; SYNCLINE Weldon (1989). 10 . 10 Location of Stations CONCEALED; INFERRED SW NE D CLEGHORN FAULT D' 4000 i DEEP CREEK 2000
SL ' ' N30E-
D Pleistocene Younger Alluvium D Miocene Crowder Formation Pliocene Phelan Peak Deposits Mesozoic crystalline basement D D of San Bernardino Mountains
Figure 2-5. Simplified cross-section trending northeast through the Silverwood Lake area and Cedar Springs fault system to Deep Creek. Note the high density of faulting in the Silverwood Lake area, many of which are used in this study. Cross-section adapted from Meisling and Weldon (1989). (a) S N A A' feet above meters above sea level sea level depositional contact of 3800 ------:-· 1160 Crowder F'm on basement 3600 1100
3400 1040
3200 Eastwood fault 980
3000 920
Lakeview fault
Lakeview fault (St. 47) 500' No vertical Eastwood fault (St. 17) 47 o Rake from the west exaggeration 90o rake (pure dip-slip) left-lateral reverse fault slip = 183 m W E 47o
180 m slip = 180 m
(b) S N B B' feet above meters above sea level depositional contact of sea level 3800 Crowder F'm on basement -, ------1160 3600 1100 ' Westwood fault l Silverwood ' 3400 Lake 1040 ' Cleghorn fault 3200 980
3000 920
No vertical Younger Alluvium (Pleistocene) 500' Westwood fault (St. 54) D exaggeration 55 o Rake from the east; left lateral reverse fault; Miocene Crowder Formation slip is unconstrained due Cleghorn fault (St. 3) D to lack of hanging wall formerly a reverse fault; Mesozoic crystalline basement or footwall cutoff dip-slip component = 200 m of San Bernardino Mountains D throw = 150 m
Figure 2-6. Cross-sections showing estimates of minimum slip constraints based on lack of the Crowder Formation at higher topographic elevations. 23 degrees of NE tilting from the western San Bernardino arch has been removed. See Figure 2-4 for locations of sections. (a) Cross section A-A' through the Lakeview and Eastwood reverse faults with slip calculations beneath. (b) Cross section B-B' through the Cleghorn and Westwood faults. The Cleghorn fault has been reactivated as a left-lateral strike-slip fault accumulating 3.5 - 4 km of offset throughout the Quaternary (Meisling and Weldon, 1989). 83
12 11 10 9 8 7 6 5 4 3 NumberFaultsGroupof in 2 1 0-10 cm 10-100 cm 1-10 m 10-100 m 100-1000 m 0 Slip
Figure 2-7. Histogram of amount of slip on all 31 slip-constrained faults studied ranging from 2 cm to 183 m of slip. The mean for these faults is 15.5 m while the median is only 50 cm, showing the skewed nature of the data set towards small- displacement faults. 84
(a)
(b) fault plane
vertical separation
dike
Figure 2-8. Outcrop photographs showing elements used for slip calculations. (a) Slickenlines exposed on fault 2 at Station 58 (Figure 2-4). Diagram shows orientation and rake of fault. The slip vector on this fault rakes 60° from the east. (b) Outcrop photo showing offsets of dike forming a mini-graben at Station 56. Note hammer for scale. Using fault orientation, offset marker orientation, slip vector, and separation, net slip was calculated when possible. 85
Group 1 average: 179/81 W 1% area contour of poles to Group 2 average: 099/60 S planes of all fractures (n=253) Footwall Contour int = Hanging wall W E 2% per 1% area
fractures (n=26) fractures (n=35) 2 faults (n=8) fractures (n=23) distance = -2 m distance = 1 m distance = 4 m
1
Main fractures (n=12) fractures (n=47) fault fractures (n=25) faults (n=3) faults (n=2) faults (n=4) fractures (n=23) fractures (n=25) distance = -4 m distance = -0.2 m distance = 0.2 m distance = 2 m distance = 8 m
Covered 6 11 4 8 9 16 7 12 15 Covered 1 11 8 Covered 12 3 2 4
3 9 4 6 16 1 2 6 4 3 5
6 4 6 16 Legend: main fault subsidiary fault fracture 5 m 1 dike 2 3
Figure 2-9. Photomosaic and simplified sketch of the Grass Valley fault zone with view to the south. Stereonets are lower hemisphere equal area projections. Each stereonet shows the main fault in green, number of planes, and distance perpendicular from the main fault. Hanging wall fractures have a bimodal distribution as seen best in the 2, 4, and 8 m plots. Footwall fractures have a more random orientation. Most subsidiary faults are roughly parallel to the main fault except for the -4 m plot in the footwall. The contoured stereonet shows poles to all fractures across the entire fault zone with best fit great circles.
(a)
Damage Elements - Grass Valley
N footwall hanging wall S 50
• 40•
D D
• •
• • 30 • •
• • faults
fractures
• • 20 • •
• •
• • • • • • 10 Density (per 70 cm) Density 70 (per 0 -10 -5 0 5 10 Distance (m) (b) Grass Valley fault zone
N S 60
positions of dominant
• • I
50 • subsidiary faults t t t t t t t t y = 7.8826x + 48.98 n 2 R = 0.9812 40 HW FW
30 HWDZ
• • • • • • Linear (FW) Linear (HWDZ) 20 # of damage elements
10 y = -12.889x + 49.479 R2 = 0.9826
0 -6 -4 -2 0 2 4 6 8 10 distance from fault (m)
Figure 2-10. Transect data from the Grass Valley reverse right-lateral fault. Distances are perpendicular to the main fault. (a) Graph of subsidiary faults and fractures plotted separately. Main fault is at 0 m; hanging wall is positive distance and footwall is negative distance. The majority of damage elements are fractures. No veins were observed at this fault zone. (b) Plot of combined damage elements (subsidiary faults and fractures) per 70 cm scans across the fault zone. Note the linear decrease in density away from the main fault. Increase in damage element density from 4 to 8 m in the hanging wall is attributed to subsidiary faults. Thus, the green trendline represents the hanging wall damage zone. HW: hanging wall, FW: footwall, HWDZ: hanging wall damage zone. 87
fault
pegmatite dike
granodiorite
Figure 2-11. View to the southeast of a subsidiary fault in the Grass Valley fault zone. The fault in the granodiorite is manifested as a well-defined fault core composed of clay gouge and microbreccia. When the fault cuts the harder dike, it becomes a broader zone of fractures and faults. This shows the control that lithology has on fault geometry. Shovel for scale is approximately 0.5 m long. Equal Area 1% area contour of poles to fractures (n=103) in secondary damage zone Close-up view of Contour Int = 2.0% per 1% area Eastwood fault core. Compass is 7 cm on Group 1: 121/84 S each side. Group 2: 026/66 E
Main fault in red Cretaceous crystalline fault core basement in hanging wall relatively undeformed Miocene Crowder N Formation in footwall S
HW secondary damage zone FW HW
primary damage zone
main FW fault
Figure 2-12. Photomosaic and simplified sketch of the Eastwood reverse fault with 1 m view to the east. Stereonet is of all the fractures in secondary damage zone (n = 103) and contours poles to the planes. HW: hanging wall; FW: footwall.
(a)
Damage Elements - Eastwood Reverse
S hanging wall N 25 • 20 15 • • faults • • fractures 10 • • Dveins 5
Density (per 70 cm) Density 70 (per ~ 0 - • -• ....A .....A - 0 2 4 6 8 10 12 Distance (m)
(b)
Eastwood Reverse S mm to cm size fault breccia up to 4.75 m N into hanging wall - unable to inventory 30 • 25 y = -2.0136x + 31.635 R2 = 0.5667 20
15
10 • # of damage elements
5
0 0 2 4 6 8 10 12 distance from fault (m)
Figure 2-13. Transect data from the Eastwood reverse fault hanging wall. Distances are perpendicular to the main fault. (a) Graph of subsidiary faults, fractures, and veins plotted separately. The dominant damage element is the fractures. (b) Plot of combined damage elements (subsidiary faults, fractures, and veins) per 70 cm scans across the hanging wall damage zone. The brecciated and altered rock in the first 5 m is defined as the primary damage zone.
SW NE
Crystalline Basement (hanging wall) fault Crowder core Formation (footwall)
fault core
maroon layer
Figure 2-14. Photographs of the Cleghorn fault (reverse left-lateral slip) juxtaposing crystalline basement on top of the Miocene Crowder Formation. Total fault core thickness is 32 cm, including a 2 cm thick foliated, maroon clay layer against the Crowder Formation fault wall.
(a)
Damage Elements - Cleghorn Fault
S hanging wall N 16 14 • • 12 10 • faults 8 • • • fractures 6 • D 4
Density (per 70 cm) Density 70 (per 2 0 0 5 10 15 20 Distance (m)
(b)
Cleghorn Fault S N 25
• • .....! this area highly damaged and covered 20 with vegetation; unable to inventory - damage elements y = 0.25x + 17.625 • R2 = 0.0667 15
10 # of damage elements
5
0 0 2 4 6 8 10 12 14 16 18 distance from fault (m)
Figure 2-15. Transect data from the Cleghorn reverse left-lateral fault hanging wall. Distances are perpendicular to the main fault. (a) Graph of subsidiary faults and fractures plotted separately. Note the higher density of faults in the Cleghorn damage zone as compared to the other fault zones. This may be attributed to the greater amount of slip on the main fault. (b) Plot of combined damage elements (subsidiary faults and fractures) per 70 cm scans across the hanging wall damage zone.
(a)
Damage Elements - Powell Canyon
SW footwall NE 35 30 • • 25 • • 20 • faults 15 • • Dfractures 10 • 5 Density (per 70 cm) Density 70 (per • 0 • • - - • 0 100 200 300 400 Distance (m)
(b)
Powell Canyon SW NE 40
35 • • 30 •
25 • • All points 20 DZ This is probably the extent of damage associated with the main Powell Canyon fault. Linear (DZ) Fluctuating damage to the NE can be attributed to many mapped faults at a high angle 15 --to the main fault. D
# of damage elements y = -0.4674x + 43.986 10 R2 = 0.841
5
0 0 50 100 150 200 250 300 350 distance from fault (m)
Figure 2-16. Transect data from the Powell Canyon right-lateral fault footwall. Distances are perpendicular to the main fault. (a) Graph of subsidiary faults and fractures plotted separately. The dominant damage element is the fractures. (b) Plot of combined damage elements (subsidiary faults and fractures) per 70 cm scans across the footwall damage zone (DZ). First three data points are interpreted as defining the extent of damage (approximately 75 m thick) associated with the main Powell Canyon fault.
Secondary DZ Primary DZ north Primary DZ south 1% AREA CONTOUR OF POLES fractures (n = 4): TO FAULTS (n = 34): fractures (n = 3): faults (n = 6): faults (n = 7): Contour Int = 2.0% per 1% area main fault 2 main fault Group 1: 081/57 S Group 2: 264/80 N 1 Main fault in green
NNW SSE north Primary Primary fault zone Secondary DZ south DZ north fault DZ south
25 gouge/microbreccia
8 13 breccia highly a breccia breccia 30 highly many small 22 29 damaged 15 17 24 damaged 5 9 27 faults a 10 20 16 b 28 47-1 a 11 21 23 meters 6 18 34 bush b 19 12 31 3 4 7 14 gouge 1 2 c 32 33 35 b covered covered
5 meters 10 15 20 mafic slivers 11 fault trace;ID located where orientation was taken N schist 47-1 sample location
south north fault zone fault
footwall hanging wall
Figure 2-17. Photomosaic and simplified sketch of two faults in the Lakeview reverse left-lateral fault zone. Note hammer for scale and yellow measuring tape. South fault appears to be larger than north fault. Lower hemisphere equal area projections of the primary and secondary damage zones (DZ) are shown. The primary damage zone plots show the main fault ("south fault") orientation in green and the dominant fault and fracture orientations (not all discontinuities are shown) in red and blue, respectively. The secondary damage zone plot contours the poles to the planes of the dominant faults (n = 34) and provides average great circles.
10 19 22 11 18 12b 29 254-1 12a 20 N 12 a a 23 25 31 32 7 8 15 21 27 b 54-2 14 24 26 28 54-5 3 54-4 meters 4 b 30 33 34 35 5 6 9 13 17 b b 54-3 1 a a 16 5 10 15 20
54 64 bush 65 footwall 51 52 50 63 A hanging wall 46 53 66 67 D C B 41 47 48 59 37 40 43b b 55 60 61
62 meters 36 39 56 54-7 38 45 a 58 44 43a 49 68 54-8 42 57 I I I I 30 54-6 35 40 45 ~ 25 I I~ fault trace; ID N mafic sliver gneiss meters A: white fault core D: pinkish brown breccia D 4 located where grus-forming B: brown breccia 54-7: sample location blocky granite orientation was granodiorite D - C: pinkish white breccia - taken N
30 35 19 9
N D 53 C 49 main fault 36
Figure 2-18. Photomosaic and simplified sketch of the Westwood reverse left-lateral fault (Station 54). Although slip is not constrained on the main fault, this fault zone provides good exposure of the footwall (once tilt from the western San Bernardino arch is removed) damage zone with many subsidiary faults. Sketch only shows faults, not fractures. A: fault core; B-D: primary damage zone; the rest of the outcrop is secondary damage zone. 95 (a)
9 8 • 7 • 6 5 4 3 y = 0.0002x + 1.7463 2 R2 = 0.1514 1 Fault Core Thickness (cm) 0 1 10 5000 10000 15000 20000 1 1 1 Slip (cm) 1 1 1 1 1 1 1 (b) 1 ------
4 3.5 • 3 • • 2.5 • 2 .. • • 1.5 • • • • • 1 •• • 0.5 Fault Core Thickness (cm) 0 0 100 200 300 400 500 Slip (cm)
Figure 2-19. Plots of fault core thickness with respect to slip. (a) Plot of all 26 slip-constrained faults. The trendline is linear and the equation of the line and R2 value are shown. Due to the low R2 value, faults do not appear to thicken with increased slip. (b) Close up of 23 smaller faults from the box in plot (a).
a subsidiary fault showing c shear fabric c
b
1 mm b PSS d d
1 mm
1 mm 5 mm
Figure 2-20. Microstructure of the Grass Valley fault. (a) Scan of thin section from main fault core. Locations of photomicrographs shown in red. PSS = primary slip surface (b) Photomicrograph of fault core in PPL showing open fractures and alteration. (c) Photomicrograph in CPL of PSS. (d) Photomicrograph in CPL of damage surrounding PSS. CPL = cross polarized light; PPL = plane polarized light. a
b
5 mm
b 1 mm
Figure 2-21. Microstructure of the Grass Valley fault zone hanging wall. (a) Scan of thin section 1 m from the main fault in the hanging wall. Note the marked decrease in damage from Figure 2-20. (b) Photomicrograph in CPL showing lack of damage at this distance from the fault. CPL: cross polarized light. a
5 mm
b c 1 mm
c
d
d 1 mm Fault Plane
5 mm
Figure 2-22. Microstructure of the Eastwood fault. (a) Scan of thin section parallel to fault approximately 5 cm from edge of fault core. (b) Scan of thin section perpendicular to Eastwood fault (right edge of image). Locations of photomicrographs in upper right corner. (c) Photomicrograph in CPL showing open fractures and chlorite. (d) Photomicrograph of microbreccia in PPL. CPL: cross polarized light; PPL: plane polarized light. a b 1 mm
b
open fracture 5 mm
c d chlorite 1 mm
clay and 5 mm microbreccia fill
Fe2O3 staining d feldspars
e f biotite
f
OPX
5 mm 1 mm
Figure 2-23. Microstructure of the Eastwood fault zone hanging wall. (a) Scan of thin section 30 cm into hanging wall from edge of fault core. Sample characterized by open fractures, feldspar alteration, and comminution. Location of photomicrograph shown in red. (b) Photomicrograph in CPL of microbreccia and open fractures. (c) Scan of thin section 2.5 m into the hanging wall. Green mineral is chlorite. Location of photomicrograph shown in red. (d) Photomicrograph in PPL. (e) Scan of thin section 5.5 m into hanging wall. Note the drastic decrease in damage. Location of photomicrograph shown in red. (f) Photomicrograph in CPL showing coherent granodiorite composed of quartz, feldspars, biotite, and orthopyroxene. CPL: cross polarized light; PPL: plane polarized light. a
c
b
5 mm
b c
0.5 mm 1 mm
Figure 2-24. Microstructure of the Cleghorn fault 30 cm into the hanging wall from the fault core. (a) Scan of thin section perpendicular to fault. Note grain comminution and veining. Locations of photomicrographs shown in red. (b) Photomicrograph of twinned calcite and shattered grains in CPL. (c) Photomicrograph of typical damage near the Cleghorn fault in CPL. CPL: cross polarized light. a
b
calcite vein
open fracture
b 1 mm
5 mm d
c
biotite
chlorite
altered d K-spar
5 mm 1 mm
Figure 2-25. Microstructure of Cleghorn fault zone hanging wall. (a) Scan of thin section 55 cm into hanging wall from fault core. Note the brecciation and large open fractures; some small fractures have calcite fill. Location of photomicrograph shown in red. (b) Photomicrograph in CPL. (c) Scan of thin section 9.5 m into hanging wall from fault core. Have a marked decrease in damage. Sample characterized by mostly open fractures and feldspar alteration. Location of photomicrograph shown in red. (d) Photomicrograph in CPL. CPL: cross polarized light. a
c
b 5 mm
b c
fine matrix
microbreccia
Fe2O3 lined fracture Fe2O3 fractures
1 mm 1 mm
Figure 2-26. Microstructure of the Lakeview fault. (a) Scan of thin section from breccia in fault core. Location of photomicrographs are shown in red. (b) Photomicrograph in CPL showing reduced grain size and a fracture partially lined with iron oxide. (c) Photomicrograph in CPL showing fractured nature of sample. Difference in color between B and C caused by blue toned light filter on microscope. CPL: cross polarized light. a
b
b
comminuted grains 1 mm
5 mm
Figure 2-27. Microstructure of the Westwood fault. (a) Scan of thin section from footwall; 1.2 m from fault core. (b) Photomicrograph of open fracture (or fault) lined with comminuted grains in CPL. CPL: cross polarized light.
Riedel shears a
b 5 mm
c
Fault b c
edge of fault core
calcite veins
1 mm 1 mm
Figure 2-28. Microstructure of the Westwood fault footwall damage zone. (a) Scan of thin section from footwall; perpendicular to subsidiary fault 57 (Figure 2-18). Fault along bottom edge of section. (b) Photomicrograph of calcite veins in CPL. (c) Photomicrograph of edge of fault core in CPL. CPL: cross polarized light. 105 (a) 17-3-clay-gl 17-3-cla -o 3000 shifted smectite peak Intensity (cps) Intensity 2500
2000
1500
1000 palygorskite 500
0 5 10 15 20 25 30 2Theta (°) (b) 3-1-clay-gl 600 shifted smectite peak 3-1-cla -o
Intensity (cps) Intensity 500
400 palygorskite
300
200 kaolinite
100
0 5 10 15 20 25 30 2Theta (°)
Figure 2-29. Diffraction patterns of fault cores showing original sample (blue) and glycolated sample (red). (a) Pattern for the Eastwood fault core showing the shifted smectite peak and the palygorskite peak. (b) Pattern for the Cleghorn fault core showing the shifted smectite peak, palygorskite peak, and the kaolinite peak. These results are representative of fault cores from the Westwood and Grass Valley faults as well. 106
(a)
Grass Valley PCA variable loadi ngs Na20 & Al203 & o., P••W~ao MnO Si0:2 & 01 N & ~ C 03
.{) ,6 .{)4 .{).3 .,,, ·0.2 01 o& 0.3 o., 0.6 TF£i02 .,,, & .{) ,2
.{).3
LOI .,,, &
.{) Alls1
(b)
Grass Valley Fault Zone • Na2O 18 MgO 16 • •• & Al2O3 14 & 12 ••• X K2O 10 X CaO 8 • XXX X XX + TiO2 6 X Weight Percent 4 •• X Fe2O3 2 •• • LOI 0 • -10 -5 0 5 10 15 20 25 X SiO2/10 Distance From Fault (m)
Figure 2-30. Whole-rock geochemistry of the Grass Valley fault zone. (a) Principal Component Analysis of all oxides and LOI. Variables furthest away from the origin of the plot are the most significant. (b) Plot of weight percent of selected oxides and LOI with respect to distance from main fault. Negative distance values are in the footwall and positive values are in the hanging wall.
107 (a)
Eastwood PCA variable loadings Na20 • 06 0.5 Al203
• 0.4
P205 02 •Cao Si02 "' 01 ~ • K20 ... Ti02 C 03 • --0.5 ... --0.2 --01 01 0.2 0.4 0.5 D.6
--01 ,MnO...,,
--02 •MgO • .... LOI• s Aos1
(b)
Eastwood Reverse Fault
Na2O 18 • MgO 16 • • • • • • • Al2O3 14 • • • 12 X K2O 10 CaO 8 • + - + 6 I + + Fe2O3
Weight Percent • 4 • • • I, • LOI 2 * • I •X I X TiO2 0 ~ I X ' 0 2 " 4 6 8 10 + SiO2/10 Distance From Fault (m)
Figure 2-31. Whole-rock geochemistry of the Eastwood fault zone. (a) Principal Component Analysis of all oxides and LOI. Variables furthest away from the origin of the plot are the most significant. (b) Plot of weight percent of selected oxides and LOI with respect to distance from main fault. Values are from the hanging wall.
108 (a)
Cleghorn PCA variable loadings "'103 o. ... To02 ... o., Fe:3P205... Si02.. Moo ... o_, "' ~ C 03 -0.5 -04 .Q.J -0.2 -01 01 02 0.3 ..0 .4 0.5 -01 K20... LOI -0.2 ...MnO .. -0.3
-04 cao... -0. Aos1
(b)
Cleghorn Fault • Na2O 16 MgO 14 • Al2O3 12 10 X K2O 8 CaO X X • 6 + TiO2 X Weight Percent 4 • Fe2O3 2 • • • LOI 0 • 0 5 10 15 20 X SiO2/10 Distance From Fault (m)
Figure 2-32. Whole-rock geochemistry of the Cleghorn fault zone. (a) Principal Component Analysis of all oxides and LOI. Variables furthest away from the origin of the plot are the most significant. (b) Plot of weight percent of selected oxides and LOI with respect to distance from main fault. Values are from the hanging wall.
109 (a)
Powell Canyon PCA variable loadings o. P...205 K20... o., l...01
Al203 ... Fe203 o_, ....0 C 03 TI02 -0.5 -04 -0.3 -0.2 -0 1 01 02 M...nO 03 0.4 0.5 -0 1 SI02... -0.2
cao -0.:J ...
-04
Na20 -0. ... Aos1
(b)
Powell Canyon Fault
Na2O 18 • MgO 16 •• • • 14 • • •Al2O3 12 )I( K2O 10 CaO 8 • X X X 6 ~ X X X + TiO2 Weight Percent 4 I Fe2O3 ~ • I • • 2 • • • • + LOI 0 "' + 0 50 100 150' 200 250 300 350 X SiO2/10 Distance From Fault (m)
Figure 2-33. Whole-rock geochemistry of the Powell Canyon fault zone. (a) Principal Component Analysis of all oxides and LOI. Variables furthest away from the origin of the plot are the most significant. (b) Plot of weight percent of selected oxides and LOI with respect to distance from main fault. Values are from the footwall.
110 (a)
Grass Valley Fault Zone
18 16 14 12 10 8 6 4 Weight Percent 2 0
K2O CaO LOI Na2O Al2O3 Cr2O3 Fe2O3 MgO*10 SiO2/10 TiO2*10 P2O5*10 MnO*100
fault core 4 m (HW DZ) 8 m (HW DZ) 23.2 m (HW DZ) -4 m (FW DZ)
(b)
Grass Valley Fault Zone
800 700 600 500 400 300 200
parts per millionparts 100 0
Sr Zr Ba Y*10 Nb*10 Sc*10
fault core 4 m (HW DZ) 8 m (HW DZ) 23.2 m (HW DZ) -4 m (FW DZ)
Figure 2-34. Spider diagrams at different positions throughout the Grass Valley fault zone. HW DZ: hanging wall damage zone; FW DZ: footwall damage zone. (a) Concentrations of oxides and LOI in weight percent. (b) Trace element concentrations in parts per million.
111 (a)
Eastwood Reverse Fault
18 16 14 12 10 8 6 4 Weight Percent 2 0
LOI MgO K2O CaO Na2O Al2O3 Cr2O3 Fe2O3 SiO2/10 TiO2*10 P2O5*10 MnO*100
1~ fault core ...... 3 m (PDZ) ...... 9 m (SDZ) I
(b)
Eastwood Reverse Fault
1200 1000 800 600 400
parts per millionparts 200 0
Sr Zr Ba Y*10 Nb*10 Sc*10
1~ fault core ...... 3 m (PDZ) -+- 9 m (SDZ) I
Figure 2-35. Spider diagrams at different positions throughout the Eastwood fault zone. PDZ: primary damage zone; SDZ: secondary damage zone. (a) Concentrations of oxides and LOI in weight percent. (b) Trace element concentrations in parts per million.
112 (a)
Cleghorn Fault
16 14 12 10 8 6 4 Weight Percent 2 0
LOI MgO K2O CaO Na2O Al2O3 Cr2O3 Fe2O3 SiO2/10 TiO2*10 P2O5*10 MnO*100
fault core 0.55 m (PDZ) 16 m (SDZ) I-+- ---
(b)
Cleghorn Fault
1800 1600 1400 1200 1000 800 600 400 parts per millionparts 200 0
Sr Zr Ba Y*10 Nb*10 Sc*10
fault core 0.55 m (PDZ) 16 m (SDZ)
---
Figure 2-36. Spider diagrams at different positions throughout the Cleghorn fault zone. PDZ: primary damage zone; SDZ: secondary damage zone. (a) Concentrations of oxides and LOI in weight percent. (b) Trace element concentrations in parts per million.
113 (a)
Powell Canyon Fault
18 16 14 12 10 8 6 4 Weight Percent 2 0
LOI MgO K2O CaO Na2O Al2O3 Cr2O3 Fe2O3 SiO2/10 TiO2*10 P2O5*10 MnO*100
25 m 60 m 150 m I-+- ----
(b)
Powell Canyon Fault
2500 2000 1500 1000
partsmillion per 500 0 = -
Sr Zr Ba Y*10 Nb*10 Sc*10
25 m 60 m 150 m I-+- ----
Figure 2-37. Spider diagrams at different positions throughout the Powell Canyon fault zone. (a) Concentrations of oxides and LOI in weight percent. (b) Trace element concentrations in parts per million.
114
V V V b I a 1111 b I a 1111b I a IPI b I < i.. /l_/l___/l )" ; I I 1... ~ u _r __ .1 o l__ _1· · . z 0 · r..__-----....1 I I z I Ill -4 m > 0 -< > Ill m iii I: ·r_ 0
.l I I I - ! ·r~ .....i
Figure 2-38. Variations in shear stress (τ) and displacement (u) over time for seismic and aseismic crustal fault zones. The earthquake stress cycle is divided into four phases: α-phase: secular strain accumulation; β-phase: possible preseismic anelastic deformation; γ -phase: coseismic phase of mainshock rupturing; and δ -phase: afterslip and decaying aftershock activity. Figure from Sibson (1989). 115
Activated complex
C •0 ·....u- C,:$ Q) I-< Q) r:n Reactants I-< Q) > >-,t Q) b.O I-< I-< I-< 0 C llE ..8 UJ i
Progressof reaction~
Figure 2-39. Diagram showing the concept of activation energy. Reactions can go forward only at the rate at which some of its particles acquire enough energy to get over the “energy hump”, or activation energy (Ea). The change in energy is shown as ΔE. From Krauskopf and Bird (2003). 116
(a) 10 21 ,,v.. 8.0 ,c::sy ,,,__,~,, ,,,,/ ...... • Cajon Pass Borehole ,, ,,.:', ,, ,, E z 1019 • Various Combined Studies ,, xi",,.:'°".;"~ '··/ 6.7 ...... / • -.,;.Y.,,.; C / ., ."../ Q) 1017 ,,( ·1.'l -,,I ,,;, E ..;;.-r...,,,.. .. #/ • 5.3 0 ..),-:• ,,, ~~~.. : .l•./.,~ . Mw ~ 1015 ..... • • Jy 4.0 (.) ,11.• •• >~. • > E ,,.~· -~ 1013 ,{ 2.7 /. . Q) . Cf) 1011 1.3
--- Constant stress drop (MP a) ...... ;...... c.... ___ _..______.. ____ _.______.0.0 5 10 10 2 10 3 104 10 5 Source Dimens ion (m)
(b) 8.00E+l5
7.00E+l5
6.00E+15
5.00E+l5 g !:E4.00E+l5 Tecton ic 3.00E+l5
2.00E+l5
I.OOE+15 Seismic O.OOE+OO.js:i:~ ~:::...:~ t- - -+ _:_..:_+-_L-+----1 3.00 4.00 5.00 6.00 7.00 8.00 9.00 mL
Figure 2-40. Plots showing seismic relationships used to calculate total moment energy for the Eastwood fault. (a) Plot showing linear relationship between source dimension and seismic moment. This relationship justifies the following calculation for total moment. From Abercrombie (1995). (b) Plot showing the contribution of each magnitude interval to the total seismic moment release for the Gutenberg-Richter law. The rate of seismic moment release is equal to the area under the curve and is shaded in gray based on a maximum magnitude of 5.0 estimated for the Eastwood fault. Due to the relatively low magnitude, the area can be approximated as a triangle (1/2 bh). The total seismic moment release is approximately 2 x 10^15 N-m [(1/2) 5(0.8 x 10^15)], or 2 x 10^12 kJ. Figure adapted from Main et al. (1999). For clarity of presentation, only positive error bars are shown. MF(m) = moment release rate; mL = earthquake magnitude. 117
fault
secondary damage primary damage primary damgage secondary damage zone zone zone zone
10000
Cleghorn Powell Canyon 1000 ?
Eastwood ? Lakeview 100 Grass Valley Increasing Slip (meters) 10 damage zone is essentially absent at less than ten meters of slip
100 10 1 10 100 footwall (meters) hanging wall
primary damage zone secondary damage zone Figure 2-41. Diagram showing the extent of the primary and secondary damage zones for five major fault zones studied. The Westwood fault is not included since displacement is not known. The footwall is to the left of center and the hanging wall is to the right. The Grass Valley fault zone is the only one with data from both fault blocks. Eastwood fault data come from the hanging wall and Lakeview fault data come from the footwall. Primary damage zones typically have a half-thickness of approximately 5 m while secondary damage zones have a half-thickness on the order of tens of meters. 118 CHAPTER 3
THE CEDAR SPRINGS FAULT SYSTEM: ANALYSIS OF MICROSEISMICITY
AND COMPARISON TO THE PUENTE HILLS BLIND-THRUST SYSTEM2
Abstract
The Cedar Springs fault system is a late Miocene oblique reverse fault system that has been exhumed from 1-2 km depth due to uplift of the San Bernardino Mountains in southern California. Three topics regarding the Cedar Springs fault system are explored.
Firstly, there is a lack of microseismicity associated with the Cedar Springs fault system from 1983-2001 despite the favorable orientation of these faults with respect to the modern maximum horizontal stress. We propose that steepening of these faults due to the western San Bernardino arch is keeping them locked from failure. However, an historical earthquake and the presence of other major faults in the area (i.e., Santa Ana thrust, North Frontal thrust, and San Andreas fault) may relieve the stress field, thus decreasing the seismicity in the Silverwood Lake area. Secondly, the Eastwood fault (a thrust fault within the Cedar Springs fault system) is used as an analog to the Puente Hills blind-thrust beneath the Los Angeles basin. Uplift of the San Bernardino Mountains exposes the Eastwood fault where it juxtaposes crystalline basement against the Crowder
Formation, a Miocene sandstone and conglomerate. The fault core varies from 2 to 5 cm in thickness and is planar. The primary slip surface is against the footwall (Crowder
Formation fault wall) and creates a more pronounced foliation in the gouge, possibly due to the softer nature of the sandstone. Thirdly, thickness constraints are given for damage
2 Coauthored by Joseph R. Jacobs and James P. Evans. 119 zones from five different faults in the Silverwood Lake area and show thickening with increased slip. The highly fractured and altered rocks throughout these zones would have a lower velocity and density as compared to the undeformed host rock. This elastic heterogeneity makes it difficult to accurately image the detailed structure of fault zones in the subsurface and shows the importance of analyzing exhumed faults in addition to seismic reflection work.
3-1. Introduction
The western San Bernardino Mountains are characterized by a late Miocene high- angle reverse fault system due to the obliquity between plate motion (NW-SE) and the
WNW-ESE strike of the San Andreas fault in southern California in the “Big Bend”, thus causing oblique convergence (Meisling and Weldon, 1989; Spotila and Sieh, 2000). The transpressional fault system in the Silverwood Lake area was coined the Cedar Springs
System by Meisling and Weldon (1989) and is the topic of this paper. The faults strike
NW-SE to E-W and generally dip to the NE or N, respectively (Figure 3-1). East-west- striking segments typically exhibit a pure north-side-up dip-slip motion, whereas northwest-striking segments often have a component of strike-slip, thus making them right-oblique slip faults (Meisling and Weldon, 1989). A structural and geochemical characterization of several fault zones, along with thickness-displacement correlation testing of many faults, was done on the Cedar Springs fault system and presented in
Chapter 2 of this thesis. The focus of this paper is on the microseismicity of the study area and on the Eastwood fault zone, a thrust sheet that places crystalline bedrock on top of the Crowder Formation, a Miocene sandstone and conglomerate (Figure 3-1). The 120 frontal thrust of this fault zone propagated from the crystalline bedrock into softer sedimentary rocks, similar to blind thrust faults beneath the Los Angeles basin. Thus, the
Eastwood fault zone is a good analog for blind thrust faults at depth. Direct observations of the Eastwood fault are the result of 1-2 km of exhumation in the hanging wall of the
Santa Ana and North Frontal thrust faults in the San Bernardino Mountains (Spotila and
Sieh, 2000). In this paper we examine the mesoscopic structure of the Eastwood fault and discuss the similarities to the Puente Hills thrust system and the effects that fault damage has on seismic reflection.
3-2. Microseismicity
Historical seismic data compiled by the Southern California Earthquake Center
(SCEC) reveal a pronounced lack of microseismicity beneath the field area (Aguilar et al., 2003). Microseismicity studies in many parts of southern California typically record very large numbers of small earthquakes (Abercrombie and Brune, 1994). From August
1, 1983 to August 2, 2001, a total of 33 events were recorded beneath the field area.
These events ranged from M 0 to M 2.93 and 2.8 to 11 km in depth (Figure 3-2). There are other areas in southern California, and even areas along the San Andreas fault that are not seismically active. However, the lack of microseismicity beneath the Cedar Springs fault system is puzzling because the faults seem to be optimally oriented for failure.
Townend and Zoback (2001) show the maximum horizontal stress in the area to be NNE-
SSW. The WNW-ESE strike of the faults is favorable for reverse dip-slip motion with the given stress state (Figure 3-1) and yet there is a pronounced lack of microseismicity. 121 We propose several possibilities that may contribute to the lack of microseismicity in the field area.
1. Moderate-size earthquakes occurred in 1857, 1881, 1901, 1922, 1934, and 1966 on
the Parkfield segment of the San Andreas fault (Bakun and McEvilly, 1979). Bakun
and Lindh (1985) predicted that a moderate size earthquake would occur on the
Parkfield segment between 1985 and 1993. However, this predicted earthquake did
not occur. The Coalinga and Nunez earthquakes of 1983 have been proposed to be a
possible reason for the interruption in the cyclic seismicity by reducing the shear
stress on the Parkfield segment (Toda and Stein, 2002). The Parkfield segment finally
ruptured in 2004. Similarly, a moderate event occurred near the southwest corner of
the field area in 1899. The National Earthquake Information Center (NEIC) database
shows this as a M 6.5, while the SCEC database shows a M 5.7. This historical
earthquake may have reduced the shear stress on the Cedar Springs fault system
similar to the Coalinga and Nunez effects on the Parkfield segment.
2. The North Frontal and Santa Ana thrust faults bound the San Bernardino Mountains
to the north and south, respectively (Figure 3-3). Spotila and Sieh (2000) estimate
average vertical displacements of 1.7 km for the North Frontal thrust system and 1.0
km for the Santa Ana thrust. Displacements along these two faults may accommodate
the majority of the stress, thus isolating parts of the range. Slip on the San Andreas
fault in southern California may further decrease the amount of stress that reaches the
field area. The Big Bear area east of Silverwood Lake is seismically active (M 6.5 in
1992 related to the Landers earthquake and M 5.4 in 2003), which is not consistent
with an isolated range model. However, the Big Bear area is structurally different 122 and would not be expected to have the same seismic behavior as the Silverwood
Lake area. There are many NW-SE striking faults in Big Bear that appear to be
related to the Helendale fault, possibly causing it to be more active.
3. The axial trace of the Pleistocene western San Bernardino arch trends through the
southwest portion of the field area parallel to the San Andreas fault and increases the
dip of the north-dipping faults by 20° to 30° (Meisling and Weldon, 1989). This
steepening of the original fault geometry may cause the Cedar Springs fault system to
be locked from current failure based on Andersonian fault mechanics. Many of the
faults have a current dip of 70° N or more. This greatly decreases the resolved shear
stress on the fault plane making failure less likely to occur.
3-3. Eastwood Fault and Puente Hills Blind-Thrust Analog
3-3.1 Fault System Properties
The northern Los Angeles basin is underlain by an active blind-thrust system, termed the Puente Hills blind-thrust (Shaw et al., 2002). This thrust system is composed of three soft-linked (i.e., overlapping boundaries without tear faults) en echelon segments
(Los Angeles, Santa Fe Springs, and Coyote Hills, from west to east) that generally strike east-west and dip to the north at 25° to 30° (Shaw et al., 2002). Estimated slip at the center of these three segments ranges from 1000 m on the Santa Fe Springs segment to
2000 m on the Coyote Hills segment, with slip as low as a few hundred meters at the segment boundaries (Shaw et al., 2002). These faults create growth structures that are diagnostic of fault bend and fault-propagation folding (Suppe et al., 1992; Shaw and
Suppe, 1994; Allmendinger, 1998; Shaw et al., 2002). Allmendinger and Shaw (2002) 123 modeled some structures above the blind thrusts using trishear (e.g., Erslev, 1991;
Hardy and Ford, 1997; Allmendinger, 1998) and showed that fold shapes are consistent with the geometry of the underlying fault.
The Eastwood fault in the Silverwood Lake area is used here as an analog for the
Puente Hills blind thrust system. Surface exposure of the Eastwood fault in crystalline basement is afforded due to uplift of the San Bernardino Mountains, something which is not available for the blind thrusts (Figures 3-4 and 3-5). Although the blind-thrust segments of the Puente Hills system are larger than the Eastwood fault in terms of both trace length and slip, the two systems are comparable. Slip on the Eastwood fault is a minimum of 183 m. The orientations of these faults are also very similar. The Eastwood fault and Puente Hills thrusts both have an overall east-west strike and dip to the north.
Most importantly, both of these faults propagate from crystalline rock into softer sedimentary rocks.
3-3.2 Trishear Modeling
Fault bend and fault-propagation folding have been documented above the Puente
Hills blind-thrust system (Allmendinger and Shaw, 2002; Shaw et al., 2002). The
Eastwood reverse may have also produced fault-propagation folds in the crystalline basement and this can be modeled using trishear (e.g., Erslev, 1991). FaultFold software of Allmendinger was used to model the Eastwood fault (Figure 3-6). The model indicates the formation of a footwall syncline and hanging wall anticline as would be expected for a thrust fault. The model is then rotated clockwise by 23° to simulate tilting from the western San Bernardino arch. The normal fault to the NNE of the frontal thrust 124 is also added to the model, while the less significant thrust faults are left out for simplicity. This final model is compared with the cross-section of the Eastwood thrust sheet (Figure 3-7). Topography is superimposed on the model using the footwall cutoff of the Crowder Formation as a guide. Removal of the upper part of the model (via erosion) yields a cross-section that matches the observed geology.
3-3.3 Eastwood Fault Implications
Although the Puente Hills system has been imaged on seismic reflection profiles
(Shaw et al., 2002), the small-scale structure of these thrust faults are not known.
Exposure of the Eastwood fault at Silverwood Lake captures the basement-sandstone fault contact approximately 30 m from the base of the Crowder Formation (Figure 3-8).
The fault core varies from 2 to 5 cm in thickness and is planar. The primary slip surface appears to be against the footwall (Crowder Formation fault wall) and creates a more pronounced foliation in the gouge. This may be attributed to the lithologic contrast between fault blocks, in which the fault favors the softer sedimentary rocks.
Alternatively, this asymmetry in the fault core may be due to the mechanical processes of thrust faults, where the overriding block of basement rock is displaced while the footwall block is more stationary, thus causing slip to be focused along the sandstone fault wall.
There is much work on thrust faults that analyzes fault-propagation folds and growth strata in order to better understand the nature of thrusting (Suppe et al., 1992;
Shaw and Suppe, 1994; Schneider et al., 1996; Allmendinger, 1998; Allmendinger and
Shaw, 2002; Shaw et al., 2002). Work on the Puente Hills thrust system by Shaw et al.
(2002) is performed in Cenozoic strata and analyzes fault-related folds to produce 125 Quaternary slip profiles along each fault segment. Industry and high-resolution seismic reflection profiles are used to image the folds above the thrust faults (Shaw et al.,
2002, and references therein). The seismic reflection profile across the Santa Fe Springs segment of the Puente Hills thrust shows a hanging wall anticline and growth triangles in the Quaternary section (Figure 3-9). The sedimentary cover above the Eastwood fault, albeit much thinner than in the Los Angeles basin, has been eroded due to uplift of the
San Bernardino Mountains, exposing this thrust fault where it juxtaposes crystalline basement against the Crowder Formation (Figure 3-8). This allows us to analyze fault structure and lithology at depth and predict what occurred above the fault (“bottom-up” approach). This is different than the “top-down” approach used on the Puente Hills thrust
(Shaw et al., 2002) and helps to further constrain fault processes at depth.
Mesoscopic mapping of the Eastwood fault zone, along with microstructural analysis, suggests division of the fault zone into a fault core and damage zone as previously documented by Chester et al. (1993) for the North Branch San Gabriel and
Punchbowl faults. We further divide the damage zone into primary and secondary zones, where the primary damage zone is defined by intense microstructural damage, grain-size reduction, and chemical alteration and the secondary damage zone is characterized by an elevated density of mesoscopic damage compared to the host rock. The primary and secondary damage zones have a typical half-thickness (i.e., the thickness on one side of the fault) of approximately 5 meters and tens of meters, respectively, for faults in the
Cedar Springs system. The highly fractured and altered rocks throughout a fault zone would have a lower velocity and density as compared to the undeformed host rock. This elastic heterogeneity produces scattering of seismic waves (Chavez-Perez, 1997) and can 126 be caused by fractures and faults as small as the meter-scale (Chavez-Perez, 1997, and references therein). Fault zones can also trap waves (Ben-Zion and Malin, 1991; Li et al., 1994; Li and Vidale, 1996), creating a low-velocity layer (Chavez-Perez, 1997).
These effects make it difficult to accurately image the detailed structure of fault zones and show the importance of analyzing exhumed faults in addition to seismic reflection work.
3-4. Conclusions
The late Miocene Cedar Springs fault system is a high-angle transpressional system in the Silverwood Lake area, western San Bernardino Mountains (Meisling and
Weldon, 1989). Townend and Zoback (2001) show the maximum horizontal stress in the area to be NNE-SSW. The WNW-ESE strike of the faults is favorable for reverse dip- slip motion with the given stress state. Microseismicity from 1983 to 2001 is relatively scarce in this area (Aguilar et al., 2003) despite the favorable orientation for fault failure.
We propose three mechanisms for the lack of seismicity: (1) An historical earthquake in
1899 from the southwest corner of the field area may have reduced the shear and
Coulomb stress on the Cedar Springs fault system similar to the Coalinga and Nunez effects on the Parkfield segment of the San Andreas fault; (2) Displacements along the range-bounding faults (Santa Ana and North Frontal thrusts) and the San Andreas fault may accommodate the majority of the stress, thus isolating the Silverwood Lake area from the stress field; and (3) Steepening of the original fault geometry due to the western
San Bernardino arch may cause the Cedar Springs fault system to be locked from current failure based on Andersonian fault mechanics. 127 The Eastwood fault is used here as an analog with Puente Hills blind-thrust system beneath the Los Angeles basin. Fault bend and fault-propagation folding have been documented above the Puente Hills blind-thrust system (Allmendinger and Shaw,
2002; Shaw et al., 2002). The Eastwood reverse also appears to have fault-propagation folding of the crystalline basement and can be modeled successfully using trishear (e.g.,
Erslev, 1991; Hardy and Ford, 1997; Allmendinger, 1998). The sedimentary cover above the Eastwood fault, albeit much less than in the Los Angeles basin, has been eroded due to uplift of the San Bernardino Mountains, exposing this thrust fault where it juxtaposes crystalline basement against the Crowder Formation. The fault core varies from 2 to 5 cm in thickness and is planar. The primary slip surface appears to be against the footwall
(Crowder Formation fault wall) and creates a more pronounced foliation in the gouge, possibly due to the softer nature of the sandstone.
Thickness constraints are given for damage zones from five different faults in the
Silverwood Lake area and show thickening with increased slip. The primary and secondary damage zones have a typical half-thickness of approximately 5 meters and tens of meters, respectively. The highly fractured and altered rocks throughout these zones would have a lower velocity and density as compared to the undeformed host rock. This elastic heterogeneity produces scattering (Chavez-Perez, 1997) and trapping (Ben-Zion and Malin, 1991; Li et al., 1994; Li and Vidale, 1996) of seismic waves. These effects make it difficult to accurately image the detailed structure of fault zones and show the importance of analyzing exhumed faults in addition to seismic reflection work.
128 References
Abercrombie, R. E., and J. Brune (1994), Microseismicity rates in California and Nevada; does the b-value change at small values, Seis. Res. Lett., 65, 33.
Aguilar, E., J. P. Evans, J. R. Jacobs, S. M. Kirby, and S. U. Janecke (2003), Analysis of regional seismicity in two areas related to the Pacific-North American transform plate boundary, poster presented at SCEC Annual Meeting, Oxnard, Calif., 7-11 September.
Allmendinger, R. W. (1998), Inverse and forward numerical modeling of trishear fault- propagation folds, Tectonics, 17, 640-656.
Allmendinger, R. W., and J. H. Shaw (2002), Estimation of fault-propagation distance from fold shape: implications for earthquake hazards assessment, Geology, 28, 1099-1102.
Bakun, W. H., and A. G. Lindh (1985), The Parkfield, California earthquake prediction experiment, Science, 229, 619-624.
Bakun, W. H., and T. V. McEvilly (1979), Earthquakes near Parkfield, California: Comparing the 1934 and 1966 sequences, Science, 205, 1375-1377.
Ben-Zion, Y., and P. Malin (1991), San Andreas fault zone head waves near Parkfield, California, Science, 251, 1592-1594.
Chavez-Perez, S. (1997), Enhanced imaging of fault zones in southern California from seismic reflection studies, Ph.D. thesis, 130 pp., University of Nevada, Reno.
Chester, F. M., J. P. Evans, and R. L. Biegel (1993), Internal structure and weakening mechanisms of the San Andreas Fault, J. Geophys. Res., 98, 771-786.
Erslev, E. A. (1991), Trishear fault-propagation folding, Geology, 19, 617-620.
Hardy, S., and M. Ford (1997), Numerical modeling of trishear fault propagation folding, Tectonics, 16, 841-854.
Li, Y-G., K. Aki, D. Adams, A. Hasemi, and W. H. K. Lee (1994), Seismic guided waves trapped in the fault zone of the Landers, California, earthquake of 1992, J. Geophys. Res., 99, 11705-11722.
Li, Y-G., and J. E. Vidale (1996), Low-velocity fault-zone guided waves: numerical investigations of trapping efficiency, B. Seismol. Soc. Am., 86, 371-378.
129 Meisling, K. E., and R. J. Weldon (1989), Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California, Geol. Soc. Am. Bull., 101, 106-128.
Schneider, C. L., C. Hummon, R. S. Yeats, and G. Huftile (1996), Structural evolution of the northern Los Angeles basin, California, based on growth strata, Tectonics, 15, 341-355.
Shaw, J. H., and J. Suppe (1994), Active faulting and growth folding in the eastern Santa Barbara Channel, California, Geol. Soc. Am. Bull., 106, 607-626.
Shaw, J. H., A. Plesch, J. F. Dolan, T. L. Pratt, and P. Fiore (2002), Puente Hills blind- thrust system, Los Angeles, California, B. Seismol. Soc. Am., 92, 2946-2960.
Spotila, J. A., and K. Sieh (2000), Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface, Tectonics, 19, 589-615.
Suppe, J., G. T. Chou, and S. C. Hook (1992), Rates of folding and faulting determined from growth strata, in Thrust Tectonics, edited by K. R. McClay, pp. 105-121, Chapman Hall, London.
Toda, S., and R. S. Stein (2002), Response of the San Andreas Fault to the 1983 Coalinga-Nunez earthquakes: an application of interaction based probabilities for Parkfield, J. Geophys. Res., 107(B6), ESE 6, doi: 10.1029/2001JB000172.
Townend, J., and M. D. Zoback (2001), Implications of earthquake focal mechanisms for the frictional strength of the San Andreas fault system, in The Nature and Tectonic Significance of Fault Zone Weakening, edited by R. E. Holdsworth, R. A. Strachan, J. F. Magloughlin, and R. J. Knipe, pp. 13-21, Geol. Soc. London, Spec. Pub., 186.
130
SHmax 34o 20' 40'' I 34 o 20' 40''
Eastwood fault
,....,__ _._l o 34 o 15' 34 15' o 117 o 14' 08'' 117 26'15'' SHmax ! 1 5 km D Younger Alluvium D Pliocene Phelan Peak Deposits Older Alluvium Miocene Crowder Formation Pleistocene D D Shoemaker Gravel D D Mesozoic crystalline basement complex D Harold Formation
60 ___L_ BEDDING ATT ITUDE
~-6..::..::'.. FOLIATION ATIITUDE
Figure 3-1. Simplified geologic map of the ---- ...... ? Silverwood Lake area showing orientation of CONTACT; CONCEA LED; INFERRED maximum horizontal stress (SHmax) from Townend and Zoback (2001). Faults have a favorable strike 45 t u _. • ... .. 7 . D ....- for reverse dip-slip failure. White box shows FAULT : DIP: SLIP ; THRUS T approximate extent of Figure 3-4. Adapted from CONCEA LED; INFERRED Meisling and Weldon (1989). ---!J,-···...... ? .. . --f~ _ ...... 7 .. . ANTICLINE : SYNCLIN E CONCEA LED: INFERRED
131
(a) N
o o 117 o 26' 15'' 117 14' 08'' o ~ o 34 o 20' 40'' ,_ . 34 o 20' 40'' • .
I rrn11 • W • • • E •• • II ... .. • o •• o 34 o 15' • 34 o 15'
T,...~
I ~ " " I" I I (b) ' ' I " I " " , ' , • I • I ~ • •• I • \ I • j •·-~ •• • . • I - J I s: w I '\ ' ' J '/ I I \ / I I ' 0 I I I I ~ I '\ J. I I I I \\- ....{)\- - (\ \\] - - \ \ \ J\P\1'~ \\/\ i\ 'fj\ -
Figure 3-2. Microseismicity plots of 33 events from 1983-2001 for the Silverwood Lake area. Events range from M 0 to M 2.93 and from 2.8 to 11 km in depth. (a) Map view of plot. (b) Oblique 3-D view of plot. 132
Mohave Desert San Gabriel Los Angeles Mountains Helendale fault
North Frontal thrust
San Andreas fault
San Bernardino Mill Creek faultMountains 5 km Santa Ana thrust
Figure 3-3. Oblique along-strike view to the northwest of the San Andreas fault. The fault divides the San Gabriel Mountains to the west from the San Bernardino Mountains to the east. White box shows approximate location of field area. Since view is oblique, scale bar applies only to its present location. Base map from Bowen (2003), the Electronic Map Library, http://130.166.124.2/library.html. 133
20 N 30 55
D U
C'
48 150 m
87 80 81 88
80 Pleistocene ,,,,,,,,,,,,,,.,,.,,.,, 70 .. 73 Younger Alluvium Eastwood fault
25 Silverwood Miocene Lake 58 U Crowder Formation 23 D
C Cretaceous Crystalline Basement Complex
Trace of fault with dip amount 48 U fault block has moved up Dashed where inferred 23 Strike and dip of bedding fault block has moved down Dotted where covered D
Figure 3-4. Geologic map of the Eastwood fault area combining work from Weldon (1986) with field mapping performed as part of this research. See Figure 3-1 for the location of this map and Figure 3-5 for cross section C-C'. 134
(a) C' C(SSW) (NNE)
1-2 km below sea level
D Miocene Crowder Formation D Cretaceous Crystalline Basement (b)
C (SSW) (NNE) C' feet meters above above sea level sea level
3500' 1065
3000' 915
100' No vertical exaggeration
Figure 3-5. Cross section C-C' to accompany Figure 3-4. (a) Cross- section showing proposed original fault geometries after removing the northeast tilt from the western San Bernardino arch. (b) Cross-section showing present fault geometry without removal of the western San Bernardino arch. 135
(a) ground surface
?
000 0 0000 0 0000 0 0000 0 00 -•- Crowder Formation
8888888888888888888 ____ Cretaceous Crystalline Basement Complex
Eastwood fault
(b)
ground surface
00 0000 0 0000 0 0000 0 0000 0 0000 0 0000 0 0000 0 000 1-2 km 8888888888888888888888888888888888888
Eastwood fault Slip = 301.55 Propagation = 452.33 Ramp angle = -34.000 trishear angle = 60.000 P/S ratio = 1.5000
Figure 3-6. Trishear model of the Eastwood fault using FaultFold software of Allmendinger. Original fault geometry (after removing NE tilt from western San Bernardino arch) is used. (a) Pre-fault rupture model showing the Crowder Formation on basement rock at unknown depth. (b) Model results showing rupture of Eastwood fault through the basement rock and Crowder Formation. Input parameters are shown. P/S is the ratio of propagation to slip. Model shows base of the Crowder Formation only, not the entire thickness of the unit. 136
(a)
C (SSW) (NNE) C' feet meters above above sea level sea level I 3500' \ 1065 \, \ \ \ \ \ \ ~ I. 3000' ' \ 915 ' ,, ' ' 100' No vertical exaggeration Miocene Crowder Formation (b) D D Cretaceous Crystalline Basement
(SSW) (NNE)
topography
crystalline basement
Figure 3-7. Comparison of trishear model with present geology. (a) Cross- section through the Eastwood thrust sheet showing current fault geometries. Less significant faults are shown as dashed lines. (b) Trishear model of Eastwood thrust sheet using only the frontal thrust and normal fault with FaultFold software of Allmendinger. Strain elipses are shown for reference. Topography from cross-section is overlain on the model using the footwall cutoff of the frontal thrust as a guide. Once the upper half of the model is removed by erosion, it matches the observed geology. Model shows lower 30 m of the Crowder Formation, not entire thickness. 137
N S (a)
granite hanging wall
sandstone footwall
(b) (c)
fault fault core core
Figure 3-8. Photographs of the Eastwood fault. At least 183 m of slip has occurred on this 2-5 cm thick fault core. (a) View to the east showing granite thrust upon the Miocene Crowder Formation. Note the variation in fault core thickness. (b) Thicker portion of the fault core. Note the more foliated gouge against the footwall. (c) Thin portion of the fault core. The compass for scale is 7 cm on a side. 138
Fault fault (slip)
secondary damage primary damage primary damgage secondary damage zone zone zone zone
Cleghorn (3.5 km)
Powell Canyon (1.2 km) ?
Eastwood (183 m) Increasing Slip Lakeview ? (180 m)
Grass Valley (39 m)
100 10 1 10 100 footwall (meters) hanging wall
primary damage zone secondary damage zone
Figure 3-9. Diagram showing the extent of the primary and secondary damage zones for five major fault zones in the Silverwood Lake area. The footwall is to the left of center and the hanging wall is to the right. Faults increase upward with slip and range from 39 m on the Grass Valley fault to 3.5 km on the Cleghorn fault. Primary damage zones typically have a half-thickness of approximately 5 m while secondary damage zones have a half-thickness on the order of tens of meters. The damage zone would have a decreased velocity and density relative to the undamaged host rock and may therefore cause scattering or trapping of seismic waves. 139 CHAPTER 4
CONCLUSIONS
The late Miocene Cedar Springs fault system is a high-angle transpressional system in the Silverwood Lake area, western San Bernardino Mountains, southern
California (Meisling and Weldon, 1989). A total of 35 predominantly reverse faults are examined in the Silverwood Lake area in order to continue the structural and compositional approach (e.g., Chester and Logan, 1986, 1987; Chester et al., 1993; Mitra,
1993) to understanding fault growth processes. The topic of fault growth and evolution is of much interest to the seismological community because it relates to numerous subjects of earthquake mechanics, such as the width of the fault surface that slips during an earthquake (Kanamori and Heaton, 2000), fault zone structure (Rubin, 1999), and rupture propagation processes (Heaton, 1990; Andrews and Ben-Zion, 1997; Harris and Day,
1999). This thesis studies faults with modest amounts of slip (meters to hundreds of meters), which allows for characterization at early stages of fault development using slip as a proxy for time. A detailed structural and geochemical characterization is provided for six fault zones with various amounts of slip. Basic geometric and kinematic results are provided for an additional 29 small-displacement (cm- to m-scale) faults. The fault exposures studied here have been exhumed from 1-2 km depth (Spotila and Sieh, 2000) and therefore have not been affected by the “free face,” or surface of the earth, which can drastically change fault geometry (e.g., surface expression of the San Andreas fault).
Therefore, these faults are good analogs for faults at seismic depth. 140 The main faults of this study can be divided into the fault core, primary damage zone, and secondary damage zone, which fit the combined conduit-barrier model of Caine et al. (1996). Fault cores are usually well-defined and consist of clay gouge and/or microbreccia. Fault cores range from millimeters to tens of centimeters in thickness for the faults studied. Based on microscopic and geochemical analysis, the fault cores are characterized by a mixture of comminuted protolith and alteration products, such as smectite, laumontite, and palygorskite. The primary damage zone
(“gouge and breccia zone” of Biegel and Sammis, manuscript in preparation, 2004) can be defined as the area immediately surrounding the fault core that is characterized by microstructural damage, grain-size reduction, and chemical alteration. This zone is typically several meters in half-thickness and is recognized with optical microscopy and geochemical analysis, as well as outcrop observation. In general, both the fault core and primary damage zone have a decrease in mobile elements relative to the host rock, indicating enhanced fluid-rock interactions in these zones. The secondary damage zone
(“wall rock damage zone” of Biegel and Sammis, manuscript in preparation, 2004) has a similar microstructure and mineralogy as the host rock, but has an increased fracture density. This is a thicker, more gradational zone that is on the order of tens of meters in half-thickness. The highly fractured and altered rocks throughout the primary and secondary damage zones would have a lower velocity and density as compared to the undeformed host rock. This elastic heterogeneity may produce scattering (Chavez-Perez,
1997) and trapping (Ben-Zion and Malin, 1991; Li et al., 1994; Li and Vidale, 1996) of seismic waves. These effects make it difficult to accurately image the detailed structure 141 of fault zones and show the importance of analyzing exhumed faults in addition to
seismic reflection work.
Although there is a general increase in fault core thickness with slip, there is not a
significant correlation when analyzing all faults. Both the primary and secondary
damage zones appear to thicken with increased slip on the main fault. Overall, the
structure and composition of the relatively small-displacement faults (< 200 m slip)
studied here are similar to larger strike-slip and reverse faults (Chester et al., 1993; Mitra,
1993; Yonkee and Mitra, 1993), indicating that the fault core and damage zone develop early in a fault’s history. However, the fault cores studied here are composed of non- cohesive gouge and breccia, unlike the cohesive (ultra-)cataclasites from the North
Branch San Gabriel and Punchbowl faults (Chester and Logan, 1986; Chester et al.,
1993) and the Laramide-style reverse faults in northwestern Wyoming (Mitra, 1993;
Yonkee and Mitra, 1993). This difference can be attributed to the more shallow formation depth (1-2 km) of the faults in the Silverwood Lake area (e.g., Reed, 1964;
Sibson 1977).
Lachenbruch and McGarr (1990) developed a model of the energy balance for earthquakes, where the total earthquake energy must equal the radiated seismic energy, plus the energy lost to heat dissipation, plus unknown sinks of energy. More recent energy budgets have been proposed that include frictional energy loss and fracture energy associated with fault propagation (e.g., Kanamori and Heaton, 2000), but it is still agreed upon that there are unknown or unconstrained sinks of energy. Chemical alteration may be one of these sinks of energy in fault zones (Evans and Chester, 1995; Lay and
Wallace, 1995, pp. 394-395; Schulz and Evans, 1998). Although it is well documented 142 that mineral alterations do occur in fault zones, particularly in the fault core (Vrolijk
and van der Pluijm, 1999; Choo and Chang, 2000; Solum et al., 2003), estimates of how
much energy may go into the alteration process were not found in the literature. Rough
calculations made in this thesis indicate that the total energy associated with chemical
alteration in the fault core over the entire life of the Eastwood fault is 4.6 x 1011 kJ. A total energy release of 2 x 1012 kJ is estimated for the history of the Eastwood fault based on the Gutenburg-Richter law (Main et al., 1999) and the fact that there is a linear relationship between magnitude and source dimension (Abercrombie, 1995). Our calculations suggest that syn-tectonic alteration in the fault core may consume approximately 20% of the total earthquake energy.
Three mechanisms are proposed for the scarce microseismicity of the Cedar
Springs fault system (Aguilar et al., 2003) despite the favorable orientation for failure: (1)
An historical earthquake in 1899 from the southwest corner of the field area may have reduced the shear and Coulomb stress on the Cedar Springs fault system similar to the
Coalinga and Nunez effects on the Parkfield segment of the San Andreas fault; (2)
Displacements along the range-bounding faults (Santa Ana and North Frontal thrusts) and the San Andreas fault may accommodate the majority of the stress, thus isolating the
Silverwood Lake area from the stress field; and (3) Steepening of the original fault geometry due to the western San Bernardino arch may cause the Cedar Springs fault system to be locked from current failure based on Andersonian fault mechanics.
The Eastwood fault is used as an analog to the Puente Hills blind-thrust system beneath the Los Angeles basin. Fault bend and fault-propagation folding have been documented above the Puente Hills blind thrust system (Allmendinger and Shaw, 2002; 143 Shaw et al., 2002). The Eastwood fault also appears to have fault-propagation folding involving the crystalline basement and can be modeled successfully using trishear. Uplift of the San Bernardino Mountains and subsequent erosion of the sedimentary cover exposes the Eastwood fault where it juxtaposes crystalline basement against the Crowder
Formation, a Miocene sandstone and conglomerate. The fault core varies from 2 to 5 cm in thickness and is planar. A slip preference to the sandstone (footwall) fault wall is observed as a foliated clay gouge layer and creates an asymmetry in the fault core. This is also seen in the Cleghorn fault core and is probably due to the softer nature of the
Crowder Formation.
The results of this work indicate that the narrow, clay-rich cores form early in the development of a fault. Subsequent slip appears to be focused along these narrow zones, with some deformation accumulating in the damage zone. The lack of observed pseudotachylyte and the increased LOI values (indicating trapped fluids) in the fault core imply that fluid pressurization is an important mechanism for reducing shear resistance during fault rupture. Fault processes are similar throughout the different stages of development, and large amounts of slip can occur on very narrow slip surfaces.
Therefore, the study of relatively small-displacement faults can be used to understand fault evolution through time and the processes of larger faults in the brittle crust.
144 References
Abercrombie, R. E. (1995), Earthquake source scaling relationships from -1 to 5 ML using seismograms recorded at 2.5-km depth, J. Geophys. Res., 100, 24,015- 24,036.
Aguilar, E., J. P. Evans, J. R. Jacobs, S. M. Kirby, and S. U. Janecke (2003), Analysis of regional seismicity in two areas related to the Pacific-North American transform plate boundary, poster presented at SCEC Annual Meeting, Oxnard, Calif., 7-11 September.
Allmendinger, R. W., and J. H. Shaw (2002), Estimation of fault-propagation distance from fold shape: implications for earthquake hazards assessment, Geology, 28, 1099-1102.
Andrews, D. J., and Y. Ben-Zion (1997), Wrinkle-like slip pulse on a fault between different materials, J. Geophys. Res., 102, 552-571.
Ben-Zion, Y., and P. Malin (1991), San Andreas fault zone head waves near Parkfield, California, Science, 251, 1592-1594.
Caine, J. S., J. P. Evans, and C. B. Forster (1996), Fault zone architecture and permeability structure, Geology, 24, 1025-1028.
Chavez-Perez, S. (1997), Enhanced imaging of fault zones in southern California from seismic reflection studies, Ph.D. thesis, 130 pp., Univ. of Nevada, Reno.
Chester, F. M., and J. M. Logan (1986), Implications for mechanical properties of brittle faults from observation of the Punchbowl fault zone, California, Pageoph, 124, 79-106.
Chester, F. M., and J. M. Logan (1987), Composite planar fabric of gouge from the Punchbowl Fault, California, J. Struct. Geol., 9, 621-634.
Chester, F. M., J. P. Evans, and R. L. Biegel (1993), Internal structure and weakening mechanisms of the San Andreas Fault, J. Geophys. Res., 98, 771-786.
Choo, C. O., and T. W. Chang (2000), Characteristics of clay minerals in gouges of the Dongrae fault, southeastern Korea, and implications for fault activity, Clay. Clay Miner., 48, 204-212.
Evans, J. P., and F. M. Chester (1995), Fluid-rock interaction in faults of the San Andreas system: Inferences from San Gabriel fault rock geochemistry and microstructures, J. Geophys. Res., 100, 13,007-13,020.
145 Harris, R. A., and S. M. Day (1999), Dynamic three-dimensional simulations of earthquakes on en echelon faults, Geophys. Res. Lett., 26, 2089-2092.
Heaton, T. H. (1990), Evidence for and implications of self-healing pulses of slip in earthquake rupture, Phys. Earth Planet In., 64, 1-20.
Kanamori, H., and T. H. Heaton (2000), Microscopic and macroscopic physics of earthquakes, Am. Geoph. Un. Mono., 120, 147-161.
Lachenbruch, A. H., and A. McGarr (1990), Stress and heat flow, in The San Andreas Fault System, California, edited by R. E. Wallace, pp. 261-277, U. S. Geol. Surv. Prof. Pap. 1515, Denver, Colo.
Lay, T., and T. C. Wallace (1995), Modern Global Seismology, edited by R. Dmowska and J. R. Holton, 521 pp., International Geophysics Series, vol. 58, Academic Press, San Diego, Calif.
Li, Y-G., K. Aki, D. Adams, A. Hasemi, and W. H. K. Lee (1994), Seismic guided waves trapped in the fault zone of the Landers, California, earthquake of 1992, J. Geophys. Res., 99, 11705-11722.
Li, Y-G., and J. E. Vidale (1996), Low-velocity fault-zone guided waves: numerical investigations of trapping efficiency, B. Seismol. Soc. Am., 86, 371-378.
Main, I., D. Irving, R. Musson, and A. Reading (1999), Constraints on the frequency- magnitude relation and maximum magnitudes in the UK from observed seismicity and glacio-isostatic recovery rates, Geophys. J. Int., 137, 535-550.
Meisling, K. E., and R. J. Weldon (1989), Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California, Geol. Soc. Am. Bull., 101, 106- 128.
Mitra, G. (1993), Deformation processes in brittle deformation zones in granitic basement rocks: A case study from the Torrey Creek area, Wind River Mountains, in Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, edited by C. J. Schmidt, R. B. Chase, and E. A. Erslev, pp. 177- 195, Geol. Soc. Am. Spec. Pap. 280, Boulder, Colo.
Reed, J. J. (1964), Mylonites, cataclasites, and associated rocks along the Alpine fault, South Island, New Zealand, New Zealand J. Geol. Geophys., 7, 645-684.
Rubin, A. M. (1999), Streaks of microearthquakes along creeping faults, Nature, 400, 635-641.
146 Schulz, S. E., and J. P. Evans (1998), Spatial variability in microscopic deformation and composition of the Punchbowl fault, southern California: Implications for mechanisms, fluid-rock interaction, and fault morphology, Tectonophys., 295, 223-244.
Shaw, J. H., A. Plesch, J. F. Dolan, T. L. Pratt, and P. Fiore (2002), Puente Hills blind- thrust system, Los Angeles, California, B. Seismol. Soc. Am., 92, 2946-2960.
Sibson, R. H. (1977), Fault rocks and fault mechanisms, J. Geol. Soc. London, 133, 191- 213.
Solum, J. G., B. A. van der Pluijm, and D. R. Peacor (2003), Influence of phyllosilicate mineral assemblages, fabrics, and fluids on the behavior of the Punchbowl fault, southern California, J. Geophys. Res., 108(B5), 2233, doi: 10.1029/2002 JB001858.
Spotila, J. A., and K. Sieh (2000), Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface, Tectonics, 19, 589-615.
Vrolijk, P., and B. A. van der Pluijm (1999), Clay gouge, J. Struct. Geol., 21, 1039-1048.
Yonkee, W. A., and G. Mitra (1993), Comparison of basement deformation styles in parts of the Rocky Mountain foreland, Wyoming, and the Sevier orogenic belt, northern Utah, in Laramide Basement Deformation in the Rocky Mountain Foreland of the Western United States, edited by C. J. Schmidt, R. B. Chase, and E. A. Erslev, pp. 197-228, Geol. Soc. Am. Spec. Pap. 280, Boulder, Colo.
147
APPENDICES
148
APPENDIX A: Slip calculations for constrained faults
149 Station 15 Water Tank fault zone Area 2 fault 5: 108/90 dike: 320/23 NE separation = 10 cm 08 R W trace angle = 13 E
10 cm Station 9 Clipper reverse
fault 9: 282/65 N 90 rake slip = 6 cm 80 cm 50 cm fault 8: 230/87 NW trace angle = 75 E 75 282
horizontal separation = 80 cm trace of offset fault in HW slip = 3 m net slip
trace of offset
fault in FW
Station 15 Water Tank fault zone Area 1
fault 5: 278/65 N dike: 349/46 E horizontal 30 R W trace angle = 52 E separation = 13 m
13 m 278 2 m 52 30 slip = 10.5 m net slip trace of dike
in HW
trace of dike
in FW 150
Station 15 Water Tank fault zone Area 2
dike: 288/ 52 N horizontal fault 2: 345/13 E trace angle = 67 S separation = 75 cm 50 R N 50 cm 75 cm 345 trace of dike 67
in FW
trace of dike net slip
in HW 50 slip = 78 cm
fault 5: 108/90 dike: 320/23 NE vertical 07 R W trace angle: 13 E separation = 10 cm
5 cm
108
13 trace of dike 10 cm
07 net slip
trace of dike slip = 28.5 cm 151
Station 17 Eastwood reverse fault zone fault 6: 266/88 N dike: 035/65 E horizontal 45 R W trace angle: 58 E separation = 1.7 m
1.7 m 50 cm 58 266 trace of dike net slip
in FW 45 trace of dike
in HW
slip = 1.5 m 152
Station 32 Main Fault Grass Valley fault zone fault 1: 285/45 S dike A: 320/61 S horizontal 55 R E trace angle = 73 E separation = 12.2 m
5 m
10 topography 285 12.2 m
73
slip = 39 m
55
trace of dike
in footwall
net slip
in hanging wall trace of dike 153
Station 32 Hanging Wall Grass Valley fault zone
fault 2: 105/46 S dike A: 140/61 S horizontal 20 R E trace angle: 73 E separation = 1.5 m
0.5 m 105 topography 10
trace of dike
in FW
net slip 73
trace of dike 20
in HW
slip = 1.9 m
fault 2: 105/46 S dike D: 345/45 E horizontal 20 R E trace angle: 40 E separation = 1.2 m
1 m topography 10
40 105 trace of dike
net slip in FW trace of dike slip = 2.7 m in HW 20 154
Station 32 Hanging Wall Grass Valley fault zone fault 3: 090/46 S dike D: 345/45 E horizontal 30 R E trace angle: 47 E separation = 0.5 m
0.5 m 090 topography 10 0.5 m
47 trace of dike in FW net slip
trace of dike in HW slip = 1.5 m 30
fault 11: 117/79 S dike F: 073/33 S vertical 35 R E trace angle: 27 E separation: 10 cm
10 cm 117 10 cm 27 net slip trace of dike in FW trace of dike in HW 35
slip = 58 cm 155
Station 32 Hanging Wall Grass Valley fault zone fault 4: 105/53 S dike C: 320/46 E horizontal 30 R E trace angle: 24 E separation = 0.6 m
0.5 m 105 topography10
0.6 m 24 trace of dike slip = 3.3 m in HW net slip
trace of dike 30 in FW
fault 4: 105/53 S dike D: 345/45 E horizontal 30 R E trace angle: 39 E separation = 0.5 m
0.5 m topography10
0.5 m 105
39
net slip trace of dike slip = 2.3 m in FW trace of dike
in HW 30 156
Station 32 Hanging Wall Grass Valley fault zone
fault 6: 095/52 S dike F: 073/33 S vertical 30 R E trace angle: 28 E separation = 1.7 m
095 1.5 m 1.7 m trace of dike in FW
trace of dike in HW 28 slip = 36.8 m
net slip
30 157
Station 32 Footwall Grass Valley fault zone fault 4: 091/55 S dike: 141/58 SW 10 50 R E trace angle: 78 W topography 5 cm horizontal net slip50 separation = 5 cm 5 cm
091 slip = 5.7 cm
trace of dike in FW
78
trace of dike in HW
10 fault 6: 107/61 S dike: 152/60 SW 50 R W trace angle: 77 W topography 2 m 50 horizontal 2 m separation = 2 m net slip
107 slip = 4 m trace of dike in FW
trace of dike in HW 77
fault 16: 160/69 W fault 11: 106/55 S vertical 40 R N trace angle: 61 S separation: 25 cm
trace of offset
fault in FW
25 cm 10 cm 160
40
net slip
61
trace of offset slip = 13 cm fault in HW 158
Station 37 Main Sketch fault 3: 163/76 W dike A: 085/35 S vertical 63 R S trace angle: 36 S separation = 41 cm trace of dike in FW 163 50 cm 41 cm
net slip
trace of dike in HW 36 slip = 75 cm 63
fault 5: 112/81 S dike: 015/18 E horizontal 43 R W trace angle: 18 E separation = 80 cm
80 cm 50 cm 43 18 trace of dike in FW net slip
trace of dike in HW 112 slip = 28 cm
Small Sketch
fault 4: 081/81 S dike: 204/85 W vertical 57 R W trace angle: 77 W separation = 5 cm 5 cm
081 2.5 cm
77
slip = 3.3 cm
57
trace of dike in FW
net slip trace of dike in HW 159
Station 40 Seeley Junior
fault: 022/78 E dike: 093/61 S horizontal rake: 40 R N trace angle: 63 S separation = 43 cm
43 cm 50 cm
40 63 net slip trace of dike trace of dike
in HW 022 in FW slip = 40 cm
Station 55 fault: 289/70 N dike: 338/35 NE horizontal 85 R W trace angle: 34 E separation = 1.1 m
1 m
1.1 m trace of dike in HWslip = 70 cm 289
trace of dike 85in FW
net slip 34 160
Station 47 Lakeview fault fault 20: 302/30 NE fault 15: 050/65 SE horizontal 70 R W trace angle: 62 E separation = 2 cm
2 cm 1 cm 302
62 70 trace of offset trace of offset
fault in FW fault in HW slip = 2.4 cm
net slip
fault 20: 302/30 NE fault 16: 054/67 SE horizontal 70 R W trace angle: 60 E separation = 2 cm
slip ~ 2.4 cm (see above) fault 20: 302/30 NE fault 17: 052/59 SE horizontal 70 R W trace angle: 57 E separation = 2 cm
slip ~ 2.4 cm (see above) 161
Station 54 Westwood fault
fault 2: 158/18 SW fault 8: 101/73 S horizontal 45 R SE trace angle: 63 N separation = 10 cm
10 cm 5 cm 158
45 63 net slip slip = 9.4 cm trace offault offset in HW
trace offault offset in FW
fault 21: 160/23 W fault 20: 242/55 NW horizontal 55 R S trace angle: 81 N separation = 10 cm
10 cm 5 cm
trace of offset fault in FW 55 81 160 trace of offset fault in HW
net slip
slip = 14.5 cm
fault 21: 160/23 W fault 19: 278/80 N horizontal 55 R S trace angle: 60 N separation = 7 cm
7 cm 160 5 cm trace of offset fault in FW 55 60 trace of offset
fault in HW
net slip slip = 6.7 cm 162
Station 54 Westwood fault fault 56: 289/68 N mafic sliver: 195/32 W vertical 90 R trace angle: 34 W separation = 11 cm
5 cm
34 289 net slip 11 cm trace of sliver in HW
slip = 11 cm trace of sliver in FW
fault 58: 275/61 N gneiss: 172/48 W vertical 90 R trace angle: 47 W separation = 50 cm
25 cm
net slip 47
50 cm 275
trace of gneiss in HW
slip = 50 cm
trace of gneiss in FW
fault 57: 240/70 NW sliver: 156/34 SW vertical 50 R NE trace angle: 36 W separation = 5 cm
5 cm 5 cm
trace of sliver in HW 240 net slip50
trace of sliver in FW slip = 4 cm 36 163
Station 56 fault 9: 112/77 S upper dike: 235/15 NW vertical 85 R W trace angle: 13 W separation = 35 cm
15 cm
13 112 trace of dike in FW
85 35 cm
net slip slip = 35 cm
trace of dike in HW
fault 10: 112/62 S dike: 260/28 N horizontal 85 R E trace angle: 14 W separation = 30 cm
25 cm 30 cm
net slip trace of dike in FW 85 14 112
trace of dike in HW slip = 17 cm 164
Station 56
fault 11: 273/60 N gneiss: 223/07 NW vertical 70 R E trace angle: 06 W separation = 2 m
06 1 m trace of gneiss in HW
70 273 2 m
net slip
slip = 2 m
trace of gneiss in FW
(main fault sketch) fault 8: 295/71 N fault 7: 075/31 S vertical 90 R trace angle: 19 E separation = 6 cm
19 trace of offset fault in HW 5 cm
6 cm 295 net slip
trace of offset fault in FW slip = 6 cm 165
Station 58 fault 1: 010/78 E dike: 238/11 NW vertical 70 R S trace angle: 08 N separation = 25 cm
trace of dike in HW 25 cm --1 25 cm .. 010 net slip 70 slip = 25 cm
08 trace of dike in FW
fault 5: 030/66 SE upper dike: 265/36 N vertical 80 R S trace angle: 29 N separation = 14 cm
29 14 cm 25 cm 030 net slip --1 ------<~ 80 trace of dike in HW slip = 14 cm trace of dike in FW 166
,,,..__ / / / --- / -- :;'\ / / / \ / / \ / \ Block diagram illustrating (-. -- \ horizontal (H) and vertical I -----. \ I ---- \ (V) separation. These I \ measurements are used to I \ \ I calculate net slip in this \ I / / \ thesis. The dip (D) and I / \ / I / ) strike (S) separation are I / I / I also shown for reference. I / I From van der Pluijm and < .... I ...... I Marshak, 2004, Figure 8.10. .... 1, I I \ ...... I I I \ I \ I I \ I ...._..._ \ I .... -..I, Example of slip calculation for the Grass Valley fault. horizontal o topographyo 73 The fault plane is parallel to horizontal 10 73o 12.2 m the page. Horizontal
horizontal 5 m separation is measured hanging wall 55o cutoff along topographic slope and 105 the trace angle is from the horizontal. Measuring distance along direction of slip vector between dike in
trace of dike in footwall hanging wall and footwall yields the net slip of the fault 1: 105/45 S fault. 55 R E equals 55 55 R E slip vector degree rake from the east. dike A: 140/61 S The calculations that follow in hanging wall trace angle = 73 E trace of dike use this method, although vertical separation is horizontal separation = 12.2 m sometimes used. Many faults do not have a topographic slope to consider. dike footwall cutoff slip = 39 m fault o 73 167
APPENDIX B: Thin section descriptions
168
distance from Sample ID plane 1 plane 2 billet orientation arrow mineralogy fabric fault (m) medium damage; open and ultracat.-filled fractures; much 2-1 121/66 NE 011/58 W perpendicular to plane 1 031 Qz, plag, K-spar, hornblende, biotite, chlorite feldspar alteration 9.5 medium damage; open and ultracat.-filled fractures; much 2-2 005/57 W 094/85 S perpendicular to plane 2 004 Qz, plag, K-spar, hornblende, biotite, Fe2O3 feldspar alteration 16 3-1 no section fault core high damage; shattered grains and clay matrix; calcite, K- 3-2A 078/79 N 134/82 SW perpendicular to plane 1 348 Qz, K-spar, plag, calcite, clay, Fe2O3 spar, and ultracat. vein fill 0.3 high damage; shattered grains and clay matrix; calcite, K- 3-2B 078/79 N 134/82 SW parallel to plane 1 Qz, K-spar, plag, calcite, clay, Fe2O3 spar, and ultracat. vein fill 0.3 medium damage; cataclasite; open fractures and calcite and 3-3 113/76 N 014/96 E perpendicular to plane 1 023 Qz, plag, K-spar, calcite, Fe2O3 K-spar veins 0.55 7-1 300/54 NE 028/82 NW perpendicular to plane 1 210 Qz, K-spar, plag, biotite, muscovite, Fe2O3 little damage; schistosity; Fe2O3 and mica vein fill fault wall little damage except along fault wall, where there is 9-1 101/82 N 205/43 E perpendicular to plane 1 011 Qz, plag, K-spar, biotite, chlorite cataclasite and oriented biotite and chlorite; open fractures fault wall 11-1 no section fault core 11-2 065/60 S parallel to plane 1 none Qz, plag, K-spar little to medium damage; shattered clasts, feldspar alteration fault wall 11-3 no section hardened fin "fault 5" AOC 2 15-1 106/88 N 335/28 E perpendicular to plane 1 016 Qz, plag, K-spar, biotite little damage fault wall high damage; cataclasis, much alteration; open, Fe2O3, and "fault 4b" AOC 2 15-2 094/82 S 209/47 NW perpendicular to plane 1 004 Qz, plag, K-spar, biotite, Fe2O3 mica filled fractures fault wall "fault 5" AOC 1 15-3 265/68 N perpendicular to plane 1 175 Qz, plag, K-spar, biotite, hornblende medium damage; feldspar alteration; mostly open fractures fault wall "fault 5" AOC 2 15-4 no section fault core 17-10 no section 9 17-11 140/72 SW 050/62 NW perpendicular to plane 1 050 Qz, plag, hornblende, biotite, chlorite little damage 9.5 17-12 090/90 040/86 NW parallel to plane 1 ?? none Qz, plag, minor hornblende and mica medium damage; mostly open fractures "fault 6" fault wall high damage; mostly open fractures, some filled with clay, 17-1A 136/55 NE 200/34 NW perpendicular to plane 1 046 Qz, plag, biotite, muscovite, chlorite, Fe2O3 ultracat., and/or Fe2O3 0.05 high damage; mostly open fractures, some filled with clay, 17-1B 136/55 NE 200/34 NW parallel to plane 1 Qz, plag, biotite, muscovite, chlorite, Fe2O3 ultracat., and/or Fe2O3 0.05 17-2 170/45 W 305/90 perpendicular to plane 1 080 Qz, plag, K-spar, chlorite high damage; K-spar alteration and comminution 0.3 17-3 no section fault core 17-4 121/81 SW 228/90 perpendicular to plane 1 031 Qz, plag, chlorite, biotite, epidote medium damage; open fractures and cataclasite fill 2.5 17-5 no section 3 17-6 no section 5 17-7 101/80 S 202/50 E perpendicular to plane 1 011 Qz, plag, hornblende, epidote, chlorite little damage 5.5 17-8 no section 7.5 17-9 095/90 102/75 S perpendicular to plane 1 005 Qz, plag, hornblende, chlorite little damage; minor grain cracking 8 high damage; comminuted with remnant grains; no major 31-1 090/90 parallel to plane 1 090 clay with remnant Qz and feldspar clast fractures fault core high damage; clay fabric, Fe2O3 staining, remnant clasts; 31-2 parallel to plane 1 105 clay with remnant Qz and feldspar clast several open fractures fault core 31-3 260/80 S perpendicular to plane 1 170 clay, micas, chlorite, Qz, feldspar high damage; open and Fe2O3 filled fractures fault core high damage; shattered grains in clay matrix; open fractures; 31-4 205/90 parallel to plane 1 205 Qz, plag, micas, chlorite Fe2O3 staining fault core high damage; shattered grains in clay matrix; open and 31-5 parallel to plane 1 clay, micas, chlorite, Qz, feldspar Fe2O3 filled fractures fault core 31-6 080/80 N parallel to plane 1 080 Qz, plag, chlorite high damage; shattered grains in matrix; oriented chlorite fault core clay gouge; pss with surrounding damage; comminuted 32-1A 285/45 S perpendicular to plane 1 195 Qz, plag, clay, chlorite, Fe2O3 grains and clay matrix; open fractures fault core 32-1B 285/45 S parallel to plane 1 Qz, plag, clay, chlorite, Fe2O3 fault core 32-2 a,b,c no section 0.5 medium damage; a few open fractures and minor 32-3 283/53 S perpendicular to plane 1 193 K-spar, Qz, plag, biotite comminution 7 169
distance from Sample ID plane 1 plane 2 billet orientation arrow mineralogy fabric fault (m) 32-4 300/42 SW 005/65 W perpendicular to plane 1 210 K-spar, Qz, plag, biotite, opaque mineral? little damage; mostly open fractures; altered plag 1 little to medium damage; several good fractures partially filled 32-5 274/69 S 001/83 W perpendicular to plane 1 184 K-spar, Qz, plag, biotite, clay/chlorite? with clay or chlorite 2 32-6 270/55 S 161/66 W perpendicular to plane 1 180 K-spar, Qz, plag, biotite little damage; some altered plag 3 little to medium damage; open and Fe2O3 filled fractures; 32-7 285/46 S 177/87 W perpendicular to plane 1 195 Qz, K-spar, plag, biotite, hornblende, muscovite altered plag 4 32-8 273/56 S 198/65 W perpendicular to plane 1 183 Qz, K-spar, plag, biotite little damage; altered feldspars and minor grain cracking 6 32-9 285/65 S 175/86 E perpendicular to plane 1 195 Qz, K-spar, plag, biotite little damage; altered feldspars and minor grain cracking 8 32-10 283/64 S 250/60 N perpendicular to plane 1 193 Qz, K-spar, plag, biotite, Fe2O3 little damage; some fractures and altered feldspars 0.25 32-11 293/61 S 183/52 E perpendicular to plane 1 203 plag, Qz little damage; large altered plag crystal 2 32-12 no section 4 32-13 no section 5 32-14 293/40 S 320/56 NE perpendicular to plane 1 203 Qz, feldspar, muscovite little damage; altered feldspar "fault 12" fault wall medium damage; feldspar alteration; open and ultracat.-filled 32-15 283/56 S 231/32 SE perpendicular to plane 1 193 Qz, plag, K-spar, biotite, muscovite fractures; Fe2O3 staining on some fracture walls "fault 6" fault wall 37-1 162/77 W 235/75 SE perpendicular to plane 1 072 Qz little damage; several open fractures "fault 3" hard fin
37-2 113/77 S 214/64 SE perpendicular to plane 1 023 Qz, plag, micas, K-spar medium damage; Qz vein, open fractures and ground mica fill "fault 5" fault wall 37-3 no section "fault 5" core 37-4 no section "fault 4" core little to medium damage; mostly open fractures with some 40-1 194/79 E 125/82 SW perpendicular to plane 1 104 Qz, plag, hornblende, micas, K-spar, Fe2O3 cataclasite fill; feldspar alteration; Fe2O3 staining fault wall comminution and alteration along fault wall; mostly open 43-1 322/83 SW 047/31 SE perpendicular to plane 1 232 Qz, K-spar, plag, biotite, chlorite, muscovite fractures < 10? 43-2 342/47 SW 231/74 SE perpendicular to plane 1 252 Qz, plag, K-spar, micas, hornblende, chlorite little damage; minor grain cracking < 10? 43-3 312/86 SW 187/32 W perpendicular to plane 1 222 Qz, plag, biotite, K-spar little damage < 10? high damage; comminution; open and Fe2O3 stained 47-1 205/66 E 084/88 N perpendicular to plane 2 354 Qz, feldspars, clay, Fe2O3 fractures 0.1 49-1 258/77 S 296/90 perpendicular to plane 1 168 Qz, plag, K-spar, opx? little damage; a few open fractures "fault 1" fault wall 49-2 244/48 SE 309/77 S perpendicular to plane 1 154 Qz, plag, K-spar, biotite, chlorite, hornblende little damage; open fractures "fault 2" fault wall 51-1 no section fault core 52-1 113\57 S 032/79 NW perpendicular to plane 1 023 Qz, K-spar, plag, opx, hornblende little damage; some feldspar alteration "fault 1" fault wall little to medium damage; biotite foliation with perpendicular 54-1 342/16 SW 284/90 perpendicular to plane 1 252 Qz, plag, biotite, K-spar fractures "fault 2" fault wall 54-2 071/77 S 265/46 N perpendicular to plane 1 341 Qz, plag, K-spar, biotite, hornblende, chlorite medium damage; ultracat. fault core; foliated; open fractures "fault 10" fault wall 54-3 340/21 W 283/87 N perpendicular to plane 1 250 Qz, plag, K-spar, biotite, hornblende, chlorite little damage; weathered/altered feldspars; open fractures "fault 21" fault wall medium damage; cataclasite fault core; some alteration; open 54-4 276/67 S 010/24 W perpendicular to plane 1 186 Qz, feldspars, biotite, chlorite and Fe2O3 lined fractures; weathered; drag fold? "fault 29" fault wall medium high damage; altered and comminuted; weathered; 54-5 077/90 000/75 E perpendicular to plane 1 347 Qz, feldspars, biotite, chlorite open and cataclasite lined fractures "fault 30" fault wall medium high damage; altered and comminuted; cataclasite 54-6 061/72 NW 268/90 perpendicular to plane 1 331 Qz, feldspars, biotite, chlorite fault core with Riedel shears "fault 57" fault wall high damage; comminution and alteration; open and 54-7 320/77 NE 188/09 W perpendicular to plane 1 230 Qz, plag, K-spar, biotite cataclasite-lined fractures, some vein fill; thru-going fracs 1.2 54-8 no section fault core medium high damage; feldspar alteration and Fe2O3 staining; 55-1 114/73 N 057/86 SE perpendicular to plane 1 024 Qz, K-spar, plag, biotite, hornblende, Fe2O3 parallel fracture set fault wall 56-1 290/80 S 002/77 W perpendicular to plane 1 200 Qz, plag, K-spar, biotite medium damage; weathered/altered feldspar; open fractures "fault 9" fault wall medium damage; thru-going fractures, open and cataclasite 56-2 294/64 S 026/87 W perpendicular to plane 1 204 Qz, plag, K-spar, hornblende, chlorite, biotite filled fault wall medium high damage; comminution, open fractures, feldspar 58-1 186/78 E 089/52 S perpendicular to plane 1 096 Qz, K-spar, biotite, plag, Fe2O3, hornblende alteration, Fe2O3 staining "fault 1" fault wall 58-2 100/73 N 023/87 E perpendicular to plane 1 010 Qz, K-spar, plag, biotite, chlorite, hornblende medium damage; open fractures and feldspar alteration "fault 4" fault wall 58-3 207/69 SE 012/55 E perpendicular to plane 1 117 Qz, plag, K-spar, cpx, biotite, hornblende medium damage; open fractures and feldspar alteration "fault 5" fault wall PC-1 107/60 S 197/63 W perpendicular to plane 1 017 Qz, K-spar, plag, biotite, chlorite? little damage; few through-going fractures, grain cracking 25 PC-2 120/65 SW 332/63 SW perpendicular to plane 1 030 Qz, K-spar, plag, biotite, cpx, hornblende little damage; feldspar alteration and grain cracking 40 PC-3 274/48 N 186/82 W perpendicular to plane 2 096 Qz, K-spar, plag, muscovite, biotite little damage; altered feldspars 60 PC-4 100/62 N 007/57 E perpendicular to plane 1 010 Qz, K-spar, plag, biotite, cpx little damage; foliated 115 PC-5 155/43 SW 047/90 perpendicular to plane 1 065 Qz, K-spar, plag, biotite, cpx, muscovite little damage 150 PC-6 096/61 N 026/85 W perpendicular to plane 1 006 Qz, K-spar, plag, biotite, cpx, hornblende little damage 230 PC-7 020/59 W 294/72 N perpendicular to plane 2 204 Qz, plag, K-spar, biotite, chlorite little damage; foliated, chlorite vein fill, grain cracking 305 170
APPENDIX C: X-ray diffraction patterns
Diffraction patterns are in order of station number. The annotated patterns are from the 1970’s Philips machine with Jade software and the non-annotated plots are from the newer Philips X-Pert PRO PANalytical machine with the accompanying X-Pert Highscore software. Sample numbers are in the upper left portion of each plot.
171
Counts 2-1.CAF 1600
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 2-2.CAF
1600
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 3-1.CAF
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
172
Counts 3-1-clay-o.CAF 225
100
25
0 5 10 15 20 25 30 Position [∞2Theta]
Counts 900 3-2.CAF
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 3-3.CAF
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
173
Counts 900 7-1.CAF
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 9-1.CAF
1600
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 1600 11-1.CAF
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
174
Counts 11-1-clay-o.CAF
225
100
25
0 5 10 15 20 25 30 Position [∞2Theta]
Counts 11-2.CAF
1600
400
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 11-3.CAF
3600
1600
400
0 10 20 30 40 50 60 Position [∞2Theta]
175
Counts 15-1.CAF
1600
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 15-2.CAF
1600
400
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 15-3.CAF 1600
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
176
Counts 15-4.CAF
900
400
100
0 10 20 30 40 50 60 Position [∞2Theta]
Counts 15-4-clay-o.CAF
100
25
0 5 10 15 20 25 30 Position [∞2Theta]
<17-1.CPI:> 1162
996 I 830 ~ 664 -~ 498 E 332
166 ,~L..J
,11 1,11 I I ,1,J "'""1.;;:-11,1'" .i~..:g ~- ~r .:ld~Ll9lS.Sl'J0:2Q;r.01-tl• .. l.d.1100 I , I ,, I ,, I, t 11 , h, 1 , , I , 1 , , , . 10 20 30 40 50 60 2-Theta(deg) UtahState Universltv
177
17-2 1992
1660 "'c 6 1328
t 996 E 664
332
Cli,=:,1$~-C:;QP.t):Sj:)o.)(S007)0:0>l)-<'-l -10"'" I ii J I I .
10 20 30 40 50 60 2-Theta( deg)
3320
2656 "'§ i1992
il 1328
664
MliR-2'-lM'1· ( 1(.ol::JO)At2Sao.lOt(\O>l]2-<~l"lo"'" I I , . 10 20 30 40 50 60 2-Theta( deg)
<17-4.CPI>
1992
~ 1660
i 1328
·_i 996 664
332
QI I ::-,O.z.:,.y~· SO:::c,~ ~ 1 :=I======,"""11======,== •.=- ~=,=,~=...=,= ..... = =_ "'=.== .=••=_=:I
10 20 30 40 50 60 2-Theta (de g) LIiah state l.tiversitv
178
Counts 17-3-clay-o.CAF 900
400
100
0 5 10 15 20 25 30 Position [∞2Theta]
<17- Pl>
2656
I 1992 ~ 11328
664
I II
, I ,1, I 1 1
I I' I I~ 'II I I, 10 20 30 40 50 60 2-Theta(deg) utah State Universilv
17-6 1162
996
I 830 ~ 664 l 498 332
166
l.bQ.,,.,..,..11w,mr..,,_~-c~ ...i'" ... 2f 179 17-7 1660 1328 ~ C ~ 0 E 996 J 664 332 0 I 10 20 30 40 50 60 2-Theta( deg) utah state Universitv 17-8 1328 ~ ~ 996 E J 664 332 10 20 30 40 50 60 2-Theta(deg) utah state Universitv 17-9 1328 i 996 1 664 332 10 20 30 40 50 60 2-Theta(deg) utah state Universitv 180 17-10'I> 1162 996 ~ C 830 ~ 0 E 664 J 498 332 166 0 I I , ,I 11111t I I ,, ,,II I ' I Ii 10 20 3() 40 50 60 2-Theta( deg) utah state Universitv <17-11.CPI> 3984 "'c 5 2656 t E 132e l,1, 1,1....:a,kw,a:IGI ~ ba•GI~· (l(. Ba,ja)0 .?S(AI.IA,a.C• \/)~ SiA I.I/J40 10 .,L.1aio.-.. I I I . I • 10 20 ~ w m ~ 2-Theta(deg) Utah State Universilv 17-12I> 3984 I i2656 _£ 1328 ,i ' 1. 10 20 30 40 50 60 2-Theta(deg) Utah State Universilv 181 Counts 31-1.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 31-1-clay-o(3).CAF 64 36 16 4 0 5 10 15 20 25 30 Position [∞2Theta] Counts 31-2.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 182 Counts 31-3.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 900 31-5.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 31-6.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 183 32-1 512 c00 ~ 384 ~ 1256 128 1 • 1 • 1 lu , , t 11 10 20 30 40 50 60 2-Theta(deg) utah State LKliversitv c; 2-2A. Pl> 1328 i 996 j 664 :;; 332 10 20 30 40 50 60 2-Theta(deg) LnahState Universitv Counts 32-2A-clay-o.CAF 100 25 0 5 10 15 20 25 30 Position [∞2Theta] 184 32-3 5312 if! 3984 ! Se, ·i2656 :,; 1328 10 20 30 40 50 60 2-Theta(deg) utah state Universitv <32-4.CPI> 4648 3984 I 3320 i 2656 J1992 1328 664 1 ,I 10 20 30 40 50 60 2-Theta(deg) utah state Universitv 32-5 5312 i 3984 ~ 12 656 1328 I'' 1, 11 'I' 10 20 30 40 2-Theta(deg) utah state Universitv 185 32-6 2656 I ~ 1992 ·~ i 1328 664 10 20 30 40 50 60 2-Theta(deg) utah state Universitv 32-7 3984 ~ ! ~2656 J 1328 J 10 20 30 40 50 60 2-Theta(deg) utah state Universitv 32-8 3984 I 3320 ~2656 -~ 1992 1a 1328 664 I II 10 20 30 40 50 60 2-Theta(deg) utah state Unlversitv 186 32-9 3984 if! 3320 c ~ 2656 ·i1992 :,; 1328 664 " """"" w~ s ~ .. ~•""c,.. Albkcak:iai~.c'Oa.....:I· (l.l:a.Co1)A~SiA~:::OS- I Ill l.4i,::,o;:li~ .c,O,,~·~ .ISOCl9-<~ii,.,..;,.I j l 11111 11, 111 10 20 30 40 50 60 2-Theta(deg) utah state Universitv 32-10 3984 3320 '"~ 2 656 ~ ·j'1992 _E 1328 664 i.lic:.-.xli"".i :,.,,oji::al<,. o:;o.lSOCifl- Albk.,:;ak:i:a~.c,.;1.;:.....:I· (N:a.C;:i)AlSi.A~:xxl-<~i.. o•:.- 1 10 20 30 40 50 60 2-Theta(deg) utah state Universitv ..:32-11.CPI> 5312 4648 3984 '"~ 332 0 0 F 26s 6 -~ i 1992 1328 664 0 I a..u, ,, '"'""¥•1 ' "k Qeoo .oao.a - i,aCa) .. S' AOXO< •ao o I I 111 10 20 ' 30 40 50 60 2-Theta(deg) utah state Unfvers ilv 187 ...... , .... ,., ... ,"-,------~ 2656 32-12 I 1992 11328~ 664 P'lrl(Pll1~::rr v11- o(l.4:1)S~k:ll O)r'Z~ld=-ia..,_I I I I . 10 20 30 40 50 60 2-Theta(deg) utah state Universilv 32-13 3320 2656 I i 1992 -~ i 1328 664 10 20 30 40 50 60 2-Theta(deg) utah state Universilv 'I> 3984 32-14 3320 '"~ 2 656 ~ ·i1992 c - 1328 664 I II I ,I 10 20 30 40 50 60 2-Theta(deg) utah st ate Univers ilv 188 32-15 3984 oo"3320 § i 2656 11992 1328 664 l I• I ,11 10 20 30 40 50 60 2-Theta(deg) LnahState Universitv Counts 6400 37-1.CAF 3600 1600 400 0 10 20 30 40 50 60 Position [∞2Theta] Counts 37-2.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 189 Counts 37-3.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 1600 37-4.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 40-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 190 Counts 1600 43-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 1600 43-2.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 43-3.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 191 Counts 47-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 47-1-clay-o.CAF 100 25 0 5 10 15 20 25 30 Position [∞2Theta] Counts 49-1.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 192 Counts 49-2.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 51-1.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 51-1-clay-o.CAF 400 100 0 5 10 15 20 25 30 Position [∞2Theta] 193 Counts 52-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 54-1.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 54-2.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 194 Counts 225 54-2-clay-o.CAF 100 25 0 5 10 15 20 25 30 Position [∞2Theta] Counts 900 54-3.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 54-4.CAF 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 195 Counts 54-4-clay-o.CAF 400 100 0 5 10 15 20 25 30 Position [∞2Theta] Counts 54-5.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 54-6.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 196 Counts 54-7.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 1600 54-8.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 54-8-clay-o.CAF 225 100 25 0 5 10 15 20 25 30 Position [∞2Theta] 197 Counts 1600 55-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 56-1.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 56-2.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 198 Counts 58-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 1600 58-2.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 58-3.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 199 Counts PC-1.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts PC-2.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts PC-3.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 200 Counts PC-4.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts 1600 PC-5.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] Counts PC-6.CAF 1600 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 201 Counts 1600 PC-7.CAF 900 400 100 0 10 20 30 40 50 60 Position [∞2Theta] 202 APPENDIX D: Whole-rock geochemistry raw data 203 Sample Ident Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 Sr Y Zr Nb Ba Sc LOI Sum Scheme Code ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A ICP95A Analysis Unit % % % % % % % % % % % ppm ppm ppm ppm ppm ppm % % Detection Limit 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 10 10 10 10 10 10 0.01 0.01 32- 1 0.52 1.65 10.82 68.13 <0.01 0.93 1.01 0.41 <0.01 0.02 3.63 171 <10 92 13 279 <10 12.9 100.1 32- 2 0.91 1.42 13.24 66.43 0.01 2.9 2.14 0.56 <0.01 0.02 4.09 293 11 149 14 490 <10 9.3 101.1 32- 3 2.5 0.02 13.5 70 <0.01 9.19 0.07 <0.01 <0.01 <0.01 0.07 341 <10 <10 <10 1160 <10 1 96.53 32- 4 3.52 0.05 14.38 73.31 <0.01 3.77 1.49 0.03 <0.01 <0.01 0.33 289 <10 <10 <10 225 <10 0.6 97.56 32- 5 3.05 0.12 13.96 68.79 <0.01 6.8 0.84 0.08 <0.01 0.01 0.78 255 <10 54 <10 863 <10 0.8 95.38 32- 6 2.28 0.03 12.52 77.01 <0.01 6.15 0.89 0.05 <0.01 <0.01 0.46 243 <10 38 <10 485 <10 0.7 100.2 32- 7 3.11 0.05 12.34 77.04 <0.01 4.93 1.26 0.05 <0.01 <0.01 0.67 237 <10 29 <10 372 <10 0.5 100 32- 8 3.05 0.05 13.48 75.84 <0.01 3.39 1.29 0.05 <0.01 <0.01 0.47 339 <10 <10 <10 479 <10 2.7 100.4 32- 9 2.89 0.04 12.92 76.34 <0.01 5.63 1 0.02 <0.01 <0.01 0.33 271 <10 <10 <10 375 <10 0.8 100 32-10 2.02 0.04 12.23 77.3 <0.01 8.09 0.36 0.02 <0.01 <0.01 0.32 179 <10 <10 <10 348 <10 0.9 101.4 32-11 4.34 0.04 14.5 77.03 <0.01 0.56 2.53 0.03 <0.01 <0.01 0.54 303 <10 58 <10 54 <10 0.6 100.3 32-12 3.6 1.24 15.77 68.87 0.15 1.46 4.65 0.5 <0.01 0.06 3.72 666 15 136 13 718 <10 0.1 100.3 32-13 3.7 1.46 16.06 63.16 0.16 1.54 4.71 0.6 <0.01 0.07 4.45 677 16 167 <10 734 <10 1.7 97.81 32-14 5.07 0.04 13.14 77.53 <0.01 1.01 2.42 0.02 <0.01 <0.01 0.13 201 23 <10 <10 66 <10 2.1 101.5 32-15 3.21 0.04 12.95 77.31 <0.01 4.32 1.23 0.03 <0.01 <0.01 0.41 278 <10 <10 <10 292 <10 0.5 100.1 17- 1 2.86 1.37 16.2 60.63 0.3 2.18 7.04 0.87 <0.01 0.09 5.79 819 35 252 18 661 11 0.4 97.94 17- 2 3.64 1.02 13.98 64.54 0.2 2.25 5.16 0.58 <0.01 0.07 4.28 588 37 155 14 488 10 4.4 100.3 17- 3 0.87 1.92 12.12 66.78 0.08 2.39 2.17 0.46 <0.01 0.07 4.7 286 14 162 <10 638 <10 3.7 95.41 17- 4 3.48 1.18 13.4 60.73 0.2 3.32 5.92 0.59 <0.01 0.08 4.57 765 17 170 15 885 <10 9 102.7 17- 5 3.13 0.92 14.63 67.54 0.2 2.61 3.22 0.52 <0.01 0.04 2.78 399 39 203 16 1080 <10 2.5 98.29 17- 6 2.76 2.54 15.03 59.56 0.33 1.75 5.86 1.07 <0.01 0.11 7.18 706 19 164 11 488 13 5.3 101.6 17- 7 3.66 2.71 15.8 59.48 0.31 1.72 5.64 1.09 <0.01 0.1 7.28 665 21 165 12 504 12 3.1 101 17- 8 3.58 2.58 16.16 60.77 0.28 1.76 4.94 0.98 <0.01 0.11 6.84 529 23 135 13 334 14 2.5 100.6 17- 9 3.5 2.73 16.16 60.28 0.26 1.75 4.83 1.04 <0.01 0.11 7.24 544 20 141 <10 412 15 3.4 101.4 17-10 3.56 2.57 16.31 57.54 0.3 1.85 5.22 1.02 <0.01 0.12 6.98 572 32 143 <10 641 14 3.9 99.54 17-11 3.62 1.29 15.27 69.19 0.13 2.32 3.77 0.54 <0.01 0.06 3.43 545 20 120 <10 846 <10 1.6 101.4 17-12 3.38 0.54 13.02 74.11 0.15 3.57 1.55 0.36 <0.01 0.04 2.51 422 14 215 <10 1470 <10 2 101.5 3-1 0.26 4.43 13.21 57.38 0.18 1.95 4.73 0.6 <0.01 0.07 4.34 307 13 153 <10 617 11 14.4 101.7 3-2 1.69 1.41 11.61 54.9 0.11 3.21 13.2 0.33 <0.01 0.09 2.55 431 17 120 <10 1060 <10 12.1 101.4 3-3 3.02 0.85 11.96 69.81 0.13 3.26 5.32 0.3 <0.01 0.04 2.06 481 11 116 <10 1180 <10 4.2 101.2 2-1 3.93 0.89 14.92 69.94 0.14 2.98 2.93 0.48 <0.01 0.05 2.96 453 16 159 <10 748 <10 1.7 101.1 2-2 3.04 0.64 13.9 71.99 0.1 5.1 1.91 0.35 <0.01 0.03 2.12 478 19 229 <10 1530 <10 1 100.4 PC- 1 3.11 0.82 15.72 67.55 0.28 6.36 1.94 0.21 <0.01 0.05 2.68 555 51 63 <10 2100 17 0.9 99.94 PC- 2 4.26 1.66 16.44 62.96 0.17 1.82 4.55 0.54 <0.01 0.1 4.13 592 47 120 16 508 <10 0.7 97.47 PC- 3 3.77 0.34 14.25 72.4 0.03 3.24 2.67 0.12 <0.01 0.03 1.28 629 <10 50 <10 1260 <10 0.4 98.75 PC- 4 4.03 1.6 15.58 67.98 0.17 1.6 4.38 0.56 <0.01 0.06 4.37 578 11 120 <10 453 <10 0.8 101.3 PC- 5 3.32 1.54 14.87 65.86 0.16 3.16 3.36 0.55 <0.01 0.06 4.01 783 15 112 <10 1470 <10 1 98.16 PC- 6 3.66 1.84 15.29 68.25 0.17 1.82 3.79 0.6 <0.01 0.06 4.07 435 11 134 11 350 <10 1.2 100.9 PC- 7 3.49 2.08 16.06 64.25 0.14 1.92 4.23 0.75 <0.01 0.05 5.46 565 <10 118 12 551 <10 1.6 100.2 DUP-32- 1 0.54 1.66 10.7 68.07 <0.01 0.93 0.99 0.41 <0.01 0.01 3.58 169 <10 93 13 285 <10 12.8 99.76 DUP-32-13 3.84 1.42 16.4 63.6 0.17 1.57 4.76 0.62 <0.01 0.07 4.48 695 16 175 10 718 <10 1.7 98.81 DUP-17-10 3.7 2.55 16.08 56.73 0.29 1.83 5.15 1.02 <0.01 0.12 6.93 574 32 148 <10 636 14 4.1 98.65 DUP-PC- 5 3.33 1.55 15.2 66.47 0.16 3.21 3.38 0.55 <0.01 0.06 4.03 750 15 116 <10 1470 <10 1.1 99.32 204 APPENDIX E: Principal component analysis 205 PRINCIPAL COMPONENTS ANALYSIS FOR THE GRASS VALLEY FAULT ZONE Imported data Analysis begun: Wednesday, June 09, 2004 10:01:56 AM Analysing 12 variables x 15 cases Data log(10) transformed Tolerance of eigenanalysis set at 1E-007 Data standardized Transformed data 32- 1 32- 2 32- 4 32- 5 32- 6 32- 7 32- 8 32- 3 32- 9 32-15 32-10 32-11 32-12 32-13 32-14 Na2O 0.182 0.281 0.655 0.607 0.516 0.614 0.607 0.544 0.590 0.624 0.480 0.728 0.663 0.672 0.783 MgO 0.423 0.384 0.021 0.049 0.013 0.021 0.021 0.009 0.017 0.017 0.017 0.017 0.350 0.391 0.017 Al2O3 1.073 1.154 1.187 1.175 1.131 1.125 1.161 1.161 1.144 1.145 1.122 1.190 1.225 1.232 1.150 SiO2 1.840 1.829 1.871 1.844 1.892 1.892 1.886 1.851 1.888 1.894 1.894 1.892 1.844 1.807 1.895 P2O5 0.000 0.004 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.061 0.064 0.000 K2O 0.286 0.591 0.679 0.892 0.854 0.773 0.642 1.008 0.822 0.726 0.959 0.193 0.391 0.405 0.303 CaO 0.303 0.497 0.396 0.265 0.276 0.354 0.360 0.029 0.301 0.348 0.134 0.548 0.752 0.757 0.534 TiO2 0.149 0.193 0.013 0.033 0.021 0.021 0.021 0.000 0.009 0.013 0.009 0.013 0.176 0.204 0.009 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO 0.009 0.009 0.000 0.004 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.025 0.029 0.000 Fe2O3 0.666 0.707 0.124 0.250 0.164 0.223 0.167 0.029 0.124 0.149 0.121 0.188 0.674 0.736 0.053 LOI 1.143 1.013 0.204 0.255 0.230 0.176 0.568 0.301 0.255 0.176 0.279 0.204 0.041 0.431 0.491 206 Similarity matrix Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Na2O 1.000 MgO -0.513 1.000 Al2O3 0.636 0.144 1.000 SiO2 0.357 -0.823 -0.376 1.000 P2O5 0.226 0.634 0.704 -0.613 1.000 K2O -0.104 -0.516 -0.230 0.210 -0.370 1.000 CaO 0.352 0.548 0.652 -0.357 0.745 -0.764 1.000 TiO2 -0.407 0.979 0.285 -0.835 0.719 -0.482 0.635 1.000 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO -0.014 0.819 0.578 -0.781 0.955 -0.432 0.725 0.875 0.000 1.000 Fe2O3 -0.449 0.978 0.221 -0.806 0.675 -0.498 0.613 0.986 0.000 0.847 1.000 LOI -0.760 0.594 -0.459 -0.427 -0.158 -0.337 -0.008 0.496 0.000 0.097 0.501 1.000 Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Eigenvalues Axis 1 Axis 2 Axis 3 Eigenvalues 6.279 2.940 1.119 Percentage 57.079 26.728 10.169 Cum. Percentage 57.079 83.807 93.976 PCA variable loadings Axis 1 Axis 2 Axis 3 Na2O 0.083 0.550 -0.145 MgO -0.375 -0.190 0.001 Al2O3 -0.180 0.465 0.165 SiO2 0.332 0.107 -0.355 P2O5 -0.328 0.270 0.200 K2O 0.234 -0.067 0.733 CaO -0.300 0.282 -0.374 207 TiO2 -0.388 -0.114 0.055 Cr2O3 0.000 0.000 0.000 MnO -0.375 0.129 0.181 Fe2O3 -0.380 -0.139 0.024 LOI -0.149 -0.477 -0.269 PCA case scores Axis 1 Axis 2 Axis 3 32- 1 -0.636 -1.166 -0.275 32- 2 -0.808 -0.729 0.006 32- 4 0.291 0.268 0.012 32- 5 0.174 0.056 0.341 32- 6 0.466 -0.087 0.104 32- 7 0.416 0.033 -0.012 32- 8 0.291 -0.012 -0.142 32- 3 0.520 -0.110 0.476 32- 9 0.460 0.028 0.066 32-15 0.433 0.120 -0.034 32-10 0.584 -0.218 0.231 32-11 0.163 0.449 -0.507 32-12 -1.161 0.632 0.105 32-13 -1.473 0.464 0.186 32-14 0.280 0.272 -0.554 208 PRINCIPAL COMPONENTS ANALYSIS FOR THE EASTWOOD FAULT ZONE Imported data Analysis begun: Wednesday, June 09, 2004 9:55:08 AM Analysing 12 variables x 12 cases Data log(10) transformed Tolerance of eigenanalysis set at 1E-007 Data standardized Transformed data 17- 1 17- 2 17- 3 17- 4 17- 5 17- 6 17- 7 17- 8 17- 9 17-10 17-11 17-12 Na2O 0.587 0.667 0.272 0.651 0.616 0.575 0.668 0.661 0.653 0.659 0.665 0.641 MgO 0.375 0.305 0.465 0.338 0.283 0.549 0.569 0.554 0.572 0.553 0.360 0.188 Al2O3 1.236 1.176 1.118 1.158 1.194 1.205 1.225 1.235 1.235 1.238 1.211 1.147 SiO2 1.790 1.817 1.831 1.790 1.836 1.782 1.782 1.791 1.787 1.767 1.846 1.876 P2O5 0.114 0.079 0.033 0.079 0.079 0.124 0.117 0.107 0.100 0.114 0.053 0.061 K2O 0.502 0.512 0.530 0.635 0.558 0.439 0.435 0.441 0.439 0.455 0.521 0.660 CaO 0.905 0.790 0.501 0.840 0.625 0.836 0.822 0.774 0.766 0.794 0.679 0.407 TiO2 0.272 0.199 0.164 0.201 0.182 0.316 0.320 0.297 0.310 0.305 0.188 0.134 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO 0.037 0.029 0.029 0.033 0.017 0.045 0.041 0.045 0.045 0.049 0.025 0.017 Fe2O3 0.832 0.723 0.756 0.746 0.577 0.913 0.918 0.894 0.916 0.902 0.646 0.545 LOI 0.146 0.732 0.672 1.000 0.544 0.799 0.613 0.544 0.643 0.690 0.415 0.477 209 Similarity matrix Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Na2O 1.000 MgO -0.070 1.000 Al2O3 0.594 0.555 1.000 SiO2 -0.191 -0.780 -0.622 1.000 P2O5 0.494 0.578 0.790 -0.809 1.000 K2O -0.075 -0.876 -0.723 0.689 -0.645 1.000 CaO 0.419 0.482 0.671 -0.856 0.772 -0.554 1.000 TiO2 0.304 0.864 0.811 -0.888 0.899 -0.861 0.746 1.000 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO 0.142 0.898 0.641 -0.916 0.753 -0.790 0.699 0.923 0.000 1.000 Fe2O3 0.045 0.925 0.602 -0.912 0.742 -0.831 0.697 0.931 0.000 0.976 1.000 LOI -0.026 0.197 -0.349 -0.288 -0.012 0.072 0.111 0.044 0.000 0.209 0.179 1.000 Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Eigenvalues Axis 1 Axis 2 Axis 3 Eigenvalues 7.281 1.660 1.145 Percentage 66.192 15.094 10.409 Cum. Percentage 66.192 81.286 91.695 PCA variable loadings Axis 1 Axis 2 Axis 3 Na2O -0.113 0.594 0.434 MgO -0.318 -0.304 -0.199 Al2O3 -0.297 0.411 -0.166 SiO2 0.346 0.122 -0.189 P2O5 -0.325 0.222 0.118 K2O 0.318 0.072 0.336 CaO -0.301 0.149 0.274 210 TiO2 -0.366 0.019 -0.053 Cr2O3 0.000 0.000 0.000 MnO -0.351 -0.171 0.007 Fe2O3 -0.350 -0.216 -0.064 LOI -0.034 -0.467 0.707 PCA case scores Axis 1 Axis 2 Axis 3 17- 1 -0.396 0.474 -0.377 17- 2 0.298 0.077 0.303 17- 3 0.865 -0.998 -0.439 17- 4 0.285 -0.210 0.794 17- 5 0.772 0.302 -0.002 17- 6 -0.784 -0.287 0.110 17- 7 -0.814 0.031 -0.026 17- 8 -0.698 0.069 -0.146 17- 9 -0.736 -0.054 -0.078 17-10 -0.861 -0.041 0.071 17-11 0.573 0.354 -0.209 17-12 1.495 0.284 0.001 211 PRINCIPAL COMPONENTS ANALYSIS FOR THE CLEGHORN FAULT ZONE Imported data Analysis begun: Wednesday, June 09, 2004 9:48:17 AM Analysing 12 variables x 5 cases Data log(10) transformed Tolerance of eigenanalysis set at 1E-007 Data standardized Transformed data 3-1 3-2 3-3 2-1 2-2 Na2O 0.100 0.430 0.604 0.693 0.606 MgO 0.735 0.382 0.267 0.276 0.215 Al2O3 1.153 1.101 1.113 1.202 1.173 SiO2 1.766 1.747 1.850 1.851 1.863 P2O5 0.072 0.045 0.053 0.057 0.041 K2O 0.470 0.624 0.629 0.600 0.785 CaO 0.758 1.152 0.801 0.594 0.464 TiO2 0.204 0.124 0.114 0.170 0.130 Cr2O3 0.000 0.000 0.000 0.000 0.000 MnO 0.029 0.037 0.017 0.021 0.013 Fe2O3 0.728 0.550 0.486 0.598 0.494 LOI 1.188 1.117 0.716 0.431 0.301 212 Similarity matrix Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Na2O 1.000 MgO -0.965 1.000 Al2O3 0.275 -0.119 1.000 SiO2 0.779 -0.725 0.535 1.000 P2O5 -0.663 0.823 0.183 -0.319 1.000 K2O 0.703 -0.843 0.098 0.600 -0.915 1.000 CaO -0.352 0.281 -0.833 -0.801 -0.019 -0.370 1.000 TiO2 -0.612 0.767 0.530 -0.287 0.856 -0.721 -0.240 1.000 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO -0.598 0.585 -0.480 -0.958 0.252 -0.587 0.851 0.236 0.000 1.000 Fe2O3 -0.784 0.907 0.255 -0.559 0.878 -0.851 0.063 0.951 0.000 0.499 1.000 LOI -0.835 0.800 -0.640 -0.933 0.499 -0.742 0.793 0.295 0.000 0.857 0.574 1.000 Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Eigenvalues Axis 1 Axis 2 Eigenvalues 6.825 3.189 Percentage 62.042 28.990 Cum. Percentage 62.042 91.032 213 PCA variable loadings Axis 1 Axis 2 Na2O 0.348 -0.028 MgO -0.364 0.122 Al2O3 0.106 0.495 SiO2 0.330 0.240 P2O5 -0.288 0.309 K2O 0.343 -0.122 CaO -0.205 -0.455 TiO2 -0.251 0.414 Cr2O3 0.000 0.000 MnO -0.302 -0.259 Fe2O3 -0.327 0.279 LOI -0.350 -0.218 PCA case scores Axis 1 Axis 2 3-1 -1.921 0.690 3-2 -0.624 -1.332 3-3 0.613 -0.440 2-1 0.464 0.823 2-2 1.468 0.260 214 PRINCIPAL COMPONENTS ANALYSIS FOR THE POWELL CANYON FAULT ZONE Imported data Analysis begun: Wednesday, June 09, 2004 9:40:40 AM Analysing 12 variables x 7 cases Data log(10) transformed Tolerance of eigenanalysis set at 1E-007 Data standardized Transformed data PC- 1 PC- 2 PC- 3 PC- 4 PC- 5 PC- 6 PC- 7 Na2O 0.614 0.721 0.679 0.702 0.635 0.668 0.652 MgO 0.260 0.425 0.127 0.415 0.405 0.453 0.489 Al2O3 1.223 1.242 1.183 1.220 1.201 1.212 1.232 SiO2 1.836 1.806 1.866 1.839 1.825 1.840 1.815 P2O5 0.107 0.068 0.013 0.068 0.064 0.068 0.057 K2O 0.867 0.450 0.627 0.415 0.619 0.450 0.465 CaO 0.468 0.744 0.565 0.731 0.639 0.680 0.719 TiO2 0.083 0.188 0.049 0.193 0.190 0.204 0.243 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO 0.021 0.041 0.013 0.025 0.025 0.025 0.021 Fe2O3 0.566 0.710 0.358 0.730 0.700 0.705 0.810 LOI 0.279 0.230 0.146 0.255 0.301 0.342 0.415 215 Similarity matrix Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Na2O 1.000 MgO 0.170 1.000 Al2O3 0.226 0.648 1.000 SiO2 -0.119 -0.755 -0.837 1.000 P2O5 -0.392 0.359 0.607 -0.454 1.000 K2O -0.753 -0.654 -0.279 0.302 0.335 1.000 CaO 0.706 0.769 0.447 -0.538 -0.186 -0.960 1.000 TiO2 0.212 0.981 0.553 -0.706 0.189 -0.731 0.830 1.000 Cr2O3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MnO 0.524 0.584 0.715 -0.774 0.389 -0.419 0.582 0.500 0.000 1.000 Fe2O3 0.098 0.982 0.704 -0.793 0.445 -0.567 0.712 0.957 0.000 0.560 1.000 LOI -0.399 0.776 0.459 -0.536 0.410 -0.227 0.308 0.759 0.000 0.074 0.791 1.000 Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 Cr2O3 MnO Fe2O3 LOI Eigenvalues Axis 1 Axis 2 Axis 3 Eigenvalues 6.390 2.562 1.430 Percentage 58.095 23.289 13.004 Cum. Percentage 58.095 81.383 94.387 216 PCA variable loadings Axis 1 Axis 2 Axis 3 Na2O 0.134 -0.536 0.293 MgO 0.382 0.060 -0.174 Al2O3 0.307 0.165 0.362 SiO2 -0.335 -0.156 -0.229 P2O5 0.135 0.491 0.309 K2O -0.273 0.420 0.188 CaO 0.328 -0.340 -0.096 TiO2 0.372 -0.013 -0.273 Cr2O3 0.000 0.000 0.000 MnO 0.287 -0.066 0.521 Fe2O3 0.378 0.123 -0.143 LOI 0.257 0.323 -0.441 PCA case scores Axis 1 Axis 2 Axis 3 PC- 1 -0.874 1.185 0.424 PC- 2 0.978 -0.456 0.856 PC- 3 -1.864 -0.774 -0.109 PC- 4 0.418 -0.450 -0.002 PC- 5 0.031 0.293 -0.253 PC - 6 0.369 -0.046 -0.351 PC- 7 0.941 0.249 -0.565