Geophysical Investigations of Structures Within Southern Fish Lake Valley, Western Great Basin

Home , Fish Lake Valley, Last Chance Range

GEOPHYSICAL INVESTIGATIONS OF SOUTHERN FISH LAKE VALLEY,

WESTERN GREAT BASIN, CALIFORNIA

Kyle A. McBride

APPROVED BY SUPERVISORY COMMITTEE:

______Dr. John F. Ferguson, Chair

______Dr. Tom H. Brikowski

______Dr. John S. Oldow

Kyle A. McBride

I dedicate this thesis to my grandfather, Bill McMullin, with whom I would have enjoyed to have

the time to discuss geology and the earth sciences.

GEOPHYSICAL INVESTIGATIONS OF SOUTHERN FISH LAKE VALLEY, WESTERN

GREAT BASIN, CALIFORNIA

KYLE A. MCBRIDE, BS, BBA

THESIS

Presented to the Faculty of

The University of Texas at Dallas

in Partial Fulfillment

of the Requirements

for the Degree of

MASTER OF SCIENCE IN

GEOSCIENCES

THE UNIVERSITY OF TEXAS AT DALLAS

December 2016

ACKNOWLEDGMENTS

I would like to thank Dr. John Ferguson for his guidance and support throughout the duration of this project; our numerous conversations and his provided insights being crucial for the development of this thesis. I would also like to thank Dr. Ferguson for the opportunities extended to me while at UT Dallas, such as SAGE and the Denbury gravity surveys. I also want to thank Dr. John Geissman for his encouragement and mentoring during my time at UT Dallas. I want to thank my committee members for taking the time to review and comment on this document, and of course, I want to thank my wife, Denise, and my family, for their patience and support.

November 2016

GEOPHYSICAL INVESTIGATIONS OF SOUTHERN FISH LAKE VALLEY, WESTERN

GREAT BASIN, CALIFORNIA

Publication No. ______

Kyle A. McBride, MS The University of Texas at Dallas, 2016

ABSTRACT

Supervising Professor: John F. Ferguson, PhD

Growth of the Cucomungo Canyon restraining bend along the Fish Lake Valley-Furnace Creek fault zone has resulted in localized uplift at the southern end of Fish Lake Valley in eastern California. This thesis is part of an integrated study by the Miles Geoscience Center to develop a model of this recent deformation. This study focuses on the southernmost section of Fish Lake Valley, where Paleozoic sedimentary rocks are juxtaposed with Cenozoic sediments on multiple faulted boundaries. Structural constraints are poorly known as the faults are locally obscured by Quaternary alluvial deposits of various ages. A near surface geophysical survey utilizing high-resolution seismic refraction and microGal gravity measurements was done to explore the subsurface beneath the alluvium. Forward models were created to identify faults and ascertain vertical offsets and orientations. The geophysical models indicate a zone of extensional deformation north of the restraining bend.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... v

ABSTRACT ...... vi

LIST OF FIGURES ...... ix

LIST OF TABLES ...... xii

CHAPTER 1 INTRODUCTION ...... 1

CHAPTER 2 GEOLOGIC SETTING ...... 3

2.1 Geologic Unit Descriptions...... 6

CHAPTER 3 GEOPHYSICAL METHODS ...... 10

3.1 Seismic Refraction ...... 10

3.2 Gravity ...... 14

CHAPTER 4 PROFILE DESCRIPTIONS ...... 17

4.1 Eureka Valley Road Profile ...... 18

4.2 Fish Lake Valley Transect ...... 18

4.3 Horse Thief Canyon Profile ...... 19

4.4 Granite Arroyo Survey ...... 21

CHAPTER 5 PROFILE CONSTRUCTION ...... 22

5.1 Physical Properties ...... 22

CHAPTER 6 GEOPHYSICAL MODELS ...... 31

6.1 Eureka Valley Road Profile ...... 31

6.2 Fish Lake Valley Transect ...... 35

vii

6.3 Horse Thief Canyon Profile ...... 39

CHAPTER 7 DISCUSSION ...... 42

CHAPTER 8 CONCLUSION...... 44

REFERENCES ...... 45

VITA

viii

LIST OF FIGURES

Figure 2.1 Map showing regional features along the Death Valley-Furnace Creek fault system. Major fault traces are shown in black. Physiographic locations are labeled in black and major fault systems are labeled in red. SP – Silver Peak Range, FLV – Fish Lake Valley, PM – Palmetto Mountains, WM – White Mountains, OV– Owens Valley, SM – Sylvania Mountains, DSV – Deep Springs Valley, EV – Eureka Valley, LCR – Last Chance Range, DV– Death Valley, CCRB – Cucomungo Canyon restraining bend, FLVFZ – Fish Lake Valley fault zone, SMFS – Sylvania Mountain fault system, OV- WMFZ– Owens Valley–White Mountains fault zone, PMFS – Palmetto Mountain fault system, DV-FCFS– Death Valley–Furnace Creek fault system...... 4

Figure 2.2 Map showing study location and adjacent features. Seismic refraction profile locations are shown in white in the satellite image on the right side of the figure, the location of the yellow box. Major fault traces are shown in black on the left side of the figure. Physiographic locations are labeled black and major fault locations in red. SP – Silver Peak Range, FLV – Fish Lake Valley, PM – Palmetto Mountains, SM – Sylvania Mountains, DSV – Deep Springs Valley, EV – Eureka Valley, LCR – Last Chance Range, HTH – Horse Thief Hills, CCRB – Cucomungo Canyon restraining bend, FC-FLVF – Furnace Creek and Fish Lake Valley faults, SF – Sylvania Mountain fault system, PMF – Palmetto Mountain fault system. Modified from Oldow and Geissman (2013) ...... 5

Figure 2.3 Generalized geologic map of the southern edge of FLV just west of Willow Wash with geophysical survey locations indicated in the dashed black lines which are discussed in Chapter 4. This map is in the location specified by the gold box in Figure 2.1...... 9

Figure 3.1 An example of seismic traces that have been gained. The first arrivals are marked with x’s and the airwave with the solid line. The selected first arrivals, shown with the blue x’s, form the traveltime curves. Adapted from Nisengard et al., 2008 ...... 12

Figure 3.2 Example of a simple two layer refraction model. A) Two layer model with computed traveltime curves plotted with the first arrivals determined from the observed seismic traces; B) Two layer model depicting ray path motion of waves created at the left shot point; C) Two layer model with wavefront propagation from the left shot point with refracted waves. Modified from Nisengard et al., 2008 ...... 13

Figure 4.1 Geologic Map with EVRP and FLVT profile locations. Unit descriptions can be viewed in Figure 2.2...... 19

Figure 4.2 Geologic map with the HTCP profile location ...... 20

Figure 4.3 Location of the seismic refraction survey performed to determine the P-wave velocity of the crushed quartz monzonite along the DVFCFZ ...... 20

Figure 5.1 Map of the 28 gravity stations used for Nettleton density analysis of the Pz/P metasediments shown in the top of the figure. Results of the analysis are shown in the chart on the bottom of the figure and the minimum slope 0.001798 mGal/m is associated with a density of 2500 kg/m3...... 27

Figure 5.2 Map of the 46 gravity stations used for Nettleton density analysis of unit Qa2 is shown in the top of the figure. Results of the analysis are shown in the chart on the bottom of the figure and the minimum slope 0.000596 mGal/m is associated with a density of 1800 kg/m3 ...... 28

Figure 5.3 Example of a P-wave velocity model created using seismic refraction data in geomod. Observed traveltime curves are shown in red and synthetic traveltime curves shown in blue on the upper sections of the figure. Synthetic traveltime curves are theoretical traveltimes based on the assigned velocities and geometry of the polygons in the model. The model on the left side of the figure shows the beginning of the model building process and adjustments must be made for the model to be an accurate representation of the subsurface. The model on the right side of the figure is complete, and the synthetic traveltime curves match the observed data. Wave-fronts are shown in white in the model on the right. This is the P-wave model developed from the Granite Arroyo survey discussed in section 4.4 ...... 30

Figure 6.1 Cross section along the Eureka Valley Road profile shown in the lower section of the figure. Observed traveltime curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure...... 31

Figure 6.2 EVRP with wavefront plotted in white for a single shot point at 124.81 m. Observed traveltime curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure ...... 32

Figure 6.3 Graph showing gravity stations collected along the EVRP. Complete Bouguer Anomaly (CBA) gravity data is shown in red, with the change in CBA between stations shown in blue. There is a noticeable change in the CBA values in the first 100 m of the profile that coincides with the location of the fault shown in Figures 2.2 and 4.1...... 33

Figure 6.4 Cross section along the Eureka Valley Road Profile created from seismic refraction data shown in the bottom of the figure with density values assigned to the polygons in the model. Complete Bouguer Anomaly is shown in red with computed gravity response shown in blue in the top section of the figure ...... 34

Figure 6.5 Cross section along the Fish Lake Valley Transect shown in the lower section of the figure. Observed travel time curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure ...... 35

Figure 6.6 Graph showing gravity stations collected along the FLVT. Complete Bouguer Anomaly (CBA) gravity shown in red, with the change in CBA between stations shown in blue ...... 37

Figure 6.7 Cross section along the Fish Lake Valley Transect created from seismic refraction and gravity data shown in the bottom section of the figure with density values assigned to the polygons in the model. Complete Bouguer Anomaly is shown in red with computed gravity response shown in blue in the top section of the figure ...... 37

Figure 6.8 Cross section along the Horse Thief Canyon Profile shown in the lower section of the figure. Observed travel time curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure ...... 39

Figure 6.9 Graph showing gravity stations collected along the HTCP. Complete Bouguer Anomaly (CBA) gravity shown in red, with the change in CBA between stations shown in blue ...... 40

Figure 6.10 Cross section along the Horse Thief Canyon profile created from seismic refraction and gravity data shown in the bottom section of the figure with density values assigned to polygons in the model. Complete Bouguer Anomaly is shown in red with computed gravity response shown in blue in the top section of the figure...... 41

Figure 7.1 Modified geologic map from Figure 2.3. The fault at 418 m along the FLVT is presumed to align with the fault at 169 m along the HTCP...... 43

LIST OF TABLES

Table 5.1 Observed P-wave velocities from the traveltime curves used in the creation of the forward models and associated lithologic unit...... 23

Table 5.2 Rock types with a range of observed P-wave velocities. Adapted from Reynolds, (2011) ...... 24

Table 5.3 Measured densities of Paleozoic/Proterozoic metasediment and Jurassic quartz monzonite samples collected from survey sites ...... 25

xii

CHAPTER 1 INTRODUCTION

The development of the Cucomungo Canyon restraining bend (CCRB) along the Furnace Creek/Death Valley fault system (DVFCFS) has resulted in localized shortening within the southern margin of Fish Lake Valley (FLV) near the California/Nevada border. The establishment of timing relationships of the deformation occurring from this uplift is the focus of a study performed by students at the UT Dallas Miles Geoscience Center for Integrated Field Studies. The goal of the Miles Center study is to develop a chronological geologic model related to the development of the CCRB. In this thesis, structural constraints are established for the model through the use of near surface seismic refraction and gravity surveys that were performed to ascertain subsurface geometries of structures with subtle or no surface expression. Specifically, the aim of this study is to determine the structural geometry north of the CCRB along the FLV fault zone. The geophysical surveys were located in a fault trough at the southernmost extent of FLV just north of a 15 km left bend in the right lateral Furnace Creek/Fish Lake Valley fault. This bend along the fault has resulted in the development of the 15 km wide and 27 km long CCRB, which is located within the Sylvania Mountains and the Last Chance Range in western Nevada and eastern California (Oldow, personal communication, 2012). This uplift is deeply incised and reveals an extended history of syntectonic deposition of Pleistocene to late Miocene sedimentary and volcanic rocks overlying a basement of highly deformed Paleozoic-Proterozoic metasedimentary rocks and Jurassic quartz monzonite, which are juxtaposed across the FLV fault zone (Oldow, personal communication, 2012). North of the CCRB within the study area (Fig 1.) displacement is being transferred from an environment characterized by contraction into an environment undergoing transtension. Within this location there are subtle surface expressions of faults with most traces being covered and obscured by Quaternary alluvium of various ages. Many of these fault traces have been previously mapped (Reheis, 1992; McKee, 1967), but with true orientations unknown. It is important to understand

1 2 the geometry of these buried structures in order to reliably and accurately create an interpretation of deformation. Understanding the geometry of these structures will aid in the establishment of timing relationships of events when coupled with geologic mapping and fluvial geomorphology of the sedimentary units exposed at the surface and within Willow Wash, a well-exposed drainage just east of the survey sites in this study.

The geologic model of deformation relating to the growth of the CCRB is the integration of independent studies conducted by the Miles Center students. Given the largely buried nature of these faults, near surface geophysical surveys were utilized in an effort to strip away the alluvial sediments and reveal the underlying structures. High resolution seismic refraction surveys were conducted, utilizing clustered and redundant shot points that can be used to model complicated subsurface features. Seismic refraction surveys utilizing similar designs have been used before to successfully model complex subsurface structures in shallow archaeological investigations (Nisengard et al. 2008). Therefore, this technique is believed to be viable for accomplishing the goals of this study. In addition to refraction data, gravity data was collected along the seismic profiles to enhance the accuracy of the resultant models. 2D cross sections have been prepared that extend across and along the southern end of Fish Lake Valley and reveal the geometries of subsurface structures.

CHAPTER 2 GEOLOGIC SETTING

The San Andreas fault system accounts for approximately 80 percent of the lateral motion between the North American and Pacific plates, with the remainder being accommodated by 10 - 14mm/yr of movement along the western edge of the Great Basin within a tectonic boundary zone consisting of the Eastern California Shear Zone and Walker Lane (Oldow et al., 2008; Sauber et al., 1994; Bennet et al., 1999, Miller et al., 2001). The 80 km long Fish Lake Valley fault zone (FLVFZ) is the northern extent of the Death Valley - Furnace Creek fault system (DV- FCFS), which is an active, 250 km long right lateral oblique system that strikes north northwest along the California/Nevada border (Reheis and Sawyer, 1997). The Death Valley – Furnace Creek – Fish Lake Valley fault zone accounts for 65 to over 100 km of right lateral displacement, and is the fastest moving segment of the Eastern California Shear Zone (ECSZ), accommodating up to half its total motion (Reheis and Sawyer, 1997; McKee, 1968, Stewart, 1967). Reheis and Sawyer (1997) state that strike slip movement along the FLVFZ began around 10 Ma, with extension occurring later at 5 Ma. However, recent examination by the Miles Center suggests that right lateral displacement began prior to 15 Ma (Mueller et al., 2016). Displacement from the FLV fault zone was transferred eastward through the Silver Peak – Lone Mountain (SPLM) extensional complex, a region (Fig. 1) that extends 85 km northeast from the north end of the FLVFZ to the east of Lone Mountain (Oldow, 1992; Oldow et al. 1994). The SPLM was active during the late Miocene to early Pliocene, and accommodated up to 25 km of right lateral displacement from the FLV fault system through localized extension by low angle detachment faults (Oldow, 1992; Oldow, 2008). Extension within the LMSP extensional complex ceased during the mid-Pliocene as displacement was taken up by the Mina Deflection during a kinematic reorganization (Oldow, 2008). The Mina Deflection is a 120 km by 75 km zone of east northeast striking faults of Cenozoic age that kinematically links Cenozoic northwest oriented faults of the FLVFZ with those of the Walker Lane (Oldow 1992; Oldow et. al, 1994).

3 4

Despite movement of these faults having occurred during Cenozoic time, the geometry of the structures and faults within the Mina Deflection and Walker Lane are influenced by an older crustal boundary (Stewart, 1988; Oldow, 1992).

Figure 2.1. Map showing regional features along the Death Valley-Furnace Creek fault system. Major fault traces are shown in black. Physiographic locations are labeled in black and major fault systems are labeled in red. SP – Silver Peak Range, FLV – Fish Lake Valley, PM – Palmetto Mountains, WM – White Mountains, OV– Owens Valley, SM – Sylvania Mountains, DSV – Deep Springs Valley, EV – Eureka Valley, LCR – Last Chance Range, DV– Death Valley, CCRB – Cucomungo Canyon restraining bend, FLVFZ – Fish Lake Valley fault zone, SMFS – Sylvania Mountain fault system, OV-WMFZ– Owens Valley–White Mountains fault zone, PMFS – Palmetto Mountain fault system, DV-FCFS– Death Valley–Furnace Creek fault system.

Figure 2.2. Map showing study location and adjacent features. Seismic refraction profile locations are shown in white in the satellite image on the right side of the figure, the location of the yellow box. Major fault traces are shown in black on the left side of the figure. Physiographic locations are labeled black and major fault locations in red. SP – Silver Peak Range, FLV – Fish Lake Valley, PM – Palmetto Mountains, SM – Sylvania Mountains, DSV – Deep Springs Valley, EV – Eureka Valley, LCR – Last Chance Range, HTH – Horse Thief Hills, CCRB – Cucomungo Canyon restraining bend, FC-FLVF – Furnace Creek and Fish Lake Valley faults, SF – Sylvania Mountain fault system, PMF – Palmetto Mountain fault system. Modified from Oldow and Geissman (2013).

The dimensions of FLV vary, with a width ranging from 8 km in the south up to 25 km wide in the north (Reheis and Sawyer, 1997). The FLVFZ serves as the western edge of FLV separating the valley from the White Mountains in the north, and the Horse Thief Hills in the south. The Volcanic Hills border FLV to the north where the right lateral strike slip displacement of the FLV fault zone is transferred to the east through the east-northeast striking faults of the Mina Deflection in an extensional right step over into the Walker Lane (Sawyer,

1991; Oldow et al., 1994; Reheis and Sawyer, 1997). In the east, FLV is bordered by the Silver Peak Range, and to the south are the Sylvania Mountains and the Last Chance Range.

Reheis and Sawyer (1997) determined that the slip rate along the FLV fault zone was variable from the late Neogene through the Pleistocene. Their long-term lateral slip rate for this segment of the fault is an average of 5 mm/yr with a range of 3 -12 mm/yr. Lateral slip rates declined in the Pliocene, increased to 11 mm/yr in the middle Pleistocene and declined to 2-3 mm/yr in the late Pleistocene (Reheis and Sawyer, 1997). The decline in slip rates within the Pliocene are believed to coincide with the inception of extensional displacement along the FLVFZ and the opening of Fish Lake Valley (Reheis and Sawyer, 1997). This extensional component along the fault zone created the accommodation space necessary for the deposition of the fluvial and lacustrine sedimentary sequences that have been exhumed in southern Fish Lake Valley (Willow Wash, shown as unit Qa1/Ns in Figure 2.3). Tectonic shearing along the DV- FCFS has led to the formation of crush zones within the granitic bed rock of the Sylvania Mountains, which has produced high volumes of sediments that are currently transported westward and deposited into the Cucomungo alluvial fan within Eureka Valley (Blair, 2003). The Cucomungo Canyon Restraining Bend is located along a large, approximately 10 km, left step along the DVFLV fault system at the southern end of FLV where the highly deformed Paleozoic and Proterozoic meta-sedimentary sequences of the Last Chance Range are juxtaposed with the Jurassic quartz monzonite of the Sylvania Mountains (McKee, 1968; Reheis, 1992; Blair, 2003). The CCRB uplift exceeds 1.2 km and serves as the topographical high, 15 km wide and 27 km long, separating Fish Lake Valley and Death Valley (Blair, 2003; Oldow, personal communication, 2012).

2.1 Geologic Unit Descriptions

The survey site is the narrow fault trough at the southern end of FLV north of Cucomungo Canyon and the Last Chance Range (Figure 2.2). The western border of this narrow valley is bounded by the Horse Thief Hills which are composed of Paleozoic and Proterozoic metamorphosed sedimentary rocks consisting of dolomite, limestone, marble, quartzite, siltstone and shale and that are highly deformed (Reheis and Sawyer, 1997; Reheis, 1992). These are the

7 rock units that compose the basement of FLV except where intruded by Mesozoic quartz monzonite and are sporadically overlain by Pliocene and Miocene volcanic rocks (Reheis and Sawyer, 1997). These units have been detailed in length by McKee (1968) and are designated as Pz/P in the geologic map in Figure 2.3. Immediately east of the study area is Willow Wash, an important drainage that is also the focus of the Miles Center study of the CCRB. Units within Willow Wash are shown here as undivided as the stratigraphy and deformation of Willow Wash are discussed in more detail by another study conducted by the Miles Center for the CCRB project. The Sylvania Mountains lie to the south and east of the study area and are largely composed of porphyritic quartz monzonite (Jq) with distinctive euhedral potassium feldspar phenocrysts with diorite inclusions (McKee, 1968; Reheis, 1992). This quartz monzonite is derived from a mid-Jurassic pluton that has been dated at 155± 3 Ma through K-Ar dating of biotite crystals found within collected samples (McKee, 1968). The White Mountains, located to the west of FLV, are composed of Jurassic and Cretaceous granitic plutons partially overlain by Pliocene and Miocene volcanic rocks (Stockli et al., 2003). Quartz monzonite found in the southern end of the White Mountains in the Inyo batholith is of similar age, 161 ± 4 Ma, and composition to that found in the Sylvania Mountains, 8 mi to the southeast (McKee, 1968). This suggests that the quartz monzonite was once a continuous unit and has been displaced by movement along the FLV fault. During the Pleistocene, the White Mountains were glaciated and served as a source for much of the sedimentation within northern FLV (Sawyer 1990; Reheis et al., 1996). The Quaternary aged sediments that comprise the eastern hills bordering the southern end of FLV along the exposed fault scarps northwest of Willow Wash are composed mainly of quartz monzonite. The section does not contain clasts from the now adjacent Paleozoic and Proterozoic metasediments and has compositions, bedding, grain size and angularity that indicate derivation from the Sylvania Mountains (Robinson et al., 1968; Reheis and McKee, 1991; Reheis et al., 1991; Reheis and Sawyer, 1997). This unit is a buff-gray, moderately sorted, consolidated sandstone conglomerate with an angular feldspar matrix (K. Gibson, personal communication, 2016) and is designated as Quaternary alluvium 2 (Qa2 in Fig. 1).

The Quaternary alluvium in the study area can be divided into four main units that are primarily bounded by faults. Quaternary alluvium 3 (Qa3) is a middle Pleistocene alluvial fan

8 sequence of Indian Creek composed of gravel, sand, and silt that is poorly sorted and stratified (Reheis, 1992). This unit is similar in composition, but of greater age, to more recent alluvial sequences and outcrops as a chain of exposures (shown along the partially concealed fault in Figure 2.3) in the southernmost section of FLV. Quaternary alluvium 4 (Qa4) are late Pleistocene to mid Holocene age alluvial fans of Marble, Leidy and Indian Creek and cover much of southern FLV (Reheis, 1992). This unit forms broad, low angle alluvial fans west of Willow Wash and is composed of poorly sorted and consolidated, matrix supported, intermixed and interbedded gravel, sand and silt (Reheis, 1992). The main contact between Qa2 and Qa4 follows a fault line that is revealed by numerous fault scarps with a NW trend on the eastern side of the small fault trough that makes up the southern end of FLV (Figure 2.1). Qa1/Ns represent the undivided Quaternary and Neogene fluvial and lacustrine sediments found within Willow Wash. Nv represents Neogene volcanic deposits consisting of ash-fall tuff and basalt. These units are described in detail by Reheis and Sawyer (1997).

by specified in is map This . location the

which are discussed in Chapter in whichdiscussed 4 are

survey of Willow of southern map geophysical just FLV the of west Wash edge Generalized . with geologic

gold box 2.1. gold Figure in

Figure 2.3 Figure lines black the in indicated dashed locations the

CHAPTER 3 GEOPHYSICAL METHODS

Despite several of the faults in the study area exhibiting subtle surface expressions, the orientations of these faults in the subsurface are largely speculative. In order to ascertain the geometries of these faults, near surface geophysical surveys utilizing high resolution seismic refraction and microgravimetry were performed. The locations of the surveys were chosen based on the likelihood of crossing important structures informed from prior mapping and from visible surface expressions. A total of three seismic surveys and three gravity surveys were conducted for the purpose of creating forward models.

3.1 Seismic Refraction

The seismic refraction method (Reynolds, 2011; Pelton, 2005) is a commonly used geophysical exploration technique used to determine the depth and position of subsurface structures through the analysis of seismic waves as they propagate through materials with varying elastic properties (density, porosity, mineralogy, etc.) that determine seismic wave velocity. Seismic refraction surveys require the use of a controlled energy source at a known location that is synchronized with a recorder, or seismograph. The source imparts seismic energy into the ground as a seismic wavefront (Pelton, 2005) that is detected by geophones arranged along a profile line at equal spacing within a spread. Geophones are seismometers that detect vertical ground motion from transient seismic waves and convert this motion into an electrical signal that is sent along a cable to the seismograph. The movement of each geophone along the active spread is recorded as a seismogram. A seismogram is recorded for each geophone that is active along the spread for each shot, producing a seismic record called a shot gather. The shot gather has each seismogram recorded for a single shot point on a plot of geophone position verses traveltime. Multiple shot points are used at spaced intervals along the

10 11 spread to obtain redundant data with consistent traveltime curves used to resolve subsurface ambiguities (Ackermann, 1986) that may otherwise be indeterminable.

The most important information in seismic refraction data is the time of the first arrival of the P-wave energy imparted by the source at each geophone. The selection of the first arrival is done manually and care must be taken due to the presence of background seismic noise such as wind or unintended ground vibrations. Determining the first arrivals can be difficult for geophones adjacent to the source location given that the airwave, approximately 330m/s, can be faster than the seismic velocity through the surficial layer; referred to as the low velocity layer. Seismic energy from the source attenuates rapidly with increasing offset requiring that the plot traces be amplified (Figure 3.1). This will also amplify noise which can be problematic given that the signal to noise ratio decreases with increasing offset, decreasing the overall validity of first arrival picks with greater offset. The manually selected first arrivals are then plotted with geophone location against time, and this is called the traveltime curve (Figure 3.2). Seismic refraction relies on the analysis of waves that are transmitted through layers of the Earth as opposed to waves that are reflected at interfaces between layers with contrasting acoustic impendences. Seismic velocities tend to increase with greater depth, and when seismic waves happen upon an interface separating layers with different seismic velocities, the waves will refract. At a certain incident angle, the seismic wave will critically refract and travel parallel to the surface along the interface of two layers with a seismic velocity in the second layer that is greater than the first. This critically refracted wave will leak energy back to the surface in the form of a head wave. The head waves are recognized as the first arriving energy with linear traveltime in seismograms. Waves that are critically refracted at greater depths travel with an overall higher average velocity than waves that refract within shallower, slower layers and will be detected by geophones with a greater offset from the source. This is due to the airwave and direct wave having a much slower average velocity than the waves refracting from faster mediums. The velocities of the critically refracted waves depend upon the velocity and topography of the refracting layers. The apparent velocity of the refracted arrivals is determined from the slopes of the traveltime curves, which is the slowness, or the reciprocal of the P-wave

12 velocity. The use of multiple shot points with waves traveling in multiple directions allows for the determination of seismic layer velocities through averaging (Nisengard et al., 2008).

Figure 3.1. An example of seismic traces that have been gained. The first arrivals are marked with x’s and the airwave with the solid line. The selected first arrivals, shown with the blue x’s, form the traveltime curves. Adapted from Nisengard et al., 2008.

In order to model complex subsurface structures, closely spaced shot points and geophones are necessary to obtain redundant data and consistent traveltime curves. The surveys were designed to cross structures with subtle surface expressions orthogonal to their strike direction to best ascertain their subsurface geometries. Two-dimensional velocity models were created from the traveltime curves associated with each of the profile lines surveyed.

An airless jackhammer was used as the energy source, and the stacking of multiple shots per shot point was needed to improve the signal to noise ratio given the low amount of energy imparted and high attenuation of the seismic energy within the alluvial sedimentary units. Depending on the offset distance from the source to the spread, the number of stacked shots ranged from 6 to 144 per shot point. The recorder was a Geometrics StrataVisor NZ 48 channel seismograph, connected to two cables with forty-eight 30 Hz geophones each. Only one spread

13 of 48 geophones was active for each shot, with both split-spread and off-spread records made for each shot point through the rotation of the active spread. Despite the energy provided by the

Figure 3.2. Example of a simple two layer refraction model. A) Two layer model with computed traveltime curves plotted with the first arrivals determined from the observed seismic traces; B) Two layer model depicting ray path motion of waves created at the left shot point; C) Two layer model with wavefront propagation from the left shot point with refracted waves. Modified from Nisengard et al., 2008.

airless jackhammer being less than desirable, with a penetration depth of approximately 45 m, it was the best available source given the survey sites are within the Sylvania Mountains Wilderness Area where vehicles and wheeled devices of any kind are prohibited from leaving the

14 roads. All seismic equipment needed to be carried from point to point on foot, which limited the extent of the surveys.

3.2 Gravity Gravity surveys can reveal variations in the Earth’s gravity field based on local contrasts in the density of subsurface features, and have been used to ascertain the extent and depth of sedimentary basins, for structural analysis, petroleum exploration, and for small scale engineering applications (LeFehr and Nabighian 2012; Reynolds, 2011). The physical characteristics of rocks influence their density: mineralogy, level of compaction, age and burial depth (diagenesis), porosity and pore fluid type (Reynolds, 2011). High-resolution gravity data can reveal subtle changes in lateral densities within the subsurface. These changes in density along the profile can be used in conjunction with the seismic refraction data to mark the location of subsurface structures.

Gravity data was collected with Scintrex CG-5 gravimeters, a relative gravimeter that makes use of an electrostatically nulled fused quartz spring that can achieve microGal resolution (Scintrex, 2006). The location of each gravity station was measured using Leica GPS receivers with RTK and rapid static that can achieve accuracy below 2 cm with post processing. In order to determine the local effects of density variations in the subsurface, the gravity data was reduced following the methods outlined by LeFehr and Nabighian (2012). When taking gravity measurements, there are several factors that influence the observed gravity field: Earth tides, instrumental drift, latitude, elevation, and terrain. These factors need to be removed in order to observe the response caused by local density contrasts in the near surface. To do this, the

Complete Bouguer Anomaly, GCBA, is calculated using the following formula, with each term explained in the subsequent paragraph:

GCBA = GS – GT + CFA – CBC

Instrumental drift is the inherent within the gravimeter and is caused by the relaxation of physical components. Drift is accounted for by the use of survey loops, where gravity stations

15 are reoccupied during the survey in order to fit the drift to a linear curve. The gravitational fields of the sun and moon affect the Earth’s gravity field as a function of time and location. Similar to instrumental drift, these tidal effects are removed through reoccupation of gravity stations over time. Over a period of a few hours, the observed tidal effect is usually linear, and is removed. Since the CG-5 is a relative meter, an absolute gravity station in Tonopah, NV was occupied to convert the measured relative data into absolute gravity values. After these corrections, we have the station gravity, GS.

The Earth’s gravity field varies with latitude, increasing toward the poles. This is due to the ellipsoidal shape of the Earth, and the squarely inverse relationship of the gravity field strength with distance from the center of mass. To correct for latitude, the theoretical gravity value, GT, is calculated on the Geodetic Reference System 1980 ellipsoid, where φ = latitude:

ퟏ + ퟎ . ퟎퟎퟏퟗퟑퟏퟖퟓퟏퟑퟖퟔퟑퟗ풔풊풏ퟐ흋 ퟗퟕퟖퟎퟑퟐ. ퟔퟕퟕퟏퟒ [ ] √ퟏ − ퟎ. ퟎퟎퟔퟔퟗퟒퟑퟕퟗퟗퟗퟎퟏퟑ풔풊풏ퟐ흋

Elevation has a large effect on the strength of the gravity field, approximately 0.3086 mGal per meter, and is why accurate GPS measurements were taken to ensure the accuracy of the interpretations. The free air correction, CFA, accounts for the difference in height between the measured elevation of the gravity stations and datum elevation, which for this study was the GRS 1980 ellipsoid. Free air corrections were also made to account for the gravitational attraction of the atmosphere above the gravimeter.

Associated with elevation is an increase in mass below the gravity station, and this is accounted for with the Bouguer correction, CB. This is done by approximating a laterally infinite slab of constant density, usually 2670 kg/m3, with a height equal to that of t. The equation used to determine the effect the mass of this slab will have on the observed gravity is:

CB = 2πGρt

Variations in topography result in either an increase or decrease in mass adjacent to the gravity stations, and will have an effect on the measured gravity values. A terrain correction is made to remove the effect of topography around each of the gravity stations through the use of

16 digital elevation models with 1/3 arc second resolution (~10 m). An inner correction is made from 10 m to 5 km, and an outer correction is made from 5 km to 166.7 km according to the Hayford-Bowie method. In addition, a Bullard correction is made to account for the curvature of the Earth using the spherical cap formula (LeFehr and Nabighian, 2012). With the terrain corrections applied to the Bouguer correction, we obtain the Bullard correction, GBC. With the removal of the theoretical gravity, GT, and the Bullard correction, GBC, and the addition of the free air correction, GFA, we now have the Complete Bouguer Anomaly, GCBA, which is representative of gravity variations based on local density contrasts in the subsurface.

CHAPTER 4 PROFILE DESCRIPTIONS

Four seismic surveys and three gravity surveys were conducted in this study. Of these surveys, three profiles were created from the seismic and gravity data to model structures in the subsurface. These surveys were conducted in the southernmost extent of FLV within the narrow fault trough just west of Willow Wash and the Sylvania Mountains in the region shown in the geologic map in Figure 2.2. Moving north to south in Figure 2.2 the profiles are: the Horse Thief Canyon Profile (HTCP), the Fish Lake Valley Transect (FLVT), and the Eureka Valley Road Profile (EVRP). These three primary surveys were positioned to cross previously mapped and inferred faults. The fourth seismic survey (explained in detail in section 4.4) was performed only to determine the P-wave velocity of the pulverized quartz monzonite exposed along the FLV fault zone.

4.1 Eureka Valley Road Profile

Figure 4.1 shows the locations of the EVRP and the FLVT profiles at larger scale. The Eureka Valley Road Profile (EVRP) extends 520 m northwest from the Paleozoic/Proterozoic meta-sediments and follows Eureka Valley Road. This profile was designed to cross the contact between the Paleozoic/Proterozoic metasediments and the recent Quaternary sediments (Qa4) that has been previously mapped as a normal fault with a northward dip by Reheis (1992) and McKee (1967). This survey was designed with eight spreads of 48 geophones, with a geophone spacing of 0.5 m used for the first 48 m of the profile to obtain higher resolution data across the mapped fault location. Due to available equipment, receivers were spaced at 1 yd (.91 m) for spreads 3 and 4. For the remaining spreads, receivers were spaced by 2 m. Shot points were spaced with a 6 m interval for the first 66 m of the profile, a 12 yd interval from 58.97 m to 135.78 m, and a 24 m interval for the remainder of the line.

17 18

4.2 Fish Lake Valley Transect

The Fish Lake Valley Transect extends 576 m across the width of the narrow fault trough at the southern end of FLV. This profile crosses three previously mapped faults which are targeted with this survey (Figure 4.1). 20 m from the southwest end of the FLVT is the contact between the Pz/P metasediments and Qa4 which has been mapped as a high angle normal fault by Oldow (2012). Continuing to the northeast, the FLVT intersects with the end of the EVRP at 120 m. From 333 to 392 m the profile crosses an outcrop of Qa3. There are three outcrops of Qa3 in the center of the valley that are aligned to the northwest. McKee (1967) and Reheis (1992) mapped a concealed fault along the northeastern side of these outcrops. 507 m along the profile is the third targeted fault that is partially exposed from intermittent fault scarps that separate Qa4 from the older Qa2. The seismic refraction survey was designed using six 96 m spreads with 48 geophones at 2 m intervals. Shot points were spaced at 12 m for the first 5 shots, and 24 m spacing for the remainder of the line. Gravity stations were positioned every 24 m reoccupying shot point locations.

Figure 4.1. Geologic Map with EVRP and FLVT profile locations. Unit descriptions can be viewed in Figure 2.2.

4.3 Horse Thief Canyon Profile The Horse Thief Canyon Profile begins within the Pz/P metasediments at the head of the Horse Thief Canyon and extends northeast into the valley for a length of 384 m (Figure 4.2). This survey was performed to analyze the mapped fault along the contact between the Pz/P metasediments and the recent alluvium of Qa4, similar to the FLVT survey. This seismic

20 refraction survey was designed with four 48 geophone spreads with 2 m intervals. Shot points were spaced with 12 m intervals for the first 144 m to gain higher resolution data across the mapped fault. Shot points were spaced with 24 intervals for the remainder of the line. Gravity data was collected by reoccupying shot point locations every 24 m.

Figure 4.2. Geologic map with the HTCP profile location.

4.4 Granite Arroyo Survey

Figure 4.3. Location of the seismic refraction survey performed to determine the P-wave velocity of the crushed quartz monzonite along the DVFCFZ.

This seismic survey was conducted to the south of FLV, 3.62 km southeast from the EVRP, within an arroyo composed of weathered granite. This was a small scale, 48 m refraction survey performed to determine the P-wave velocity of the crushed Jurassic quartz monzonite on the eastern side of the FLV fault. This was done in an attempt to distinguish the crushed granite from the Paleozoic/Proterozoic metasediments on the western side of the fault. The geophone spacing was 0.5 m, and gravity data was not collected along this transect. Two refraction models were created from the Granite Arroyo survey, which revealed a P-wave velocity that was similar to that of the Paleozoic/Proterozoic metasediments. The resolvable depth was very shallow given a maximum spread length of 24 m for each of the profiles. The highest observed P-wave velocity in the granite outcrop was 1350 m/s.

CHAPTER 5 PROFILE CONSTRUCTION

Two-dimensional forward models were created with data acquired from the seismic refraction and gravity surveys. The models are first constructed using the refraction data given its capability to resolve subsurface structures at a higher resolution than the gravity data. The gravity data enhances the interpretation of the seismic data by exposing subtle deviations in lateral densities along the profile lines, and is applicable at greater depths. Together, the data allows for identifying the locations and relative depths of structures within the subsurface that may not be readily identifiable through the analysis of seismic data alone. The models were created with the MATLAB script geomod written by John Ferguson and Emily Hinz at the University of Texas at Dallas. The geomod script makes use of a measured topography obtained from GPS measurements of the locations of shot points and gravity stations as the basis for a model where synthetic traveltimes are computed and tested against the traveltime curves generated from analysis of the recorded data (Nisengard et al., 2008; Smith and Ferguson, 2000). An assemblage of user specified polygons are created beneath the displayed topographic surface and their vertices are assigned and adjusted manually. The adjustments made to the polygons are made to resolve anomalies observed within the traveltime curves, which can be associated with subsurface structures or lateral variances in seismic velocity. Assigned to the polygons are physical properties, such as P-wave velocity and density, and a theoretical model is run iteratively against the observed data until the model achieves a satisfactory fit (Figure 5.3). When the theoretical model provides an acceptable match to the observed data, the model can be considered a reasonable interpretation of the subsurface structure and geology.

5.1 Physical Properties Six P-wave velocities were used to characterize geologic units within the models. These velocities were determined from averaging slopes in the traveltime curves. The 330 m/s layer

22 23 represents the thin layer of loosely consolidated alluvium at the surface that comprises the low velocity layer (LVL). It is not unusual for the P-wave velocity of the LVL to be slower than that of sound in air, therefore making it difficult at times to distinguish between the direct wave and the airwave at receivers adjacent to the shot point. The 500, 800 and 1200 m/s layers represent alluvium in increasing states of compaction and age. These layers are representative of alluvial fan sequences deposited from the Sylvania Mountains. The 1500 and 2500 m/s layers are the Precambrian/ Paleozoic basement.

Table 5.1. Observed P-wave velocities from the traveltime curves used in the creation of the forward models and associated lithologic unit.

Observed P-wave velocities were slower than commonly observed values for the denser bedrock units of metamorphosed sedimentary rock and granite. Rock types have a wide range of observed seismic velocities (Table 5.2) that are dependent upon density, porosity, saturation, foliation, compaction, size and shape of grains, and burial depth. The slower velocities observed in the Pz/P and Jq basement is attributed to intense shearing and pulverization of the bedrock within this region due to continued strike slip motion on the FLV fault zone. Rempe et al. (2013) conducted a seismic refraction study within granitic plutons along the San Andreas Fault zone that had been pulverized by continued translational fault motion. The study revealed similar low seismic velocities, 1 – 2 km/s, within the damaged granite. While 1500 m/s is the P-wave velocity of water saturated sedimentary rocks, this is not believed to be evidence of the water table. The 1500 m/s velocity was confirmed for Paleozoic/Proterozoic sequence by placing shot points and receivers within and adjacent to outcrops of this unit. While the polygons in the model may be representative of geologic layers, they are modeled as zones of P-wave velocity and specific velocities may not necessarily correspond to a specific lithology. The difference in

24 seismic velocities observed in the Palozoic/Proterozoic metasediments and the Jurassic quartz monzonite was small, thus making a distinction between the two layers in modeling difficult and speculative at best.

Table 5.2. Rock types with a range of observed P-wave velocities. Adapted from Reynolds (2011).

After the creation of the P-wave velocity models, the velocity layers are assigned density values to test whether the refraction models are agreeable with the gravity data. The gravity data are entered into geomod as a plot of observed CBA values over the range of the profile. The density models have a small range of CBA values (mGal), which reflects the relatively small extent of the surveys and the relatively subtle changes in density within the subsurface over the range of the profiles. Density values are assigned to the polygons and synthetic gravity values are calculated and plotted against the observed values. The regional gravity anomaly, a long wavelength anomaly stemming from large scale geologic features (Reynolds, 2011), is calculated from the difference between the observed and calculated values and fit to a second-degree polynomial.

Rock samples from the Pz/P metasediments and Jurassic quartz monzonite (Jq) were collected from the field to calculate density. The mass of each sample was measured on an electronic

25 scale, and then placed within a graduated cylinder filled with water to determine sample volume. The calculated densities are shown in Table 5.3. Cataclastic samples of Jq had a tendency to disintegrate when placed within a beaker of water. To obtain a better bulk density, two samples of the cataclastic Jq, Jq7 and Jq8, were coated with a polyurethane varnish prior to measuring their volumes in an effort to preserve their air-filled porosity. The cataclastic samples were collected along the contact between the Pz/P metasediments and the Jq along the FLV fault. Jq samples that are described as un-weathered were recovered from an outcrop adjacent to the location of the Granite Arroyo seismic profile location shown in Figure 4.3. These samples were more intact than cataclastic samples and did not disintegrate when placed into water. The average density for the quartz monzonite samples is 2.393 g/cm3. Three samples of the Pz/P metasediments were collected and measured for density as well. Samples P1 and P4 were collected from an outcrop at the southern end of the EVRP. Samples P2 and P3 were collected from the hills just to the west of the southwest end of the FLVT. These samples were intact, and measurements serve as a valid estimate of the matrix density of this unit. The average density for Pz/P samples is 2.632 g/cm3.

Table 5.3. Measured densities of Paleozoic/Proterozoic metasediment and Jurassic quartz monzonite samples collected from survey sites.

A Nettleton density analysis was performed using gravity stations collected within the Pz/P and Qa2 for the purpose of identifying the proper densities for use within the models

(Nettleton, 1939). To create the Nettleton profiles, a range of densities are used in the calculation of the Bouguer anomaly and compared to the topography. Figures 5.1 and 5.2 show the Nettleton profiles for the Pz/P and Qa2 units and the locations of the gravity stations used in the analyses. The density values that correlate least with the topography (slope of the mGal/m is nearest to zero) are the density values selected for use within the gravity models. Twenty-eight gravity measurements within the Pz/P metasediments were used for the Nettleton density analysis which resulted with the minimum slope value correlating to a density of 2500 kg/m3 (Figure 5.1). This density value had been assigned to Paleozoic/Proterozoic metasedimentary bedrock in prior gravity modeling of the Yucca Flat location within the Nevada Test Site (Ferguson et al., 1988). Forty-six gravity measurements were used for the analysis of Qa2, with the minimum slope correlating with a density of 1800 kg/m3 (Figure 5.2).

Figure 5.1. Map of the 28 gravity stations used for Nettleton density analysis of the Pz/P metasediments shown in the top of the figure. Results of the analysis are shown in the chart on the bottom of the figure and the minimum slope 0.001798 mGal/m is associated with a density of 2500 kg/m3.

Figure 5.2. Map of the 46 gravity stations used for Nettleton density analysis of unit Qa2 is shown in the top of the figure. Results of the analysis are shown in the chart on the bottom of the figure and the minimum slope 0.000596 mGal/m is associated with a density of 1800 kg/m3.

With the measured densities from the collected rock samples and the densities determined from the Nettleton analysis, the porosity of the Pz/P and Jq units can be analyzed. If we assume that the bulk density of these units is represented by the density values realized from the Nettle analysis, we can use the following equation that relates mass density to porosity:

흆 = ∅흆푭 + (ퟏ − ∅)흆푴 The bulk density of the material is denoted by, 흆, with fluid density within the pore space denoted as, 흆푭, matrix density as, 흆푴, and the porosity as ∅. The average density value of 2.632 g/cm3 calculated for the Pz/P samples in the lab was used for the matrix density of the Pz/P given the lack of porosity within the collected samples. The average density of granite was used as the matrix density for the Qa2 given the granitic provenance of these sediments. These units are assumed to possess air filled porosity and therefore the fluid density is that of air: 1.2928 g/cm3. These values result in a calculated porosity of 32.09% for Qa2, and 5.02% for the Pz/P.

shown in white in the model on the right. the in white in shown right. the on model

fronts are frontsare

the model building process adjustments model the and process must building made be

curves shown in blue on the upper sections of the figure. Synthetic the on figure. blue in shown sections curves the of upper

wave velocity model created using seismic refraction data in geomod. Observed traveltime in data seismic using Observed refraction geomod. created model velocity wave

wave model developed from the Granite Arroyo survey discussed survey 4.4. model Arroyo from section in Granite the developed wave

curves are shown in red and synthetic traveltime traveltime red in shown are and synthetic curves polygons in model. the the of theoretical velocities geometry traveltimes and based assigned the on are curves traveltime of side the shows figure left the on model the of beginning The of side complete, is the The the of figure right the on model subsurface. an to representation be accurate model the for match data. Wave observed the curves traveltime synthetic the and is This P the Example 5.3. a of P Figure

CHAPTER 6 GEOPHYSICAL MODELS

6.1 Eureka Valley Road Profile

Figure 6.1. Cross section along the Eureka Valley Road profile shown in the lower section of the figure. Observed traveltime curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure.

Figure 6.1 is the forward model developed from seismic refraction data along the EVRP. For this profile, 87 traveltime curves were analyzed across eight spreads. South directed traveltime curves originating from shot points 10-20 in spreads 3 and 4 show a marked decrease

31 32 in traveltime at 50 m, and a marked bowing effect (Figure 6.2). This is evidence of P-waves moving into a zone of higher velocity across a vertical interface. This feature is modeled as a normal fault with an 84 dip to the northwest, and offsets the Pz/P basement by 19 m. This confirms the location of the mapped normal fault discussed in section 4.1. The incisions into the 800 m/s and 1200 m/s layers from 375-495 m are modeled as buried channels. The exact nature of these structures is difficult to determine given the presence of a shallow high velocity layer observed within shot records from spread 8. This anomaly, in addition to high winds that decreased the signal to noise ratio, made the selection of first arrivals difficult. The 2500 m/s layer from 154-361m is representative of more consolidated Pz/P metasediments.

Figure 6.2. EVRP with wavefront plotted in white for a single shot point at 124.81 m. Observed traveltime curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure.

Figure 6.2 shows interaction of the wavefront as it transgresses the fault plane at 50 m. The faster traveltimes observed on the south side of the fault plane are caused by seismic waves critically refracting along the fault plane and traversing this interface back towards the geophones at the surface resulting in the bowing of the traveltime curve seen in Figure 6.2. This anomaly is observed in traveltime curves from each of the refraction profiles (though most noticeable in the EVRP and FLVT) and is used to accurately model the locations and orientations of structures.

Figure 6.3. Graph showing gravity stations collected along the EVRP. Complete Bouguer Anomaly (CBA) gravity data is shown in red, with the change in CBA between stations shown in blue. There is a noticeable change in the CBA values in the first 100 m of the profile that coincides with the location of the fault shown in Figures 2.2 and 4.1.

Gravity data collected along the EVRP is charted in Figure 6.3 and applied to the seismic refraction model in Figure 6.3. The gravity data plotted in Figures 6.3 and 6.4 shows a relatively large decrease in CBA values over the first 100 m of the profile, confirming the presence of a lateral density contrast over the location of the fault.

Figure 6.4. Cross section along the Eureka Valley Road profile created from seismic refraction data shown in the bottom section of the figure with density values assigned to polygons in the model. Complete Bouguer Anomaly is shown in red with computed gravity response shown in blue in the top section of the figure.

6.2 Fish Lake Valley Transect

Figure 6.5. Cross section along the Fish Lake Valley Transect shown in the lower section of the figure. Observed traveltime curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure.

Figure 6.5 is the forward model developed from seismic refraction data along the FLVT and reveals subsurface structures at the locations of the three targeted faults discussed in secton 4.2. A total of 58 travel time curves were analyzed across the six spreads. The gray 1500 m/s layer is interpreted as the Paleozoic/Proterozoic metasedimentary basement, as the southwest end of the survey line was positioned within an outcrop of this unit. This relatively fast P-wave velocity was evident at shallow depths from 0-18 m where the Paleozoic/Proterozoic metasediments come into contact with Qa4. Southwest directed traveltime curves emanating from shot points 3-10 exhibit a decrease in traveltime at 18 m along the profile. This decrease in traveltime, observable at the same position across multiple traveltime curves, indicates that

36 southwest bound P-waves are moving across a vertical interface and into higher velocity material. This boundary is modeled in Figure 6.5 as a high angle normal fault that offsets the Pz/P basement by 5 m. From 147-180 m there is a gradual increase in traveltimes observed from northeast directed traveltime curves from shot points 1-9. This is modeled as a buried channel within the 1200 m/s layer. Depths to the 1200 m/s and 1500 m/s velocity layers outside of 147 and 180 m are similar, arguing against the presence of faulting without greater evidence. There is a gap in the coverage of traveltime curves across this location stemming from an error in survey design. However, the gravity data shows a decrease in the residual anomaly across this part of the profile indicating a thicker zone of lower density material. From 333-392 m the survey line crosses the southern outcrop of Qa3 that forms a chain of three small mounds in the center of the valley (Fig 4.1). This coincides with the location in the refraction model where the 800 m/s layer is exposed at the near surface. The most prominent structure in the refraction model is the large normal fault at 418 m with an approximate 45 dip to the northeast. Southwest directed traveltime curves originating from shot points 22-27 show a decrease in traveltimes within spread 5, and bowing of traveltime curves indicative of seismic waves refracting along the fault plane and returning to the surface much like the traveltime curve shown in Figure 6.2. This is evidence of P-waves transitioning across an interface and into material of higher velocity, and also reveals P-waves critically refracting along the fault plane. 510 m along the profile another normal fault is modeled. Northeast directed traveltime curves originating from shot points 19-21 exhibit a decrease in traveltimes within spread 6 that are characteristic of a blind step.

Figure 6.6. Graph showing gravity stations collected along the FLVT. Complete Bouguer Anomaly (CBA) gravity data is shown in red, with the change in CBA between stations shown in blue.

Figure 6.7. Cross section along the Fish Lake Valley Transect created from seismic refraction and gravity data shown in the bottom section of the figure with density values assigned to polygons in the model. Complete Bouguer Anomaly is shown in red with computed gravity response shown in blue in the top section of the figure.

Gravity data collected along the FLVT is charted in Figure 6.6 and applied to the seismic refraction model in Figure 6.7. There is an abrupt change in the CBA value adjacent to the modeled buried channel at 147-180 m. Gravity anomalies have greater effect on the residual when near the surface, and this is interpreted as a shallow density contrast within the alluvial layers. CBA values along the FLVT decrease across the location of the large normal fault at 418 m indicating a thicker sequence of lower density material on the northeastern side of the fault. In the location of the fault at 510 m, Figure 6.5 shows a slight increase in the CBA value, which correlates with the blind step in the seismic refraction model. The depth to the Pz/P basement varied from 2-30 m on the southwest side of the normal fault at 418 m. Depths on the northeast side of this fault were indeterminable but the gravity model indicates that downward throw of the Pz/P basement is approximately 100 m. However, it is difficult to confirm the lithology of the basement on the downthrown side of this fault without more information.

6.3 Horse Thief Canyon Profile

Figure 6.8. Cross section along the Horse Thief Canyon profile shown in the lower section of the figure. Observed traveltime curves are shown in red with synthetic traveltime curves shown in blue on the upper section of figure.

Figure 6.8 shows the forward model for the seismic refraction survey performed at the head of the Horse Thief Canyon. The signal to noise ratio was poor for many of the shot records collected for this survey and made the determination of first arrivals difficult while increasing the potential for errors. Within spreads 1 and 2, the poor quality was observed within the shots with longer offset. In spreads 3 and 4, the shorter offset split-spread shots were plagued with noise and an anomalous shallow high velocity layer. Despite the shortcomings of the data, interpretations of the subsurface structures were still viable. 54 traveltime curves were analyzed across 4 spreads revealing several subsurface structures. Lateral variations within the travel times reveal the presence and locations of several buried normal faults. Fault locations were

40 identified by subtle changes within the travel time curves that indicated either an increase or decrease in velocity as the P-waves laterally traverse a vertical interface between two contrasting velocities. There are two normal faults dipping toward the east modeled at 43 and 67 m, with Pz/P basement offsets of 9 and 3 m respectively. Southwest directed traveltime curves originating from shot points 7-10 show a subtle decrease in traveltimes when crossing these locations. Modeled at 114 m is another normal fault with a westward dip and offsets the Pz/P basement by 3 m. Southwest directed traveltime curves from shot points 11-15 exhibit a blind step at 114 m indicative of P-waves transitioning into a lower velocity zone. This is supported by an evidenced decrease in traveltimes across this location in northeast oriented traveltime curves originating from shot points 4-9. The faults at 67 and 114 m appear to form a small graben. At 169 m is the fourth and largest of the faults modeled along the HTCP with a 47 dip to the northeast, and a 28 m offset of the Pz/P basement. Northeast directed traveltime curves from shot points 9-13 display a subtle blind step in this location, which coincides with a thickening of lower velocity material on the downthrown side. The fault at 169 m is aligned with the northwest trending fault on the eastern edge of the Pz/P outcrop shown in Figure 4.2. Figure 6.9 shows a decreasing trend in CBA values moving northeastward along the HTCP into the valley. With the gravity data assigned to the refraction model in Figure 6.10, we can determine a general depth to the Pz/P basement based with single contrast density model.

Figure 6.9. Graph showing gravity stations collected along the HTCP. Complete Bouguer Anomaly (CBA) gravity data is shown in red, with the change in CBA between stations shown in blue.

Figure 6.10. Cross section along the Horse Thief Canyon profile created from seismic refraction and gravity data shown in the bottom section of the figure with density values assigned to polygons in the model. Complete Bouguer Anomaly is shown in red with computed gravity response shown in blue in the top section of the figure.

CHAPTER 7 DISCUSSION

The geophysical models presented reveal the geometries of several faulted boundaries within the southern end of FLV. The dominant form of deformation in the southern end of FLV has been due to oblique right lateral movement along the FLV fault zone. While the fault zone has had an active history of translational motion, an extensional component responsible for the opening of FLV is evident in the geophysical models presented. In the EVRP seismic refraction model, the fault at 50 m confirms that the previously mapped fault in Figure 4.1 that forms the contact between the Pz/P and Qa4 is a normal fault with a northward dip. The HTCP reveals that the Pz/P is overlain by a thin covering of alluvium at the head of the Horse Thief Canyon with a depth to basement ranging from 2 – 17 m west of the fault shown at 169 m. The FLVT models reveal that the concealed fault at 418 m mapped by McKee (1967) and Reheis (1992) is a normal fault with a dip to the northeast. Combined with visible surface expressions, it can be stated with confidence that this fault follows the northeast edge of the Qa3 outcrops exposed in the center of the valley, in similar northwest orientation to faults within the FLV fault zone. The depth to the Pz/P basement, though indeterminable on the eastern side of this fault with the surveys conducted, is relatively shallow on the western side, where the alluvial sequence is much thinner and rests atop a pediment of the Pz/P with a depth to the Pz/P layer ranging from 2-30 m. The easternmost fault in the FLVT is the fault that forms the boundary between Qa2 and Qa4 and serves as the eastern margin of the narrow fault trough that makes up the southern end of FLV. The older alluvial deposits of Qa2 are elevated along this fault, though the fault follows the trend of oblique strike slip faults along the FLV fault zone. This suggests that uplift from the CCRB is causing inversion along previously faulted boundaries. The seismic refraction profile for the HTCP shows convincing evidence that the fault that borders the Qa3 outcrops and aligns with the Pz/P metasediments is a continuation of the fault 418 m along the FLVT. The two modeled

42 43 faults at 418 and 510 m are bounding faults for a major sedimentary basin to the northeast that is now tilted with recent sedimentation on the now topographically low Pz/P basement. The profiles also indicate that the fault that borders the northeast side of the Qa3 outcrops can be extended to align with the northeast side of the Pz/P outcrop at the head of Horse Thief Canyon (Figure 7.1).

Figure 7.1. Modified geologic map from Figure 2.3. The fault at 418 m along the FLVT is presumed to align with the fault at 169 m along the HTCP.

CHAPTER 8 CONCLUSION

The work performed in this study has revealed the geometry of faults obscured by alluvium in the southern end of Fish Lake Valley. Forward models developed from the analysis of seismic refraction and gravity data reveal structures that are the result of extensional deformation along the FLV fault zone. Through determining the orientation of faults within the subsurface adjacent to the CCRB uplift, structural constraints for the collaborative model of deformation detailing the growth of the CCRB have been established.

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VITA

Kyle McBride was born in Hayward, CA in 1986. After moving to Texas in the 1990’s, Kyle graduated from Bridgeport High School in Bridgeport, TX in 2004 and attended business school at the University of North Texas. Feeling intellectually unfulfilled with a business degree, Kyle decided to obtain a master’s degree in geology, and enrolled at UT Dallas in the fall of 2012. In the winter of 2014, Kyle began working with the first research group at the Ellison Miles Geoscience Center at UTD, where he worked with Dr. John Ferguson on near surface geophysics. Kyle graduated with his Master of Science degree in December 2016.