GEOMETRY OF OWENS VALLEY IN VICINITY OF LONE PINE AND INDEPENDENCE, CALIFORNIA BASED ON GRAVITY ANALYSIS

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1 1 GEOMETRY OF OWENS VALLEY IN VICINITY OF LONE PINE AND INDEPENDENCE, CALIFORNIA BASED ON GRAVITY ANALYSIS An Undergraduate Thesis Presented to The Faculty of California State University, Fullerton Department of Geological Sciences In Partial Fulfillment of the Requirements for the Degree Bachelor of Science in Geology By Dominic Chaulk August 16, 2016 PHIL ARMSTRONG, Faculty Advisor

2 2 Geometry of Owens Valley in vicinity of Lone Pine and Independence, California based on gravity analysis California State University of Fullerton Undergraduate Thesis By Dominic Chaulk Advised by Dr. Phil Armstrong

3 3 Abstract The subsurface geometry of Owens Valley in the vicinity of Lone Pine is controlled by faults. Although the surficial aspects of the faults are well constrained, subsurface features are not. Owens valley is bound on the west by the Sierra Nevada Mountains and on the east by the White and Inyo Mountains. The Sierra Nevada Frontal Fault System (SNFFS) on the eastern edge of Owens Valley has a shallow dip of ~25-35 at the surface. The Owens Valley Fault system strikes through study area and the eastern half of Owens Valley. In order to evaluate the subsurface geometry, including constraining the fault systems, a gravity survey was performed between Manzanar and Independence. The Independence and Manzanar transects comprise 57 and 45 gravity stations and are 19.5 and 18 km long, respectively. The Independence transect displays little to no isostatic gravity anomaly changes for the western ~10 km followed by an abrupt increase to +12 mgal east of the Owens Valley Fault in the vicinity of the West Inyo fault. For the Manzanar transect, anomalies increase to +5 mgal over the northward projection of the Alabama Hill, but decrease abruptly to -8 mgal east of the Owens Valley Fault. Modeling of regionally corrected isostatic anomalies suggest a graben structure between the Owens Valley and West Inyo faults, with basin depths ranging from ~1 to 3 km depending on basin fill density. The lack of gravity anomalies at the SNFFS on the western side of Owens Valley indicates that the faults there place granitic rocks in the footwall against shallowly buried (~200 m) granitic rocks in the hanging wall. Despite being unable to determine SNFFS dip from gravity data, the shallow depth of the granitic rock on the western edge of Owens Valley suggests ~2.5 km of vertical offset across the SNFFS.

4 4 Introduction Owens Valley is the westernmost part of the Basin and Range Province (Figure 1). It is considered a tectonically controlled basin located in a westward moving microplate between the Pacific Plate and the North American Plate (Unruh et al., 2003). This microplate is referred to by Stevens et al. (2013) as the Sierra Nevada-Great Valley microplate. In order to fully understand the tectonics of the valley it is important to understand its basin geometry. Owens Valley is located ~135 km east of Fresno, CA and ~80 km west of the California-Nevada border, between the Sierra Nevada Mountains on the west and the White Mountains on the east (Figure 2). Several earlier studies evaluated different structural and tectonic aspects of the Owens Valley structural system. For example, Unruh et al. (2003) studied the area to determine whether normal faulting was a result of accommodating microplate movement rather than Basin and Range extension. Earlier work on the Sierra Nevada Frontal Fault System (SNFFS) shows dips of ~ 25 to 35 east (Gadbois, 2015; Mottle, 2015; Shagham, 2011; Phillips and Majkowski, 2011). A gravity survey in the area revealed a north-south oriented region of low gravity anomalies that help define the overall basin geometry (Pakiser et al., 1964) (Figure 3). Lee et al. (2001) determined the age of pre-1872 earthquakes along the Owens Valley fault, to better understand its role in the accommodation of right-lateral shear within the Eastern California Shear Zone. Gravity surveys can be used as a geophysical tool to help determine the subsurface geometry in areas such as Owens in the Lone Pine/Independence area. In this study, a gravity survey was conducted in Owens Valley in order to map the depth of the alluvial basin fill and determine the location and orientation of faults in the subsurface. These faults include the Sierra Nevada Frontal Fault System (SNFFS) and the Owens Valley Fault System (Figure 4a). These

5 5 data can be used to help determine the basin depth of Owens Valley and possibly to place constraints on the dip of the SNFFS in the Lone Pine and Independence area. Geologic Background The Sierra Nevada Mountains have a NNW-SSE orientation, and border the west side of Owens Valley. The Sierra Nevada Mountains are characterized as an uplifted fault block that was tilted westward and are cored by mostly Mesozoic granitic plutons (Unruh, 1991). The plutons are granodiorite-granite in composition and form the bulk of the Sierra Nevada Batholith (Stevens et al., 2013). This granodiorite-granite intruded the preexisting Paleozoic and Mesozoic volcanic and metasedimentary rocks that now occur as pendants between intrusions (Matthews and Burnett, 1965). The overall geology can be seen in Figure 5. The Alabama Hills are located east of the Sierra Nevada Mountains immediately west of the town of Lone Pine and have a NNW-SSE orientation. The eastern base of the Alabama Hills is defined by the Lone Pine segment of the Owens Valley fault system, which ruptured the 1872 earthquake (Pakiser et al., 1964; Beanland and Clark, 1994). The Alabama Hills contain Late Cretaceous granite and middle Jurassic metavolcanics. The White and Inyo Mountains bound the east side of Owens Valley. These mountains are made up of a variety of Precambrian to Jurassic metasedimentary, metavolcanic, and granitic rocks (Matthews and Burnett, 1965) including Precambrian sandstone and dolomite, Early Cambrian limestone and shale, and Middle and Late Cambrian limestone, quartzite, and calcareous sandstone (Pakiser et al., 1964; Matthews and Burnett, 1965). The granitic rocks of the White and Inyo Mountains are Jurassic and Cretaceous in age. The Jurassic intrusive rocks are monzonite and granite, whereas the Cretaceous rocks are leucocratic biotite granite.

6 6 The alluvial sediment filling the basin of Owens Valley is dominated by Quaternary alluvial and glacial deposits and lacustrine sediments. Quaternary cinder cones and lava flows are present near Big Pine (Gillespie, 1982). The alluvial fans along the Sierra Nevada Mountain front are composed of Quaternary sediment derived from the Sierra Nevada Mountains. The alluvial deposits contain as rounded to subrounded boulders, cobbles and pebbles of weathered granitic rocks (Le et al., 2007). Much of the valley floor is flat and underlain by lake sediments. South of independence, the valley floor sediment has an age of approximately 10ka (Beanland and Clark, 1994). Owens Valley contains both normal and strike-slip faults (Figure 6). The east side of the Sierra Nevada Mountains is bound by east dipping and NNW striking SNFFS (Unruh, 1991). The SNFFS is defined by Le et al. (2007) in the Lone Pine to Independence area as a fault zone composed of multiple fault scarps along the eastern piedmont of the Sierra Nevada. The vertical displacement for the SNFFS is ~3000 m (Hollett et al., 1991; Beanland and Clark, 1994). The Owens Valley Fault Zone extends from southern Owens Valley to Lone Pine near the eastern base of the Alabama Hills, and farther north crosses the Owens Valley floor to the Poverty Hills (Beanland and Clark, 1994) (Figure 6). Strike-slip deformation along the Owens Valley Fault in the Lone Pine/Alabama Hills area is concentrated along a narrow fault zone. North and south of this area, the Owens Valley Fault zone strike-slip deformation is distributed across several small faults. The fault system as a whole is a right-lateral strike-slip fault (Stevens et al., 2013). The total right lateral displacement across the Owens Valley fault zone is ~2500 m, with the east side slightly shifted down (Beanland and Clark, 1994).

7 7 Earlier Gravity Work Earlier gravity surveys were conducted in the same area by Pakiser et al. (1964), Stevens et al. (2013), Saltus & Jachens (1995) and Langenheim & Kirchoff-Stein (1991). Pakiser et al. (1964) shows that the gravity fields are low in the valleys and basins in the Owens Valley region. The gravity values in the valleys are ~780,000 mgal after removal of the regional trend. The highest gravity value is ~805,000 mgal. The paper interprets a linear gravity low extending through the entire valley, but further interpretation showed a much more complex geologic history of the valley. The Pakiser et al. (1964) gravity data show a regional gravity trend increasing to the east, implying the crust is thicker on the east side. In Figure 3 the correction for this regional trend is shown along with the model of the subsurface structure found in Pakiser et al. (1964). The regional trend correction was used to make the data agree with elevations more closely to aid in modeling and discussion of the area. Pakiser et al. (1964) calculated the densities for the rock types in the area by collecting about 20 samples of each rock unit, and then finding the densities of each sample. The maximum density for the metamorphic rocks is 2940 kg/m 3 and the minimum is 2630 kg/m 3 with an average density of 2780 kg/m 3. The alluvium density has a calculated average density of 2200 kg/m 3. Densities of Cenozoic clastic and volcanic rocks in the area were found to be variable and not very well known. The densities of pre-tertiary plutonic and metamorphic rocks were well known however. Stevens et al. (2013) shows the regional isostatic residual gravity in the Owens Valley area (Figure 4a), which can provide insight about the area before more detailed work is completed. The isostatic gravity of the Sierra Nevada is ~ -13 mgal, ~ -22 mgals in Owens

8 8 valley, and ~ -10 mgals for the Inyo Mountains. The ~ 10 mgal lower gravity in Owens valley with respect to surrounding mountains suggest basin depths of ~1 2 km (Figure 4b). Saltus and Jachens (1995) work consisted of creating gravity and basin depth maps of the Basin and Range Province. Langenheim & Kirchoff-Stein (1991) created a map of the complete Bouguer gravity anomalies and isostatic anomalies of the Basin and Range Province. Methods Field Procedures & Equipment The gravity survey of Owens valley was conducted on November 1-2, Two eastwest transects across the valley were included in this gravity survey (Figure 7). The Independence transect extends from the Sierra Nevada Mountain bedrock on the west to the White Mountains bedrock on the east, and transects the town of Independence (Figure 7). The Manzanar transect extends from the Sierra Nevada bedrock to the White Mountains bedrock at the latitude of Manzanar (Figure 7). A local gravity base station was set up along each transect. The local base stations were revisited twice during the day for a total of three measurements each day to evaluate instrument drift. The Independence transect base station was located ~1 km southwest of the town of Independence. The base station was located approximately half way through the transect near two large white tanks (Figure 8), ~1 meter away from the fence to the west and ~13 meters away from the gate to the south (Figure 9). The Manzanar transect base station was located ~1.5 km west of Highway 395 at the corner of a dirt road (Figure 10). The measurements were taken ~1.3 m away from the fence to the east and ~10 m away from the gate to the north (Figure 11). Local base station readings are corrected to a nearly absolute gravity station (see below). This allows all the relative gravity readings along the transect to be corrected to absolute g values.

9 9 All gravity measurements were recorded using the California state University of Fullerton geology department Scintrex CG-5 gravity meter. The Scintrex gravity meter is accurate to less than mgals. The internal drift correction of the CG-5 was calibrated by running the meter for 48 hours to monitor drift. The meter also applies an internal tide correction to data. This tide correction is generated using the latitude, longitude and time and is based on Longman s formula (Longman, 1959). All gravity measurements were conducted consistently. The CG-5 was placed on a leveling plate and leveled. The meter was set to have a read time of 60 seconds and with one measurement every second. The final gravity reading was an average of the 60 readings. During the leveling, the meter had to be shielded from the wind to enhance stable and consistent readings. During the minute it spent reading we had to step away from the meter in order to not interfere with the gravity measurements. The gravity readings and the standard deviation of the readings were then recorded in a field notebook. If a standard deviation was larger than 0.05 then the Scintrex was re-leveled and the reading was taken again. The coordinates of each gravity station and each base station were recorded using the California State University of Fullerton geology department Topcon GPS system, including both a rover unit and a base station unit. The base station unit was set as close to the gravity base station reading location as possible. The rover unit was placed on the roof of the vehicle during travel between locations and then placed on a handheld pole to get accurate locations. Each unit had to remain in constant view of the sky. A Garmin handheld GPS unit was also used as a precaution against the potential malfunctions of the Topcon system. All GPS data were recorded using the datum NAD27 in degree decimal minutes. These GPS readings were later post-

10 10 processed using the Topcon Tools Version 7.5 software package. The post-processing of the data consisted of changing the datum of the latitudes and longitudes and exporting the information as a file compatible with Google Earth in order to plot the appropriate locations. Both transects were chosen before going into the field by evaluating Google Earth images. The Independence transect includes a paved road the entire distance across the valley. The Manzanar transect was started on foot to get to the bedrock of the Sierra Nevada Mountains on the west and continues along a dirt and paved roads eastward across the valley. The Independence transect includes 55 gravity stations, plus two recordings at the local base station. Each gravity station was approximately 200 meters apart. The Manzanar transect includes 43 recorded gravity stations, plus additional readings at the local base station. Data Processing and Calculations A station was created in Big Pine at a location with a known absolute gravity. The exact location of this station is in the city of Big Pine in Inyo County at latitude N and longitude W, at the Owens Valley Radio Observatory guest cottage. The absolute gravity at this location is 979, / mgal. The data collected at this location was used to calculate absolute gravity values using the relative gravity values collected at all of the other gravity stations. Once a conversion had been calculated using the known absolute gravity station the conversion was simply applied to all of the gravity stations. The Big Pine gravity location was on the floor living room of a guest home on the premises of the observatory indicated by a small round marker in the floor underneath the chairs. The predicted gravity at each station is determined from the international gravity reference formula (IGRF: g n = (1 + A sin φ 2 + B sin φ 4 ) where A = and B = ), which allows the calculations of the theoretical gravity based on

11 11 latitude. Several corrections were made to the predicted gravity at each location to the data before gravity anomalies were evaluated and modeled. The gravity corrections given below are summarized from Burger et al. (2006). The first of the corrections is the free-air correction (formula: g = mgal/m* h, where Δh is the elevation of the station). This corrects for elevation differences between each observation station and the base station. The Bouguer correction (formula: g = mgal/m* h) corrects for the rock mass between the current station and the established base station, but assumes an infinite horizontal slab for the rock present or missing between the two elevations. Because the rock mass between the elevations is not an infinite slab, a terrain correction must be applied. Field terrain correction for multiple locations along each transect were calculated based on the half-slope method of Sandberg (1958) as described in Plouff (2000). The field terrain corrections were calculated by drawing paths across each gravity station in Google Earth, and then using the elevation profile provided by each path to find the change in elevation relative to the distance of that change. The distance on either side of the station was considered out to 223 feet away. Once the elevation change and distance of the slope had been recorded the equation Θ = tan 1 ( change in elevation change in distance ). This value along with the flat distance before the slope and after the slope were used in a table to find the appropriate inner-zone terrain correction for the area. (See appendix B for calculations of field terrain corrections). A terrain correction beyond 223 feet from each station was determined using the USGS DEM based terrain correction, using the latitudes, longitudes and elevations received during the survey itself (Vicki Langenheim, personal communication, March 2016). In addition to the terrain corrections, the USGS also provided the isostatic corrections for each location. The isostatic correction corrects for assumed crustal thickness variations.

12 12 Results The gravity survey data reveal important information needed to understand the subsurface geometry of the area and provide constraints on subsurface structures in the area. The trend of the absolute gravity data from the Independence transect can be seen in appendix C. The data show that the absolute gravity values greatly increase to the east for about 7.5 km, followed by values still increasing but more slowly for the next 4.8 km. Absolute gravity values flatten for about 4.1 km and then decreases over the remaining 3 km. These data show only how the gravity was read by the gravity meter without taking into account any gravity corrections that may affect the reading. Absolute gravity data across the Manzanar transect have a few small differences compared to the Independence transect. Absolute gravity in the western 1.5 km increases slowly, but increases abruptly over the next 6 km. Farther east, absolute gravity flattens out for the remaining 10.5 km (appendix c). The isostatic anomaly values for the Independence transect are near zero for the western ~ 12 km (Figure 12). Though the anomalies are near zero, they fluctuate up to 2.5 mgal. At 2.5 km, anomalies increase to ~ +2.5 mgal. Between 8 and 12 km anomaly values decrease to -1 mgal. From 13 to 14 km the values decrease from 0 mgal to about -2 mgal. From 14 to 17 km anomaly values increase from -2 mgal to +12 mgal. Farther east between 17 and 18 km, anomaly values decrease abruptly by ~5mGal, and increase by ~2 mgal at the easternmost 1 km of the transect. The Manzanar transect isostatic anomaly decrease from 0 to -3.5 mgal in the western 2 km of the transect (Figure 13). From 2 km to 6 km the values increase from -3.5 to +5 mgal. The values start to decrease steadily from 6 to 11 km from values of +5 mgal to values of 0 mgal. A

13 13 steeper decrease in values occurs from 11 to 14 km with a change in values form 0 to -7.5 mgal. Isostatic anomaly values increase from -7.5 mgal to +14 mgal between 14 km and 18 km. Isostatic anomalies for both transects display an increasing regional trend across both transects. Following Pakiser et al. (1964), the trends were corrected in order evaluate local, valley wide anomaly changes. The correction for each was made determining the regional gradient between west and east data points. The gradient (slope) was used to calculate corrections for each of the other data points. Once all points were corrected, a new plot was made for each transect (Figures 12 and 13). After the regional trend correction was applied to the data the plot had the same shape but contained different values for the Independence transect. The western 2 km still remained close to 0 mgal. From 2 km to 3 km there was an increase in the values between 1 and 2 mgal. From 3 to 11 km the values gradually and consistently decreased from -2 mgal to around -6 mgal. Following this decrease is a relatively constant set of values from 11 km to 13 km at -6 mgal. From 13 to 14 km the vales decrease from -6 mgal to -9 mgal followed by an abrupt increase from 14 km to 17 km from values of -9 mgal to 4 mgal. The values then decrease again but slightly from 4 mgal to -2 mgal from 17 km to 18 km, and finally leveling back out to 0 mgal at 20 km. The Manzanar transect underwent the same regional trend correction and also resulted with a similar shape but with different values. The westernmost 2 km undergo an immediate decrease in values from 0 mgal to -5 mgal and then a large increase in values from 2 km to 6 km with values increasing from -5 mgal to just over 0 mgal. The data then decreases again from 6 km to 11 km where values go from 0 mgal to -7 mgal. Following this slight decrease in values is a more abrupt decrease in values of -7 mgal to -20 mgal from 11 km to 14 km. This large

14 14 decrease in values can be an indication of the subsurface geology changing from one rock to another rock or even a structural change such as a fault. The remainder of the plot is an abrupt increase in values from -20 mgal to 0 mgal from 14 km to 18 km. Modeling Methods In order to model the regionally corrected isostatic anomaly variations across both transects, the program GravMag from Burger et al. (2006) was used. This program allows the user to generate polygons of various sizes, shapes and density contrasts, which then are used to calculate the 2D gravitational effect across the transects. The computed (modeled) gravity can then be compared to actual isostatic anomaly values. The goal is to utilize as much geologic knowledge as possible, such as local density changes where bedrock changes to alluvium or locations of known faults, to build model polygons whose calculated gravity matches the observed gravity anomaly values. It should be noted that, because both density and shape/size of polygons can be adjusted, the resulting computed gravity result is non-unique. Model parameters include rock and alluvium densities, depths of the created model polygons, locations of faults and rock types from published geologic maps (e.g., Matthews and Burnett, 1965). The easiest of these parameters to manipulate are the densities of the bedrock and alluvium. The densities of the bedrock on either side of the valley are known due to the work done by Pakiser et al. (1964), while the alluvium density is variable. Geologic data from Matthews and Burnett (1965), USGS (2016), and Stone et al. (2000) were used to constrain the models (Figure 5). For modeling the isostatic anomalies, the regionally corrected data were used (tables 1 and 2). Low density bodies are assumed to underlie regions of low gravity anomalies, while

15 15 regions of high gravity anomalies are consistent with high density bodies in the subsurface. In order to capture the range of possible lower density alluvium bodies, a few different densities were tried. Alluvium end-member density values between 2370 kg/m 3 and 2170 kg/m 3 were evaluated in order to get a range of different models. Using a range of density values for alluvium provides arrange of acceptable basin depths. The kg/m 3 range is consistent with the range of alluvium densities from Pakiser et al. (1964). The density of the granite in the Sierra Nevada Mountains is ~2670 kg/m 3. The White Mountains consist of metamorphic rocks such as gneiss, schist, and marble (Pakiser et al., 1964) that have overall higher densities than the mostly granitic rocks of the Sierra Nevada Mountains. Therefore, a higher density of 2770 kg/m 3 was used for the White Mountains. This higher density is consistent with the density used by Pakiser et al. (1964). Each polygon in the GravMag models was given vertices in order to have many areas that can be manipulated to change the shape of the polygon. I constantly adjusted the vertices of the polygons to fit modeled anomaly values to the regionally corrected isostatic anomalies. Due to the White Mountains having a higher density than the granite of the Sierra Nevada Mountains, an additional polygon was created with the appropriate gravity set to the polygon to get the most accurate prediction possible. Best-fit models are shown in Figures 14 and 15 for Independence and Figure 16 and 17 for Manzanar. Results The Independence transect contains several small-scale local perturbations in the gravity anomaly profile, such as at 6-8 km and 13 km. These local, though potentially important at a local scale, were ignored in the modeling so that larger scale features could be evaluated. The models show that the area contains three alluvium filled valleys, the westernmost and

16 16 easternmost valleys being shallower than the center valley in both models for the Independence transect (Figure 14). For the model with the alluvium having a density of 2370 kg/m 3 the deepest valley for this transect is 2.2 km deep with the two smaller valleys being about 0.7 km and 0.1 km deep. For the model with the alluvial density of 2170 kg/m 3 the deepest valley has a depth of about 0.8 km, and the two smaller valleys having depths of about 0.4 km and 0.05 km. The deepest valley is located about 5 km from the eastern boundary of the transect, whereas the smallest valley is at the western boundary and the valley with the middle depth is at the eastern boundary. This is interesting in relation to the Manzanar models because these models only contain two valleys. The deepest Manzanar valley with an alluvium density of 2370 kg/m 3 has a depth of 3 km and the shallow valley has a depth of about 0.5 km. On the Manzanar model with an alluvium density of 2170 kg/m 3 the deepest valley is about 1.3 km deep and the shallow valley is about 0.3 km deep. The deep valley is located 3 km west of the eastern boundary of the transect while the shallow valley is located at the western boundary. As mentioned earlier, the model results shown in Figures are non-unique. Changes in density or size/depth/location of the density changes can result in differences in computed gravity. Additional data, such as well data or seismic data, are needed to accurately constrain models. However, the results presented provide broad geometries and constraints and allow interpretations regarding general features of the subsurface. Discussion The gravity survey of this study agrees with the study of Stevens et al. (2013) (Figure 18). The gravity plot of Stevens et al (2013) is a close match to the plots created through this research. Both sets of plots show the largest gravity anomalies between the Owens Valley Fault and the West Inyo Fault. The valley in Figure 18 is about 1.5 km deep which matches the

17 17 Manzanar Model with the alluvium density set to 2170 kg/m 3. Just like the models of this research, the models of Stevens et al. (2013) are also non-unique interpretations. The models show signs of faulting, which makes sense with the known faults in the area. The modeled fault locations are consistent with locations of mapped faults (Stone et al., 2000; Beanland and Clark, 1994). The model produced by Stevens et al. (2013) also shows faults that match up with locations of known faults in the area. According to the fault locations and slip directions, it can be determined that the bedrock at the base of the deepest valley moved downward causing the valley to be deeper than the other valleys in the immediate area. The major faults taking a role in this are the Owens Valley fault and the West Inyo fault. The Owens Valley fault is an east dipping normal fault with the hanging wall being the block that sank in the valley. The West Inyo fault is a west dipping normal fault that down-dropped Owens Valley. This pattern follows through both transects in this survey and is similar to the results of Stevens et al. (2013). These results place the alluvium next to the Sierra Nevada bedrock on the east and the alluvium next to the White/Inyo Mountains bedrock on the west. There are other smaller faults along the entirety of each transect, but the two faults mentioned are the largest and most important in term of modeling. Modeling results in this study suggest that the Owens Valley fault and West Inyo fault have steep dips (>45 ). Stevens et al. (2013) suggest that these faults dip ~60. Using the models in this study, the variations in the models make it impossible to unequivocally determine fault dips. However, the best fits occur with steep dips (>45 deg.) and with east dip for the Owens Valley fault and steep west dip for the West Inyo fault. One of the overall goals of this study was to constrain the dip of the SNFFS. Previous studies (Gadbois, 2015; Mottle, 2015; Shagham, 2011; Phillips and Majkowski, 2011) suggest

18 18 the SNFFS dips east. Gravity results from this study are not able to constrain SNFFS dip. Gravity analysis indicates that the granitic rock of the Sierra Nevada is juxtaposed against largely granitic rock below a thin veneer of alluvium east of the SNFFS. Though the lack of a density contrast across the SNFFS prohibits a dip calculation for the SNFFS, the gravity analysis does constrain overall offset estimates across the SNFFS. Figure 19 shows elevation change from the top of the Sierra Nevada Mountains to the top of bedrock (base of alluvium) east of the SNFFS based on gravity models. Assuming high elevations of the Sierra Nevada were once continuous with the top of the bed rock, the maximum vertical offset across the SNFFS is ~2500 m (Figure 19). The gravity data and modeling in the Manzanar transect suggest the Alabama Hills may project northward in the subsurface. The Manzanar transect (Figures 16 and 17) show a km deep alluvial filled valley to the west and a 3 km deep alluvial valley to the east. These alluvial valleys are separated by an antiformal structure with a thin veneer of alluvium between. The Alabama Hills comprise an island of granitic and metamorphic rocks that crop out in the middle of Owens Valley in the vicinity of Lone Pine (Matthews and Barnett, 1965; Stone et al, 2000). The Alabama Hills are located ~5 km to east of Sierra Nevada fault and SNFFS. Geologic maps (e.g., Stone et al., 2000) show that the Alabama Hills granitic rocks crop out ~10 km south of the Manzanar transect (Figure 5). Modeling from this study show that the Alabama Hills probably project northward and are covered by <100 m of alluvium in the vicinity of the Manzanar transect. The Independence transect shows that granitic rocks are only shallowly buried (up to 500 m) farther north. Thus everywhere between the SNFFS and the Owens Valley fault may have shallow alluvium and relatively high standing (but buried) granitic bedrock and the Alabama Hills are where the bedrock is exposed above the alluvium.

19 19 The basement rocks of the Alabama Hills appear to project northward, based on geologic maps and Google Earth analysis, into the shallow subsurface of the Manzanar transect. This is consistent with the results of the modeling because of the different density of the Alabama Hills, potentially causing gravity anomalies to be present in the data. The rock is denser than the alluvium on top of it, so gravity is slightly increased in the area locally. It does not extend as far north as the Independence transect, and therefore is not apparent in the data for that transect. Summary Owens Valley is located between the Sierra Nevada Mountains on the west and the White Mountains on the east. A gravity survey of the valley was conducted totaling 57 gravity stations recorded along the Independence transect and 45 gravity station recorded along the Manzanar transect. The Independence transect was ~20 Km in length and the Manzanar transect was ~18 Km in length. Regionally corrected gravity anomalies generally decrease eastward from the SNFFS and reach lows of -10 to -20 mgal just east of the Owens Valley fault. Modeling of the gravity data based on end-member density values for valley fill alluvium are consistent with locations of the Owens Valley and west Inyo faults near east end of the transects. The deepest parts of the Owens Valley are located between the Owens Valley and West Inyo Faults. Importantly, the gravity data indicate that the west side of Owens Valley contains only a thin veneer (generally <200 m) of alluvial fill; these results are consistent with the mostly granite rocks of the Alabama Hills projecting northward beneath western Owens Valley between Lone Pine and Independence. The dip of the SNFFS could not be determined from the gravity data because the fault system separates granitic rock of Sierra Nevada from shallow granitic rock in the western part of Owens Valley. Assuming the top of modern Sierra Nevada Mountain coincides with the top of the

20 20 shallowly buried granitic rocks east of the SNFFS, the maximum vertical offset across the SNFFS is ~2500 m.

21 21 References Beanland, S., Clark, M.M., 1994, The Owens Valley Fault Zone, Eastern California, and Surface Faulting Associated with the 1872 Earthquake: U.S. Geological Survey Bulletin Bierman, P., Gillespie, A., Whipple, K., Clark, D., 1991, Quaternary Geomorphology and Geochronology of Owens Valley, California: Geological Society of America Field Trip, p Burger, H. R., Sheehan, A. F., Jones, C. H., 2006, Introduction to Applied Geophysics: W. W. Norton & Company. Gadbois, B., 2015, Fault Orientations of the Sierra Nevada Frontal Fault Zone in the Vicinity of Lone Pine, California: Unpublished Undergraduate Thesis, California State University, Fullerton. Gillespie, A.R., 1982, Quaternary glaciation and tectonism in the southwestern Sierra Nevada, Inyo County, California: Pasadena, Calif., California Institute of Technology, Ph.D. thesis, 695 p. Hollett, K.J., Danskin, W.R., McCaffrey, W.F., and Walti, C.L., 1991, Geology and water resources of Owens Valley, California: U.S. Geological Survey Water-Supply Paper 2370-B, 77 p. Langenheim, V. E., Kirchoff-Stein, K. S., Complete Bouguer Anomaly and Isostatic Gravity maps of the Fresno 1 x 2 Quadrangle, California, 1991, Scale 1:250,000. Le, K., Lee, J., Owen, L. A., Finkell, R., 2007, Late Quaternary slip rates along the Sierra Nevada frontal fault zone, California: Slip partitioning across the western margin of the Eastern California shear Zone-Basin and Range Province: Geological Society of America. doi: /B Lee, J., Spencer, J., Owen, L., 2001, Holocene slip rates along the Owens Valley fault, California: Implications for the recent evolution of the Eastern California Shear Zone, v. 29; no. 9; p Longman, I.M., Formulas for computing the tidal acceleration due to the moon and the sun. Journal of Geophysical Research 64, Matthews, R.A., and Burnett, J.L., 1965, Fresno sheet, geologic map of California (Olaf P. Jenkins edition): California Division of Mines and Geology, scale 1:250,000.

22 22 Mottle, G., 2015, Evaluation of the Sierra Nevada Frontal Fault System at Bairs Creek in the Vicinity of Manzanar, California: Unpublished Undergraduate Thesis, California State University, Fullerton. Pakiser L. C., Kane M. F., Jackson W. H., 1964, Structural Geology and Volcanism of Owens Valley Region, California - A Geophysical Study. Phillips, F. M., Majkowski, L., 2011, The role of low-angle normal faulting in active tectonics of the northern Owens Valley, California: v. 3; no. 1; p , doi: /L73.1. Plouff D., 2000, Field Estimates of Gravity Terrain Corrections and Y2K-Compatible Method to Convert from Gravity Readings with Multiple Base Stations to Tide- and Long-Term Drift-Corrected Observations, California. Saltus, R.W.; Jachens, R.C. Gravity and Basin Depth Maps for the Basin and Range Province, Western United States; U.S. Geological Survey Geophysical Map GP-1012; U.S. Government Printing Office: Washington, DC, USA, 1995; Scale 1:250,000. Sandberg, C.H., 1958, Terrain corrections for an inclined plane in gravity computation: Geophysics, v. 23, no. 4, p Shagham, G., 2011, Orientation of the Sierra Nevada Frontal Fault Zone near Independence and Lone Pine, California: Unpublished Undergraduate Thesis, California State University, Fullerton. Stevens, C. H., Stone, P., Blakely, R. J., 2013, Structural Evolution of the East Sierra Valley System (Owens Valley and Vicinity), California: A Geologic and Geophysical Synthesis: Geosciences, v. 3, p , doi: /geosciences Stone, P., Dunne, G.C., Moore, J.G., Smith, G.I, Geologic Map of the Lone Pine 15 Quadrangle 1:62,500, Inyo County, California: U.S. Geological Survey, Unruh, J. R., 1991, The uplift of the Sierra Nevada and implications for late Cenozoic epeirogeny in the western Cordillera, v. 103, p Unruh, J, Humphrey, J. Barron, A., 2003, Transtensional model for the Sierra Nevada frontal fault system, eastern California: Geological Society of America, v. 31; no. 4; p USGS, 2016, Quaternary Fault and Fold Database of the United States, Accessed on July 29, 2016.

23 23 Figures Figure 1: Map of the Basin and Range Province from the USGS website. Owens Valley is shown on the map using a red box. Modified from

24 24 Figure 2: Map showing the location of Owens Valley between the Sierra Nevada Mountains and the White and Inyo Mountains. Modified from Stevens et al. (2013).

25 25 Figure 3: Interpreted model of a possible subsurface geometry of Owens Valley. Bouguer gravity data (upper) and model of subsurface geology (lower). Figure by Pakiser et al. (1964).

26 26 Figure 4a: Isostatic anomaly map showing isostatic residual gravity across both transects (marked as red lines). Figure modified form Stevens et al. (2013).

27 27 Figure 4b: Alluvium thickness map showing the thickness of the valley fill. Modified from Stevens et al. (2013).

28 28 Independence Transect N Manzanar Transect 10 km Figure 5: Geologic map of the Owens Valley Area with the gravity transects shown by the black lines. The blue and red lines indicted faults in the area. Faults are from USGS (2016) and geologic map is from Matthews and Burnett (1965).

29 29 N Independence Transect Manzanar Transect 10 km Figure 6: Map of Owens Valley containing the Sierra Nevada Frontal Fault System (SNFFS) and the Owens Valley Fault Zone, showing faults in relation to the two transects in the area. Faults are from USGS Quaternary Fault and Fold Data Base (USGS, 2016).

30 30 N Independence Independence Transect Figure 8 10 km N Manzanar Manzanar Transect Figure km Figure 7: Map with the Independence transect (upper) and Manzanar transect (lower) across Owens Valley. Numbers are station numbers for each of the transects.

31 31 N 1 km Figure 8: The location of the base station along the Independence transect. The base station is indicated by the red box.

32 32 Figure 9: Field photos noting Independence base station location. The gravity meter (small blue and gray box) was placed approximately 1 m away from the fence to the west, the base station GPS unit (yellow tripod) was placed as close to the gravity meter as possible.

33 33 N 1 km Figure 10: The location of the base station along the Manzanar transect. The base station is indicated by the red box.

34 34 Figure 11: Field photos noting Manzanar base station. The base station was on the corner of a dirt road just off of the main road.

35 Relative Isostatic Anomaly (mgal) 35 Relative Isosatic Anomaly vs Distance from the West Distance From West (km) Figure 12: Plot of the relative isostatic anomaly values along the Independence transect relative to distance of the gravity station from the west. The blue line is the original data, the orange line represents the slope from the westernmost point to the easternmost point, and the gray line is the data same data with a regional trend correction applied.

36 Relative isostatic Anomaly (mgal) 36 Relative Isostatic Anomaly vs Distance from the West Distance From West (km) Figure 13: Plot of the relative isostatic anomaly values along the Manzanar transect relative to distance of the gravity station from the west. The blue line is the original data, the orange line represents the slope from the westernmost point to the easternmost point, and the gray line is the data same data with a regional trend correction applied.

37 37 Owens Valley Fault West Inyo Fault Figure 14: Interpreted model of the geometry of Owens Valley along the Independence transect with a density difference of 300 kg/m 3 between the bedrock of the Sierra Nevada and the alluvial fill. The white space has a density of 2670 kg/m 3, the red has a density of 2370 kg/m 3, the yellow has a density of 2770 kg/m 3.

38 38 Owens Valley Fault West Inyo Fault Figure 15: Interpreted model of the geometry of Owens Valley along the Independence transect with a density difference of 500 kg/m 3 between the bedrock of the Sierra Nevada and the alluvial fill. The white space has a density of 2670 kg/m 3, the red has a density of 2170 kg/m 3, the yellow has a density of 2770 kg/m 3.

39 39 Owens Valley Fault West Inyo Fault Figure 16: Interpreted model of the geometry of Owens Valley along the Manzanar transect with a density difference of 300 kg/m 3 between the bedrock of the Sierra Nevada and the alluvial fill. The white space has a density of 2670 kg/m 3, the red has a density of 2370 kg/m 3, the orange has a density of 2770 kg/m 3.

40 40 Owens Valley Fault West Inyo Fault Figure 17: Interpreted model of the geometry of Owens Valley along the Manzanar transect with a density difference of 500 kg/m 3 between the bedrock of the Sierra Nevada and the alluvial fill. The white space has a density of 2670 kg/m 3, the red has a density of 2170 kg/m 3, the orange has a density of 2770 kg/m 3.

41 41 Figure 18: Gravity data and model from Stevens et al. (2013) of a transect spanning across Owens Valley from the Sierra Nevada Mountains to the White Mountains.

42 Elevation (m) km N ~ 2500 (m) Distance (km) Figure 19: Elevation profile (plot) across the Sierra Nevada Mountains and Owens Valley. The location is shown by the white path on the map. Elevation profile used to calculate approximate vertical offset from the average maximum elevation of the Sierra Nevada to the top of the bedrock below the alluvium adjacent to the Sierra Nevada Frontal Fault System in westernmost Owens Valley.

43 43 Data Tables Table 1: Data for Manzanar transect distance from west (km) Free Air Correction (mgal) Bouguer Correction (mgal) Terrain Correction (mgal) Free Air Anomaly (mgal) Complete Bouguer Anomaly (mgal) Isostatic Anomaly (mgal) Relative Isotatic Anomaly (mgal) GPS station elevation (m) latitude longitude absolute g g MANZBAS MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ MANZ

44 44 Table 2: Data for Independence transect distance from west (km) Free Air Correction (mgal) Bouguer Correction (mgal) Terrain Correction (mgal) Free Air Anomaly (mgal) Complete Bouguer Anomaly( mgal) Isostatic Anomaly (mgal) Relative Isostatic Anomaly (mgal) GPS station elevation (m) latitude longitude absolute g g INDBASE IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND IND