The Stanislaus Group in a Beheaded Drainage: Tectonics at the Margin of the Sierra Nevada Microplate

ABSTRACT

THE STANISLAUS GROUP IN A BEHEADED DRAINAGE: TECTONICS AT THE MARGIN OF THE SIERRA NEVADA MICROPLATE

West of Bridgeport Valley near the Sierra Nevada crest, the Little Walker Caldera erupted Late Miocene Stanislaus Group lavas (Table Mountain Formation) and ignimbrites (Eureka Valley Tuff). Remnants of these rocks are now distributed from the western Sierra Nevada foothills across the range and into the Walker Lane. This wide distribution is attributed to the lavas flowing down paleochannels, and provides an excellent marker for post-emplacement deformation in the region. Priest (1978) documented a thick section of these lavas along Flatiron Ridge and other peaks surrounding Buckeye Canyon, including four members in stratigraphic order: Lower, Large Plagioclase, Two-Pyroxene, and Upper Member. 40Ar/39Ar geochronology indicates these Table Mountain Formation Lavas erupted from 10.4-9.5 Ma. Lithologically similar lavas have been identified near Rancheria Mountain, geochemical and paleomagnetic data support this correlation. The lavas flowed down a now-beheaded late Miocene drainage, supporting a westward shift of the Sierra Nevada crest since the Late Miocene. Paleomagnetic data from Priest’s Measured Section reveal that the Upper Member and the Two-Pyroxene Member are reversed polarity. The Large Plagioclase Member and the Lower Member flows are normal polarity. Based on remanence, lithology, and geochemistry, we correlate the Upper Member with the Rancheria Mountain lavas, requiring the presence of a 30 km long paleochannel for the lava to flow down. The Sierra Nevada crest currently divides these ii outcrops of Stanislaus Group lavas. Since lava flows downhill from its source, and Priest’s Measured Section is the most proximal locality, the crest of the Sierra Nevada must have been near to or east of Priest’s Measured Section in the late Miocene. Using the established paleomagnetic reference direction for the Eureka Valley Tuff members, we demonstrate ~15° clockwise vertical-axis rotation at sites stepping from Mono Basin towards the Sierra Crest graben. A cross section that passes from Rancheria Mountain through the Sierra Crest, Priest’s Measured Section and Boone Canyon reveals total down-to-the-northeast offset of about 3500 meters. Numerous north-trending faults in the area cutting Stanislaus Group rocks indicate that deformation through the area was accommodated by a combination of fault offset and vertical-axis rotation.

Rosalie Power Schubert May 2017

THE STANISLAUS GROUP IN A BEHEADED DRAINAGE: TECTONICS AT THE MARGIN OF THE SIERRA NEVADA MICROPLATE

by Rosalie Power Schubert

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno May 2017 APPROVED For the Department of Earth and Environmental Sciences:

We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree.

Rosalie Power Schubert Thesis Author

Christopher Pluhar (Chair) Earth and Environmental Sciences

John Wakabayashi Earth and Environmental Sciences

Keith Putirka Earth and Environmental Sciences

For the University Graduate Committee:

Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS

x I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship.

Permission to reproduce this thesis in part or in its entirety must be obtained from me.

Signature of thesis author: ACKNOWLEDGMENTS Thank you to my advisor Christopher Pluhar for his insight and guidance throughout this project. Thanks to my committee members John Wakabayashi and Keith Putirka for their thoughtful feedback and advice. Thanks to my intrepid field assistants: Julie Reith, Kou Yang and Trevor Gledhill. Your assistance with sample collecting and packing was in invaluable. And thanks to my fiancé Andrew and my parents for always being there for me.

Funding provided by: Fresno State University College of Science and Math Fresno Gem and Mineral Society Fresno State Graduate Net Initiative RGrant

TABLE OF CONTENTS Page

LIST OF TABLES ...... vii

LIST OF FIGURES ...... viii

LIST OF PLATES ...... x

INTRODUCTION ...... 1

LITERATURE REVIEW ...... 6

Farallon Subduction ...... 6

Present-day Tectonics of North America ...... 7

Uplift of the Sierra Nevada ...... 8

Sierra Nevada Drainage Divide ...... 12

Stratigraphy ...... 12

METHODOLOGY ...... 16

Stratigraphy, Lithology, and Petrography ...... 16

Geologic Mapping ...... 16

Geochemistry ...... 18

ARGUS VI Mass Spectrometer – OSU Argon Geochronology Laboratory .. 19

Paleomagnetism ...... 21

RESULTS ...... 25

Lithology ...... 25

Geochemistry ...... 27

Mapping and Cross-Section ...... 28

40Ar/39Ar Geochronology ...... 33

Paleomagnetism ...... 36

DISCUSSION ...... 46 vi vi Page

Tilt Corrections for Paleomagnetic Data ...... 46

40Ar/39Ar Geochronology ...... 47

Paleochannel of the Tuolumne River ...... 47

Vertical-Axis Rotation ...... 55

Kinematics ...... 57

CONCLUSIONS ...... 59

REFERENCES ...... 61

APPENDICES ...... 72

APPENDIX A: PLATES ...... 73

APPENDIX B: PALEOMAGNETISM STEREONETS ...... 78

LIST OF TABLES

Page

Table 1: Major element composition of Table Mountain Formation Samples. Data from this study (PMS samples) and Jones (2014) including Rancheria Mountain (EE), Boone Canyon (BOC) and Priest's Measured Section (PMS)...... 29 Table 2: Trace element composition of Table Mountain Formation samples. Includes data from Jones (2014) and this study (PMS samples)...... 30

Table 3: Summary of 40Ar/39Ar Results ...... 34

Table 4: Mean Flow ChRM Paleomagnetic Results ...... 37 Table 5: Sample site rotation results. A positive rotation indicates clockwise sense. A positive flattening indicates shallower inclination...... 38

LIST OF FIGURES

Page

Figure 1: Fault map of western United States with color blocks representing geologic provinces. Modified from Stockli et al. (2003). Faults from the USGS’s Geologic Map of North America...... 2 Figure 2: Original extent of the Stanislaus Group and current mapped extent. Modified from Pluhar et al. (2009) and King et al. (2007)...... 3

Figure 3: Stanislaus Group nomenclature from Priest (1979)...... 13 Figure 4: LeBas (1985) diagram showing the relative silica and alkali composition of Table Mountain Formation samples. Typical Table Mountain Formation and typical Eureka Valley Tuff chemistry generalized from Asami (2014), King et al. (2007), and Koerner et al. (2009). Filled symbols represent normal polarity and hollow symbols represent reversed polarity...... 31 Figure 5: Ti-Zr-Y ternary plot with Table Mountain Formation samples. The colors are similar to the above LeBas (1986) diagram and the hollow symbols represent reversed polarity and the filled symbols represent normal polarity...... 32 Figure 6: 40Ar/39Ar Results. Incremental heating apparent-age spectra for all experiments...... 35 Figure 7: Representative Zijderveld diagrams, showing overprints and ChRM for selected specimens ...... 39 Figure 8: a) Tilt corrected mean ChRM's of all Priest's Measured Section flows color coded by directional group. b) Mean ChRM for each directional group ...... 41

Figure 9: Buckeye Overlook Tilt Correction ...... 43

Figure 10: Eagle Creek Tilt Correction ...... 45 Figure 11: Composite Magnetostratigraphy. Colors correspond to directional groups...... 48 Figure 12: Magnetostratigraphy highlighting potential correlation across Table Mountain Formation Upper Member flows. Colors correspond to directional groups...... 49 Figure 13: All reversed polarity Table Mountain Formation lava flow mean ChRM's. Colors correspond to directional groups...... 50 ix ix Page

Figure 14: Flow path and topographic profile from Priest's Measured Section to Rancheria Mountain ...... 52 Figure 15: Rotation Map. Vertical-axis rotation magnitude depicted as arrows referenced from geographic north. Wedges bounding rotation arrows represent statistical error (α95) for each rotation calculation. Dashed lines and names are rotation domains modified from Carlson et al. (2013). Sites not included in Table 4 of this study can be found in King et al. (2007), Pluhar et al. (2009), Jones (2014), and Carlson et al. (2013)...... 56

LIST OF PLATES

Page

Plate 1: Stratigraphic column of Priest's Measured Section ...... 74 Plate 2: Geologic map of the southeastern margin of the Little Walker Caldera...... 75 Plate 3: Cross Section (a) Hetch Hetchy to Sierra Crest, (B) Sierra Crest to Priest’s Measured Section, (c) Priest’s Measured Section to Boone Canyon. See appendix for annotated sections with graphic measurement of offsets. (d) schematic demonstrating deformation by faulting and tilting and the resulting vertical offset. The schematic cross section is only a cartoon; elevations and distances along the profile are fictitious...... 76 Plate 4: Annotated Cross Sections. Numbers above faults are meters of vertical separation on that fault. Green dashed lines are projections of the unit contact to the nearest fault...... 77

INTRODUCTION

The present-day Sierra Nevada is thought to have been the western shoulder of a large, high plateau, the Nevadaplano, whose extensional collapse isolated the Sierra Nevada Range (DeCelles, 2004; Wolfe et al., 1997). After the collapse of the Nevadaplano, the eastern margin of the Sierra Nevada microplate experienced progressive westward encroachment of deformation into the microplate (Dilles and Gans, 1995; Surpless et al., 2002). As the microplate moved northwest relative to North America, blocks of crust broke off the east escarpment causing the drainage divide to shift westward (Dixon et al., 1995; Oldow et al., 2008; Stockli et al., 2003) (Figure 1). Stanislaus Group lavas and ignimbrites are distributed across the microplate’s eastern margin and are ideally suited to quantifying this deformation process (Figure 2). Identifying where the central Sierra Nevada drainage divide has been located over time would provide evidence for blocks breaking off the eastern margin of the Sierra Nevada. Studying offsets on the Frontal Fault Zone on the eastern margin of the Sierra Nevada and vertical-axis rotation in the region can provide some clues about how deformation on the margin of the microplate occurred. Slemmons (1953) estimated at least 1000 feet of vertical separation of the Eureka Valley Tuff across the Sierra Nevada Frontal Fault Zone based on differences in elevation. This study uses a detailed cross section in the same area to quantify vertical separation on each fault and cumulatively across the range front. To describe deformation completely, vertical-axis rotation in addition to tilting and ordinary fault motion must be accounted for. Deformation along the Sierra Nevada Frontal Fault Zone is almost exclusively described as normal or strike-slip faulting (Rood et al., 2011). However, there is evidence of

Figure 1: Fault map of western United States with color blocks representing geologic provinces. Modified from Stockli

et al. (2003). Faults from the USGS’s Geologic Map of North America.

Figure 2: Original extent of the Stanislaus Group and current mapped extent. Modified from Pluhar et al. (2009) and King et al. (2007).

4 additional rotational component (Bormann et al., 2016). Carlson et al. (2013) documented extensive vertical-axis rotation in the Sweetwater Range and Bodie Hills. This study expands Carlson et al. (2013) study area to the southwest, across the present-day Sierra Nevada range crest. The additional data changes the location of previously inferred domain boundaries and supports the magnitude of rotation within the Buckeye domain. Mapping and paleomagnetic data confirm that there is a rotational component to deformation in our study area, in addition to dip-slip motion on the Sierra Nevada Frontal Fault System. In seeking to constrain the evolution of the central Sierra Nevada range front, we employed the Stanislaus Group as a marker for deformation (figure 3). The Stanislaus Group erupted from the Little Walker Caldera in the Late Miocene. These lavas and ignimbrites inundated the paleodrainage network of the central Sierra Nevada (Lindgren, 1911; Ransome, 1898) (Figure 2). Since the deposition of the Stanislaus Group the Sierra Nevada have undergone significant deformation. The range has uplifted, possibly through westward block tilting (Huber, 1981, 1990; Unruh, 1991; Wakabayashi and Sawyer, 2001) and specifically in the southern Sierra Nevada as a result of removal of the eclogite root (Ducea and Saleeby, 1998; Ducea and Saleeby, 1996; Jones et al., 2004; Manley et al., 2000). The Walker Lane has been stepping westward, encroaching on the Sierra Nevada and resulting in down-dropped blocks east of the Sierra Nevada crest (Dilles and Gans, 1995). Superimposed on this tectonic activity are erosional processes reducing the size of Stanislaus Group outcrops (figure 2). We identified outcrops of Stanislaus Group lavas and ignimbrites and tested to determine if they were part of the same flow. We used the correlation to reconstruct a Miocene paleochannel of the Tuolumne River crossing the present- day Sierra Nevada drainage divide ending near Hetch Hetchy reservoir. These

5 remnant outcrops indicate that the drainage network has been beheaded, with the drainage divide moving westward over time.

LITERATURE REVIEW

Farallon Subduction Beginning in the late Jurassic, shortening between the Rocky Mountains and the present-day Sierra Nevada formed the Nevadaplano (DeCelles, 2004). Proposals for the process that caused crustal thickening include low-angle subduction (Dickinson and Snyder, 1978) and collision of Baja-BC with North America in the ‘Hit and Run’ model (Maxson and Tikoff, 1996). During subduction of the Farallon plate, granitoid intrusions formed above the east- dipping subduction zone (Bateman, 1992; Chapman et al., 2012; Chen and Moore, 1982; Coleman and Glazner, 1997; Hamilton and Myers, 1967; Stern et al., 1981). After 80 Ma, flat-slab subduction shifted magmatism eastward (Coney and Reynolds, 1977; Dickinson and Snyder, 1978). After the Farallon-Pacific spreading center reached the Franciscan trench, the Pacific Plate motion was parallel to the trench forming the modern transform margin (Atwater, 1970; Atwater and Stock, 1998). The development of the transform margin was progressive, as more of the Farallon Plate subducted the Menodcino Triple Junction migrated northward and the transform boundary lengthened (Atwater, 1970; Atwater and Stock, 1998). This change in tectonic

environment resulted in extension across western North America and the collapse of the Nevadaplano (Zoback et al., 1981). Wernicke and Snow (1998) reported between 250 and 300 km of extension between the microplate and the Colorado Plateau in the last 16 Ma. This extension resulted in subsidence that seemed to reduce elevations in western Nevada from 3000 meters at 16 Ma to present elevations of 1500-2000m by 13 Ma (Wolfe et al., 1997). This extension in the late Cenozoic broke up the western North American crust into blocks and formed 7 7 the Sierra Nevada microplate (McQuarrie and Wernicke, 2005; Saleeby et al., 2009; Wernicke, 1981)

Present-day Tectonics of North America As suggested above, much of the topography in western North America is a relic of subduction along the western margin of the continent, but the youngest topography results from oblique transform motion along the plate boundary (Figure 1). GPS and modern geodesy show that currently the Pacific Plate moves northwest relative to North America about 48mm/yr (DeMets et al., 1990). The plate boundary is wide and motion between plates is diffuse and gradational in nature with the majority of motion accommodated in the San Andreas Fault system (Argus and Gordon, 2001; Atwater and Stock, 1998). The remaining motion is accommodated east of the microplate, through extension concentrated at the eastern edge of the Basin and Range and dextral deformation concentrated in the Walker Lane (Dixon et al., 1995). The Sierra Nevada and the Great Valley together constitute the Sierra Nevada microplate, a relatively-rigid block with internal deformation much smaller than deformation at the edges (Unruh et al., 2003; Wakabayashi and Sawyer, 2001) (Figure 1). The San Andreas Fault System, colloquially considered the Pacific-North American plate boundary, actually separates the Pacific plate from the Sierra Nevada microplate (Argus and Gordon, 1991; McQuarrie and Wernicke, 2005; Unruh et al., 2003). Eastern Coast Range faults are likely the actual western edge of the rigid microplate. The Pacific Plate moves northwest relative to the Sierra Nevada microplate at about 39mm/yr (Argus and Gordon, 2001).

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About 80% of Pacific-North American plate motion is taken up west of the microplate (Argus and Gordon, 2001). The remaining 20% is accommodated east of the microplate (Argus and Gordon, 2001). The Walker Lane is a 100km wide zone of normal and strike-slip faulting forming the eastern boundary of the microplate, separate from the westward-extending Basin and Range (Stewart, 1988; Unruh et al., 2003) (Figure 1, p. 2). It partially accommodates the 12mm/yr of motion between the microplate and stable North America (Argus and Gordon, 1991; Dixon et al., 1995). The Basin and Range Province spans about 800km east of the Walker Lane with typical topography featuring alternating ranges and valleys (Figure 1, p. 2). It is a zone of continental extension with extension rates varying in space and time. In general, extension is concentrated in the Wasatch Fault Zone near the Colorado Plateau (Dixon et al., 1995; Hammond and Thatcher, 2004).

Uplift of the Sierra Nevada The asymmetric shape of the Sierra Nevada, has long led researchers to believe that the range uplifted by block tilting and east-down normal fault motion on the Sierra Nevada Frontal Fault Zone. Some data indicates that the Late Cretaceous southern Sierra Nevada had a moderate elevation, and underwent significant Neogene uplift to attain present elevations. (U-Th)/He data has been interpreted to indicate that the elevation of the Late Cretaceous southern Sierra Nevada was about 1500 meters (Clark et al., 2005). Wakabayashi and Sawyer (2001) used the difference between basement topographic highs and the base of Cenozoic strata to determine the minimum amount of pre-Late Cenozoic relief (200-1500 meters, decreasing to the north). Wakabayashi and Sawyer (2001) used paleo-incision rates determined by elevation differences between Late Cenozoic

9 9 deposits and present-day channels to show ~1000 meters of incision since 5 Ma. Tilted strata in the San Joaquin and Tuolumne drainages indicate that uplift rate was slow before 10 Ma and increased significantly between 10 and 3 Ma (Huber, 1981, 1990). Agreeing with this, tilted Cenozoic strata indicate that westward block tilting of the Sierra Nevada began around 5 Ma (Huber, 1981, 1990; Unruh, 1991; Wakabayashi and Sawyer, 2001). While uplift can be related to tilted strata, uplift could not be related to fault displacement unless a reference frame for elevation of the hanging wall or footwall exists. Evidence for a moderate- elevation Sierra Nevada in the Late Cenozoic in combination with tilted strata and evidence of stream incision indicates that Neogene uplift could have occurred because of westward block tilting. Researchers have proposed several mechanisms for significant Neogene uplift. South of the San Joaquin River drainage, the Sierra Nevada is thought to have experienced uplift driven by removal of an eclogite root (Ducea and Saleeby, 1998; Ducea and Saleeby, 1996; Jones et al., 2004; Manley et al., 2000). This part of the range is now thought to be supported by lateral-density variations in the upper mantle (Wernicke et al., 1996). Others have suggested that uplift occurred as a result of the northward migration of the Mendocino triple junction. Warming after the passage of the triple junction converted lithosphere to more buoyant asthenosphere causing uplift (Crough and Thompson, 1977). The progressive nature of the migrating triple junction lead to more exhumation in the southern Sierra than the northern Sierra (Mahéo et al., 2009). Thompson and Parsons (2009) explored the idea that with large movement on the Sierra Nevada Frontal Fault System, small-magnitude isostatic uplift of the footwall block could result. Combining two of these processes, Wakabayashi (2013) suggested that one phase

10 10 of uplift occurred following the passage of the Mendocino Triple Junction, and a second phase of uplift occurred in the southern Sierra Nevada after delamination. Contrary to the previous statements, some research suggests that the Sierra Nevada had high elevations prior to the Neogene. House et al. (1998) interpreted (U-Th)/He cooling ages to mean that the Late Cretaceous Sierra Nevada were as high as 4500 meters and mean elevations have decreased since 70 Ma. House et al. (2001) interpreted (U-Th)/He data to indicate 1500±500 meters of relief across the Late Cretaceous southern Sierra and an elevation of about 3000 meters. Wernicke et al. (1996) also suggested that the Sierra Nevada might have been 4000-5500 meters high prior to 20 Ma. Mulch et al. (2006) used hydrogen isotopes from Eocene Yuba River gravels to suggest that 40-50 Ma elevations were greater than 2200 meters. Similarly, Cassel et al. (2009b) showed that hydrated volcanic glass had isotopic composition values that decreased with distance from the edge of the Ione Formation, similar to the present-day isotopic gradient. Using a modeled Oligocene lapse rate, Cassel et al. (2009b) interpreted a change in elevation across the Oligocene transect to be 3200 m +1100/–2000 m. Wakabayashi (2013) calls into question the assumption in stable isotope paleoaltimetry that volcanic glasses are a closed system, suggesting they instead undergo progressive hydration. The elevation profile produced by Cassel et al. (2009b) is too similar to the present-day profile, and it cannot account for restoration of post-Oligocene faulting in the eastern Sierra Nevada, indicating that the volcanic glasses underwent progressive hydration. While these studies imply that Late Cenozoic uplift was not necessary because the Sierra Nevada were already at a high elevation, high elevations prior to the Neogene do not necessarily preclude additional uplift during the Neogene.

11 11 Motion on SNFFZ: Is it oblique? The Sierra Nevada microplate translates northwest, oblique to the Sierra Nevada Frontal Fault Zone, which forms the microplate boundary, resulting in transtension (Bormann et al., 2016; Dixon et al., 2000; Unruh et al., 2003). The character of the Sierra Nevada Frontal Fault Zone varies along strike. Faulting is partitioned between north-trending normal faults and northwest-trending dextral strike-slip faults (Oldow et al., 2001; Surpless, 2008; Unruh et al., 2003). The expression of these normal faults is most dramatic in the escarpment on the east side of the southern Sierra near Lone Pine, CA. For this area Le et al. (2007) determined that slip is partitioned between dextral slip on the Owens Valley Fault Zone, oblique slip on the Lone Pine Fault, and normal slip on the Sierra Nevada Frontal Fault Zone. Bormann et al. (2016) showed that GPS data requires right- lateral oblique extension. Their model predicted right-lateral oblique slip on north-striking normal faults along the Sierra Nevada Frontal Fault System. Though models predict oblique slip on the Frontal Fault System, little field evidence for it has been found (Busby et al., 2013; Phillips and Majkowski, 2010).

Vertical separation across the SNFFZ. Several studies have made estimates of vertical separation across the Sierra Nevada Frontal Fault System. In the northern Sierra Nevada vertical separation ranges from 600-1500 meters (Wakabayashi and Sawyer, 2001). In the central Sierra Nevada, Slemmons (1953) used elevation differences in the Eureka Valley Tuff across the Frontal Fault Zone to estimate at least 1000 feet of vertical separation. This estimate did not account for tilt of individual fault blocks. For the southern Sierra Nevada across the classic range front, estimates of vertical separation range from 2200-5500 meters (Jayko, 2009; Phillips et al., 2011; Wakabayashi, 2013). The vertical separation estimate for the central Sierra Nevada could be vastly improved by newly

12 12 available information about the structure and distribution of Stanislaus Group outcrops.

Sierra Nevada Drainage Divide When the Nevadaplano collapsed, it topographically isolated the Sierra Nevada, forming the microplate (Argus and Gordon, 1991; McQuarrie and Wernicke, 2005). The eastern edge of the microplate is equivalent to the drainage divide everywhere except the Feather River drainage (Wakabayashi, 2013). The Oligocene Sierra Nevada drainage divide was far east of its present location evidenced by the Valley Springs Formation, which records a much larger, now beheaded, western North American drainage network whose remnants stretch across the Basin and Range province, Walker Lane and Sierra Nevada (Busby et al., 2016; Cassel et al., 2009a; Dalrymple, 1963). Since the Oligocene, westward migration of normal fault activity has reduced the width of the Sierra Nevada and The microplate (Dilles and Gans, 1995; Slemmons et al., 1979). As normal faulting removes blocks from the eastern edge of the microplate, the drainage divide may migrate westward as a response to the changing topography. In that case, the Sierra Nevada drainage divide could be a proxy for the rate of encroachment of faulting on the microplate.

Stratigraphy The Sierra Nevada is composed of metamorphosed Paleozoic and Mesozoic sedimentary and volcanic rocks and intruded by granitoid batholith during 120-80 Ma Farallon subduction (Bateman and Eaton, 1967). Eocene-Oligocene gravels and Cenozoic volcanics unconformably overlie the batholith. The oldest of the Cenozoic volcanics is the Oligocene rhyolite Valley Springs Formation, which is as much as 4000 feet thick in some places (Slemmons, 1966). In the early

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Miocene, as a result of flat-slab subduction, volcanism shifted eastward (Coney and Reynolds, 1977). This shift resulted in the eruption of the Mehrten Formation, andesitic volcanic mudflows (Piper et al., 1939). The Stanislaus Group lavas and ignimbrites, distinctive for their high K2O content, erupted from the Little Walker Caldera about 10 Ma (Busby et al., 2008; Busby and Putirka, 2009; Putirka and Busby, 2007) (Figure 3). The Stanislaus Group is sometimes sandwiched between two phases of Mehrten Formation. When the Mehrten Formation is divided, the lower portion is termed the Relief peak formation and the upper portion is termed Disaster Peak (Dalrymple, 1963; Priest, 1979).

Figure 3: Stanislaus Group nomenclature from Priest (1979).

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The Stanislaus Group includes three formations: Table Mountain, Eureka Valley Tuff, and Dardanelles Formation (Noble et al., 1974) (Figure 3). The dominantly trachyandesitic Table Mountain Formation erupted from the Little Walker Caldera about 10.4 Ma (Busby et al., 2008; Gorny et al., 2009; Halsey, 1953; Pluhar et al., 2009; Slemmons, 1966) and filled central Sierra Nevada paleodrainage network (Lindgren, 1911; Ransome, 1898) (Figure 2, p. 3). Its type section is located in the Bald Peak – Red Peak area of the high Sierra and is part of the “cataract paleochannel” (Slemmons, 1966). The remaining Stanislaus Group outcrops are laterally extensive, occurring in the Walker Lane, across the Sierra Nevada Frontal Fault, and into the Sierra Nevada Foothills (Figure 2, p. 3). Because of their emplacement across the active eastern margin of the microplate, they record deformation at that margin relative to the stable Sierra Nevada since the Late Miocene. Priest (1979) subdivided the Table Mountain Formation in the vicinity of the Little Walker Caldera into four members: Upper Member, Two-Pyroxene Member, Large Plagioclase Member, and Lower Member (Figure 3). He also produced geochemical analyses of the Table Mountain Formation and a very detailed map of the vicinity of the Little Walker Caldera. Brem (1984) produced a map covering the Sweetwater Roadless Area, northeast of Priest’s (1979) map, including minor undivided Table Mountain Formation. Over the past 10 Ma much of the Table Mountain Formation lava has eroded. Remaining outcrops are preserved on peaks and ridges with present-day rivers nearby but far below (Figure 2, p. 3). The Eureka Valley Tuff overlies the Table Mountain Formation and can be divided into three members: Tollhouse Flat Member, By-Day Member, and Upper Member (Noble et al., 1974) (Figure 3). Tollhouse Flat Member is the most

15 15 widespread unit, distinguished by its reversed polarity and biotite phenocrysts (Noble et al., 1974). By-Day member is normal polarity and lacks phenocrystic biotite but is otherwise similar in appearance to Tollhouse Flat Member (Noble et al., 1974). The Upper Member is usually not welded, has abundant biotite, and a normal polarity (Noble et al., 1974). Eureka Valley Tuff is laterally extensive, stretching from the western Sierra Nevada Foothills into the Walker Lane, and is very useful for measuring vertical-axis rotation (Figure 2, p. 3).

METHODOLOGY

Stratigraphy, Lithology, and Petrography We used petrography to establish whether lavas at Priest’s Measured Section (locality on the southeast margin of the Little Walker Caldera) and Rancheria Mountain (locality near Hetch Hetchy Reservoir) had similar mineral composition and abundances. This information will assist in correlating lavas, determining if the lavas represent the same cooling unit, but have been separated by erosion and faulting. National Petrographic Service in Houston, Texas produced six thin sections by standard methods, each from different flows at Priest’s Measured Section. Previous workers produced and analyzed thin sections from the flows at Rancheria Mountain (Jones, 2014) and Boone Canyon (Carlson et al., 2013). Using a Zeiss petrographic microscope, we identified the minerals present in flows at Priest’s Measured Section to compare to mineralogy at Rancheria Mountain and Boone Canyon. To accurately characterize the thickness and lithology of each individual lava flow and the thickness of the section overall, we measured and constructed a stratigraphic column at Priest’s Measured Section. We used a Jacob’s Staff and

Brunton compass with the strike and dip of the contact between lava flows. In the field I sketched a graphic column with a weathering profile, and described the lithology, physical volcanology, weathering patterns, and distinctive features of the rocks.

Geologic Mapping Several researchers have produced geologic maps of parts of the central Sierra Nevada (Brem, 1984; Chesterman, 1975; Priest, 1979). We combined these 17 17 three maps in an ArcGIS geodatabase. Since much of the mapping was done before the units of the Stanislaus Group were formally identified and named, many of the volcanic units are described very generally as andesite. We suspected that much of what had been mapped as ‘Andesite of Walker River’ was actually undivided Stanislaus Group. One goal of our mapping was to determine whether these units were Stanislaus Group. At each paleomagnetism sample locality in this study (Eagle Creek, Priest’s Measured Section, Buckeye Overlook and McMillan Lake) we field checked the published maps and updated the contacts and lithology and added structural data for the Stanislaus Group Units. Using the correlative outcrops, updated geologic map, and digital elevation model, we propose a flow path for Table Mountain Formation Upper Member flows defining a paleochannel of the Tuolumne River. The Table Mountain Formation Lavas near this paleochannel are preserved in isolated outcrops, often at high elevation with canyons incised around them. We assume that the flow path is most likely to be the lowest-possible elevation path between outcrops.

Constructing a Cross-Section Outcrops of the Table Mountain Formation and the location of the proposed paleochannel are cut by numerous northwest and northeast trending faults. We drew a cross section across the Sierra Nevada that passed through three paleomagnetism sample sites; Rancheria Mountain (Jones, 2014), Priest’s Measured Section, and Boone Canyon (Carlson et al., 2013) and used ArcGIS to generate the topographic profile from 9m resolution digital elevation model data. Since the mapped faults did not have any orientation data, I used the topographic lines and surface expression of the faults to estimate their strike and dip for the cross section. To determine strike, I drew structure contours where the

18 18 fault crossed the same elevation contour two times. The orientation of the structure contours is the strike direction. To estimate dip, I measured the map distance between structure contours. Then, I divided the elevation change between structure contours by the map distance to get a gradient. The dip angle is the inverse tangent of the gradient. If the structure contours were not parallel, then the fault was not planar and thus it was not possible to estimate a strike and dip using this method. If the fault cut across topography indiscriminately we assumed it was vertical, although this is geologically unlikely. When structures crossed the section line at an angle we calculated the apparent dip. In this way, we were able to interpret aspects of the 3D geometry and determine total vertical separation.

Geochemistry Isolated outcrops with similar lithology are good candidates for geochemical correlation. Table Mountain Formation outcrops are possibly remnants of formerly-extensive lava flows inundating the Late Miocene drainage network that spanned much of the Sierra Nevada. There may have been one such paleochannel facilitating flow southwest from the Little Walker Caldera to Rancheria Mountain. To identify the presence of a paleochannel, we need to establish whether the remnant outcrops are the same flow. The geochemistry provides a chemical signature that in combination with lithology, geochronology, and paleomagnetism strengthens the correlation. We analyzed and tested correlation of six samples from Priest’s Measured Section, combined with previously-collected geochemical data from lavas at Rancheria Mountain (Jones, 2014) and Boone Canyon (Carlson, 2012). The University of Massachusetts Ronald B. Gilmore XRF Laboratory analyzed six samples for major and trace elements. This lab uses a Siemens MRS-

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400 multi-channel, simultaneous X-ray spectrometer on fused La-bearing lithium borate glass disk to conduct major element analysis (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P), using the methods of Norrish and Hutton (1969). The lab also obtained trace-element data (Nb, Zr, Y, Sr, Rb, Th, Pb, Ga, Zn, Ni, Cr, V, Ba, Ce, La) using a Philips PW2400 sequential spectrometer and pressed powdered pellets (Norrish and Chappell, 1967; Reynolds, 1967; Walker, 1973). We geochemically correlate samples from Rancheria Mountain or Boone

Canyon semi-quantitatively by plotting results on a LeBas diagram of alkali (Na2O

+ K2O) abundance versus SiO2 (Le Bas et al., 1986) and Harker diagrams of element oxides compared to SiO2 (Harker, 1909). Those samples that plotted close together on the diagrams have similar composition and thus potentially correlate.

ARGUS VI Mass Spectrometer – OSU Argon Geochronology Laboratory Ar/Ar radioisotopic dating provides absolute ages for the lavas in the study area. The ages of the oldest and youngest members of Table Mountain Formation at this locality will allow us to constrain the timing of deformation and establish a rate for vertical-axis rotation and a rate of offset on faults. We sent two samples to Oregon State University Geochronology Laboratory for incremental-heating 40Ar/39Ar dating experiments on groundmass and plagioclase separates. One sample each of the Table Mountain Formation Upper Member (PMS0104) and Table Mountain Lower Member (PMS1501) collected from Priest’s Measured Section (Figure 2, p. 3). These two samples were chosen for their relatively un-weathered quality and their position within the section.

20 20

Personal Communication from Anthony Koppers (October 15, 2016) describes the methods Oregon State University Argon Geochronology Laboratory used. “Two new 40Ar/39Ar ages were obtained by incremental heating methods using the ARGUS-VI mass spectrometer. Mineral separates were prepared by crushing and sieving bulk rock to 40–60 mesh, followed by magnetic separation of phenocrysts, ultrasonic cleaning in distilled water, and handpicking to remove impurities or grains with crystal or melt inclusions. PMS0104 and PMS 1501 samples were irradiated for 6 hours (17-OSU-01 1A29-17) in the TRIGA CLICIT nuclear reactor at Oregon State University, along with the FCT sanidine (28.201 ± 0.023 Ma, 1σ) flux monitor (Kuiper et al., 2008). Individual J-values for each sample were calculated by parabolic extrapolation of the measured flux gradient against irradiation height and typically give 0.1-0.2% uncertainties (1σ). The 40Ar/39Ar incremental heating age determinations were performed on a multi-collector ARGUS-VI mass spectrometer at Oregon State University that has 5 Faraday collectors (all fitted with 1012 Ohm resistors) and 1 ion- counting CuBe electron multiplier (located in a position next to the lowest mass Faraday collector). This allows us to measure simultaneously all argon isotopes, with mass 36 on the multiplier and masses 37 through 40 on the four adjacent Faradays. This configuration provides the advantages of running in a full multi-collector mode while measuring the lowest peak (on mass 36) on the highly sensitive electron multiplier (which has an extremely low dark-noise and a very high peak/noise ratio). Irradiated samples were loaded into Cu-planchettes in an ultra-high vacuum sample

chamber and incrementally heated by scanning a defocused 25 W CO2 laser

21 21

beam in preset patterns across the sample, in order to release the argon evenly. After heating, reactive gases were cleaned up using an SAES Zr-Al ST101 getter operated at 400°C and two SAES Fe-V-Zr ST172 getters operated at 200°C and room temperature, respectively. All ages were calculated using the corrected Steiger and Jäger (1977) decay constant of 5.530 ± 0.097 x 10-10 1/yr (2σ) as reported by Min et al. (2000). Incremental heating plateau ages and isochron ages were calculated as weighted means with 1/σ2 as weighting factor (Taylor, 1997) and as YORK2 least-square fits with correlated errors (York, 1968) using the ArArCALC v2.7.2 software from Koppers (2002) available from the http://earthref.org/ArArCALC/ website.”

Paleomagnetism The polarity of each sampled unit provides broad limits to potential correlations, while a detailed comparison of direction permits more precise quantitative test of correlation and/or measurement of vertical-axis rotation. For example, we used polarity (in combination with mineralogy) of Eureka Valley Tuff to determine what member each site included. If the sample had biotite and was reversed polarity then it was Tollhouse Flat Member, if it did not have biotite and was normal polarity it was By-Day Member, and if it had biotite and was normal polarity it was Upper Member. Paleomagnetism is the only method that measures vertical-axis rotation. If we sample enough flows spanning enough time to average out secular variation we can measure rotation relative to paleo-north. Otherwise, assuming we have an un-rotated reference direction, we can measure relative rotation. We also assembled a magnetostratigraphy at Priest’s Measured

22 22

Section, which can provide age constraints on units in stratigraphic context, especially if an approximate independent age measure is available. We collected a total of 115 samples. Three to ten oriented hand samples each from fourteen Table Mountain Formation lava flows at Priest’s Measured Section, two flows at McMillan Lake, and one flow at Eagle Creek. We also collected eight samples from each of two Eureka Valley Tuff outcrops at Eagle Creek and Buckeye Overlook localities. On average we collected six samples per site. This number of samples optimizes the balance between precision and sampling and analysis effort. In the field, we marked and recorded strike and dip of a flat face of each hand sample, and then the sample was removed from the outcrop with a sledgehammer and chisel. In the lab each hand sample was cored using a 1” diamond core bit on the drill press, and re-oriented as it was in the field. We recorded and marked each core orientation, and then removed each sample for trimming into specimens, followed by sample analysis. To analyze the samples we used paleomagnetism laboratories at the USGS Menlo Park and at Occidental College with their RAPID (Rock and Paleomagnetism Instrument Development) automated sample changing systems and cryogenic magnetometers (Kirschvink et al., 2008). Both of these magnetometers are housed in shielded rooms that reduce the ambient magnetic field to less than 1% of Earth’s ambient field. We performed 14-step alternating field demagnetization experiment (2, 4, 7, 10, 13, 16, 20, 25, 30, 40, 50, 60, 70, and 80mT) to identify the characteristic remnant magnetization (ChRM). We used Paleomac to fit great circles or least squares lines to demagnetization results to reveal any overprints and the ChRM (Cogné, 2003; Kirschvink, 1980). Assuming

23 23 a Fisher distribution, we calculated mean VGP’s, cones of confidence, dispersion and precision for each lava flow (Fisher, 1953). We applied a locality-specific tilt correction to each flow. At Priest’s Measured Section we used the orientation of pipe vesicles, which indicate original vertical, and the strike and dip of lava flow 8, calculated from a digital elevation model (DEM) and map of the base of the flow. The tilt corrections for the three remaining sites were calculated from the eutaxitic textures, or fiamme, in the Eureka Valley Tuff. We measured the apparent trend and plunge of a dozen fiamme in each outcrop. Then we plotted these apparent-dip data on a stereonet and calculated a best-fit plane, which we used as the strike and dip of the locality. To test correlation between lavas at Rancheria Mountain, Priest’s Measured Section, and Boone Canyon we compared flow ChRM’s. Using Paleomac (Cogné, 2003) we compared the two reversed-polarity lavas at Priest’s Measured Section to two reversed-polarity lavas at Rancheria Mountain and one reversed- polarity lava at Boone Canyon. If the flows were statistically indistinguishable, then they were correlative. To determine whether there is a vertical-axis rotational component to deformation in the eastern Sierra Nevada, we used results from Table Mountain Formation, and Eureka Valley Tuff By-Day and Tollhouse Flat Members. Using the methods of Demarest (1983), we compared the mean direction from each cooling unit to the appropriate reference direction. For Eureka Valley Tuff Tollhouse Flat Member, we compared the mean direction with reference direction established by King et al. (2007) from five sites on the stable Sierra Nevada block (I = −62.8°, D = 159.9°, α95 = 2.6°). For By-Day member we compared the mean direction with reference direction established by Pluhar (personal communication, 2016) (I = 51.8°, D = 349.6°, α95 = 3.0°). At Priest’s Measured Section, we

24 24 averaged the mean direction for all flows, creating a site mean direction. We calculated a relative rotation by comparing the Priest’s Measured Section site mean direction to the Table Mountain Formation lavas site mean direction at Sonora Peak (I=52.4°, D=355.0°, α95=4.3°) (Carlson et al., 2013). We compared Priest’s Measured Section to Sonora Peak because Sonora Peak is on the microplate and west of most of the Eastern Sierra faulting.

RESULTS

Lithology

Priest’s Measured Section Often described as latite, our geochemical data indicates that Table Mountain Formation lavas at Priest’s Measured Section range in composition from basaltic trachyandesite to trachyte. Priest’s Measured Section exposes basement Sierran granitoid uncomformably overlain by 416 meters of volcanics (Plate 1). These volcanics feature an un-named tuff, Table Mountain Formation Lower Member, Large Plagioclase Member and Upper Member. Lava flows at this site dip shallowly northwestward. The number of flows decreases with distance from the Little Walker Caldera. Only Upper Member is preserved at the farthest sites, Boone Canyon and Rancheria Mountain. The thickest member of Table Mountain Formation at Priest’s Measured Section, which also contains the most lava flows, is the Lower Member (Tmtl) (Plate 1). Vegetation and soil covers the bottom 65 meters of Tmtl, so it is difficult to determine how many flows are present. The 11 overlying Tmtl flows range in thickness from 4-30 meters. Lower Member lava flows are vesicular throughout, with vesicles increasing at flow tops. Each flow exhibits an 25

autobrecciated flow top and a core with irregular jointing. Near the top of Tmtl, 5- 20 cm calcite- and chalcedony-filled amigdules are present. Flow PMS08 has rare pipe vesicles. The presence of augite phenocrysts distinguishes Tmtl from other Table Mountain Formation members. In thin section, the Lower Member features a fine-grained groundmass of plagioclase microlites with large (<5mm) labradorite phenocrysts and smaller (~2mm) augite phenocrysts. 26 26

The Large Plagioclase (Tmtp) member at Priest’s Measured Section contains three flows, ranging in thickness from 6-22m (Plate 1). It can be distinguished from other Table Mountain Formation members in hand sample by large (~1 cm) plagioclase and occasional small olivine phenocrysts. Flows are vesicular throughout, but vesicles are more abundant near autobrecciated flow tops. The Upper Member (Tmtu) totals 116 meters thick over five flows at Priest’s Measured Section (Plate 1). Vesicle abundance varies, but tends to decrease up section. The flow cores are often platy. The Upper Member exhibits a very fine-grained groundmass with rare, small (<5mm) plagioclase. In thin section, labradorite to bytownite phenocrysts account for <5% of the Upper Member at Priest’s Measured Section. At Boone Canyon, the mineralogy appears very similar in thin section to the Upper Member at Priest’s Measured Section with plagioclase microlite groundmass and <5% plagioclase phenocrysts. At Rancheria Mountain, the presence of plagioclase phenocrysts is more dominant, accounting for ~30% of the total rock.

Buckeye Overlook This site features Eureka Valley Tuff By-Day Member at the top of a peak southwest of Priest’s Measured Section. Here, By-Day Member appears similar to Tollhouse Flat Member but lacks biotite phenocrysts. We collected 8 samples from a south-facing outcrop near the top of the peak.

McMillian Lake This site hosted several Table Mountain Formation flows capped by eroded Eureka Valley Tuff. Eureka Valley Tuff existed only as float at the top of a hillock. There were two Table Mountain Formation Upper Member flows

27 27 overlying one Table Mountain Formation Two Pyroxene Member (Tmtx) flow downslope from the Eureka Valley Tuff outcrop. In hand sample at McMillan Lake locality, Two-pyroxene member appeared very similar to Upper Member. Priest (1979) differentiated the Two-Pyroxene Member by the presence of orthopyroxene not found in other members. Thin sections confirm that rare orthopyroxene phenocrysts are present among the more abundant plagioclase and olivine in MML03 samples.

Eagle Creek Eagle Creek is our only locality on the south side of Buckeye Canyon. It includes a Table Mountain Formation lava flow adjacent to a Eureka Valley Tuff Tollhouse Flat Member outcrop. These outcrops were mapped as ‘Andesite of Walker River’ on the Matterhorn Peak 15’ Quadrangle (Chesterman, 1975). The unit ‘Andesite of Walker River’ may actually represent undivided Stanislaus Group. Here, the Table Mountain Formation outcrop was a heavily weathered lava flow with plagioclase and weathered olivine phenocrysts visible in hand sample. The Eureka Valley Tuff outcrop has a welded eutaxitic texture and biotite phenocrysts. The lithology and welded nature of the outcrop in combination with reversed polarity paleomagnetic results indicate it is Tollhouse Flat Member.

Geochemistry The University of Massachusetts analyzed six Table Mountain Formation samples from Priest’s Measured Section for major and trace elements. We combined these new results with geochemical data from previous Table Mountain Formation samples (Tables 1, 2, Figures 4 & 5), including the lavas at Rancheria Mountain (EE02B, EE01A) and Boone Canyon (BOC0503A). The geochemical data for all Table Mountain Formation localities are plotted on a Le Bas et al.

28 28

(1986) diagram (Figure 4). Table Mountain Formation lavas are high-K. Tmtl flows plot at the boundary between basaltic trachyandesite and trachyandesite. Tmtu flows plot at the boundary between trachyandesite and trachyte. Priest’s Measured Section Samples (PMS0201, PMS0203) both Upper Member, plot closest to EE01A and BOC0503A on the Le Bas et al. (1986) diagram indicating compositional similarity. In terms of major oxides, these four samples have slightly higher Silica and very similar Titainum, Iron, Manganese, Magnesium, Sodium and Potassium. The Ti-Zr-Y ternary plot also indicates that the samples are close in composition (Figure 5).

Mapping and Cross-Section I have combined and digitized data from three older maps to generate a map of the study area (Plate 2). The Matterhorn Peak 15’ Quadrangle (Chesterman, 1975) is a generalized map with volcanic units lumped into general categories. Chesterman (1975) included an extensive unit called ‘Andesite of Walker River’ after Halsey (1953). I field checked the map only in the vicinity of paleomagnetism sampling localities. Based on our visit to the Eagle Creek locality near the mouth of Buckeye Canyon, at least one site Chesterman (1975) mapped as Andesite of Walker River is actually undivided Stanislaus Group. A cross-section that passes from Rancheria Mountain near Hetch Hetchy reservoir, through the Sierra Crest, Priest’s Measured Section, and Boone Canyon (Plate 3a, b, c) shows down-to-the-northeast offset east of the present-day crest. We used Table Mountain Formation lavas as markers to measure offset because they were emplaced either on a horizontal or down gradient from their source, as opposed to the emplacement of a tuff. Because the lavas are thickest and all of the

Table 2: Trace element composition of Table Mountain Formation samples. Includes data from Jones (2014) and this study (PMS samples).

Figure 4: LeBas (1985) diagram showing the relative silica and alkali composition of Table Mountain Formation samples. Typical Table Mountain Formation and typical Eureka Valley Tuff chemistry generalized from Asami (2014), King et al. (2007), and Koerner et al. (2009). Filled symbols represent normal polarity and hollow symbols represent

reversed polarity. 31

Figure 5: Ti-Zr-Y ternary plot with Table Mountain Formation samples. The colors are similar to the above LeBas (1986) diagram and the hollow symbols represent reversed polarity and the filled symbols represent normal polarity. 32

33 members are present near Priest’s Measured Section, we determined that this is the most proximal site to the lava source, likely the Little Walker Caldera. We generated the schematic cross-section (Plate 3d) to demonstrate the method used to calculate vertical separation. It reveals that vertical separation when accounting for fault stratigraphic geometry results in a different amount than vertical separation based on comparative elevation of stratigraphic horizons. Vertical separation equals elevation difference only in the case where there are horizontal strata, not when strata are tilted. Field measurements of dip of Stanislaus Group units (≈10 degrees) were projected into the schematic cross-section to demonstrate that faulting and tilting of around 10 degrees could result in 3500 meters of total vertical offset but an elevation difference of only about 800 meters. We measured about 3500 meters of vertical separation between the Sierra Crest and Boone Canyon. A minimum of 275 meters of vertical separation occurred between the eruptive source area and the present-day Sierra crest. An additional ~3200 meters of vertical separation occurred between the source area and Boone Canyon (Plate 4). This vertical separation is significantly larger than previous estimates of about 1100 meters based on comparative elevations (Noble et al., 1974; Slemmons, 1966).

40Ar/39Ar Geochronology Oregon State University analyzed one sample each from Table Mountain Formation Upper Member and Lower Member. The Upper Member sample (PMS0104) resulted in a plagioclase plateau age of 9.52 ± 0.03 Ma (Table 3, Figure 6). The Lower Member sample (PMS 1501) yielded a plagioclase plateau age of 10.38 ± 0.03 Ma. The plagioclase separates are a better estimate of the crystallization age because the groundmass separates show significant argon recoil.

Table 3: Summary of 40Ar/39Ar Results

Figure 6: 40Ar/39Ar Results. Incremental heating apparent-age spectra for all experiments. 36 Paleomagnetism Our paleomagnetic results come from localities around Buckeye Canyon south and west of previous work (Carlson et al., 2013; King et al., 2007; Pluhar et al., 2009). We analyzed 115 samples from 19 sites across four localities, capturing two Eureka Valley Tuff members and four Table Mountain Formation members. Zijderveld diagrams revealed that two components of magnetization were common, but sometimes samples were univectoral (Figure 7). Some specimens had a lightning overprint that we removed through AF demagnetization. Zijderveld diagrams for most Eureka Valley Tuff specimens had curved demagnetization paths indicating the specimens had overlapping coercivities (Figure 7). Eureka Valley Tuff specimens ChRM’s were well grouped and resulted in lower α95’s than the Table Mountain Formation lava flows. Table Mountain Formation specimen ChRM’s were not well grouped and the resulting flow means had higher α95’s than those of the Eureka Valley Tuff (Table 4). We compared polarity of lavas and ignimbrites at our sites to other sites in the Sierra Nevada to identify possible correlations. We used the mean ChRM for Eureka Valley Tuff Tollhouse Flat and By-Day Members compared to the respective established reference directions to calculate rotation. We compared the mean direction of the 14 flows at Priest’s Measured Section to the mean direction of 23 Table Mountain Formation lavas at Sonora Peak (Pluhar et al., 2009) to get a relative rotation compared to the stable Sierra Nevada (Table 5).

Priest’s Measured Section Results from 14 sites across three Table Mountain Formation members reveal that the Upper Member is reversed polarity and the Large Plagioclase and Lower Member are normal polarity (Table 4, 5, Appendix B). These flows can be

Table 4: Mean Flow ChRM Paleomagnetic Results

Table 5: Sample site rotation results. A positive rotation indicates clockwise sense. A positive flattening indicates shallower inclination.

Figure 7: Representative Zijderveld diagrams, showing overprints and ChRM for selected specimens 39

40 divided into four paleomagnetic directional groups (Figure 8). Using the method of McFadden and McElhinny (1990), we tested all flow directions to determine whether each of their directions were statistically distinguishable from the flow above and below and to defensibly group them into directional groups. Upper Member flows, PMS01 and PMS02, are both reversed polarity but not the same direction; we consider them two different directional groups. The third directional group is made up of PMS03 and 04 (Large Plagioclase Member) and PMS05, 06,

07, and 08 (Lower Member). Because PMS09 has an exceptionally large α95, it is statistically indistinguishable from the flows above and below it and could have been included in both directional groups, so we excluded it. The last group includes PMS10, 12, 13, 14, and 15, all Lower Member and statistically indistinguishable from each other. For each directional group at Priest’s Measured Section we calculated a mean ChRM (Figure 8). Using the four directional group means, we calculated a locality mean. To calculate the amount of rotation relative to the stable Sierra Nevada, we compared the Priest’s Measured Section locality mean to Sonora Peak. Sonora Peak is located west of the Sierra Nevada Frontal Fault Zone, and therefore represents only the motion of the microplate. This comparison displays 20.6°±24.5° of clockwise rotation (Table 5). Flattening was insignificant with a value of 6.9°±12.3° (Table 5).

McMillan Lake McMillan Lake locality spans part of the Table Mountain Formation including the Two Pyroxene Member, which is missing from Priest’s Measured Section. Sample ChRM’s from the Table Mountain Formation Two Pyroxene Member were moderately well grouped and exhibited a reversed polarity (Table 4,

Figure 8: a) Tilt corrected mean ChRM's of all Priest's Measured Section flows color coded by directional group. b) Mean ChRM for each directional group

Appendix B). Sample ChRM’s from the Table Mountain Formation Upper Member were not well grouped and produced an unexpected locality mean. The locality mean was an up-and-to-the-southeast direction (D=116, I=39, α95=28) (Table 4, Appendix B). This unexpected result could be caused by an incorrect tilt correction. We approximated the structural correction from the strike and dip of the overlying Eureka Valley Tuff By-Day Member and flattened vesicles in Table Mountain Formation Upper Member. The Two Pyroxene Member locality mean was statistically indistinguishable from the Table Mountain Formation Upper Member at McMillan Lake. Neither of the flows at the McMillan Lake locality correlate based on paleomagnetism with any flows at Priest’s Measured Section. An accurate tilt correction may produce a paleomagnetic correlation between these two sites.

Buckeye Overlook Buckeye Overlook is east of McMillan Lake locality and hosts the Eureka Valley Tuff By-Day Member. Sample ChRMs from Buckeye Overlook were well grouped and produced a locality mean that clearly demonstrates site vertical-axis rotation relative to the stable Sierra Nevada. Alternating field demagnetization removed most of the lightning overprint present on these samples. We fitted great circles to sample demagnetization paths, and then found the best-fit intersection of great circles, which yielded a normal-polarity unit mean for the locality. (Table 4, Appendix B). Using this site mean we calculated a vertical-axis rotation of 19.7°±4.4° clockwise with 6.8°±2.8° of flattening from the established reference direction (Pluhar, unpublished data) (Table 5). Significant flattening suggests an inadequate tilt correction for this site (Table 5). The tilt correction came from the

Figure 9: Buckeye Overlook Tilt Correction

44 orientation of a dozen fiamme in the outcrop (Figure 8). However, this is still a significant rotation because flattening is smaller than rotation.

Eagle Creek Sample ChRM’s from Eureka Valley Tuff Tollhouse Flat Member at Eagle Creek were reversed polarity and well grouped, but produced a locality mean with excessive flattening compared to the expected direction. We compared the Eureka Valley Tuff Tollhouse Flat direction from Eagle Creek to the Eureka Valley Tuff Tollhouse Flat reference direction from King et al. (2007) revealing 10.6°±6.5° of clockwise rotation with 9.6°±3.4° of flattening (Table 5). Excessive flattening implies that we used an inadequate tilt correction. The tilt correction came from the orientation of a dozen fiamme in the outcrop (Figure 9). Because flattening is smaller than rotation, we can still consider this a significant rotation. Adjacent to the Eureka Valley Tuff outcrop at Eagle Creek there is a Table Mountain Formation lava flow outcrop. Sample ChRM’s from Table Mountain

Formation were not well grouped and produced a locality mean with an α95 of 40° (Table 4, Appendix B). The resulting mean is normal polarity, but it is not possible to correlate it with any other flows due to the large error.

Figure 10: Eagle Creek Tilt Correction

DISCUSSION

Tilt Corrections for Paleomagnetic Data The paleomagnetic rotation calculation revealed significant inclination flattening at two localities. Eagle Creek had flattening of 9.6°±3.4° and Buckeye Overlook had flattening of 6.8°±2.8° (Table 5). Significant flattening in this case suggests inaccurate tilt correction. This inaccuracy could be caused by post- emplacement compaction or an original dip inherited from slopes dipping away from the eruptive center. For both these sites we calculated the tilt correction from measurements of the trend and plunge (apparent dips) of a dozen fiamme (figure 9, 10). At Priest’s Measured Section the rotation calculation revealed insignificant flattening, indicating an accurate tilt correction (Table 5). Dispersion of the flow mean ChRM’s at Priest’s Measured Section (S=27.7) was higher than expected (Table 5). Expected dispersion is determined by age of the formation and latitude, for this site we expect it to be <12° (Butler, 1992). High dispersion is related to difficulties in determining ChRM. Including sampling an excursion, high error in general, or sampling displaced blocks. We know that this site has four distinct directional groups (Figure 8), so we averaged the directional group means to get a locality mean. Eliminating the duplicate flows through averaging across stratigraphic groups did not reduce dispersion; it actually increased, because we had a small number of stratigraphic groups. Another factor contributing to the high dispersion are the sample sites with only three ChRM’s averaged together, and very high α95’s (Table 4). These factors together are likely causing high dispersion at Priest’s Measured Section.

47 47 40Ar/39Ar Geochronology Oregon State University conducted 40Ar/39Ar radio-isotopic dating on groundmass and plagioclase separates of Table Mountain Formation samples from Priest’s Measured Section. The groundmass split of Upper Member resulted in an 36Ar/40Ar ratio that differed from atmospheric indicating inherited argon. The Lower Member groundmass split was too fine grained, causing atom loss due to argon recoil and subsequent dates were not good enough to produce reliable results. Plagioclase separates resulted in a better estimate of the crystallization age. The plagioclase dates are in stratigraphic order with the Upper Member being nearly 1 Ma younger than the Lower Member (Table 3, Figure 6). In addition, when compared with the magnetic polarity timescale (Evans et al., 2007) (Figure 11), the dates agree with expected polarity. Also, these Lower Member date for Table Mountain Formation on the southeast margin of the Little Walker Caldera agree with dates from normal polarity lavas at Sonora Peak, while the Upper Member date is very close to the overlying Eureka Valley Tuff Tollhouse Flat Member date (Figure 11).

Paleochannel of the Tuolumne River Lithological similarities indicate a potential correlation between Table Mountain Formation Upper Member lavas at Priest’s Measured Section, Rancheria Mountain, and Boone Canyon. All are reversed polarity (Figure 12), all plot in in a similar area of a Le Bas et al. (1986) diagram having more affinity for trachyte (Figure 4), and the mean ChRM’s at Boone Canyon, Rancheria Mountain and PMS01 agree when back-rotated (Figure 13). This correlation allows for comparison of vertical-axis rotations, measurement of offset on faults and validates the hypothesis that the lava flowed down a paleochannel from Priest’s

48 48

Figure 11: Composite Magnetostratigraphy. Colors correspond to directional groups.

49 49

Figure 12: Magnetostratigraphy highlighting potential correlation across Table Mountain Formation Upper Member flows. Colors correspond to directional groups.

50 50

Figure 13: All reversed polarity Table Mountain Formation lava flow mean ChRM's. Colors correspond to directional groups.

51 51

Measured Section to Rancheria Mountain. This is the most southern crest- crossing paleochannel originating at the Little Walker Caldera. Table Mountain Formation lavas innundated the paleodrainage network when they erupted. Remnants of the original flow are present across much of the Sierra Nevada (Figure 2). Beginning at Rancheria Mountain near Hetch Hetchy reservoir, Huber (1990) and then Wakabayashi and Sawyer (2001) used remnant lava outcrops to trace the paleo-Tuolumne river downstream. The source of these lavas at the time was unknown. The Rancheria Mountain site is about 30 km southwest from Priest’s Measured Section and an additional 30 km from Boone Canyon. Based on the thickness and number of members present, we determined that Priest’s Measured Section is the most proximal locality to the lava source. Using our map in combination with a digital elevation model, we identified a potential paleochannel that connected the correlative sites at Priest’s Measured Section and Rancheria Mountain (Figure 14). This paleochannel is the lowest elevation route possible between Priest’s Measured Section and Rancheria Mountain. It crosses the modern Sierra Nevada crest at Buckeye Pass and continues southwest to Rancheria Mountain (Figure 14). The topographic profile of this paleochannel reveals that there are no peaks that rise above the gradient (~29m/km) between the two sites (figure 14). Since the lava flowed down a paleochannel to Rancheria Mountain the basal contact sloped away from the eruptive center with a concave up profile and a gradient that represents the paleostream gradient plus any increase in gradient that resulted from block tilting (Figure 14). This correlation and paleochannel profile confirm that the source of the lavas was probably the Little Walker Caldera. This paleochannel is the

52 52

Figure 14: Flow path and topographic profile from Priest's Measured Section to Rancheria Mountain

53 53 southern-most crossing of lava from the Little Walker Caldera over the Sierra crest. The lava source is likely the Little Walker Caldera, proximal to Priest’s Measured Section, which helps interpret the vertical separation data (Figure 8, appendix). The lava must have flowed downhill from the source, which means that Victoria Peak (present Sierra Nevada Crest) must have been lower elevation than Priest’s Measured Section at the time of eruption. The vertical separation measurement (figure 8b, Appendix) must represent a minimum offset because downhill from the lava source we have relative uplift. Vertical separation on the east side of Priest’s Measured Section is a combination of the original gradient of the lavas flow path and any uplift (Plate 4). Offset is large and blocks are tilted upstream, so there has likely been considerable offset. We cannot provide a minimum vertical separation because we do not know what the original gradient was.

Vertical Separation across the Sierra Nevada Frontal Fault System Previous work has estimated vertical separation across the Sierra Nevada Frontal Fault System finding that it ranges widely based on location. Across the Mohawk Valley Fault Zone and faults in the Feather River drainage in the northern Sierra Nevada, vertical separation ranged from 600-1500 meters (Wakabayashi and Sawyer, 2001). In the central Sierra using the Eureka Valley Tuff as a marker both Slemmons (1966) and Noble et al. (1974) found ~1100 feet based on differences in elevation. Vertical separation appears to be greatest in the southern Sierra Nevada ranging from 2200 to 5500 meters (Jayko, 2009; Phillips et al., 2011; Wakabayashi, 2013).

54 54

Cartoon. Prior to this study, measure of offset on Stanislaus Group units has only considered elevation difference, but that method fails to account for the dip of strata and faults. Comparing elevation exclusively misses an important horizontal component of deformation. The schematic cross section (Plate 3) is intended purely as a visual example demonstrating that vertical separation accounting for tilted strata is significantly larger than vertical separation comparing elevations between distant outcrops. The strike and dip of the outcrops at Priest’s Measured Section (240/11.7) averaged over the length of the cross section C-D (28 km) produces 3500 meters of vertical separation (Plate 3, 4). If all of the strata were horizontal and the elevations were the same across it, then the vertical separation would be the difference in elevation between the farthest outcrops. When you use the dip of the strata to reverse the motion on each fault it increases the vertical separation. Dipping units indicate that there has been a horizontal component to deformation. This cartoon demonstrates that the elevation differences across the section vastly under estimate the total vertical separation.

Cross sections. From our mapping, we were able to draw a cross-section passing through all three correlative sites (Plate 3). Projecting the dipping outcrops to the nearest faults revealed offset across each fault and across the whole section (Plate 4). The tilts for the units in each block were determined first by any structural data collected in the field, then by any structural data present on any published maps, and lastly by a strike and dip determined by the relationship of the contacts to topography. The tilts for the faults were determined using the same method, but in the absence of structural data when the fault cut topography indiscriminately it was interpreted to be vertical. This cross section line from C-D is 35° clockwise from the trend of the Sierra Nevada Frontal Fault System,

55 55 however the cross section is roughly perpendicular to the faults it crosses (Plate 4). The total vertical separation between Boone Canyon and the highest Table Mountain Formation Upper Member outcrop is ~3500 meters (Plate 4). This estimated vertical separation is huge compared to other estimates, though it still falls between the estimates farther north (600-1500 m) and estimates farther south (>2200-5500 m). Uplift can only be related to fault displacement and vertical separation if there is an elevation datum to attach the hanging wall or footwall to. In this case, we cannot determine uplift from vertical separation because we do not know what elevation the outcrops were at initially or if both the hanging wall and footwall blocks were moving relative to sea-level. If we could establish that say the eastern fault block was stable relative to sea level, then we could attribute vertical separation to uplift plus erosion.

Vertical-Axis Rotation Carlson et al. (2013) identified several rotational domains in the central eastern Sierra Nevada and our sites are within the Buckeye domain (figure 15). Our three additional sites west and south of previous data demonstrate that there has been 16.9° of rotation as marked by the Eureka Valley Tuff in the Buckeye domain (Table 5). These numbers agree with Carlson et al. (2013) who described 18° of clockwise rotation in the Buckeye domain and the neighboring Frontal Fault domain separated by an inferred boundary. The additional data provides no justification for keeping the inferred boundary between the Buckeye domain and the Frontal Fault domain, which we merged into a single domain (Figure 15).

56 56

Figure 15: Rotation Map. Vertical-axis rotation magnitude depicted as arrows referenced from geographic north. Wedges bounding rotation arrows represent statistical error (α95) for each rotation calculation. Dashed lines and names are rotation domains modified from Carlson et al. (2013). Sites not included in Table 4 of this study can be found in King et al. (2007), Pluhar et al. (2009), Jones (2014), and Carlson et al. (2013).

57 57

These rotated sites are within the part of the cross section that has experienced significant vertical offset. It is likely that any locality east of the Sierra Nevada crest has experienced some amount of vertical-axis rotation.

Kinematics For the lava to have erupted from the Little Walker Caldera and flowed across Priest’s Measured Section down a paleochannel to Rancheria Mountain, there would have had to be a continuous down slope between the two sites. Presently, the Sierra Nevada drainge divide is part way between these sites causing the flow path to be uphill near Priest’s Measured Section. For the lava to travel this path and distance, in the past the drainage divide must have been east of Priest’s Measured Section. The postulated collapse of the Nevadaplano and subsequent crustal blocks calving off the microplate could have resulted in the eastern sierra losing elevation relative to the western slope of the Sierra. This process must have occurred after emplacement of the Table Mountain Formation Upper Member at 9.52±0.03 Ma (Figures 6, 11). The paleomagnetic results and implications of those results reveal the kinematics of a shifting drainage divide. Eureka Valley Tuff sites have experienced about 15° of clockwise rotation. Many north trending faults cut Table Mountain Formation outcrops with a significant amount of vertical separation. Previous research has indicated that Walker Lane slip is accommodated on normal faults and strike-slip faults but not oblique faults. Busby et al. (2013) reported some field evidence for oblique slip on faults in the central Sierra Nevada but north of this study area. This in combination with our paleomagnetism data indicate that oblique slip could accommodate the vertical axis rotation.

58 58

Some research suggests the Walker Lane is stepping west over time (Dilles and Gans, 1995; Surpless et al., 2002). Rotation Magnitude at sites along this cross-section decrease to the west indicating that the deformation is progressive in nature affecting sites in the east for a longer time than sites in the west. Faulting is concentrated east of the present Sierra Nevada crest indicating that brittle deformation does not extend any farther west than that. Presently the western margin of the Walker Lane in this area is west of Priest’s Measured Section near the northeast end of Flatiron Ridge.

CONCLUSIONS

This work demonstrates that lavas at Rancheria Mountain are Table Mountain Formation Upper Member and correlative to lavas at Priest’s Measured Section. Table Mountain Formation lava flows are thickest, and all members are present around Priest’s Measured Section signifying that it is proximal to the source. New 40Ar/39Ar geochronology yielded ages of 9.52±0.03 Ma for Upper Member and 10.38±0.03 Ma for Lower Member Table Mountain Formation at Priest’s Measured Section (Figure 6, Table 3). These lavas filled the paleodrainage network. The original distribution would have consisted of many fingers of lava tracking the paleodrainage network (figure 2). We assume that a paleochannel connected Priest’s Measured Section to Rancheria Mountain (Figure 2, 14). This paleochannel shows that the site east of the current Sierra Nevada drainage divide must have been at a higher elevation at the time of eruption to facilitate lava flow. A topographic profile of the paleochannel reveals that the gradient of the channel is currently about 29 m/km (Figure 14), which is comparable to other estimates of the gradient of the Miocene Tuolumne River at 20.8 m/km (Huber, 1990) and the Miocene San Joaquin River at 24.3 m/km (Huber, 1981). We conclude from this that the drainage divide must have migrated west over time. We used the Table Mountain Formation lava flows with the mapped faults in the region to measure vertical separation across a cross section. From the Sierra Crest to Boone Canyon there is a total of ~3500 meters of vertical separation (Plate 3, 4). From the source area near Priest’s Measured Section west to the Sierra crest a minimum of 275 meters of vertical separation has occurred since 60 60

Stanislaus Group emplacement. East of Priest’s Measured Section to Boone Canyon we estimate about 3200 meters of vertical separation. Previous studies demonstrated that vertical-axis rotation was partially accommodating deformation at the margin of the microplate. The Eureka Valley Tuff provides a marker to measure any deformation accommodated as vertical- axis rotation. So, we compared Eureka Valley Tuff mean directions to the established reference direction from the stable Sierra Nevada and determined that Eureka Valley Tuff experienced about 15° of clockwise rotation in the Buckeye block of Carlson et al. (2013) (Figure 15). Evidence for both vertical separation and vertical-axis rotation in the area indicates that contrary to previous studies, deformation is accommodated through oblique slip on the faults in the region. This deformation accommodated the shift west over time of the edge of the Walker Lane Belt. This shift westward could be part of the process of the collapse of the Nevadaplano. Future work could investigate areas identified as Andesite of Walker River to explore to better identify the maximum extent of the Stanislaus Group. Assuming that the vast areas mapped in the Matterhorn Peak Quadrangle are indeed undivided Stanislaus Group, they could be used to further the understanding of how far west vertical-axis rotation is accommodating deformation. They could also be used to determine if there is a sharp break between the Sierra Nevada microplate and Walker Lane deformation, or if the division is more gradational.

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APPENDICES

APPENDIX A: PLATES 74

Plate 1: Stratigraphic column of Priest's Measured Section

Plate 2: Geologic map of the southeastern margin of the Little Walker Caldera.

Plate 3: Cross Section (a) Hetch Hetchy to Sierra Crest, (B) Sierra Crest to Priest’s Measured Section, (c) Priest’s Measured Section to Boone Canyon. See appendix for annotated sections with graphic measurement of offsets. (d) schematic demonstrating deformation by faulting and tilting and the resulting vertical offset. The schematic cross section is only a cartoon; elevations and distances along the profile are fictitious.

Plate 4: Annotated Cross Sections. Numbers above faults are meters of vertical separation on that fault. Green dashed lines are projections of the unit contact to the nearest fault.

APPENDIX B: PALEOMAGNETISM STEREONETS 79 79

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