University of Nevada Reno

!Late Quaternary deformation and seismic risk vl in the northern Sierra Nevada-Great Basin Boundary Zone near the Sweetwater Mountains, California and Nevada

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology

by

Garry Fallis Hayes W\

April 1985 i MINIS 1 LIBRARY

University of Nevada Reno

April 1985 ii

ABSTRACT Remote-sensing, seismic and field studies indi­ cate three major zones of Quaternary deformation near the Sweetwater Mountains. Holocene fault scarps are present in the Antelope, Little Ante­ lope, Smith and Bridgeport Valleys, and in the Sonora Basin. Two other vaguely defined zones, between Carson and Antelope valleys, and from the Bridgeport Valley east to Bald Mountain, may repre­ sent Mio-Pliocene zones of faulting which more recently have acted as conjugate shears releasing stress between fault basins in the Western Great Basin between the Sierra Nevada and Walker Lane shear zone. The northern portion of the Sierra Nevada-Great Basin Boundary Zone is less active than the south­ ern part in Owens Valley, as shown by lower slip rates, shorter fault lengths and lower levels of historical seismicity. Maximum Credible Earthquake magnitudes for the fault basins range from 6.3 to 7.2, with expected displacements of 3 meters or more. iii

ACKNOWLEDGEMENTS

The author would like to thank Dr. D.B. Slemmons, Craig DePolo and J.O. Davis for helpful discussions during the course of this study. Special thanks to Craig DePolo, Susan Hciyss and Ron Smith, who assisted with the field studies, and to Glenn Hayes who assisted with the manuscript preparation. The Nevada Bureau of Mines and Geology and the Reno office of Toiyabe National Forest kindly provided aerial photography, while U.R. Vetter and the University of Nevada Seismology Laboratory provided data and assistance on the seismology of the region. The Photogeology Laboratory at the University of Nevada, Reno, provided materials and eguipment. Special thanks are due to Naomi Sullwold of Santa Barbara City College, who assisted with many of the illustrations.

This study was supported by grants from Chevron Inc., the Graduate Student Association of University of Nevada, Reno, and the Sigma Xi Scholarship Foundation, and a teaching assistantship and loans from the Geology Department of the University of Nevada, Reno.

No thanks are offered to the thief who stole my thesis notes the night of Dec. 29, 1983, or the drivers of Upland, California, who gleefully continued to drive by as my thesis blew across Euclid Avenue on July 1, 1984. TABLE OF CONTENTS

INTRODUCTION...... x LOCATION AND EXTENT OF STUDY AREA...... INVESTIGATIVE PROCEDURE...... 3 seismic study...... !!!!!!!!!!!! 4 AERIAL PHOTOGRAPHS AND TOPOGRAPHIC MAPS 4 FIELD STUDIES...... ' 6 PREVIOUS INVESTIGATIONS...... !!!!!!!!!!!!!* 7 REGIONAL STRATIGRAPHY...... !!!!!!!!.".*! 9 PRE-TERTIARY STRATIGRAPHY.... ! !!!!!!."! ! 9 TERTIARY STRATIGRAPHY...... !!!!."! 10 QUATERNARY STRATIGRAPHY...... !!!!!!!!!!! 13 Glacial Stratigraphy...... 13 Grouse Meadows-McGee Glaciation... 18 Sherwin-Deep Creek Glaciation.... 19 Mono Basin and Tahoe Glaciations.. 20 Tenaya and Tioga Glaciations..... 22 Alluvial Stratigraphy...... 23 Pediment Deposits...... 23 Gravel Deposits...... 24 Alluvial Fan and Basinal Sediments 25 Landslide Deposits...... 28 TECTONIC HISTORY...... ’’ 29 PRE-QUATERNARY TECTONIC DEVELOPMENT.... 29 QUATERNARY TECTONIC HISTORY...... * 33 PRESENT TECTONIC STRUCTURE...... 36 ANALYTICAL RESULTS...... 41 REGIONAL SEISMICITY...... ’ j * 41 PHOTO-ANALYSIS...... * " 49 FIELD STUDIES...... ' 53 Fault Scarp Morphology...... 53 Stratigraphic Age Constraints on Faulting...... 58 DISCUSSION OF RESULTS...... !.'.*![ 60 ZONATION OF FAULTING...... 60 SIERRA NEVADA-GREAT BASIN BOUNDARY ZONE. 64 Introduction...... 64 Antelope Valley Fault Zone...... 66 Slinkard Fault System...... 69 Antelope Valley-East Side...... 70 North Antelope Valley...... 71 Lost Cannon Creek/Walker River Gorge. 75 Sonora Basin/Mt. Emma Zone...... 77 West Bridgeport Valley and New Range. 78 WESTERN GREAT BASIN...... 80 Introduction...... 80 V

Smith Valley Fault Zone...... 82 Desert Creek Peak/Sweetwater Valley.. 84 The Sweetwater Trough...... 87 Bridgeport Valley Fault Zone...... 88 HOLOCENE FAULTS AND SEISMICITY...... 91 INTRODUCTION...... 91 ANTELOPE VALLEY...... 91 SONORA BASIN...... 96 SMITH VALLEY/DESERT CREEK PEAK ZONE.... 98 BRIDGEPORT VALLEY FAULT ZONE...... 101 SEISMIC RISK ANALYSIS...... 104 INTRODUCTION...... 104 SLIP RATES...... 104 RECURRENCE INTERVALS...... 107 MAXIMUM CREDIBLE EARTHQUAKES...... 109 References Cited...... Ill

List of Figures Figure 1: Index map of study area...... 2 Figure 2: Map of glacial deposits in the study area...... 15 Figure 3: a) Uplift at crest of central Sierra Nevada; b)Rate of uplift of crest of central Sierra Nevada...... 35 Figure 4: Map showing regional tectonic structures, CA-NV...... 3 8 Figure 5: Map showing distribution of earthquake epicenters, 1970-1983...... 43 Figure 6 : a) Focal plane solutions for selected quakes in the SNGBZ; b) Stress axes for focal mechanisms.... 44 Figure 7: Spatial-temporal variations of seismicity near the Sweetwater Mountains, CA-NV...... 48 Figure 8: Lineaments and lineament zones in the west half of the Walker Lake sheet...... 50 Figure 9: Diagram showing the principal features of a recently formed normal fault scarp...... 54 Figure 10: Limits of principal slope angle versus age of fault scarp...... 54 Figure 11: Diagram of buried fault scarp in the central part of the AVFZ... 56 Figure 12: Map showing major tectonic subdivisions and structural blocks within the study area...... 61 Figure 13: Tectonic map of study area showing distribution of faults, vol­ canic centers, alluvial basins, and possible zones of strike-slip faulting.. 63 vi Figure 14: Explanation of map symbols used in figures 15, 16 and 17...... 67 Figure 15: Generalized geologic map of surficial deposits in the Antelope Valley and vicinity...... 6g Figure 16: Geologic map of surficial deposits in the north and east parts of Antelope Valley...... Figure 17: Geologic map of surficial deposits in the Desert Creek Peak

List of Tables Table 1: Tertiary stratigraphic units and regional correlations, SNGBZ...... n Table 2: Surficial stratigraphic units, SNGBZ...... 14 Table 3: Proposed seguence and timing of glacial advances in the eastern Sierra Nevada...... 17 Table 4: Outline of the sequence, mag­ nitude, and distribution of Quatern­ ary tectonic deformation near the Sweetwater Mountains, CA-NV...... 37 Table 5: Focal plane solutions for selected earthquakes in the western Great Basin...... 45 Table 6 : Table showing location, height and principal slope of Holocene scarps in the study area...... 92 Table 7: Slip rates for faults in selected parts of the SNGBZ...... 105 Table 8: Postulated fault rupture lengths, maximum credible earth­ quakes, and recurrence intervals in selected parts of the SNGBZ...... 108 List of Plates Plate 1: Tectonic map of the study area showing distribution of faults, vol­ canic centers, alluvial basins, and possible zones of strike-slip motion...... in pocket

List of Appendices Appendix A: Seismology data...... 120 Appendix B: Scarp profiles of selected Holocene faults, and scarp height calculations 130 INTRODUCTION This study has been undertaken in an effort to define the tectonic framework of a portion of the Sierra Nevada- Great Basin Boundary Zone (SNGBZ) in and around the Sweet­ water Mountains of California and Nevada. The goals of this study were twofold: to locate and define the tectonic domains in the region as indicated by faults, erosion surfaces and stratigraphy; and to place age constraints on the most recent tectonic activity in the region, for the purpose of develop­ ing a seismic risk analysis.

LOCATION AND EXTENT OF THE STUDY AREA

The Sweetwater Mountains of California and Nevada lie just east of the Sierra Nevada Range north of the town of

Bridgeport in Mono County. The range is a large tilted fault block which is bounded by five structurally distinct basins or valleys (fig. 1, plate 1). These basins include Antelope, Smith, Sweetwater and Bridgeport Valleys and the basin con­ taining Sonora Junction. The region is drained by the East with with major range front faults indicated by heavier of of the Sierra Nevada-Great Zone Boundary Basin lines, and basins alluvial shown by lined pattern

-LAKE TAHOE- Figure 1: parts adjacent and area study the showing map Index 3

and West forks of the Walker River and Desert Creek. Eleva­ tions range from about 1,600 m (5,000 ft) in the lower valley floors to about 3,600 m (11,700 ft) in the highest part of the Sweetwater Range. The climate ranges from semi-arid in the valley areas to sub-alpine at the higher elevations.

Most of the region is dominated by Pinyon-Sage plant commun­ ities, with local areas of Ponderosa and Fir woodlands.

The region presently supports agriculture and tourism. Communities include Bridgeport (the seat of Mono County), Walker, Coleville, Topaz Lake, Smith and Wellington. The total population in the study area is about 3,500.

INVESTIGATIVE PROCEDURE

This study was conducted in four parts. A detailed library research project allowed the scope and intent of the project to be defined, as well as enabling a comparision of published literature with the results of the field obser­ vations. The literature research led to the proposal for a project to be pursued along three avenues of investigation: a seismic study, an analysis of topographic maps and aerial photographs, and field observations consisting of fault morphology measurements and analysis of features identified in the aerial photographs. 4

SEISMIC STUDY

The records of the University of Nevada Seismology Laboratory at Reno were searched to assess seismic activity in the study area during the years 1970-1983. The more than 200 quakes recorded by the Nevada Seismic Array provided valuable information concerning the distribution, depth and frequency of earthquakes and their relation to tectonic boundaries in the study area. Six focal mechanisms were calculated, providing regional stress data, and the spatial- temporal variations in seismicity were diagrammed to chart patterns of earthquake Occurrence. v

AERIAL PHOTOGRAPHS AND TOPOGRAPHIC MAPS

Topographic maps at a scale of 1:62,500 and 1:24,000 were used to determine the location and elevation of erosion surfaces, and as a preliminary guide to the location of major faults and other tectonic features.

Aerial photograph coverage of the study area was avail­ able at two scales. The Nevada Bureau of Mines and Geology made available the Army Mapping Service Series 145 photo­ graphs for most of the study area at a scale of about

1:62,500, while the Reno office of Toiyabe National Forest provided color aerial photographs at a scale of 1:15,000, which covered about 85% of the study area. The quality of the photographs ranged from good to excellent. A Nikon 700 Mirror Stereoscope was used to analyze the photographs. i i 5

The photographs were first studied at the 1:62,500 scale to confirm the location of faults and other features reported in earlier studies and to determine the presence of pre­ viously unmapped lineaments and related features. Pertinent data was traced onto a Mylar overlay and transferred to topo­ graphic maps. These photographs were useful for determining the location and nature of the major fault zones but were less helpful for locating minor faults and young scarps. The 1:15,000 scale photographs were used to locate smaller features such as young fault scarps in alluvium in the valley floors or along the major range front faults. Observable structural features were traced on Mylar overlays to be used as guides for the field work.

It was soon discovered during the field studies that many of the scarps found in the field could not be discerned in the aerial photographs despite the good quality of the pictures. Low-sun-angle photography, which would have increased the visibility of these features, was not available for this area. Conversely, many lineaments seen in the photos could not be seen in the field. Despite this problem, these lineaments were mapped and compared to the mapping of Rowan and Purdy (1984) for the purpose of locating lineament zones as a possible guide to tectonic activity. I i 6

FIELD STUDIES

Field work was completed between February 1984 and Feb­

ruary 1985, and included approximately 6 weeks in the field. The main emphasis of the field studies was to confirm the findings of the photo study and to determine the location and distribution of late Pleistocene to Holocene fault features. The methods used were those of Wallace (1977) and Bucknam and Anderson (1979), which assert that the age of a scarp could be determined from the its morphology. Measurements were made using a Brunton Compass and a stadia rod. Data were recorded in a field notebook and later transferred to a data sheet for use in organizing the report. The field deter­ minations of total offset and fault length could be used to approximate the size of the causative event using the regres­ sion relationships of Slemmons (1977, 1980).

To get the best possible analytical results, all major faults in the study area with possible offsets in alluvium were investigated, with special emphasis given to the south­ ern terminations of the Antelope and Smith Valleys, and to faults which appeared to diverge from the regional north- south pattern. Profiles were measured at 24 locations, and maximum slopes were recorded at other locations where profiles could not be measured due to thick vegetation or other complicating factors. 7

PREVIOUS INVESTIGATIONS

The region around the Sweetwater Mountains has been the site for a large number of studies. Pioneering reconnais­ sance studies include those by Halsey (1953) in the Sweet­ water Mountains area, by Curtis (1951) in the Markleeville- Topaz Lake area, and by Slemmons (1953) in the Sonora Pass region. More recently, Gilbert and Reynolds (1973) studied the late Tertiary stratigraphy and the tectonic history of the region immediately to the east of present study area. They postulated the presence of a tectonic boundary which divides the region into areas dominated either by faulting or warping. Proffett (1977) studied the seguence and style of faulting in the Yerington District on the northeast boundary of the area and described probable listric faulting and extreme extension during late Tertiary and Quaternary time. Hardyman (1978) described volcanic stratigraphy and Tertiary deformation in the portion of the Walker Lane immediately east of the study area, finding evidence of strike-slip and detachment faulting amounting to as much as 48 km of movement.

The West Walker River region was briefly described in the pioneering report on the Sierra Nevada glacial sequence by Blackwelder (1931). More recently, Clark (1967) and Sharp (1972) describe the glacial deposits and related tectonic activity in Quaternary time. 8

The Walker Lake 1x2 degree Quadrangle was the scene for an intensive mineral investigation by the U.S. Geologic Survey (Stewart et al., 1984). For this project, Dohrenwend (1982a, 1982b) mapped the distribution of surficial sediments and late Cenozoic faults, while Stewart et.al. (1982) mapped the regional bedrock geology. Rowan and Purdy (1984) mapped the regional lineament patterns. The surficial and bedrock geology at a scale of 1:62,500 is available for the Bridge­ port, Sonora Pass, Wellington, and Topaz Lake Quadrangles (Dohrenwend and Brem, 1982; Guisso, 1980; Stewart and Dohrenwend, 1984; John, et al., 1980). The central part of the Sweetwater Range was mapped at a scale of 1:62,500 by Brem (1984), and portions of the Bridgeport Valley peripheral to the study area were mapped by Chesterman (1975) and Chesterman and Gray (1975). An excellent bibliography of geologic studies in the region is given in Stewart et al. (1984). Bryant (1983, 1984) outlined the distribution of Holocene faults in the Antelope and Bridgeport Valleys.

VanWormer and Ryall (1980) and Vetter and Ryall (1982) briefly review seismic and geophysical data pertaining to the present study area. Kleimpahl et al.(1975) discuss the geo­ physical characteristics of the Bodie Hills, adjoining the area to the southeast. 9

REGIONAL STRATIGRAPHY

PRE-TERTIARY STRATIGRAPHY

Pre-Mesozoic metamorphic rocks in the study area are exposed as roof pendants in the Mesozoic Sierra Nevada gran­ itic batholith. These rocks include slate, argillite, horn- fels, metasiltstone, quartzite, limestone and metaconglomer­ ate of the Upper Paleozoic in the Bridgeport area (Chester- man, 1975; Brook et al., 1979), and a variety of early Meso­ zoic rocks, including the West Antelope Sequence of Schweick- ert (1976) consisting of tuffaceous siltstone and sandstone interbedded with conglomerate, limestone and tuff. Noble (1962) and Halsey (1953) described the early Mesozoic strati­ graphy of the Pine Nut and Sweetwater Ranges, noting the presence of rhyolite to dacite lapilli tuff and tuff breccia, andesite flows, calcareous tuff, and lime'stones, silts and sandstones. The metavolcanic rocks seem to be slightly more abundant than the metasedimentary rocks (Moore, 1969).

The metamorphic rocks are sparsely distributed across the central part of the Sweetwater Range in a roughly east- west trending belt, and along the western side of the Ante­ lope Valley. Other sequences are exposed in the Wellington Hills and southern Pine Nut Range, and as a major belt in the eastern Sierra Nevada southwest of Bridgeport Valley. Expo­ sures are limited in many parts of the study area by a thick j i j 10

covering of Tertiary volcanic rocks and Pleistocene glacial till.

Granitic rocks of the Sierra Nevada Batholith underlie large portions of the study area as exposed bedrock or be­ neath erosion surfaces onto which Tertiary volcanic rocks have been deposited. These have been described by Schweick- ert (1976) and John (1982), and include granites, diorites, gabbros and granodiorites which range in age from Jurassic to Cretaceous, indicating multiple stages of intrusion.

TERTIARY STRATIGRAPHY

Tertiary volcanic and sedimentary rocks are widely distributed across the study area and provide a record of the stratigraphy and tectonic activity from about 30 my to about 5 my (Gilbert and Reynolds, 1973; Slemmons, 1966). The stratigraphic column and regional correlations are shown in Table 1.

Slemmons (1966) and Noble et al.(1974) describe the Tertiary stratigraphy of the central Sierra Nevada near the study area. In this region, the earliest Tertiary rocks are the rhyolite tuffs of the Valley Springs Formation which range in age from 21 to 29 my. The extrusion of the Valley Springs was followed by a period of andesite volcanism which resulted in the deposition of the mudflows and volcaniclastic sediments of the Relief Peak Formation (9.5-20 my), and by the late Miocene extrusion of the Stanislaus Group from a Sierra Nevada - .Sweetwater Sonora Pass Region Mountains Smith Valiev - Pine Grove Hills

Source: Source: Slemmons, 1966 Source: Brem, 1984 Noble, et a!.,1974 Gilbert and Reynolds, 1973

Markleeville Surface Markleeville Surface Pliocene Lewis Surface ?_ - Dlatomite Disaster Peak 7-- Younger Sed. Rx5.0my Formation 7-- Olivine Basalt Bald Mtn.Andesite 5my^ 5 - 9 m.y. Bush Mountain Rhyolite 6.5 my Trachyandeslte Basalt 6.5-7.0 my Dardanelle Fm, 9.3 m.y. Morgan Ranch Fm. Eureka Valley 7.0 m.y. Miocene Tuff 9-10 m.y. Table Mtn. Latite ------7 Coal Valley Fm. 9.0 m.y. ?------10.3 m.y. A'drlch Station Fm. Relief Peak Formation 1 1.5 - 12.0 m.y. Andesitic Rocks 9.5 - 20 m.y. Older Andesites 15 m.y.

Sed. Breccias Valley Springs Fm, and Cgis. Oligocene Rhyolite Tuff 25 - 29 m.y. Valley Springs Rhyolite Ignlmbrite Formation 23 - 29 m.y. ‘ .'.Igneous and'. * • Y Igneous and /, Y ' • Metamorphic \Y/ Igneous and' Pre-Tertiary Metamorphic ' '/, Metamorp/ifc •/%'/ •' Basement Rocks'.' • Basement Rocks ;• Basement Rocks ;

Table 1: Tertiary Stratigraphic Units and Regional Correlations, Sierra Nevada/Great Basin Boundary Zone { i1 12

caldera complex in the Sonora Junction area. This group included latite tuffs and ash flows which were extruded dis­ continuous^ over the southern part of the study area.

Halsey (1953) and Brem (1984) correlated some of the units m the Sweetwater Mountains to the Sierra Nevada, but described older andesites in the Sweetwater Summit and Mt. Emma areas as well as rhyolite units in the central Sweet­ water Mountains. The Desert Creek Peak Andesites were ex­ truded near the south edge of the Miocene basin, forming the bulk of the Wellington Hills and Desert Creek Peak, which reportedly was the eruptive center. The unit is overlain by the Stanislaus Group.

The region immediately to the east of the study area was described by Gilbert and Reynolds (1973). From 22 to 18 my the region from Smith Valley east to the Wassuk Range was a highland from which ignimbrite flows were being eroded. Subsequent andesite volcanism blanketed the region with flows

and breccias. By 9 or 8 my the entire region had developed into a single integrated basin of sedimentation into which some 2,500 m of strata accumulated, including the Aldrich Station, Coal Valley, and Morgan Ranch Formations. These units included fluvial and lacustrine sediments as well as tuffs and tuff breccias. A long period of tectonic quies­ cence lasting from 7.5 to 3-4 my ensued, and extensive ero­ sion surfaces developed. Local eruptions dacites to basalts between 5 and 7 my covered portions of these surfaces. r w a m m m m m m 1

I 13

QUATERNARY STRATIGRAPHY

The Quaternary stratigraphy of the study area can be broadly divided into four groups: glacial tills, pediment and gravel deposits, alluvial and basinal deposits, and land- slides. For the most part, the terminology of Dohrenwend (1982a) has been adopted for this study. Table 2 outlines the surficial stratigraphic units for this region.

Glacial Stratigraphy Glacial tills and moraines are especially useful in defining and placing time constraints on tectonic activity during Quaternary time. Ice flows at several times com­ pletely filled Sonora Junction Basin and extended well into Bridgeport Valley. Lobes of ice filled West Walker Canyon as far north as Antelope Valley and small glaciers formed in the highest parts of the Sweetwater Range (Fig. 2). The defin­ itive works in this region include those of Clark (1967) in the West Walker River Basin, and Sharp (1972) in the Bridge­ port Valley region.

Blackwelder (1931) proposed a four-fold glacial sequence in the eastern Sierra Nevada which included the Tioga, Tahoe, Sherwin and McGee glaciations in order of increasing age. Later studies resulted in the addition of other advances, including the Tenaya, and at least three pre-Tahoe advances, the Mono Basin, the Casa Diablo, and the TABLE 2: Surflclal stratigraphic units In the northern part of the Sierra Nevada-Great Basin Boundary Zone. After Oohrenwend (1982a)

Time Glacial Deposit s Alluvial Fan/Lacustrine Pedlments/Gravels Landslides

Matthes -Y°un9 Alluvial Fan deposit Holocene Recess Peak Qls- recent Hllgard (?) slides Tioga Late 0 ^3 _ 2 ' foung and Intermediate alluvial fan deposits Tenaya Qly - Younger lake sediments Qls- older si Ides Tahoe Qfg- Intermediate alluvial fan deposits gio - Older lake sediments

Middle Mono Basin (?) Pleistocene Qp2-Intermed1ate Pediment gravels

Sherwln Qf2_i * Intermediate to old alluvial fan deposits

QP2 _j-Intermediate Early and older Pleistocene pediment McGee gravels

QPj - Older pediment Pllo-Plelstocene gravels Qtg - Older terrace gravels

i ij 16

Deadman Pass (Sharp and Birman, 1963; Curry, 1971). Recent studies suggest that the Sierra Nevada region has been sub­ jected to repeated glaciations during the last 1.5 my (Dube1, 1984; Gillespie, 1982). The proposed sequence and timing of glaciations in the eastern Sierra region is shown in table 3. Burke and Birkeland (1979) have questioned the desig­ nation of a number of the stages, i.e. the Tenaya, Mono Basin and Casa Diablo, since they could not discern any major dif­ ferences between these tills and those of the Tioga or Tahoe. Huber (1981) argued that the Sierra Nevada was not high enough to support glaciation at 2.6 my, and believes the Deadman Pass deposits are non-glacial in origin.

Seven distinct glacial episodes have been postulated in the drainage basins of the forks of the Walker River (Sharp, 1972; Clark, 1967), including the Tioga, Tenaya, Tahoe, Mono Basin, a pre-Mono post-Sherwin, the Sherwin (?)-Deep Creek, and the Grouse Meadows-McGee. Of these, the Tioga, Tahoe, Sherwin and McGee have sufficent distribution and exposure to be helpful in tectonic analysis of the region. Table 3: Proposed sequence and timing of glacial advances in the Eastern Sierra Nevada

Estimated Claclal Stage .Minimum Age Associated Absolute Reference Age Daces, BP

Matches 700 - 1,100 Birman, 1964 Recess Peak 2,000 - 2,600 Birman, 1964 9,900 * 800 Birkeland ec al 1971 Hilgard 11,000 Dube', 1984 Tioga 18,000 - 20,000 Dube', 1984 Tenaya 40,000 - 45,000 Gillespie, 1982 Tahoe 65,000 - 75,000 Dube', 1984 53,000 * 44,000 Dalrymple ec al, 1982 119.000 i 7,000 Gillespie, 1982 Casa Diablo* Curry, 1966 129.000 ± 26,000 Bailey et al, 1976 Mono Basin 125,000 Sharp, 1972 725,000 i 15,000 Dalrymple, 1980 Sherwin** 750,000 - 820,000 Dube', 1984 McCee*** l.600,000 Huber, 1981 2.700.000 Huber, 1981 Deadman Pass* Huber, 1981 3.200.000 Huber. 1981 a ■ a a a a a mmmmm ■ I « ■■■■■ ■■■aa aaaaa mmmmm mmmmm

•These glaciations have been disputed **May include Sherwin(?) of Sharp (1972), Donner Lake of Birkeland (1964) and Deep Creek of Clark (1967) ***May correlate to Grouse Meadows of Clark (1967) ( ji 13

Grouse Meadows-McGee Glaciation

The earliest advance of ice in the region was termed the Grouse Meadows glaciation by Clark (1967) and was correlated to the McGee stage by Halsey (1953) and Black- welder (1931). The only remaining physical evidence of this stage is large erratic boulders perched on the rim of the West Walker River gorge which define a plane extending to 2,100 m at the head of Antelope Valley south of Walker, and a possible till body above Deep Creek. The Grouse Meadows glaciation was the largest Pleistocene advance, having formed a pool of ice 130 sg km in extent at Sonora Junction, extend­ ing outward in four lobes, the longest of which reached Ante­ lope Valley some 22 km to the north.

Although the Grouse Meadows glaciation cannot be specifically correlated to the McGee or other pre-Sherwin glaciations, it does have characteristics which suggest that it is at least as old as the McGee. In both areas, hundreds of meters of deformation have taken place since the advance, and tills have been eroded or weathered to the extent that only erratics remain. The West Walker River has eroded a gorge 700 m deep since Grouse Meadows time.

The absolute age of the Grouse Meadows glaciation cannot be constrained, but the possibly age-related McGee tills lie on a basalt dated at 2.7 my (Huber, 1981). The deep-sea paleotemperature record indicates two cold cycles ] 19

centered at around 2.5 and 1.5 my (Beard, 1959), but Huber

(1981) favors the correlation of the McGee glaciation to the 1.5 my cycle on the basis of Sierran uplift rates which indicate that the Sierras would not have been high enough to

support glaciation at 2.5 my.

Sherwin-Deep Creek Glaciation

The most extensively exposed pre-Tahoe glacial advance in the study area was termed the Deep Creek glaciation by Clark (1957) and Sherwin (?) glaciation by Sharp (1972). Although the deposits are very similar to those of the type Sherwin locality near Rock Creek, the 55 km distance between the two areas and the lack of continuous exposures argue caution in assigning them to the Sherwin glaciation; however a correlation is tentatively assumed in this study. These tills and associated gravels provide an excellent datum for timing the rate and magnitude of tectonic deformation.

Extensive tills cover the former piedmont which extended from Conway Summit to Green and Robinson Creeks. These tills range in thickness up to 200 m and are exten­ sively weathered and dissected by stream erosion. They consist of boulder and cobble gravels with varying amounts of clays and soil-bearing profiles of varying degrees of development, and only locally is the constructional surface preserved. In the drainage of the West Walker River, the 20

tills are scattered on wide benches and terraces above Deep Creek in the Walker Gorge, and as scattered remnants high above Little Walker and Molybdenite Creeks. Possible patches of Sherwin till occur in Mill and Lost Cannon Creeks adjacent to Antelope and Little Antelope Valleys.

The age of the Sherwin glaciation is based upon strati­ graphic relationships at Rock Creek, where till is overlain by the Bishop Tuff which has been dated at about 710,000 years (Dalrymple, 1980; Dube', 1984). Dube' (1984) places the upper age limit of the Sherwin at either 750,000 or 820,000 yrs.

Mono Basin and Tahoe Glaciations

Mono Basin tills were described in both basins of the Walker River by Clark (1967) and Sharp (1972). The tills had very limited exposure, having been for the most part over­ whelmed by the subsequent, and larger Tahoe glaciation.

Burke and Birkeland (1979) believe that the Mono Basin tills represent an early stade of the Tahoe glaciation, but they did not study tills in the Walker River drainage area and the issue cannot be resolved in this study. The possibility exists that 'Mono Basin' tills may represent any number of pre-Tahoe post-Sherwin advances (Dube', 1984; Gillispie, 1982).

The tills of the Tahoe glaciation are very well 21

exposed in both the West Walker and Bridgeport Basins.

Although less extensive than the Sherwin glaciation, lobes of ice did extend into the Bridgeport basin, coalescing into a piedmont lobe at Robinson and Buckeye Creeks, while ice flows coalesced at Sonora Junction and flowed down the Walker River Gorge as far as Chris Flat (Clark, 1967). According to Burke and Birkeland (1979), moraines and tills of Tahoe age are well preserved with only minimum amounts of weathering and dissection. Weakly developed soils may be present and sub­ surface granite clasts are usually grusified and oxidized. On the moraine surface, 50% or more of the boulders are weathered and more than 50% of the granite boulders are pitted to depths of 25 mm, while mafic inclusions are wea­ thered to a relief of more than 100 mm. The moraines, while well-preserved, are generally more broad crested and more voluminous than those of the Tioga advance.

The composition of the clasts and the close association of the moraines to the present topography indicate that the Tahoe glaciation is much younger than the Sherwin. Dalrym- ple, Burke and Birkeland (1982) determined a K-Ar age date on a post-Tahoe(?) basalt of 53,000+44,000 years. Gillespie (1982), using Ar40/Ar39 methods redated the same basalt at

119,000+7,000 years, regarding it to be pre-Tahoe. Burke and

Birkeland (1979) and Dube' (1984) pi ace the Tahoe in the interval of 65,000 to 75,000 years. 22

Tenaya and Tioga Glaciations

The Tenaya glaciation is recognized as a post-Tahoe pre-Tioga glacial stage by Sharp and Birman (1963), Clark (1967) and Gillespie (1982), but Burke and Birkeland (1979) regard it to be either an early advance of the Tioga or a late surge of the Tahoe. Tenaya deposits have been mapped in the study area, but they do not provide any additional tec­ tonic data. Gillespie (1982) dates the advance at 40,000 to 45,000 years.

The Tioga glaciation was the last major.advance to occur in the study area, with the exception of minor Holocene glaciers which generally did not extend beyond their cirques (Birman, 1964; Burke and Birkeland, 1979). Tioga moraines are smaller but sharper and fresher appearing than those of the Tahoe. In general, less than 30% of the granite boulders are weathered, less than 50% of the boulders are pitted, and mafic inclusions are weathered in relief to less than 50 mm. Subsurface clasts are largely unweathered and granite cobbles are ungrusified (Burke and Birkeland, 1979).

The age of the Tioga glaciation is bracketed by the Tahoe (65,000 years) and and a post-Tioga peat near South Lake Tahoe which gave a carbon-14 date of 9,800±800 years (Adam, 1967). Shackleton and Opdyke (1973) indicate a cold interval from 13,000 to 32,000 years, while Dube1 (1984) brackets the advance between 18,000 and 20,000 years. 23

Alluvial Stratigraphy The surfxcial geology and the sequence of alluvial units in the study area were studied by Dohrenwend (1982a), and include pediment deposits, gravels, alluvial fan and basinal sediments, and landslide deposits.

Pediment Deposits

The pediment deposits in the study area consist of relatively thin layers of alluvium deposited on surfaces eroded into bedrock. The units are distinguished from one another on the basis of the degree of drainage development, dissection, and soil formation. They occur along the tilted west flanks of the Wellington Hills, the Pine Grove Hills and Desert Creek Peak, and as scattered remnants in Little Antelope Valley. The eroded bedrock generally consists of Tertiary ash flow tuffs and slightly indurated sedimentary rocks.

Pediments are associated with the stable portion of tilted blocks, and are not usually found along tectonically active mountain fronts. The pediments in the Wellington Hills are associated with tilting of the range, and are cut by antithetic faults which dominate the westward portion of the mountain mass.

It is difficult to provide age constraints on the timing of pediment cutting in the area, but they postdate the Pliocene sediments into which they are cut, and predate 24

dissection by active stream channels to a depth of 20 to 30 m and the development of antithetic faulting. They have been removed from the present depositional regime by erosion and tectonic activity. Dohrenwend (1982a) places the formation of the pediments in the early or middle Pleistocene.

The oldest of the pediment deposits mapped in the study area include poorly to moderately sorted boulder to pebble gravels, sandy gravels and gravelly sands with varying amounts of silt and clay which lie on top of the erosonal surfaces cut along the flanks of the Pine Grove Hills and along the East Walker River. This surface was termed the

Lewis Surface by Axelrod (1956), and formed between 7.5 and 4 my ago in response to the cessation of tectonic activity in that region (Gilbert and Reynolds, 1973). The surface predates the formation of the deep tectonic basins in the present study area, but in some places has been essentially unaffected by subsequent activity, except for broad upwarping which has raised the surface as much as 250 m above the lower East Walker River. In the Bridgeport area, the pediment has been dissected to a depth of 60 m (Dohrenwend and Brem, 1982).

Gravel Deposits

Plio-Pleistocene deposits of alluvial gravels and sands cover large areas of the uplifted block of the Sweetwater Mountains at elevations ranging from 1,700 to 3,200 m. 25

Portions of the constructional surface are preserved in ter­ race deposits, consisting of cobble to pebble-sized gravel, sandy gravel, and gravelly sand in a loosely indurated

matrix. At the south end of Sweetwater Valley, these depos­ its are deeply dissected along possibly fault-related sub­ parallel drainages and are unconformably overlain by alluvial fan deposits which lie nearly parallel to the constructional surfaces (Dohrenwend, 1982a). The deposits have been faulted hundreds of meters from their original position.

The clast lithologies indicate that these deposits were derived in part from the Sweetwater Range, as. well as from sources to the west in the Sierra Nevada (Brem, 1984). This suggests eastward transport of sediments across the area prior to the development of the late Quaternary tectonic basins, and uplift of the Sierra block prior to the Sweet­ water block.

Alluvial Fan and Basinal Sediments

Alluvial fan deposits are divided into three categories, young, intermediate and old, on the basis of the degree of dissection of the fan surface, drainage development, total relief and soil development (Dohrenwend, 1982a). Where the units are complexly interrelated they have been mapped as young-intermediate or intermediate-old. Except for the oldest units, the alluvial sediments are still associated with present-day drainage channels and basins. The older units have been removed from active stream deposition by subsequent faulting and uplift.

The youngest alluvial fan deposits consist of poorly sorted boulders, cobbles, pebbles, sand, and silt. The average clast size decreases down-fan towards distal margins as sorting improves. The fan surfaces are undissected to slightly dissected with few, if any, well defined drainage channels. The drainage is distributary in nature. Soil development is very weak and little evidence of weathering is present. Young and young-intermediate fan deposits are by far the most widespread and best exposed surficial unit in the study area and are especially prominent along tectonic­ ally active range fronts.

Intermediate alluvial fan deposits are distinguished by slight to moderate dissection and the presence of numerous well-defined drainage channels, with several channels which head on the fan surface rather than at the fan apex. Relief may be as high as 5 m. The constructional surfaces are slightly to moderately weathered with weak to moderate soil development and formation of desert pavement. The unit usually is removed from active fan deposition by regional

uplift or faulting, and remnants of these surfaces are often bounded by scarps or erosional terraces. The intermediate fan deposits are discontinuously present at the south end of Antelope Valley, in Little Antelope Valley, and along the southwestern margin of Smith Valley. The unit is best ex- 27

posed in the northern half of Sweetwater Valley where it is preserved as a huge fan remnant which has been dissected by Holocene stream channels to a depth of 10-20 m.

The oldest alluvial fan deposits are indicated by deep channels which head on the fan itself, and by surfaces which are strongly weathered with moderately to very well developed soils and extensive development of desert pavement. In

general, only small remnants are preserved as fault terraces adjacent to major range fronts in Sweetwater Valley and near Desert Creek Peak, and as a fairly large erosion or fault terrace in the south part of Smith Valley. Small remnants are also preserved at the south end of Antelope Valley.

The age of the alluvial fan deposits cannot be deter­ mined with confidence. The young deposits are the only unit found m areas glaciated during Tioga time, and are assumed to be Holocene in age. The older deposits range in age from lower to upper Pleistocene.

Basinal sediments generally consist of sand, silt and clay beyond the distal ends of alluvial fans. They include floodplain deposits and undifferentiated eolian sand. Most of these deposits are Holocene in age.

Lacustrine deposits consist of silt and clay deposited in the Artesia Lake area by Pleistocene Lake Wellington. The two distinct highstand shorelines at 1,460 m and 1,510 m

(Dohrenwend, 1982a) presumably correspond to the Tahoe and Tioga Glaciations. 28

Landslide Deposits

Landslides m the study area consist of chaotic masses of unsorted angular boulder to clay sized debris which form hummocky lobate masses in moderate to steep terrain. Most of the landslides occur in Tertiary volcanic rocks.

The slides are generally associated with active tectonic mountain fronts and are especially widespread along the east and south margins of the Sweetwater Mountain block. Large slides obscure important fault terminations in Slinkard and Smith Valleys. Slides are also common in the. uplands north- vest of Bridgeport, where they are associated with uplift and oversteepening along the diffuse zone of northeast trending faults which parallels the Bridgeport Valley. 29

TECTONIC HISTORY

PRE-QUATERNARY TECTONIC DEVELOPMENT

Little direct evidence exists concerning the Paleozoic tectonic history of the northern part of the Sierra Nevada-

Great Basin Boundary Zone since no rocks older than Triassic are present except near Bridgeport. Fortunately, much of the Paleozoic tectonic history can be inferred due to the loca­ tion of the study area between major tectono-stratigraphic terranes in the northern Sierra Nevada and central Nevada which have been provisionally correlated. The Pre-Mesozoic model presented here is taken mostly from Schweickert and Snyder (1981) who postulated that the Paleozoic Sierra Nevada was the site of complex interactions between the continental margin and one or more volcanic island arcs. This activity had profound effects throughout the western Cordillera.

Beginning about 850 my ago, continental rifting resulted in the development of a miogeoclinal margin along the western part of the North American Craton (Stewart, 1976). The ocean basin which opened during the rifting was subducted westward along what Schweickert (1981) believes is the Alexander Ter- rane which has since been displaced northward to Alaska. As the arc migrated towards the continent, a lower Paleozoic accretionary wedge (the Shoo Fly Complex) developed, to be followed by arc volcanism (now represented by upper Paleozoic metavolcanic rocks in the northern Sierra and Klamath Moun- 30

tains). An arc continent collision ensued and oceanic rocks were thrust eastward onto the continental margin in what has been termed the Antler Orogeny. The Roberts Mountain Alloch- thon m central Nevada is composed of the overthrusted rocks. The arc then reversed polarity and a new subduction zone formed along the margin of the continent. A marginal basin

formed along the inner edge as a result of back-arc spreading and the Calaveras and Havallah seguences were deposited.

During the late Paleozoic, a new arc-continent collision (the Sonoma Orogeny) occurred, during which the Calaveras- Havallah rocks and parts of the lower Paleozoic allochthon were again thrust to the east, above the Roberts Mountain

Allochthon. The Calaveras-Shoo Fly Thrust was active at this time.

During the Triassic, the Alexander Terrane was truncated and displaced northward. A subduction zone became active

along the scar of the truncation (the Foothills Suture Zone) and was involved in the evolution of Mesozoic structural events including the intrusion of the Sierra Nevada Batholith and the Late Jurassic Nevadan Orogeny, which was in turn responsible for the present structural grain of the Sierran metamorphic belt and the formation of the synclinorium- anticlinorium of the Sierra Nevada and White/Inyo Mountains. The emplacement of the granitic rocks occurred mainly between 140 to 80 my, although several intrusions predate the Nevadan Orogeny and range in age from Late Triassic to Early Jurassic (Bateman and Clark, 1974). Magmatism ceased abrupt- 31

ly at 80 my but compressional tectonism continued throughout late Cretaceous and early Tertiary time (80 to 40 my). The last pulse of compressional tectonism, the Laramide Orogeny, caused large basement uplifts far inland of the Sierran thrust belt, due to an anomalously shallow dip along the subducting slab (Zoback, Anderson and Taylor, 1981).

Compressional tectonism came to an end at about 40 my as a result of a transition from a strong to a weak coupling

between the Farallon and North American plates due to a slow­ down m the rate of their convergence (Engelbretson et al., 1984). The resulting lower compressive stresses allowed andesitic magmas to rise through the crust, and an early

stage of extensional tectonism took place throughout much of the Basin and Range Province in an episode of what has been described as convergence related intra-arc extension or back-arc spreading (Zoback, Anderson and Thompson, 1981).

The Farallon-Pacific Ridge, which had been part of the subducting slab, collided with the trench between 20 and 30 my ago, and strike-slip motion along the San Andreas Fault

was initiated. The fault increased in length as the triple junctions formed by the fault, trenches and ridges migrated north and south. Because there was no longer a subducting slab beneath the western margin of the continent the source of calc-alkaline volcanic melts was removed and bimodal basalt-rhyolite volcanism commenced. The change in volcan- ism occurred in the northern Great Basin at about 15-17 my (Stewart, 1979). The cold, dense subducting slab had been 32

preventing the uplift of the low density root of the sierra Nevada, but when subduction ceased, the rate of isostatic uplift increased (Hay, 1976).

The increase in the length of the San Andreas Fault had profound effects throughout the Sierra Nevada and Great Basin Provinces. Among the effects of the strike-slip faulting were a gradual change from andesitic to bimodal basalt/ rhyolite volcanism, and as the Mendicino Triple Junction

moved north, a change in the regional least principal stress from essentially perpendicular to the continental margin to an oblique angle about 45 degrees clockwise to it (Zoback, Anderson and Thompson, 1981). The tensional stresses result­ ed in the development of the present configuration of horst and graben in the northern Great Basin.

At about 10-12 my, a system of complementary strike-slip faults developed in parts of the western Great Basin, with the Walker Lane and Garlock Faults as the main components (Proffett, 1977; Wright, 1976). These systems define two

deformational fields which are both extensional in nature, but which differ in the magnitude and style of extension, due m part to the westward motion of the Sierra Nevada block and southward narrowing of the Great Basin (Wright, 1976). The early extensional stresses resulted in the development of large tectonic basins during the interval from 12.5 to 8 my in the region just to the east of the present Sierra Crest.

At about 4-5 my the San Andreas Fault broke inland of Baja California, forming the Gulf of California and trans- I i 33

ferring shear stresses entirely onto the continental litho­ sphere (Atwater, 1970). This, and an increase in the rate of motion between the plates resulted in a dramatic increase in the rate of uplift in the sierra Nevada region (Hay, 1976).

QUATERNARY TECTONIC HISTORY

The late Cenozoic tectonic activity of the study area has been the result of plate interactions which have been contin­ uous m the last 4-5 my, with extension of the length of the San Andreas Fault Zone and continued uplift of the Sierra

Nevada region. Volcanism in this interval has been mostly of the basalt-rhyolite type and has been confined mainly to the Sierra Nevada-Great Basin Boundary Zone and associated fault systems (Gilbert and Reynolds, 1973). The combination of regional uplift and eastward directed extension resulted in the formation of deep tectonic basins east of the present Sierra Crest between 2 and 4 my (Bachman, 1978; Larson, 1979; Gilbert and Reynolds, 1973).

The magnitude and rate of Sierran uplift has been the subject of a number of studies, the earliest of which were summarized in Christensen (1966). More recently, Slemmons et al.(1979), Larson (1979), and Huber (1981), described the rate of uplift m the Sonora-Carson Pass area, the Deadman-

Mammoth Pass area, and the southern Sierra Nevada respect­ ively. The results of some of these recent studies are shown in Figures 3a and 3b and indicate that while uplift has been i j< 34

continuing for 30 my or more, the rate of uplift has been increasing, with as much as a third to a half of the total amount of uplift occurring during the last 4-5 my. The

amount of uplift has been greater in the southern Sierra and generally less in the northern Sierra adjacent to the study area, perhaps amounting to as much as 2,000 m at Sonora Pass to 3,500 m at Deadman Pass to 4,000 m in the Kings River area.

Regional uplift has continued despite the development of the deep tectonic basins to the east of the Sierras. The basin floors are continuing to rise, but at a slower rate than the Sierra Nevada block (Slemmons et al., 1979).

V°lca-nism played a major part in the tectonic develop­ ment Oi the region prior to about 5 my, but only sparse volcanic activity has occurred since that time, except for Long Valley and Mono Basin just south of the study area where activity has been nearly continuous (Gilbert et al., 1968; Bailey, Dalrymple, and Lanphere, 1976). Early rhyolite ignimbrites and andesites buried much of the regional land­ scape prior to the inception of extensional faulting (Gilbert and Reynolds, 1973). Later volcanic activity tended to fill in lower areas and these deposits now provide some suggestion as to the distribution and extent of earlier basins. The

Little Walker Caldera (Noble et al., 1974), the Bald Mountain Complex (Gilbert and Reynolds, 1973), the Desert Creek Peak Complex (Halsey, 1953), and the Sweetwater Rhyolite Complex (Brem, 1982) were major volcanic centers in the region. 3 5

and in C a s s ^ L ^ S pass region are present as a ra®gT h ! U , ^ ”

l r:r'th “ r y average- s°— ; w s £

Figure 3b: Diagram showing the increase in the rate of uplift of the central Sierra Nevada. Curves are based on the graphic different- J.yol;^ ° no?fSlemmons the Upllftand others, CUrV6S 1979.±n figure 3a>! Source : Huber, j i 36

These complexes seem to be spatially related to major tectonic boundaries.

Formation of the present basins has been a relatively recent development, having occurred mostly within the last 3-4 my (Larson, 1979; Gilbert and Reynolds, 1973; Bachman,

1978). The timing, style, and sequence of basin development is outlined in Table 4 and is discussed in detail in the section on analytical results.

PRESENT TECTONIC STRUCTURE

The study area lies within deformational field II of Wright (1976) between the Walker Lane and the Sierra Nevada, which is characterized by a large amount of extension and a complementary system of strike-slip faults (northwest strik­ ing right-lateral and northeast striking left-lateral) which are major components of the structural framework (fig. 4). North or northeast striking normal faults are abundant.

The Walker Lane is a major right lateral strike-slip fault system which is sub-parallel to the San Andreas System. It extends from the Honey Lake area in Northern California southeast to at least the Tonopah area in western Nevada (Slemmons, 1980; Slemmons et al., 1979), and has experienced as much as 48 km of right lateral slip during the last 22 my

(Ekren et al., 1980). The presence of fresh scarps in late Pleistocene and Holocene sedimentary deposits indicate that the zone is still active (Bell and Slemmons, 1979; Anderson 3 7

h 2 “ f quence> ma8nitada. and distribution of Quaternary tectonic deform- AVF7 A neau S"ee5Water Mountains California and Nevada. Abbreviations used: WF 7 " ^aUvC Z°ne; BVFZ " BridgeP°rt Valley Fau11 Zone; SB - Sonora Basin; SVFZ - Smith Valley Fault Zone; SW - Sweetwater Valley.

Time Period Bridgeport Valley Sierra Nevada-Great Western Great Basin Basin Boundary Zone

Z m offset, valley floor 4-8 m offset, SB 5-8 m offset, SVFZ Holocene w/ minor tilting 3-6 m, AVFZ Possibly 3 m, SW 4 m, BVFZ Artesia Lake Level, 1380m Tioga Tributary canyons glacia­ Glaciation to Sonora Junct . Lake Wellington, 1,460 m Glaciation ted, outwash deposits on Possible glaciation at valley floor crest of Sweetwaters. Interglacial 15-30 m, offset, BVFZ 15-30 m offset, SB, SVFZ presumably active Landsliding, BVFZ AVFZ presumably active

Tahoe Glaciation reached valley Glaciation to Chris Flat, Lake Wellington, 1,510 m

Glaciation floor at Robinson and upper West Walker Canyon Sweetwaters glaciated Buckeye Creeks 200 m offset, BVFZ 150 m offset, south AVFZ Faulting and regional Interglacial Downfaulting of Hunewill Entrenchment of lower Mill uplift, entrenchment of Hills and Sierra frontal Creek, 3 m offset on east­ East Walker River. faults, drainage to Mono trending fault, Little Ant - Sweetwater range front Basin cut off. elope Valley; 30-40 m SB. faults become inactive (?) Downwarping and burial of Faulting and tilting in SW valley floor faults Sherwin tills in valley Slinkard Valley. Sherwin Glaciers filled south end Glaciers extended to Deep Unknown extent and dist­ Glaciation of valley i_ Creek in West Walker Gorge ribution of glaciers

Interglacial Presumed basinal downwarp 400 m offsets on east­ Uplift of Sweetwater Faulting along range- trending faults, s. Ante­ Range, distribution and front and BVFZ. lope Valley, 850 m on anti - extent of faulting un­ thetic faults, e. Antelope known. Valley, AVFZ active.

McGee Unknown distribution and Glaciers filled SB, Possibly unglaciated, extent of glaciers Glaciation reached south end of Ant­ gravels from Sierra glac­ elope Valley iers at Swauger Ck. 3 8

Figure 4: Map showing regional tectonic structures between the Great Basin, the Sierra Nevada, and the San Andreas Fault. Strike-slip faults are shown with darker lines, normal faults with hachured lines. Shaded area corresponds to Deformational Field II of Wright (1976) . Study area is outlined by the dotted line. Abbreviations: AV - Antelope Valley Fault Zone, CL - Carson Lineament, CV — Carson Valley Fault Zone, BV — Bridgeport Valley Fault Zone, G - Garlock Fault Zone, LV - Long Valley Caldera, MLE - Mono Lake-Excelsior Zone, ML - Mono Lake Fault Zone, OL - Olinghouse Fault Zone, OV - Owens Valley Fault Zone, SA - San Andreas Fault Zone, SN - Sierra Nevada Fault Zone, SV - Smith Valley Fault Zone, WL - Walker Lane. Source: Wright (1976) 39

and Hawkins, 1984). Movement has occurred in a broad zone 30 km wide along multiple fault strands.

The boundary zone between the Sierra Nevada and the Basin and Range Province extends for some 450 km from the left lateral Garlock Fault Zone in the Mojave Desert to the Honey Lake region in the northern Sierra Nevada, where it merges with the diffuse systems of right lateral faults of the Walker Lane and disappears beneath a thick cover of Qua­ ternary volcanic flows. The zone is well defined south of Mammoth with 3,500 to 7,000 m of offset along range-front faults in the Owens and China Lake Valleys (Gillespie, 1982), but in the northern Sierra the zone is diffuse and irreg­ ular , consisting mainly of a series of left—stepping en echelon fault basins. Total offsets in the northern zone range from about 1,700 m to 2,300 m. The en echelon pattern suggests a regional stress pattern of right lateral movement, but basin bounding faults generally display normal motion.

The region between the Walker Lane and the SNGBZ, here­ after termed the Western Great Basin, is traversed by a series of diffuse northeast trending zones of left lateral strike-slip faulting which include the Olinghouse Zone near Reno (Sanders and Slemmons, 1979), the Carson Lineament

(Slemmons, 1980), and the Mono-Excelsior Zone (Gilbert et al., 1968). A fourth zone was postulated by Gilbert and

Reynolds (1973) on the basis of the en echelon termination of fault basins in the region east of Topaz Lake in the study area. There are few faults aligned along the trend and no i j 40

lithologic or structural differences readily visible (Slem- mons, 1980), but the region south of the zone has been de­ scribed as aseismic (VanWormer and Ryall, 1980), suggesting that deformation in the region is primarily by warping rather than faulting.

A number of mountain ranges in the Western Great Basin are aligned along the zones of strike-slip faulting, but most trend to the north or northeast and are bounded by Quater­ nary normal faults. The ranges are generally shorter than those of the main Basin and Range province, and are separated by basins which are shorter and narrower than their eastern counterparts and which typically are terminated by east­

trending ranges or die out either within the Sierra block or along structural upwarps. The north trending ranges as a rule are tilted rather strongly to the west, whereas most Great Basin ranges show tilts in either direction (Stewart, 1979). The tilting and large amount of regional extension

are cited as evidence for listric faulting as an important type of deformation in the region (Proffett, 1977).

Horizontal fault planes associated with strike-slip fault zones, or detachment faults, have been described in the

Walker Lane area near the Gabbs and Gillis Ranges (Hardyman, 1978) and have been suggested as a possible reason for structural complications in the Carson Lineament and Mono- Excelsior zone (Slemmons, 1980). i » 41

ANALYTICAL RESULTS REGIONAL SEISMICITY

The seismic analysis presented in this study is based on the records of the Seismology Laboratory at the University of Nevada at Reno, for the years 1970 to 1983. This study up­

dates the work by VanWormer and Ryall (1980) and Vetter and Ryall (1983) for this part of the Sierra Nevada-Great Basin Boundary Zone.

The SNGBZ is a region of high seismicity, and has been the subject of a number of studies. Ryall (1977), Ryall and VanWormer (1980) and Slemmons (1980) discussed the seismic hazards and maximum credible earthguakes in the western Nev— ^sgion, while VanWormer and Ryall ( 1980) reviewed the anomalous patterns of earthquake occurrence and the relations between seismicity and the tectonic structure in the SNGBZ. They noted that the lack of seismic activity in and near the study area supported the contention of Gilbert and Reynolds

(1973) that deformation is taking place primarily as a result of warping rather than faulting. Vetter and Ryall (1983) noted a change in focal mechanism patterns with depth in the SNGBZ and western Nevada. They found that most quakes at depths of less than 10 km have strike-slip first motions while deeper quakes tend to have normal or oblique slip. Wallace (1980) discussed patterns of historical faulting and

'seismic gaps', which are zones of potential surface rupture. He identified the eastern Sierra region from Mammoth to Truckee as a possible gap. I 42

Figure 5 shows the location and magnitude of epicenters for all quakes in the years 1970-1983 as well as faults dis­

playing Holocene or late Pleistocene offsets. A large number of the epicenters lie within 4-5 km of active fault zones and appear to be related to movements along these zones. Some

areas containing recent faults, most notably Smith and Sweet­ water Valleys, have had little seismic activity during the study period. The 'zone of warping' postulated by Gilbert and Reynolds (1973) extends into the study area, but rela­

tively high levels of seismicity in the Bridgeport/Sweet- water area suggest that the non-seismic zone lies in and east of the Pine Grove Hills.

Focal mechanisms allow the interpretation of the style of faulting at depth. The lack of adequate station coverage and a paucity of large quakes made the determination of focal

planes within acceptable limits of precision difficult, but composite diagrams using data from spatially and temporally

related quakes made possible the determination of seven mech­ anisms which complement the five which have been previously published (Somerville, Peppin and VanWormer, 1980; Vetter and Ryall, 1983). The results are shown in Figure 6a and perti­ nent data are summarized in Table 5. It should be noted that

these solutions are based for the most part on unrelocated 'C' quality events, and that focal depths may vary as much as 100%. They are useful however for determining the style of faulting.

Normal slip is seen to be associated with the range-front 4 3

nm Figure 5: Map showing the distribution of earthquake epicenters for the years 1970-1983, and their relation to major zones of late Quaternary tectonic activity. Source of data: University of Nevada, Reno Seismology Laboratory. 4 4 120 39

Figure 6a: Focal plane solutions for selected quakes in the north part of the Sierra Nevada-Great Basin Boundary Zone and major zones of Late Cenozoic deformation. Data for source mechanisms given in Table 5. Table 5: Focal Mechanism solutions for selected earthquakes in the wes t e r n Great Basin near the Sweetwater Mountains. See figure 6 for illustrations.

Single Events: F o c a l Magni- No. Date Time, GMT Latitude L o n g i t u d e Depth tude Qu a l i t y Strike and Dip of Focal Planes Style

1 9/27/83 25637 3 8 ° 1 5 . 4 4 1 1 9 ° 2 1 .1 5 ' 7.7 8 3.8 C N 3 5 W 70° N 6 5 E 70° S S-Obl

2 5/9/82 1919 38.318° 119. 0 6 1 ° 9.7 1.9 B N 2 5 W 6 5 ° N 0 3 W 27° N

3 9/4/78 45231 38.796° 119.795° 10.01 4 . 6 B N 4 4 W 85° N 4 8 E 70° SS

4 12/8/80 1729 38.702° 119.452° 13.1 2.8 N 5 5 W 49° N 0 6 W 51° N - Obl

' * 5 10/11/79 52525 38°37.33' 119°l6.57' 1 3 .6 6 2.5 B N 0 9 W 50° N 0 9 N 40°

6 1 2 / 8/80 16 5 6 3 8 .692° 119.471° 1 5 .0 3.7 N 6 0 W 85° N 2 5 E 45° SS-Obl

7 5/13/82 1039 38.400° 119.348° 16.3 2.4 B N 4 3 W 60° N 4 8 E 90° SS

N - O b l 8 2/22/77 624 38°29.41' 119°16.77' 8. 9 9 4.9 C N 0 1 E 70° N 1 5 W 30°

Composite Evsits:

Style No. General Location No. of Events Q Depth Range , k m.** Strike and Dip of Focal Planes

9 Bridgeport Valley (1979 Sequence) 4 c /d les s than 10 N 3 5 W 80° N 5 6 E 80° SS

10 Bridgeport Valley (1979 Sequence) 7 C more than 10 N 2 6 W 50° n 60E> 85° Obi

11 North Antelope Valley 6 C .85-8.55 N 1 8 E 90° N 7 2 W 90° SS ,* 12 South Antelope Valley 3 c 3.2 -7.2 N 1 0 E 50° NIOtf 40°

SS-Obl 13 Bodie Hills 2 B 9.65-10.65 N 3 0 W 60° N 5 0 E 70°

*- Poorly constrained values. Sources: 1,5,8-13? present study? 3: S o m erville and others, 1980; fc.2,4,7: V e t t e r a n d fiyall, 1983. **- Depths have not been relocated. Quality C events may vary as much as 1 0 0 % and should not be construed as correct. m n a n s m umrmwmwnmm

46

fault systems in Antelope, Smith and Sweetwater Valleys,

while strike-slip or oblique movement is associated with the northern Antelope Valley and the northeast trending zone of faulting in the Bridgeport Valley and southern Sweetwater Mountains. These findings were among the lines of evidence which led to the inference that a zone of left-lateral

s i —s 1 iP movement exerts some control of the structure in the Bridgeport region, and that a diffuse zone of right lateral movement extends from the Carson Valley to northern Antelope Valley.

A comparision of the styles of first motion compared with the depth of the quakes generally does not conform to the findings of Vetter and Ryall (1983) that strike-slip motion dominates at depths of less than about 10 km, while normal or oblique slip dominates at greater depths (Table 5, #1-8). This relation was seen to occur, however, in the data of the Bridgeport Swarm of 1979, where motion became oblique at depths greater than 10 km (Table 5, #9 and 10).

Figure 6b shows the orientation of the stress axis for these events and indicates that the change in focal mechanism takes place along the P axis. The minimum compressional stress axes trend nearly east-west with plunges that are nearly horizontal, while the P axes trend north-south with plunges ranging from horizontal to vertical. This pattern is consistent with that which has been observed for the entire Sierra Nevada-Great Basin Boundary Zone exclusive of the

Mammoth Lakes area, which trends to the northwest (Vetter and i i 47

Ryall, 1983). The western Great Basin (east of the Walker Lane) has P axes which trend northeast. These results corre­ spond to the findings of Zoback, Anderson and Thompson (1981) and Wright (1976) regarding regional stress patterns.

Figure 7 shows the timing of seismicity in the study area as a function of latitude. The general pattern for the per­ iod from 1970 to 1977 is that of widely distributed epicent­ ers with indistinct clustering and lack of aftershock activ­ ity following larger quakes. The Bridgeport area was quies­ cent prior to 1974. Seismic activity dropped off precipit— iously in early 1977 and a period of quiescence ensued that lasted until the summer of 1978 when a series of swarms began to occur. These included the Diamond Valley Sequence of 1978 (Somerville, Peppin and VanWormer, 1980), the Bridgeport Sequence of 1979, and the Antelope Valley Sequence of 1980. Seismic activity for the period 1980-1983 consisted almost entirely of aftershocks from the swarm activity. Only seven small events were recorded during all of 1981.

The pattern of seismicity in the study area conforms closely with the Sierra Nevada-Great Basin Boundary Zone as a whole (VanWormer and Ryall, 1980). The period of quiescence in 1977-78 was followed by an increase in activity throughout the entire SNGBZ and included the powerful sequence of quakes in the Mammoth Lakes area which resulted in surface rupture within the Long Valley Caldera.

The zonation of epicenters near areas of late Quaternary surface rupture and contrasts in the style of first motions 48 indicate that in the study area seismic activity is closely related to the surface geomorphology. A discussion of the relation of seismicity to surface features is incorporated into the description of Holocene faulting later in this report.

Figure 7: Spatial-Temporal Variations of seismicity in and near the Sweetwater Mountains, California and Nevada. Longitudinal range is from 19 to 20 . Source of data: University of Nevada, Reno Seismology Laboratory. PHOTO-ANALYSIS

The careful analysis of aerial photographs at scales of 1.62,500 and li 15,000 allowed tentative confirination of the location of previously mapped tectonic features, as well as suggesting the presence of unreported faults and lineaments. The results of the photo—analysis were used in conjunction with the field studies to map the distribution of Holocene faults (Plate 1). Lineaments, whose origins were unclear, were mapped as well. These regional lineament patterns were compared to the results of Rowan and Purdy (1984) in an effort to delineate zones of tectonic activity (fig. 8).

A lineament is defined as "a simple or composite linear feature of a surface, whose parts are aligned in a recti­ linear or slightly curvilinear relationship and which differ distinctly from the patterns of adjacent features and presum­ ably reflect a subsurface phenomenon" (O'Leary et al., 1976, p.1467). Most of the lineaments mapped in the course of this study seem to have a variety of origins, including buried faults, joint patterns, bedding or related features, and in some cases, cattle trails. Lineaments were assumed to repre­ sent tectonic activity if they crossed a variety of geologic units, or if they linked zones of previously mapped faulting. Four sets of lineaments which could not clearly be related to known zones of faulting were mapped in the course of this study. They are located in the highlands around

Antelope Valley and include lineaments near the north end of

Slinkard Valley and west of Topaz Lake, in the northeast part 5Q

Figure 8: Map of the western half of the Walker Late l°x2° quadrangle showing Quaternary faults (solid lines), tectonic basins (dashed lines), lineaments (dotted lines), and lineament zones (hachured lines) as mapped by Rowan and Rirdy (15841. Lineaments referred, to in text are nunbered. 1 51

of the Antelope Valley adjacent to the Wellington Hills, and in the northern part of the Sweetwater Mountains (fig. 8, nos. 1-4).

The Slinkard/Topaz lineaments consist of four northwest trending features. The southernmost is a vegetation linea­ ment that is shown on an older geologic map of the Walker Lake Quadrangle (Koenig, 1963) as an inferred fault. It lies on strike with the northern termination of Slinkard Valley,

and parallels a series of faults on the east side of Antelope Valley at Blackwell and Risue Canyons. The other lineaments, just to the east of Topaz Lake, consist of three northwest

trending benches or scarps. The benches are parallel to the strike of the metamorphic rocks exposed nearby, but landslid- ing or faulting can also be considered possible origins.

The lineaments at the northeast corner of Antelope Valley consist of a large number of closely spaced features that for the most part lie parallel to the antithetic faults mapped in that area. These were at first thought to be related to the faulting, but were seen to have changed in location between the times that the aerial photograph series were taken (1950 and 1979), and most likely represent cattle trails.

The lineaments in the Sweetwater Mountains occur in two sets, one which trends nearly.due north, and a second that trends west to northwest. The north-trending lineaments parallel the major faults which form the southeast margin of Antelope Valley and appear to be related. The west-trending 52

features parallel the faults in Blackwell and Risue Canyons and may represent an older fault system which has been trun­ cated and cross-cut by Quaternary faulting. These lineaments lie parallel to a possible structural trough extending west from the Sweetwater Valley area that is described in a sub­ sequent section.

Rowan and Purdy (1984) used statistical methods to de­ fine lineament zones in the Walker Lake Quadrangle. In the study area, most of the zones have northwest trends, although one large zone trends east, passing through the center of the Sweetwater Mountains (Fig. 8). Their Eagle Creek Lineament Zone parallels the Bridgeport Valley Fault System, and may indicate a zone of left—lateral faulting. The Markleevillo Lineament also trends northeast, but has few, if any faults aligned along its trend. Most studies show that most faults in the area trend towards the northwest, and seismic data indicate right lateral motion along northwest directions.

The Sweetwater Mountains-Garfield Flat Zone trends WNW, and crosses the central part of the study area. The east trending lineaments found in the present study fall within this zone as well. The eastern two-thirds is identified as an important belt of mineralization. 53

FIELD STUDIES

Fault Scarp Morphology

The methods of fault morphology analysis described by Wallace (1977) and Bucknam and Anderson (1979) have been used in this study. Their studies concluded that the slope and morphology of a fault scarp in alluvium changes in a regular and observable fashion through time.

Wallace (1977) described the components of scarps in unindurated alluvium, based on his studies of late Quaternary faults in north-central Nevada. These include a free face, a debris slope, and a wash slope (Fig. 9). In alluvium, the steep free face may persist for a few tens of years, but the debris slope develops quickly by the process of spalling from the free face, and stands at the angle of repose, or about 35 degrees. The slope of the scarp declines with time and one of about 10,000 to 12,000 years will have a slope of perhaps 20 to 25 degrees. Older scarps are more progressively dom­ inated by the low angle wash slope, so that a scarp of 100,000 years will display slopes of considerably less than 20 degrees, and a scarp of 1 million years or more will be largely obliterated by erosion. This relationship is shown in Figure 10.

These components of scarp morphology are demonstrated by a buried scarp near the central part of the Antelope Valley Fault Zone (Fig. 11). The exposure, in a recently opened gravel pit, also shows the importance of slope wash and mass 5 4

upper original surface

Figure 9: Diagram showing the principal features of a recently formed normal fault scarp. Source: Wallace, 1977.

Figure 10: Limits of principal slope angle versus age of fault scarp. Source: Wallace, 1977. 5 5

2

0 Meters

I ; | | | _1 - Subangular grus and sand with occasional cobbles to 8 cm, unlayered and unindurated. Light colored. Material of original scarp.

2 - Alternating layers of coarse sand and medium sand up to 1-2 cm ' thick. Dark gray color with clasts of up to 1 cm. Unindurated. . Inclined in layers up to 35°. Probably represents spalled debris and slope wash from scarp face

2 - Angular to subangular very coarse cobble-boulder gravel containing occasional wood fragments. Has sharp contacts above and below, J but is itself unlayered. Thins towards top of scarp, and is unindurated.

2 - Coarse gravelly material similar to #3, but crudely layered.

2 - Alternating light and dark layers of coarse sand and rare cobbles up to 2 cm . More indurated than underlying materials. | Thins at top of scarp.

Figure 11: Generalized diagram of buried fault scarp in the central part part of the Antelope Valley Fault Zone. Located in gravel quarry, T9N R22E section 25 NWJ SWJ. 56 wasting in the obliteration of fault scarps, especially along steep range fronts. In this environment, many scarps seem to be buried by mass wasting processes before any significant erosion can take place. This factor also accounts for the tendency of scarps to be more discontinuous in progressively older scarp systems.

The former free face exhibits slopes of 50-60 degrees which have been buried by debris and slope wash (Unit 2, fig. 11) with slopes averaging 35-40 degrees. The unlayered angular debris interbedded with the layered sands (Unit 3) may represent a small landslide deposit derived from the adjacent scarp slope. The upper part of the scarp is buried by dark sandy material (Unit 5) which is only a few centi­ meters thick at the top of the scarp, but which increases to a depth of almost a meter over the scarp face and downfaulted fan surface, essentially eliminating surface relief.

The best preservation of scarps occurred in alluvial fan deposits at the mouth of the wineglass canyons that entered the major fault basins. A portion of the older fan material above the fault scarp has usually been uplifted and removed from active stream deposition, providing an erosion surface from which the amount of displacement can be calculated.

Bucknam and Anderson (1979) found that the slope of the scarp is proportional to the logarithm of the scarp height, so that shorter scarps often exhibit lower scarp slope angles. Dodge and Grose (1980) found that the scarp-slope angle decreases as grain size decreases, so that scarps in 57 finer sediments are more gently sloping than those in coarser sediments of the same age. This problem is illustrated well in Sonora Basin where a post Tioga scarp with a height of 4.0 m and a high slope of 25 degrees in coarse cobbly glacial till continues into a meadow comprised of mud and silt which is normally saturated. The height of the scarp declines to less than 0.5 m with a slope of perhaps 10 degrees. Probable Holocene scarps in the floor of Bridgeport Valley which occur in saturated fine sands have slopes which average little more than 12 degrees.

The climate and topography of the lower parts of the study area is for the most part similar to that of the Wallace(1977) field area in central Nevada, but the higher areas, especially Bridgeport Valley, have appreciably higher levels of precipitation (Bryant, 1984). This would suggest that erosion of fault scarps in the drier valleys would take place at about the same rate as those in the Wallace field area, while in moister areas the degradation would take place at a more rapid rate. Appendix B summarizes the results of the field studies, providing scarp slope data and profiles of selected scarps. As Wallace (1978) reported, the apparent height of scarps on originally sloping erosion surfaces, such as steep alluvial cones and fans, can be much greater than the actual amount of displacement along the fault plane. To get the best possible data on the actual displacement, the slope in the area of the scarp was recorded and the displacement calculated using the 58 nomograms shown in Wallace (1978), assuming a fault plane with a slope of 60 to 70 degrees, which is consistent with exposed fault planes in the Carson Valley (Slemmons, 1980), and Antelope Valley (fig. 11).

Stratigraphic Age Constraints on Faulting

In addition to the methods described in the previous section, three others were used in this study to place age constraints on the Quaternary faults. These were based on the association of the scarps with glacial tills, lacustrine sediments and alluvial deposits.

Glacial tills and outwash deposits provide excellent data for determining the age of associated fault scarps. Scarps offset Tahoe and Tioga tills in the Sonora Junction and Bridgeport Valley areas, while outwash deposits are present in several major canyons. As discussed earlier, the last advance of the Tioga occurred perhaps 13,000 yrs BP just prior to the beginning of the Holocene. Tahoe tills range in age from about 65,000 to 75,000 years. Sherwin tills may be about 750,000 to 820,000 years old, while McGee tills may be as old as 1.5 or 2.5 million years. Pleistocene lake sediments are found in association with fault scarps in the northern part of Smith Valley. These sediments and strand lines were deposited by Lake Wellington, the remnant of which is known as Artesia Lake. Stewart and Dohrenwend (1984) record the presence of two highstands of the lake at 1,460 m and 1,510 m, which presumably are related 59 to the Tioga and Tahoe glaciations.

Surficial alluvial deposits have been mapped in detail throughout the entire study area, and provide a useful, al­ though imprecise method of dating the age of the last move­ ment along associated faults (Dohrenwend, 1982a). Most

Holocene faults found in this study offset all alluvial fan deposits with the exception of active stream courses. Other older faults, especially those in alluvial fan environments, are only discontinuously preserved, and their approximate age can be estimated by observing which deposits are unaffected by the faulting. 60

DISCUSSION OF RESULTS

ZONATION OF FAULTS

Field observations of Holocene Faults and Late Quatern­ ary deformation as well as seismic data and photogeologic interpretation support the delineation of the study area into a series of structural blocks divided from one another by

zones of deformation. The main structural elements of the region are the Sierra Nevada block and the Western Great Basin, which are separated by a zone of faulting and deform­ ation known as the Sierra Nevada-Great Basin Boundary Zone (Fig. 12). These designations follow the usage of Dohrenwend (1982b), who also defined a zone including the Bodie Hills and Pine Grove Hills that is characterized by relatively subdued fault traces. Gilbert and Reynolds (1973) attribute the contrast to the predominance of deformation by warping in Quaternary time.

The transition between the Sierra Nevada and the Western Great Basin is characterized not by a single zone of fault­ ing, but by a wider zone of deformation consisting of inde­ pendent blocks, some of which have been active throughout Quaternary time and others that have been stable or inactive for varying periods of time. The dimensions of these blocks range from a few hundred meters to several tens of kilo­ meters, as in the case of the Sweetwater Mountains and Antelope Valley, both of which show evidence of having been tilted essentially as single units. As a whole, these m

MASON PINE NUT RANGE SMITH VALLEY VALLEY

ANTELOPE

VALLEY

[WELLING- \ TON HILLS VoESERT \ C R E E K \ PEAK PINE GROVE HILLS

S W E E T - \ \ W ATER \ VALLEY

SWEETWATER

MOUNTAINS

B O D E HILLS

BRIDGEPORT

VALLEY

Figure 12: Map showing major tectonic subdivisions and structural blocks within the study area. Stippled areas represent zones of continuous or related deformation: Dark stipple- Sierra Nevada Province; Medium- Sierra Nevada-Great Basin Boundary Zone; Light- Western Great Basin. Dotted lines represent approximate boundaries of pre-Quaternary depositional basins, modified from Halsey (1953) and Chesterman (1968). •I structural blocks provide evidence of eastward directed ex­ tension during the last 3 to 4 my. Some portions, partic­ ularly the Antelope and Little Antelope Valleys and the Sonora Basin, have been active during Holocene time.

The portion of the Western Great Basin analyzed in this study includes the Smith, Sweetwater and Bridgeport Valleys, which bound the eastern and southern flanks of the Sweetwater Mountains. Halsey (1953) considered all three valleys to be linked by a single zone of faulting, but crosscutting faults in the East Walker River area between the Sweetwater and Bridgeport Valleys and changes in the trends of the major faults suggest this is not so. The Smith and Sweetwater Valleys are separated by the Desert Creek Peak structural block, but the continuity of the fault system east of Desert Creek suggests that the two systems are connected. Holocene faulting is associated with the Bridgeport and Smith Valley Fault Zones.

The following sections discuss the magnitude, style and timing of deformation in this part of the SNGBZ and Western Great Basin, as well as the distribution of Holocene fault scarps. Figure 13 and Plate 1 show the distribution and nature of the tectonic domains and zones of deformation discussed in the following sections. The data derived in this study have made possible a preliminary seismic risk analysis for the region. 63 mmmm

64

Figure 13 (continued):

EXPLANATION OF SYMBOLS

Scarps displaying evidence of Holocene movement; Hachures on downthrown side. Scarps displaying evidence of middle or late Pleistocene activity; ball on downthrown side Older bedrock faults, inferred faults, and lineaments

r "> Approximate location of late Tertiary volcanic v . . > centers

Alluviated basins ( wm —

Zones of possible late Tertiary strike-slip displacements as indicated by northeast or ' northwest trending faults, and deflection of north trending faults. Possible Quaternary activity indicated by presence of scarps in older alluvium. Focal mechanisms indicate strike-slip motion. I i 65

THE SIERRA NEVADA/GREAT BASIN BOUNDARY ZONE

Introduction

The SNGBZ near the California/Nevada border is discon tinuous and difficult to define, especially north of Mono

Basin, where a series of fault basins, the Bridgeport, Ante­ lope and Carson Valleys, extend into the Sierra Nevada struc­ tural block. The SNGBZ in this region includes the zone of deformation that extends north from Conway Summit into

Bridgeport Valley, across the New Range, where little, if any

late Quaternary deformation has occurred, and into the Sonora Basin. This basin is the southern end of a broad zone of faulting and tilting which can be detected as far north as Topaz Lake in Antelope Valley (Fig. 12). Range-front fault­ ing ends near this point, but antithetic faults and a diffuse zone of right lateral movement may continue on into Carson Valley to the northwest.

The fault basins that flank the eastern Sierra Nevada formed in response to eastward directed extension about 3 or 4 my ago (Gilbert et al., 1968). Prior to this time, broad

basins were collecting volcanoclastic sediments and erosional debris from nearby highlands and volcanic centers. The ces­ sation of volcanic activity between 5 and 7 my was followed by widespread erosion of the entire region to a landscape of

relatively low relief. The erosion surface, which extended across the region east of the Sierra Crest, was termed the Markleeville Surface by Curtis (1951) and the Lewis Surface !i 66 by Axelrod (1956). As the Sierran escarpment began to rise, drainage to the west was cut off and sediments were trans­ ported eastward across the present location of the Sweetwater Range, leaving extensive gravel deposits as a veneer on the erosion surfaces (Brem, 1984).

Antelope Valley Fault Zone

The Antelope Valley Fault Zone (AVFZ) forms the western margin of Antelope Valley and has been the focus of late Quaternary deformation in this part of the SNGBZ (figs. 14 and 15). In general the fault has a north-northwest trend, but displays an irregular discontinuous trace, having devel­ oped as a result of reactivation of Mio-Pliocene faults which had north to northeast trends (Gilbert and Reynolds, 1973). The fault zone begins to lose definition north of Topaz Lake, eventually merging into the series of antithetic faults in the foothills of the Pine Nut Range near Gardnerville. South of Topaz, the fault is a distinctive feature that is indi­ cated by a sharp bedrock/alluvium contact, high relief, and the presence of well-defined scarps in late Quaternary allu­ vium. The zone splays near Coleville, with one branch pass­ ing into Little Antelope Valley, and the other continuing south to the termination of the valley at Walker. At Taylor Canyon the easternmost branch enters steeper terrain and can­ not be followed with certainty. The west branch in Little Antelope Valley links up with faults in the Lost Cannon and Mill Creek areas. Relief along AVFZ reaches 1,500 m. Figure 14: Explanation of units for figures 15, 16 and 17

HOLOCENE:

Qbs - Basina! sediments; includes floodplain deposits, aeolian sands and fluvial gravels.

Qf- - Young alluvial fan sediments; generally post- Tioga in age.

HOLOCENE-PLEISTOCENE:

- Qf, „ - Young and intermediate alluvial fan sediments; ____l may contain Holocene sediments.

PLEISTOCENE:

- Qf, - Intermediate alluvial fan sediments; may be middle i to late Pleistocene in age.

- Qf Intermediate to old alluvial fan sediments; may include early Pleistocene sediments.

- Qf - Older alluvial fan sediments; probably early to 1 middle Pleistocene sediments.

- Qp„ Early to middle Pleistocene pediment deposits; ^” may include Sherwin outwash in Little Antelope Valley area.

- Qp^ - Early Pleistocene pediment deposits

- Qgs - Sherwin glacial tills

PLIOCENE-PLEISTOCENE:

.Jw ;o'J - qtg - Uplifted terrace gravels

STRUCTURE: y * ' - Holocene Scarps; hachures on downthrown side sC - Middle to late Pleistocene scarps; ball on downthrown side - Older bedrock faults, lineaments, or inferred faults 68

Figure 15: Generalized geologic map of surficial deposits and faults in Antelope Valley and vicinity. Modified from Dohrenwend (1982a) and Stewart et. al. (1982). For explanation of geologic units, refer to Figure 14. 69

Slinkard Fault System

The Slinkard Valley Fault Zone parallels the Antelope Valley Fault on the west, extending south from the vicinity of Monitor Pass (fig. 15). Like the AVFZ, the irregular nature of the fault trace resulted from the reactivation of

Miocene faults in Pleistocene time (Curtis, 1951). Tertiary volcanic rocks on the downthrown block have been tilted as much as 20-30 degrees, possibly indicating listric-style faulting that may have been a gravity influenced response to downfaulting and oversteepening along the AVFZ. The north end of the fault is covered by a large landslide, but the zone may merge with the AVFZ along the northwest trending fault or lineament noted in aerial photographs and shown on older maps (Koenig, 1963)'. Slickensides on the surfaces of joints exposed in a gully just south of Slinkard Creek in Antelope Valley (near scarp 1-15-01, Plate 1) are horizontal,

indicating strike-slip motion in the vicinity of the north­ west trending fault at some time in the past.

The Slinkard Fault System continues south for more than 20 km., linking up with faults in the Lost Cannon Creek area. The amount of offset decreases in this direction, from about 750 m near Highway 89 to less than 100 m near Little Ante­ lope Pack Station. Evidence for Holocene movement is gener­ ally lacking (Bryant, 1983). i i t 70

East Antelope Valley

Antelope Valley is a large roughly triangular shaped depression which has disrupted the formerly continuous ero sion surface that consisted of the Pine Nut Mountains, the

Sweetwater Range, and the Markleeville Block. Although the AVFZ is the major fault system along which the greatest dis— placement has occurred, other faults which were active prior to and during the deformation of the structural block are exposed on the east side of the valley (Fig. 16). The block which formed the depression has been tilted to the west, as indicated by the westward facing pediment surfaces along the east side of the valley. Contrasts in the style of faulting

and other geomorphic and lithologic changes suggest that the Antelope Valley consists of a simple tilted block to the north, but that it is a more complex graben to the south, where it is bounded on both margins by major faults. The two structural blocks are separated from one another by a zone of northwest trending faults in the vicinity of Risue and Blackwell Canyons.

The northwest trending faults and lineaments seem to have been active primarily in late Tertiary or early Pleist­ ocene time, since no evidence was seen to suggest that the

faulting extends into the intermediate and older fan gravels that cover the valley floor near the fault zones. North of

the zone, the pediment surfaces have been fragmented by num­

erous north-trending faults antithetic to the Wellington Hills generally with small throws down to the east. This 71 part of the Wellington Hills is topographically more subdued, as it is underlain by easily eroded lacustrine sediments. At least two stages of pediment development have been described in this area by Dohrenwend (1982a). The pediment surfaces have subsequently been dissected by headward erosion.

South of the zone, the faults are more widely spaced, and have much greater displacements, ranging as high as 350 m with a total aggregate throw of about 850 m. Except for two minor faults in the valley floor, these faults are downthrown to the west. They displace an erosion surface underlain by resistant granite and volcanic bedrock that probably corres­ ponds to the Markleeville Surface described by Curtis (1951). These faults are deflected to the east just prior to being truncated by the northwest trending fault system, and near this termination the areas between the faults are broken into a series of smaller blocks (figure 13, p. 63). The deflec­ tion may have originated as a result of drag folding due to right lateral movement on the northwest trending fault zone.

North Antelope Valley The north Antelope Valley is a region of complex structural relationships which represents the northern terminations of the Antelope Valley basin, the system of antithetic faults that traverse the western Wellington Hills, and the Holocene range-front faulting along the AVFZ (Fig. 16). Although more than 1,000 m of relief separates the southern Pine Nut Mountains from the floor of the Antelope V*-’

Figure 16: Generalized geologic map of surficial sediments and faults in the north and east parts of Antelope Valley. For explanation of symbols, refer to Figure 14, p. 67. 73

Valley near the West Walker River, no single zone of Qua­ ternary faulting divides the structural units. The range front is instead an irregular grouping of embayments and outliers of bedrock that extend into the surrounding alluvium. The Topaz Hills and the basin containing Topaz Lake also interrupt the continuity of the valley floor east of the AVFZ. While the range front as a whole trends east, faults in older alluvium and linear bedrock/alluvium con­ tacts along the range front trend northwest. There is little evidence to suggest that Holocene movement has taken place along any of these faults.

The northwest trend of this diffuse group of faults would suggest a component of right lateral motion (Wright,

1976), a contention that is supported by focal plane solu­ tions for associated earthguakes (Fig. 6a and Table 5, pp. 44-45). Somerville, Peppin and VanWormer (1980) report a similar pattern for a swarm in the south Carson Valley, 25 km to the northwest. The intervening terrain between the two sites is characterized by subdued relief, and a general lack of continuity of north trending faults. Although specific surficial evidence for strike slip faulting is scanty, this data suggests that such a zone extends from the Carson Valley to the northern Antelope Valley. The faults in the north Antelope Valley occur only in older alluvium or bedrock. They have small displacements, but seem to interrupt the drainage patterns of streams which cross the fault trace, although no consistent direction of 74 motion could be discerned. At least one of the faults cross­ es directly into the northern Wellington Hills, interrupting the traces of the north-trending antithetic faults along the west side of the Hills. The Smith Valley Fault Zone in the adjacent valley has a prominent northwest deflection south of Hoye Canyon along the southeast extension of the zone.

According to Somerville, Peppin and VanWormer (1980) and Wright (1976), strike-slip faults in the Western Great Basin and SNGBZ formed as a result of shear stresses during Miocene time, when they exerted a great deal of influence on the re­ gional structure. Post-Miocene shearing has been a second­ ary response to regional extension along previously estab­ lished zones. In the northern SNGBZ, where the major fault basins are separated from one another, diffuse zones of strike-slip faulting accomodate the regional stresses which must occur between the basins as a result of the eastward directed extension. Although Somerville et.al. did not feel the faults in their study constituted such a zone of fault­ ing, the additional data presented in this report lends credence to this possibility. 75

Lost Cannon Creek/Walker River Gorge Zone

At the south end of Antelope Valley, the AVFZ crosses

several minor east trending faults and enters the high moun­ tainous region where traces of recent movement are lost in steep terrain. This zone, bounded by Antelope Valley and Sonora Basin and lying between Lost Cannon Creek and the

gorge of the West Walker River, displays structural blocks with varying dimensions which probably are similar in nature to the structure buried beneath the alluvium of Antelope Valley to the north. Halsey (1953) described four north trending subzones which are distinguished by contrasting tectonic structure (Plate 1).

West of Lost Cannon Creek, the Markleeville block is mostly unaffected by faulting and tilting, with more or less

continuous preservation of the Markleeville Erosion Surface. The region between Lost Cannon Creek and Mill Creek is com­ posed of strongly tilted blocks similar in nature to the Slinkard Valley, and continuous in structure with it. The narrow area between Mill Creek and the east canyon wall of the West Walker River Gorge is composed of small structural blocks which are untilted, but which are downfaulted in the direction of Antelope Valley. These blocks lie to the east of the AVFZ south of Walker. The fourth zone consists of the

continuation of the antithetic faults forming the east margin of the Antelope Valley south of Blackwell Canyon. The faults can be distinguished as far south as the Deep Creek area, 76 where traces of offset have been obliterated by Sherwin(?) glacial activity.

Tectonic activity has been relatively continuous in this area during Pleistocene time, because older glacial deposits show progressively greater amounts of deformation (Clark, 1967; Halsey, 1953). The northernmost tills or lag deposits of the Grouse Meadows(McGee) Glaciation lie on the peak be­ tween Taylor Canyon and the West Walker River, some 400 m above the Antelope Valley. The offset occurred along the relatively minor east trending cross faults, which have been mostly inactive since Sherwin time, because Clark indicated that the West Walker River is graded at most only a few tens of meters below Sherwin deposits at Deep Creek. The trace of an east-trending fault system at the south end of Little Antelope Valley for example displaces Sherwin(?) outwash,

raising the south side only 4 or 5 m, with a scarp slope of about 15 degrees, indicating an age on the order of several tens of thousands of years, and a recurrence interval of

hundreds of thousands of years. Sherwin(?) tills in Mill Creek on the west side of the

AVFZ, on the other hand, have been uplifted at least 150 m above the Antelope Valley, with the result that Mill and Lost Cannon Creeks have been perched above Antelope Valley as hanging valleys. This offset is egual in magnitude to the relief between Antelope and Little Antelope Valleys, suggest­

ing that Little Antelope Valley became separated from the

main valley primarily during post-Sherwin(?) time. 77

Sonora Basin/Mt. Emma Zone

The region encompassing Sonora Basin represents the termination of the system of faults responsible for the Antelope Valley and the tectonically deformed zone adjacent to and south of the fault basin. The system becomes inactive in the vicinity of Mt. Emma, merging into the high region linking the Sierra Nevada block and the Sweetwater Range. Most of the basin was filled with ice during the Sherwin glaciation, and Tahoe glaciers extended well into the basin area (Clark, 1967), effectively erasing most evidence of early Pleistocene fault activity, either through burial by till and outwash material or by erosion of prominent scarps. Continued tectonic activity is indicated by the presence of scarps in Wisconsin tills near Sonora Junction (Plate 1).

The coincidence of the termination of many of the faults along the east and west margins of the Antelope Valley with the boundary of the greatest advance of the Sherwin glacia­ tion suggests that most of the fault systems have not been active within the last 800,000 years or so. The active fault zone forms the eastern margin of the ridge extending south from Mt. Emma, and has offset Tahoe tills as much as 30 m, and Tioga tills 8 m (Clark, 1967). The sense of motion, trend and amount of offset is consistent with that of the Antelope Valley Fault Zone, and the two zones may be related.

The area between the two zones is extremely rugged, but 78 differences in the elevation of erosion surfaces on the two sides of the West Walker Gorge suggest that fault displace­ ments have occurred.

West Bridgeport Valley and New Range

The Sierra Nevada range front between Sonora Basin and

Robinson Creek in Bridgeport Valley is characterized by a lack of late Quaternary tectonic activity. The trend of the zone changes from south to southeast, and the relief de­ creases to less than 600 m. Tahoe tills in Molybdenite and upper Buckeye Creeks are unaffected by faulting. Clark (1967) noted however that the New Range block, the structural link between the Sweetwater Mountains and the Sierra Nevada, had rotated independently during the period between the Lit­ tle Walker Caldera eruptions and the early glacial episodes. The northwest trend of the zone of deformation would suggest the possibility of right-lateral fault movements, but there is no evidence suggesting the presence of such motion, except for focal plane solutions for quakes in the Buckeye Creek area that could be interpreted as having right lateral first motions. Most of the zone is aseismic. The Sierra Nevada structural block is continuous from Robinson Creek south into the Mono Basin, but evidence for late Quaternary faulting along the flanks of the range is discontinuous north of Conway Summit. The late Pleistocene or Holocene scarp extending northwest from Lundy Canyon veers to the north at Conway Summit where it dies out beneath a 79 veneer of Sherwin(?) glacial tills. The range-front itself becomes more subdued and gently sloping, and Tahoe tills in Virginia and Green Creeks are unaffected by faulting. Scarps and lineaments are present in older tills however, and according to Sharp (1972) the Hunewill Hills must have been faulted down in post-Sherwin time to account for the fact that no deposits of that age are present in them. The total relief averages about 1,000 to 1,500 m, increasing to the north towards Robinson Creek. 80

WESTERN GREAT BASIN

Introduction

The portion of the Western Great Basin within the study area includes the zones of faulting and deformation which lie east and south of the Sweetwater Mountains. These include the Smith and Sweetwater Valleys, the Desert Creek structural block, the East Walker Canyon, and the Bridgeport Valley (Fig. 12, p .61).

The Smith Valley and Desert Creek Peak fault zones represent the southern part of a major zone of deformation that extends northwest past Artesia Lake and into the Mt. Como area. Smith Valley is the widest, longest and deepest tectonic basin within the study area. South of the Desert Creek block, the faults merge into an irregular network of middle and late Pleistocene faults in the Sweetwater Valley along the eastern flank of the Sweetwater Range. Holocene scarps are present along much of the Smith Valley Fault System, but they do not seem to continue across the Desert Creek block. A few Holocene aged scarps may be present in the northern Sweetwater Valley. The Bridgeport Valley/East Walker River Zone includes the Bridgeport Valley Fault Zone and the wide zone of fault­ ing which extends across the southern Sweetwater Range into the valley of the East Walker River. The zone is seen to displace and truncate north-trending faults in the southern Sweetwater Valley. The northeast structural trend of this 81 zone contrasts sharply with that of other parts of the West­ ern Great Basin, which generally trend north to northwest. Probable Holocene scarps are present in the interior of Bridgeport Valley, and along the Bridgeport Fault Zone (BVFZ) at Robinson and Buckeye Creeks. Several lines of evidence suggest a component of left-lateral strike slip motion within the zone.

Prior to the development of the Quaternary basins and zones of deformation, the part of the Western Great Basin under study was the site of widespread depositional basins receiving debris from nearby highlands and volcanic eruptive centers. The youngest sediments of the basin deposition have yielded age dates of about 5 my (Gilbert and Reynolds, 1973). Deformation in the period extending from 12.5 to 4 my has been documented in the regions east and south of the study area, including left-lateral motions on northeast trending faults in the Mono Basin (Gilbert and Reynolds, 1973; Gilbert et al., 1968). Tectonic quiescence late in that time period resulted in the development of the Markleeville and Lewis Erosional Surfaces described in previous sections. South of Bridgeport, basalts ranging in age from 3 to 4 my were extruded in a continuous mantle across the Sierra Crest and were later displaced by the faults responsible for the present basin configurations (Gilbert et al., 1968). 82

Smith Valley Fault Zone

The Smith Valley Fault Zone (SVFZ), which forms the western margin of Smith Valley, has developed along previous­ ly existing Mio-Pliocene faults, resulting in an irregular

At Pipeline Canyon, a branch of the fault splays into a series of narrow horsts and grabens which extend across the interior of the Pine Nut Mountains to merge with the anti­ thetic faults on the west side of the range. The prominent embayment at Hoye Canyon coincides with the north end of Antelope Valley to the west. Both the embayment and the pronounced southeast deflection of the SVFZ south of this point apparently were influenced by the zone of northwest trending faults described in the previous section that form the north margin of Antelope Valley. The greatest amount of relief along the Smith Valley Fault System is attained north of Hoye Canyon, where it exceeds 1,800 m. South of this point, the relief declines sharply, ranging from 500 m at the mouth of Desert Creek to about 100 m at Hoye Canyon itself. The prominent difference in relief can be attributed to the downdrop of the Antelope Valley graben just to the west. The downfaulting of the graben nearly kept pace with the downdrop of the Smith Valley 83 graben. Differential erosion of the soft lacustrine sedi­ ments in this vicinity also contributed to the relative lack of relief (Gilbert and Reynolds, 1973).

Range-front style faulting ceases abruptly near the mouth of Desert Creek where the fault trace is buried by a major landslide a kilometer south of the last occurrence of Holocene scarps. The valley floor terminates against the structural block of Desert Creek Peak, but a series of en echelon scarps originating in the valley floor extend into the mountain block to the east of Desert Creek Peak and Black Mountain. These faults may represent a structural link be­ tween the Smith Valley System and the faults in the Sweet­ water Valley area, which are discussed in the following section.

Smith Valley is a broad assymetric graben which widens towards the north and narrows to the south. The valley ends to the north where the trend of the Pine Nut Mountains changes to the northeast and the tilted structural block of the Buckskin Range interrupts the valley floor. The pattern of tilting in outcrops of bedrock on the eastern margin of the valley indicate that the Smith Valley graben itself must be a large westward tilted block. Studies in the Yerington District show that the amount of tilt increases in progress­ ively older rocks indicating listric-style faulting at depth (Proffett, 1977). The continuation of tilting into recent times is indicated by the slight tilting of late Pleistocene gravels in Smith and Mason Valleys (Proffett, 1977), and by 84 the tendency of streams and lakes to flow or collect along the western margins of the valleys. Pediments are prominent­ ly developed along the east margin of the Smith Valley (Dohrenwend, 1982a).

Broad warping or tilting may have raised the southern end of the Smith Valley basin as well. Streams flowing across the alluvial fans in this area are prominently de­ flected to the north. Artesia Lake occupies the northern part of the basin at an elevation of 1378 m, several tens of meters lower than the channel of the West Walker River where it crosses the central part of the valley, suggesting the establishment of the river course prior to the latest stage of warping.

Desert Creek Peak/Sweetwater Valley The tectonic framework of the region south of the SVFZ is characterized by the lack of a single zone of rupture. The area is instead crossed by a network of faults which disrupt valley floor and mountain block alike. The zone of deformation extends from the south end of Smith Valley to the broad canyon of the East Walker River, where northeast trend­ ing faults of the Bridgeport Valley Zone truncate the system (Fig. 17). The system contains only sketchy evidence of Holocene faulting, but seems to have undergone broad warping that has uplifted much of the Sweetwater Valley, causing the entrenchment of the East Walker River Valley to a depth of more than 100 m. 8 5

trough referred to in text. Modified from Dohrenwend, 1982a 86

The faults which disrupt the Desert Creek Peak segment originate in the Smith Valley floor southeast of the promi­ nent bend in the SVFZ which was described in the previous section. They may or may not be part of the same system.

The faults form a series of small horsts and grabens which penetrate the mountainous area east of Desert Creek Peak near Dalzell Canyon. Most of the faults have relatively small throws, generally less than 150 m, although the trace which crosses through the upper part of Garden Canyon has at least 350 m of displacement. The faults disrupt a portion of the Markleeville Surface eroded onto the andesites of the Desert Creek Peak Formation, forming a series of narrow linear val­ leys and mesas with widths of a half kilometer or less and lengths of several kilometers. Many of the grabens contain ephemeral lakes. South of Desert Creek Peak, the fault system enters the Sweetwater Valley, a high intermontane depression covered by a relatively shallow veneer of alluvium. The valley floor is occasionally interrupted by bedrock outcrops bounded by nor­ mal faults. Many of the streams are entrenched, due to the lowering of base level by the erosion of the East Walker Riv­ er, so that deposition is not taking place on the alluvial fans on the valley floor. Despite their obvious appearance in aerial photos, the prominent faults in the south part of the valley do not seem to offset the younger fan sediments, but several of the faults in the north part of the valley show some evidence of I I 87

late Pleistocene activity. These faults exhibit prominent

scarps in unindurated alluvium and slope angles as high as 23 degrees. The network of faults crossing the valley floor seems to have been more active than the faults along the Sweetwater range front during late Pleistocene time. The range front is relatively subdued and at several points the mountain block is broken into several smaller structural blocks where faults from the Desert Creek block meet the range front.

The Sweetwater Trough The narrow region lying between the Sweetwater Mountains and Desert Creek Peak consists of a northwest trending trough which farther west forms the structural boundary between the Sweetwater Range and the Wellington Hills. The alluvium filled trough is traversed by the Desert Creek Peak system of

faults, which are deflected west as they enter into the fea­ ture (Fig. 17). Although few faults follow the trend of the

trough at this location, it does parallel the strike of the northwest trending faults in Blackwell and Risue Canyons and

may be related to that system. The deflection of the faults in the trough and at Black- well Canyon, the slickensides near Slinkard Valley, and the northwest trend of the faults together suggest the eastward extension of a possible late Tertiary zone of right lateral strike slip motion which has since become mostly inactive. The apparent termination of the zone near the Desert Creek 88

Peak and Bald Mountain eruptive centers recalls the conten­ tion of Wright (1976) and other workers that volcanic acti­ vity tends to be localized near the ends of en echelon strike-slip zones in regions of intense spreading such as the Western Great Basin. The Bridgeport Zone, which is discussed in the following section, likewise appears to be terminated by the Bald Mountain block.

Bridgeport Valley Fault Zone

At Robinson Creek, the Sierra Nevada frontal fault system either merges into or is truncated by the Bridgeport Valley Fault Zone (BVFZ), a major northeast trending fault that forms the northwest margin of Bridgeport Valley (Plate

1). The regional trend of the late Quaternary faulting changes from about N40W in the Hunewill Hills to N30E at the mouth of Buckeye Creek. Faults in the valley floor between

the two systems trend to the north. The Bridgeport Valley Fault Zone has been active more recently than the Sierra range front system, displacing Tioga outwash deposits in lower Buckeye Creek, and Holocene gravels on the valley floor. Sherwin(?) age gravels in Huntoon Val­ ley on the north side of the fault zone have been uplifted 200 m above the Bridgeport Valley (Sharp, 1972). The zone of

active faulting extends northeast for 20 km to the vicinity of Bridgeport Dam, although much of the fault trace is ob­ scured by landsliding. Older faults and lineaments suggest .{ i 89 the continuity of the zone as far east as Bald Mountain (Stewart, Carlson and Johannesen, 1982; Dohrenwend and Brem, 1982) .

The unusually linear trace of the BVFZ, the northeast trend, and the presence of range facing scarps near the main fault zone suggest the presence of a component of left later­ al motion along faults in this system. This contention is supported by first motion studies of local earthquakes (figure 6a and Table 5, pages 44-45), and by a north-trending fault in the eastern part of the zone near Sweetwater Valley that has been laterally displaced several tens of meters

(Dohrenwend and Brem, 1982). The floor of Bridgeport Valley is traversed by a series of short arcuate faults which offset probable Holocene grav­ els. The four faults are downthrown to the east with off­ sets of only a few meters, but they seem to divide the valley floor into a series of blocks which have been tilted slightly towards the west (Bryant, 1984). The faults lie at the west­ ern end of a large wedge shaped structural block that is defined by the BVFZ and the SNGBZ which may have moved as a unit away from the Sierra Crest along the left trending BVFZ and the possibly right trending SNGBZ (Plate 1). Deformation within the interior of the block has been primarily by warp­ ing, with only minor faulting along somewhat irregular trends

(Dohrenwend, 1982b; Sharp, 1972). The pattern of faulting strongly resembles a structural "knee", the term used by

Gilbert and Reynolds (1973) to describe the change of trend 90 of faulting at the Carson Lineament and Mono Basin.

Within the broad zone of warping, which extends south and east of Mono Lake and Mono Basin (Gilbert et al., 1968; Sharp, 1972; Higgins, Chapman, and Chase, 1981), large amplitude folds can be distinguished. The Bodie Hills represent a broad antiform structure while the Mono Basin is a synclinal downwarp. Bridgeport Valley formed the downward dipping northern axis of the Bodie Hills antiform.

The deformation is shown to have continued into late Quaternary time by several lines of the evidence (Sharp, 1972; Gilbert et al., 1968). In Bridgeport Valley, Sherwin gravels at the margins of the valley are tilted as much as 10-15 degrees basinward, and streams through the Conway Sum­ mit region have been reversed by tilting. The bulk of the Sherwin gravels within Bridgeport Valley have apparently been downwarped and buried by late Pleistocene gravels (Sharp, 1972 ) . j i 91

HOLOCENE FAULTS AND SEISMICITY

INTRODUCTION

Holocene and late Pleistocene activity has occurred along the master faults which bound the Antelope, Little Antelope, Smith and Bridgeport Valleys, and along faults within the Sonora Basin and northern Sweetwater Valley.

Several of the areas of Holocene faulting lie within contin­ uous zones of deformation but are separated by regions of high relief or structural discontinuities. The resultant un­ certainties concerning total fault length and continuity are discussed in the following sections. Data concerning scarp morphology and displacement are compiled in Table 6 and Ap­ pendix B. Plate 1 shows the distribution of these features.

Profiled scarps are indicated by reference numbers and are shown on Plate 1 as well.

ANTELOPE VALLEY Along the AVFZ, Holocene scarps can be detected from the vicinity of Topaz Lake to the mouth of Mill Creek and Taylor Canyon at Walker, a distance of 25 km. Within this zone, the fault trace is characterized by an abrupt range front which has slopes exceeding 35 degrees in many areas, and by the presence of fault scarps in alluvium. The scarps are discon­

tinuous, due primarily to the tendency of slope wash and mass movements to quickly obliterate scarps on steep slopes. Scarp measurements reveal slopes which range from 22 to 92

Table 6: Scarp morphology for selected scarps in the northern part of the Sierra Nevada-Great Basin Boundary Zone. For further details regarding location, and for scarp profiles, refer to Appendix B.

Reference Location Parent Material High Displacement number or Slope source

Antelope V al l e y 1-15-01 W of Topaz Alluvial Fan 22° 9’ - 3 m 1-28-01 S of Slinkard Ck Alluvial Fan 24° 7’ - 2.3 m 1-47-01 N of Coleville Alluvial Fan 34° 9-10' -3.3m 1-21-01 N of Walker Alluvial Fan 25° 8-11' - 3-4 m 1-34-01 Mill Creek Alluvial Fan 25° 9' - 3 m

Sonora Basin 1-36-02 Near Sonora Jet. Tioga Outwash 25° 10' -- 3.3 m 1 S of Junct. Lake Tioga Till 32° 18-19’ 6 m *

East Antelope Valley Alluvium 22 7' 2.5 m 1-63-01 Larson Lane T 1 Larson Lane Alluvium 21° 5-6 - 2 m

Little Antelope Valley 1-18-01 N Little Anti. Alluvium 19 5' - 1.7 m 1 N Little Anti. Alluvium 14° 13.5' -4m*

Smith Valley Fault Zone 1-05-01 Nevada Hot Springs Alluvial Fan 36° 11-14' - 4.5 m 1-06-01 Nevada Hot Springs Alluvial Fan 28° 12-16'- 5.0 m 1-60-01 S of Wellington Alluvial Fan 24 14' - 4.5 m 1-59-01 S of Wellington Colluvium 33 23’ 7.5m

Sweetwater Valley 1-62-01 N of Airstrip Alluvial Fan 23° 10' ■ 3 m

Bridgeport Valley 1-39-01 Valley Floor Alluvium 12 5’ - 1.5 m 2 Robinson Creek Tioga Outwash 22 13' ■4m* * Represents scarp height only; Actual displacement may be much lower. Sources: 1 - Bryant, 1983 2 - Bryant, 1984 34 degrees. The erosion surfaces across the scarps have been displaced 2.5 to 3.5 m, and seem to represent a single event that took place during the last 10,000 years, based on the conclusions of Wallace (1977). At least several thousand years have elapsed since the rupture, as streams have incised the scarps and are graded back to the original fan surface. By comparision, Holocene scarps in Carson and Smith Valleys both exhibit prominent nick points where streams cross the scarps. The Carson Valley scarp has slopes which exceed 35 degrees and may be no older than a few hundred years (Slem- mons, 1975), while scarps in Smith Valley often exceed 30 degrees. Scarps in both valleys are more continuous than those in Antelope Valley.

The northernmost scarps Antelope Valley displaying Holocene offsets are exposed at the mouth of Slinkard Creek, but the range front is relatively sharp and well-defined as far north as Holbrook Junction (fig. 16, p.72). Holocene scarps at Topaz Lake may be concealed by road-building activity and by the lake itself, the level of which has been raised for use as a storage reservoir. Just to the south, a prominent fault-line scarp above the village of Topaz that was cited by Curtis (1951) as evidence of recent movement is in actuality a composite scarp in bedrock that is an older feature. Where the bedrock scarp crosses a gully, a smaller scarp offsets the alluvial fan 2 to 3 m with a high slope of

22 degrees (scarp 1-15-01)

South of Topaz, the Holocene scarp is relatively contin- i i 94 uous but somewhat indistinct, appearing best in low angle sun conditions. It passes well up the slope at the top of the talus cones and steep alluvial fans. At the gravel quarry the fault turns west, again becoming prominent at the north end of a large landslide (Fig. 15, p.68). At this point the scarp has a height of 7 m, perhaps representing multiple events, and a maximum slope of 33 degrees (scarp 1-49-01). The buried scarp discussed in the Analytical Results section was exposed in the gravel quarry at this location, indicating rapid burial by slope processes (see figure 11, page 55). The trace of the main fault is lost in the steep terrain at the top of the landslide, and for the next kilometer cannot be followed with certainty.

The scarp reappears a kilometer northwest of Coleville, where it has a slope of 34 degrees, offsetting the fan sur­ face 3.3 m (scarp 1-47-01). The AVFZ seems to splay at this point, with one branch passing west of Centennial Bluff and entering Little Antelope Valley, and the other passing east of the bluff, continuing south into Taylor Canyon. Both branches have been active in late Pleistocene time, although the Taylor Canyon branch has been the more recently active, as scarps in alluvium on that branch have high slopes of 25 degrees, while those in the Little Antelope Valley average only 19 to 20 degrees. The greatest displacement, on the other hand, has taken place in the Little Antelope Valley, where 500 m of relief is present, compared to only 30 to 150 m in Antelope Valley. The Antelope Valley branch formed i » 95 mostly in post-Sherwin time, for the reasons discussed in the previous section. An additional fault has been mapped (John, et al., 1980) which links the two systems just south of Cen­ tennial Bluff, but no clear evidence of late Quaternary move­ ment was found at this site. As this fault forms the northern margin of Little Antelope Valley, it is inferred that the AVFZ originally passed east of Centennial Bluff, and then turned west to form the boundary of Little Antelope Valley, which at the time was continuous with the main part of Antelope Valley. Some later movement was taken up by the faults west of Centennial Bluff, but in post-Sherwin time most motion has taken place along the Taylor Canyon branch, including Holocene movements. A Holocene scarp is prominently displayed at the north end of Walker, where the alluvial fan has been offset between 2.5 and 3.5 m, with slopes as high as 25 degrees in mostly unindurated sands. The small gully at this location has not had the cutting power to completely erode through the scarp and instead has formed a smaller alluvial fan at the base. The interior of the scarp is exposed by a roadcut just to the north, and consists of probable outwash from the Tioga or

Tahoe glaciations. The fault continues south along the base of the slope which forms the west side of Mill Creek. At this point instead of continuing up Mill Creek, as is shown on several previous maps (Dohrenwend, 1982a,b), the fault crosses the

Mill Creek fan, passes over a low ridge, and enters Taylor 96

Canyon. The Mill Creek fan is displaced 3 m, with slopes as high as 25 degrees (scarps 1 — 34 —01, 1—34—02)• The scarp passing over the ridge can be seen during low sun angle con­ ditions as a series of lineaments on the west side of Taylor Canyon. Close inspection of the scarps showed them to be composed of highly weathered cobbly and bouldery sediments.

Taylor Canyon seems to have been eroded along the trace of the continuation of the AVFZ. The lineaments in the lower canyon can be followed upcanyon for a short distance but the terrain becomes too steep at this point for the preservation of fault scarps. The continuation of the zone is indicated by the presence of numerous talus and rock slides on the west side of the canyon, indicating instability on the upthrown side of the block. Rocks on the east side are far more weathered and support rich growths of lichens. Beyond the meadows at the top of the ridge the fault trace enters the extremely steep and unstable terrain of the west wall of the Walker River Gorge and is lost.

SONORA BASIN

The scarps exhibiting Holocene displacement in the So­ nora Basin were described by Clark (1967) and Bryant (1983). The fault can be followed along the east side of the ridge extending north from Mt. Emma, where Tioga deposits have been displaced 8m. A scarp near Sonora Pass Road displaces out- wash deposits about 3 m, with slopes of 22 to 25 degrees

(scarps 1-36-01, 1-36-02), while Bryant (1983) describes two scarps with heights of 6 to 7 m and slopes of 32 to 35 degrees. The fault crosses to the east side of the West Walker River and is lost in the vicinity of Burcham Flats.

The total gap between the two segments of Holocene faults is about 10 km, in extremely steep terrain. Because erosion surfaces across the gorge area have been offset (Clark, 1967), the two zones are suspected to be part of the same system.

In summary, the AVFZ is an active fault system with Holocene displacements discontinuously present over a dis­ tance of nearly 50 kilometers, with offsets on the order of 3 meters. The evidence presented here suggests that it last broke in early Holocene time, perhaps 8,000 to 10,000 years ago. Recent seismicity in the zone is concentrated in the northern portion of the zone near Topaz Lake, with sparse to moderate activity in the time period from 1970-1983. Almost no activity has been detected south of Walker. Focal plane solutions indicate a component of strike slip motion to the north, but are indeterminate to the south, with neither strike-slip nor normal motion ruled out. i I 98

SMITH VALLEY/DESERT CREEK PEAK ZONE

Holocene scarps are present along much of the Smith Valley Fault Zone, and may also be present in the northern part of the Sweetwater Valley. It is possible that these segments comprise a single system, but they are separated by the Desert Creek Peak structural block, and the continuity of the system through this zone has not been established.

The Holocene scarps of Smith Valley are more continuous than those of Antelope Valley, and because of their greater displacement, seem to record at least two late Pleistocene or Holocene rupture events. The youth of the scarps is indi­ cated by slope angles in unindurated alluvium which often exceed 25 degrees (Table 6), and by stream nickpoints that in some cases are only a few tens of yards upstream from the scarps. Lake sediments from the highstands of late Pleis­ tocene Lake Wellington are exposed within the scarp at a roadcut near Nevada Hot Springs. The scarps can be followed from the Nevada Hot Springs area south to the Hoye Canyon embayment. The scarps die out or are modified by cultural developments, but Holocene scarps are again detectable three or four kilometers south of Wellington. They continue almost without interruption to a point about a kilometer north of the mouth of Desert Creek.

At Nevada Hot Springs the 10 m high scarp splays for a short distance into two shorter scarps presumably represent­ ing two rupture events (scarps 1-06-01, 1-05-01). Both scarps displace unindurated alluvium, have slope angles of 28 and 36 degrees respectively, and each offsets the fan surface 3 to 4 m.

Where the scarp enters the study area, scarp slope an­ gles remain- high, with slopes of 28 degrees at Wedertz Can­ yon, 30 degrees at the base of Taylor Hill, and 24 degrees at the embayment near Hoye Canyon. The trace of the scarp is

lost near Wellington because of extensive canal construction, and cannot be clearly located for several kilometers.

Holocene scarps are again visible just south of the prominent deflection of the range front south of Wellington. The fault splays briefly at this point, with the younger scarp forming a small embayment into the mountain front, and an older Pleistocene scarp displacing the lower fan surface. The scarp again becomes a prominent continuous feature, with a height of about 7 m, and slopes in colluvium which average 25 degrees or more. Measurements of the scarps 1-59-01 and 1-60-01 in this zone reveal slopes of 24 and 33 degrees respectively in unindurated alluvium, heights of 4.5 and 7.5 m, and the presence of two bevels in the stream channel that crosses the scarp, indicating multiple rupture events.

Traces of active range front faulting end a kilometer north of Desert Creek, and the fault trace disappears beneath a large landslide (Dohrenwend, 1982a). The fact that Late

Quaternary deformation has continued south of the fault zone is demonstrated by the series of en echelon faults in the valley floor at the south end of Smith Valley, and the devel­ 100

opment of horsts and grabens in the region east of Desert

Creek Peak. Scarps in alluvium demonstrating Holocene offset have not been observed, but the relative youth of the bedrock scarps is indicated by the sharp, linear bedrock/alluvium contacts in late Tertiary volcanic rocks and the relative lack of dissection. A more precise estimate of the age of these faults is provided by the extensions of the faults into the valley floors north and south in Smith and Sweetwater Valleys.

In the south part of Smith Valley, the long linear scarp that parallels Desert Creek on the west has a height of 2.5 m, and a high slope of 19 degrees (scarp 1-60-02). A scarp

in north Sweetwater Valley that is continuous with the most active fault in the Desert Creek block, that which crosses the west side of Garden Valley (figure 17, page 85) has a height of about 10 m and slopes of 20 to 25 degrees. Another scarp near the airstrip has a slope of 23 degrees and a height of 2.5 m (scarp 1-62-01; see Plate 1).

Although other faults are prominent features in aerial photographs both within the valley and along the Sweetwater range front, none were found which had slopes in alluvium which exceeded 20 degrees. In most cases, the scarps are dissected by deep stream valleys, and no evidence was found to suggest that any Holocene alluvium had been offset.

Despite the plentiful evidence for Holocene offsets in the Smith Valley area, historical seismicity has been sparse. Only a dozen or so quakes were recorded during the years 101

1970 1983 along all parts of the zone. The largest, measur­ ing magnitude 4.9 in the vicinity of north Sweetwater Valley in 1977, was noted for the lack of aftershock activity (Somerville, Peppin and VanWormer, 1980). The focal plane solution for this quake indicated normal motion with an east- northeast direction of least compressional stress, and a dip of 70 degrees. Another focal plane solution for a magnitude 2.5 quake in the south part of Smith Valley was indetermin­ ate, with neither strike-slip or normal motion ruled out.

BRIDGEPORT VALLEY FAULT ZONE

The BVFZ is an extremely linear feature extending from Robinson Creek northeast to the Bridgeport Dam area, a dis­ tance of about 20 km. Much of the trace northeast of Swau- ger Creek and Huntoon Valley has been obscured by Pleistocene landslides, but scarps in Tioga outwash at Buckeye Creek have up to 4 meters of displacement, and slope angles of 18 to 26 degrees (Bryant, 1984). The high level of precipitation in this particular area contributes to the rapid degradation of scarps in alluvium, and thus the scarps most likely represent early Holocene rupture (Bryant, 1984). The combination of a moister climate and easily weathered Tertiary volcanics is probably responsible for the unusually large number of land­ slides which have obliterated many of the late Quaternary scarps.

The north to northeast trending scarps in the valley floor likewise seem to be Holocene in age despite slope 102 angles which do not exceed 12 to 15 degrees. The 1 to 2 m hi-Oh scarps displace fine-grained saturated alluvium that has been heavily grazed.

Two range-facing scarps can be observed in aerial photo­ graphs north of Bridgeport Reservoir. The scarps cut young fan sediments, and thus are probably Holocene in age (Dohrenwend, 1982a).

The Bridgeport Valley area is by far the most seismic- ally active part of the study area. Although activity was rather sparse prior to 1979, in that year a swarm of more than 60 earthquakes was recorded, with the initial shock measuring magnitude 4.9 and at least six aftershocks of magnitude 4.0 or greater. Most of the shocks were centered beneath the northwest corner of the valley, close to the con­ vergence of the BVFZ and the SNGBZ at Robinson and Buckeye Creeks. Longterm aftershock activity has been sparse, al­ though a magnitude 3.8 quake occurred in September of 1983. No surficial breaks were recorded during the swarm.

Five focal plane solutions have been determined for quakes in the general vicinity of Bridgeport Valley (Figure 6a, page 44), which indicate strike-slip and oblique slip motion with a least compressive stress direction of nearly due east, with steep dips. Selection of the fault plane of cannot be made on the basis of surficial breaks, and the presence of two possible fault trends in the immediate vicin­ ity makes the choice difficult. The most recent surficial activity has taken place on the northeast trending zone, 103 which would indicate a component of left-lateral slip motion, but the aftershock distribution defines a north to northwest trending zone about 4 km long and 1.5 km wide which would indicate right-lateral motion parallel to the Sierra Nevada range front. Given the location of the quakes at the inter­ section of the two contrasting zones, it may be that both zones were activated by the seismic activity brought on by east-west extension. 104

SEISMIC RISK ANALYSIS INTRODUCTION

Part of this study has been directed towards delineating the location and extent of Holocene and Late Quaternary fault activity in the SNGBZ and Western Great Basin for the purpose of analyzing seismic risk in the region. Three potentially active fault systems have been studied, those in the Antelope Valley/Sonora Basin area, the Smith/Sweetwater Valleys, and the Bridgeport Valley. Evidence for Holocene fault rupture was found in each of these basins.

The data derived in this study make possible a number of conclusions regarding slip rates, recurrence intervals, and seismic risk in this area, allowing comparisions to other parts of the SNGBZ.

SLIP RATES

The association of glacial tills and outwash deposits with some of the fault scarps provided a reasonably accurate method of determining long-term slip rates. Table 7 outlines the slip rates calculated for major faults in the study area, as well as those for those in other parts of the Sierra Nev- ada/Great Basin Boundary Zone.

The calculated slip rate for the Antelope Valley Fault

Zone is based upon the amount of displacement of outwash deposits from Sherwin (?) glaciers in the lower part of Mill Creek near Walker, which were described in previous sections. The rate of about 0.20 mm/yr is somewhat higher than the ■WKvar?

105

TABLE 7: Slip rates for faults in selected parts of the Sierra Nevada/Great Basin Boundary Zone, from north to south.

Location Source Slip Rates Time Interval

Antelope Valley 1 0.19 - 0.21 mm/yr post-Sherwin Smith Valley 1 0.36 - 0.41 mm/yr post-Tioga

Bridgeport Valley 2 0.21 - 0.24 mm/yr post-Sherwin 8 0.30 - 0.60 mm/yr post-Tahoe 8 0.20 - 0.70 mm/yr post-Tioga Sonora Basin 3 0.09 - 0.11 mm/yr post-Sherwin 0.40 - 0.46 mm/yr high post-Tahoe 0.20 - 0.23 mm/yr low post-Tahoe Lundy Creek 8 2.5 mm/yr post-Tioga McGee Creek 4 1.25 - 1.38 mm/yr post-Tahoe Hartley Spgs Fault 5 0.40 mm/yr 710,000 yrs Deadman Summit Fit. 5 0.37 mm/yr 710,000 yrs

Casa Diablo Fault 6 0.25 mm/yr 710,000 yrs Wilson Butte Fault 6 0.39 mm/yr 710,000 yrs Pine Creek Fault 4 0-6.5 - 0.72 mm/yr post-Tioga Mid-Valley Fault, 7 2.2 mm/yr historical Owens Valley Sawmill Ck Fault 7 0.5 mm/yr post-Tahoe Independence Fault 7 0.1 - 0.2 mm/yr post-Tahoe

Owens Valley 7 0.36 - 0.6 mm/yr post-Sherwin?

SOURCES: 1 . Present Study 2 . Sharp, 1972 3 . Clark, 1967 4. Clark, 1979 5. Huber, 1981 6. Slemmons, 1975 7. Gillespie, 198 8. Bryant, 1984 106

post—Sherwin estimate of about 0.10 mm/yr for the Sonora Basin area, which is based on data from Clark (1967). The

spectrum of higher rates for post-Tahoe deposits in this area suggests an increase in the slip rate during late Quaternary

time. Higher rates are possible in the Antelope Valley area if post-Sherwin movement in the Little Antelope Valley system is considered, but data for such estimates were lacking.

The slip rates for the Bridgeport Valley Fault Zone were based on data provided by Sharp (1972) for post-Sherwin de­ formation of the valley area, and by Bryant (1984) for dis­ placed late Pleistocene tills. The estimate for the Smith Valley fault zone is more speculative, due to the lack of associated tills and outwash gravels, and is based on the presence of probable Tioga aged lake sediments exposed in a scarp built by at least two post Tioga rupture events which resulted in about 8 meters of displacement. The long-term slip rates in this part of the SNGBZ, which average about 0.20 to 0.40 mm/yr, are comparable to many of the major fault zones to the south which range from 0.25 to 0.65 mm/yr (Table 7). It is much lower than the slip rates for the Mono Basin area faults (1.25-2.5 mm/yr) and the Mid-Valley Fault Zone in Owens Valley (2.2 mm/yr), but these rates are based on post-Tioga movements and may reflect an increasing slip rate in late Pleistocene time.

Gillespie (1982) noted that the slip rate for some fault systems varied considerably at different locations in a single zone, such as along the Independence Fault, which 107

ranged from 0 to 0.5 mm/yr. The apparent segmentation and branching nature of many of the faults in the study area

suggests that extreme variations are likely in the long-term slip rate, due to movement being taken up along different strands at different times.

RECURRENCE INTERVALS

The recurrence intervals given in Table 8 were estimated by taking the total displacement of faulted glacial or lacus­ trine deposits, and dividing by an assumed average displace­ ment of 3 meters per rupture event, which is consistent with field observations. The estimates for Antelope and Bridge­ port Valleys are based on displacements of Sherwin tills, while the Smith Valley estimate is based on a scarp recording two events that contains possible Tioga eguivalent lake sediments.

The average long-term recurrence interval for the region is on the order of 10,000 to 15,000 years, with longer inter­ vals for the Antelope and Bridgeport systems, and a somewhat shorter interval for the Smith Valley system north of Desert Creek. The faults in Sweetwater Valley appear to have longer recurrence intervals, due to their dissected appearance, and general lack of Holocene offset. These values compare to recurrence intervals in other parts of the SNGBZ which range from 2,400 to 60,000 years or more (Slemmons, 1980). 1 0 8

Table 8: Postulated fault rupture lengths, Maximum Credible Earthquakes (MCEs), and recurrence intervals in selected parts of the Sierra Nevada-Great Basin Boundary Zone and adjacent western Great Basin. Fault lengths are based upon distribution of Holocene scarps and lengths of entire zones over which Late Quaternary deformation has occurred. Recurrence intervals (R.I.) are derived using the displace­ ment of well dated stratigraphic units and an assumed average displacement of 3 meters.

Fault System Length M.C.E. R.I. Comments

Antelope Valley/ 60 km 7.0-7.4 Entire zone of Sonora Basin deformation; may be segmented

Antelope Valley/ 45 km 6.8-7.2 Holocene scarps Sonora Basin only

Antelope Valley 25 km 6.5-6.9 14,000- Holocene scarps only 15,700

Sonora Basin 10 km 5.8-6.2 13 , GOO- Holocene scarps only 15,000

Bridgeport Basin 20 km 6.3-6.7 12,300- Late Pleist­ 13,700 ocene deformation Bridgeport/East 32 km 6.6-7.0 Neogene zone of def ormation Smith Valley/ 75 km 7.2-7.6 Neogene zone of Sweetwater deformation. Prob­ Valley System ably segmented

Smith Valley 40 km 6.8-7.2 6,000- Holocene scarps only 9,000

Desert Creek/ 35 km 6.7-7.1 Neogene deform­ Sweetwater Valley ation. May consist System of smaller strands of shorter length 109

MAXIMUM CREDIBLE EARTHQUAKES

The Maximum Credible Earthquake, or MCE, is defined as the largest earthquake which can reasonably be expected to occur along a particular fault, based on observations of the total potential rupture length and/or amount of vertical or horizontal displacement occurring in a single event (Slem- mons, 1977). These values provide a semi-quantitative basis for anticipating earthquake related damages to structures and developments in quake-prone areas. The MCEs for the fault zones observed in this study were calculated using the regression relationships of Slemmons (1977, 1980), which are based on the record of historical quakes displaying surface ruptures worldwide. The assignment of an MCE to a particular fault zone can often be complicated by the fact thatjlike the AVFZ south of Taylor Canyon, the zone can be difficult to detect in an area of high relief; and,like both the Antelope Valley and Smith Valley Fault

Systems, the zones can be complex, with irregular branching and anastomosing patterns. The passage of time erases evi­ dence of fault continuity, and makes the determination of an MCE more difficult. The results of the calculations of MCEs in this study are shown in Table 8. Because of the complicating factors mentioned above, several possibilities are mentioned for each zone which take into account the uncertainties regarding . WiVJ* JJiy.LJAJJi JJJiiLLaSSBT sb mm

110

fault continuity which have been dealt with in more detail in

previous sections. The values range from about magnitude 6.3 to 7.2. These are somewhat lower than those for the southern part of the SNGBZ, reflecting the shorter length of the tec­ tonic basins and lack of longer through-going fault systems.

The southern part of the SNGBZ is characterized by long continuous fault systems, extremely deep tectonic basins, relatively high slip rates, and high MCE values (Slemmons, 1980). The northern portion of the SNGBZ reviewed in this study is by comparision less seismically active, as shown by lower slip rates and MCE values, lower total displacements, shorter fault lengths, and by lower levels of historical

seismicity. Slemmons (1980) attributes these differences to the closer proximity of the southern part of the SNGBZ to the

Garlock/San Andreas system. Nonetheless, the data indicate that the three main fault systems observed in this study are

capable of generating major quakes, with magnitudes as high as 7.2 or more with surface ruptures of 3 meters or more.

Furthermore, they lie within a possible seismic gap described by Wallace (1980) as extending from the Mammoth area to the Truckee region, within which quakes and rupturing may be expected to occur. Both the Bridgeport Valley and northern Antelope Valleys display moderate amounts of historical seismicity. Ill

References Cited Adam, D.P., 1967, Late Pleistocene and Recent palynology in the central Sierra Nevada, in Cushing, E.J., and Wright, H.E., Jr., eds., Quaternary Paleocology: INQUA Congress VII, Proceedings 7, Yale University Press, New Haven, Connecticut, p. 275-301.

Anderson, L.W., and Hawkins, F.F., 1984, Recurrent Holocene strike-slip faulting, Pyramid Lake fault zone, western Nevada: Geology, v. 12, p. 681-684.

Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geol. Soc. America Bull., v. 81, p. 3513-3536. Axelrod, D.I., 1956, Mio-Pliocene floras from west-central Nevada: California Univ. Pubs. Geol. Sci., v. 33, p . 1-322.

Bachman, S.B., 1978, Pliocene-Pleistocene break-up of the Sierra Nevada-White-Inyo Mountains block and formation of Owens Valley: Geology, v. 6, p. 461-463. Bailey, R.A., Dalrymple, G.B., and Lanphere, M.A., 1976, Volcanism, Structure and Geochronology of Long Valley Caldera, Mono County, California: Journal of Geophysical Research, v. 81, p. 725-744.

Bateman, P.C., and Clark, L.D., 1974, Stratigraphic and structural setting of the Sierra Nevada Batholith, Calif­ ornia: Pacific Geology, v. 8, p. 79-89..

Beard, J.H., 1969, Pleistocene paleotemperature record based on planktonic foraminifers, Gulf of Mexico: Gulf Coast Assoc. Geol. Soc. Trans., v. 19, p. 535-553. Bell, E.J., and Slemmons, D.B., 1979, Recent crustal move­ ments in the central Sierra Nevada-Walker Lane region of California-Nevada: Part II, The Pyramid Lake right-slip fault zone segment of the Walker Lane: Tectonophysics, v. 52, p. 571-583.

Birman, J.H., 1964, Glacial geology across the crest of the Sierra Nevada: Geol. Soc. America Special Paper 75, 80 p. Blackwelder, E., 1931, Pleistocene glaciation in the Sierra Nevada and Basin Ranges: Geol. Soc. America Bull., v. 42, p. 865-922. Brem, G.F., 1982, Sweetwater rhyolite center, Sweetwater Mountains, Mono County, California and Lyon County, Nev­ ada [abst]: Geol. Soc. America Abstracts with Programs, v. 14, p. 152.

-----, 1984, Geologic Map of the Sweetwater Roadless Area, Mono County, California, and Lyon and Douglas Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-1535-B, scale 1:62,500.

Brook, C.A., Gordon, Mackenzie, Jr., Mackey, M.J., and Chere- lat, C.R., 1979, Fossiliferous upper Paleozoic rocks and their structural setting in the Ritter Range and Saddle­ bag Lake roof pendants, central Sierra Nevada, California [abs.]: Geol. Soc. America Abstracts with Programs, v. 11, no. 3, p . 71.

Bryant, W.A., 1983, Faults in Antelope Valley, Slinkard Val­ ley, and along the West Walker River, Mono County: Calif­ ornia Division of Mines and Geology, Fault Evaluation Report FER-154, 14 p.

_____/ 1984, Faults in Bridgeport Valley and Western Mono Basin, Mono County: California Division of Mines and Geology, Fault Evaluation Report FER 155, 14 p. Bucknam, R.C., and Anderson, R.E., 1979, Estimation of fault scarp ages from a scarp height-slope-angle rela­ tionship: Geology, v. 7, p. 11-14.

Burke, R.M., and Birkeland, P.W., 1979, Reevaluation of multiparameter relative dating techniques and their ap­ plication to the glacial sequence along the eastern escarpment of the Sierra Nevada, California: Quaternary Research, v. 11, no. 1, p. 21-51.

Chesterman, C.W., 1968, Volcanic Geology of the Bodie Hills, Mono County, California: Geol. Soc. America Memoir 116, p . 45-68.

r 1975, Geology of the Matterhorn Peak quadrangle, Mono County, California: California Division of Mines and Geology Map 22, scale 1:48,000.

Chesterman, C.W., and Gray, C.H., Jr., 1975, Geology of the Bodie quadrangle, Mono County, California: California Division of Mines and Geology Map 21, scale 1:48,000.

Christensen, M.N., 1966, Late Cenozoic crustal movements in the Sierra Nevada of California: Geol. Soc. America Bull.,• / v. 77, p. 163-182. 113

Clark, M .M. , 196V, Pleistocene glaciation of the drainage of the West Walker River, Sierra Nevada, California: Ph.D. thesis, Stanford University, 170 p.

____t 1979, Range-front faulting: Cause of the difference m height between Mono Basin and Tahoe moraines at Walker Creek, in Burke, R.M., and Birkeland, P.W., eds., Field guide to relative dating methods applied to glacial depo­ sits in the third and fourth recesses and along the east­ ern Sierra Nevada, California, with supplementary notes on other Sierra localities: Friends of the Pleistocene, Pacific Cell, 131 p.

Curry, R.R., 1971, Glacial and Pleistocene history of the Mammoth Lakes Sierra, California— a geologic guidebook: Montana Department of Geology Geol. Serial Publication 11, Missoula, Montana, 49 p.

Curtis, G.H., 1951, The geology of the Topaz Lake quad- rangle and the eastern half of the Ebbetts Pass quad- rangle: University of California, Berkeley Ph.D. thesis, 315 p.

Dalrymple, G.B., 1980, K-Ar ages of the Friant Pumice Mem­ ber of the Turlock Lake Formation, the Bishop Tuff, and the tuff of Reds Meadow, central California: Isochron West, no. 28, p. 3-5. Dalrymple, G.B., Burke, R.M., and Birkeland, P.W., 1982, Concerning K-Ar dating of a flow from the Tahoe-Tioga interglaciation, Sawmill Canyon, southeastern Sierra Nevada, California: Quaternary Research, v. 17, p. 120-122.

Dodge, R.L., and Grose, L.T., 1980, Tectonic geomorphic evolution of the Black Rock fault, northwestern Nevada: U.S. Geologic Survey Open File Report 80-801. Dohrenwend, J.C., 1982a, Surficial geologic map of the Walker Lake 1 by 2 degree quadrangle, Nevada and Calif­ ornia: U.S. Geologic Survey Miscellaneous Field Studies Map MF-1382-C, scale 1:250,000.

, 1982b, Map showing late Cenozoic Faults in the Walker Lake 1 by 2 degree quadrangle, Nevada and Calif­ ornia: U.S. Geologic Survey Miscellaneous Field Studies Map MF-1382-D, scale 1:250,000. i i 114

Dohrenwend, J.C., and Brem, G.F., 1982, Reconnaissance surficial geologic map of the Bridgeport quadrangle, California and Nevada: U.S. Geological Survey Miscel­ laneous Field Studies Map MF-1371, scale 1:62,500. Dube', T.E., 1984, A tentative midcontinent-marine Qua­ ternary chronology and correlation to the Sierra Nevada: Nebraska Academy of Sciences, Lincoln, NB [TER-QUA: Inst, for Tertiary and Quaternary Studies] (in press). Engelbretson, D.C., Cox, A., and Thompson, G.A., 1984, Correlation of plate motions with continental tectonics; Laramide to Basin Range: in Correlation between plate motions and Cordilleran tectonics, Tectonics, v. 3, no. 2, p. 115-119.

Ekren, B.E., Byers, F.M., Jr., Hardyman, R.F., Marvin, R.F., and Silberman, M.L., 1980, Stratigraphy, preliminary petrology, and some structural features of Tertiary volcanic rocks in the Gabbs Valley and Gillis Ranges, Mineral County, Nevada: U.S. Geological Survey Bulletin 1464, 54 p.

Gilbert, C.M., Christensen, M.N., Al-Rawi, Y.T., and Lajoie, K.R., 1968, Structural and volcanic history of Mono Basin, California-Nevada: Geol. Soc. America Mem. 116, p .275-329.

Gilbert, C.M., and Reynolds, M.W., 1973, Character and chronology of basin development, western margin of the Basin and Range Province: Geol. Soc. America Bull., v. 84, p. 2489-2510.

Gillespie, A.R., 1982, Quaternary glaciation and tectonism in the southeastern Sierra Nevada, Inyo County, Calif­ ornia: Calif. Inst, of Technology Ph.D. Thesis, 695 p. Guisso, J.R., 1981, Reconnaissance geologic map of the Sonora Pass 15-minute quadrangle, Alpine, Mono, and Tuol­ umne Counties, California: U.S. Geological Survey Open-File Report 81-1170, scale 1:62,500. Halsey, J.H., 1953, Geology of parts of the Bridgeport, California, and Wellington, Nevada quadrangles: Univer­ sity of California, Berkeley Ph.D. thesis, 498 p. Hardyman, R.F., 1978, Volcanic stratigraphy and structural geology of the Gillis Canyon quadrangle, northern Gillis Range, Mineral County, Nevada: University of Nevada, Reno Ph.D. thesis, 248 p. w

I i 115

Hay, E.A., 1975, Cenozoic uplifting of the Sierra Nevada in isostatic response to North American and Pacific plate interactions: Geology, v. 4, p.763-766.

Higgins, C.T., Chapman, R.H., and Chase, G.W., 1983, Geo­ thermal resources of the Bridgeport-Bodie Hills region, California: California Div. of Mines and Geology Open File Report 83-14SAC, 105 p.

Huber, N.K., 1981, Amount and timing of late Cenozoic uplift and tilt of the central Sierra Nevada, California: Evi­ dence from the upper San Joaquin River Basin: U.S. Geol­ ogical Survey Prof. Paper 1197, 28 p.

John, D.A., 1983, Distribution, ages, and petrographic char­ acteristics of Mesozoic plutonic rocks, Walker Lake 1 by 2 degree quadrangle, California and Nevada: U.S. Geolog­ ical Survey Miscellaneous Field Studies Map MF-1382-B, scale 1:250,000.

John, D.A., Guisso, J.R., Moore, W.J., and Armin, R.A., 1981, Reconnaissance geologic map of the Topaz Lake’ quadrangle, California-Nevada, with Quaternary geology by J.C. Dohrenwend: U.S. Geological Survey Open File Report 81-273, scale 1:52,500.

Kleinhampl, F.J., Davis, W.E., Silberman, M.L., Chesterman, C.W., Chapman, R.H., and Gray, C.H., Jr., 1975, Aero- magnetic and limited gravity studies and generalized geology of the Bodie Hills region, Nevada and Calif­ ornia: U.S. Geological Survey Bull. 1384, 38 p.

Koenig, J.B., compiler, 1953, Geologic map of California, Olaf P. Jenkins edition-Walker Lake sheet: California Division of Mines and Geology, scale 1:250,000. Larsen, N.W., 1979, Chronology of late Cenozoic basaltic volcanism: the tectonic implications along a segment of the Sierra Nevada-Great Basin Province boundary: Brigham Young University Ph.D. thesis.

Moore, J.G., 1959, Geology and mineral deposits of Lyon, Douglas and Ormsby counties, Nevada, with a section on Industrial minerals by N.L. Archbold: Nevada Bureau of Mines Bull. 75, 44 p.

Noble, D.C., 1952, Mesozoic geology of the southern Pine Nut Range, Douglas County, Nevada: Stanford University, Ph.D. thesis, 200 p. 116

Noble, D.C., Slemmons, D.B., Korringa, M.K., Dickinson, W.R., Al-Rawi, Y.T., and McKee, E.H., 1974, Eureka Val­ ley tuff, east-central California and adjacent Nevada: Geology, v. 2, no. 3, p. 139-142.

O'Leary, D.W., Friedman, J.D., and Pohn, H.A., 1976, Linea­ ment, linear, lineation; some proposed new standards for old terms: Geol. Soc. America Bull. v. 87, p. 1463-1469.

Proffett, J.M., Jr., 1977, Cenozoic geology of the Yerington District, Nevada, and implications for the nature and origin of Basin and Range faulting: Geol. Soc. of Amer­ ica Bull., v. 88, p. 247-266.

Rowan, L.C., and Purdy, T.L., 1984, Map of the Walker Lake 1 by 2 degree quadrangle, California and Nevada showing the regional distribution of linear features: U.S. Geol­ ogical Survey Miscellaneous Field Studies Map MF-1382-P, 28 p . Ryall, A., 1977, Seismic hazard in the Nevada region, Bull. Seism. Soc. America, v. 67, p. 517-532. Ryall, A., and VanWormer, J.D., 1980, Estimation of maximum magnitude and recommended seismic zone changes in the western Great Basin: Bull. Seis. Soc. America, v. 70, p. 1573— 1581. Sanders, C.O., and Slemmons, D.B., 1979, Recent crustal movements in the central Sierra Nevada-Walker Lane region of California-Nevada: Part III, the Olinghouse fault zone: Tectonophysics, v. 52, p. 585-597. Schweickert, R.A., 1976, Shallow-level plutonic complexes in the eastern Sierra Nevada, California: Stanford Uni­ versity Ph.D. thesis, 85 p.

, 1981, Tectonic evolution of the Sierra Nevada Range, in Ernst, G., ed., Geotectonic Development of Calif­ ornia: Geol. Soc. America Rubey Volume 1, p. 88-131. Schweickert, R.A. and Snyder, W.S., 1981, Paleozoic plate tectonics of the Sierra Nevada and adjacent regions, in Ernst, G., ed., Geotectonic Development of California: Geol. Soc. America Rubey Volume 1, p. 183-201. Shackleton, N.J., and Opdyke, N.D., 1973, Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: Oxygen isotope temperature and ice volumes on a 10 5 year and 10° year scale: Quaternary Research, v. 3, p. 39-55.

Sharp, R.P., 1972, Pleistocene glaciation, Bridgeport Basin, California: Geol. Soc. America Bull., v. 83, p. 2233-2260.

Sharp, R.P., and Birman, J.H., 1963, Additions to the clas­ sic sequence of Pleistocene glaciations, Sierra Nevada, California: Geol. Soc. America Bull., v. 74, p. 1079-1086.

Slemmons, D.B., 1951, Geology of the Sonora Pass region: University of California, Berkeley, Ph.D. thesis, 201 p. ______/■ 1966, Cenozoic volcanism of the central Sierra Nevada, California, in Bailey, E.H., ed., Geology of northern California: California Div. of Mines and Geology Bull. 190, p. 199-208.

______t 1975, A field guide to Cenozoic deformation along the Sierra Nevada Province and Basin and Range Province Boundary: California Geology, v. 28, no. 5, p. 99-119. ______t 1977, State-of-the-art for assessing earthquake hazards in the United States, faults and earthquake mag­ nitude: U.S. Army Engineering Waterways Expt. Station soils and pavements lab., misc. paper S-73-1, rept. 6, 166 p.

______, 1980, Design earthquake magnitudes for the west­ ern Great Basin, Nevada and eastern California: in Pro­ ceedings of Conference X, Earthquake hazards along the Wasatch-Sierra Nevada Frontal Fault Zones: U.S. Geo­ logical Survey Open File Report 80-801, p. 62-85.

Slemmons, D.B., VanWormer, J.D., Bell, E. J., and Silberman, M.L., 1979, Recent crustal movements in the Sierra Nevada-Walker Lane region of California-Nevada: Part I, rate and style of deformation: Tectonophysics, v. 52, p. 561-570.

Somerville, M.R., Peppin, W.A., and VanWormer, J.D., 1980, An earthquake sequence in the Sierra Nevada-Great Basin boundary zone: Diamond Valley: Bull. Seism. Soc. Amer­ ica, v. 70, no. 5, p. 1547-1555. 118

Stewart, J.H , 1975, Late Precambrian evolution of North America: Plate tectonics implication: Geology, v. 4, p. 11-15

------, 1979, Geology of Nevada, a discussion to accompany the Geologic Map of Nevada: Nevada Bureau of Mines and Geology Spec. Publ. 4, 135 p.

Stewart, J.H., Carlson, J.E., and Johannesen, D.C., 1982, Geologic map of the Walker Lake 1 by 2 degree quad­ rangle: U.S. Geological Survey Miscellaneous Field Studies Map MF-1382-A, scale 1:250,000.

Stewart, J.H., Chaffee, M.A., Dohrenwend, J.C., John, D.A., Kistler, R.W., Kleinhampl, F.J., Menzie, W.D., Plouff, D., Rowan, L.C., and Silberling, N.J., 1984, The con­ terminous United States Mineral Appraisal Program: Back­ ground information to accompany folio of geologic, geochemical, geophysical, and mineral resources maps of the Walker Lake 1 by 2 degree quadrangle, California and Nevada: U.S. Geological Survey Circular 927, 21 p. Stewart, J.H., and Dohrenwend, J.C., 1984, Geologic map of the Wellington quadrangle, Nevada: U.S. Geologic Survey Open File Report 84-211, scale 1:52,500.

VanWormer, J.D., and Ryall, A.S., 1980, Sierra Nevada-Great Basin Boundary Zone: Earthquake hazard related to struc­ ture, active tectonic processes, and anomalous patterns of earthquake occurence: Bull. Seism. Soc. America, v. 70, no. 5, p. 1557-1572.

Vetter, U.R., and Ryall, A.S., 1983, Systematic change of focal mechanism with depth in the western Great Basin: Journ. of Geophysical Research, v. 88, no. B10, p. 8237-8250.

Wallace, R.E., 1977, Profiles and ages of young fault scarps, north-central Nevada: Geo. Soc. America Bull., v. 88, p. 1257-1281.

______r 1978, Patterns of faulting and seismic gaps in the Great Basin province: U.S. Geologic Survey Open File Report 78-943, 12 p.

______i 1980, Discussion-Nomograms for estimating compon­ ents of fault displacement from measured height of fault scarp: Bull. Assoc. Engineering Geologists, v. 17, no. 1, p. 39-45. raEEHK m sm sm m sm

119

Wright, L., 1976, Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block: Geology, v. 4, p. 489-494. Zoback, M.L., Anderson, R.E., and Thompson, G.A., 1981, Cainozoic evolution of the state of stress and style of tectonism of the Basin and Range province of the western United States: Phil. Trans. R. Soc. Lond., A 300, p. 407-434. 1 2 0

APPENDIX A:

PART 1

Catalogue of earthquakes in the northern Sierra Nevada- Great Basin Boundary Zone for the years 1970 to 1982, from latitude 38° 00' to 39°00' and longitude 119°001 to 120°00‘. Source: University of Nevada, Reno Seismology Laboratory.

Headings are as follows:

A: Year date and time of quake B: Latitude and longitude C: RMS D: Number of stations reporting quake E: Magnitude F: Quality of recording (A best, D worst) G: Depth of focus lfflJJW ^k].,-U...I--ULLjL'.UU+U....U------ULUm'LUIIMi^MWW—

1 2 1

B D E 1 ^ 0 —V r?—03fl231"72' 32,773 ITTTTT^- 1970 1 ~ 7T 2,2 *""" ' 1? 053452.3 33.764- 119,198 10 2.5 1270____1 15L .-060.625.1 33.779 USL.19.fl- .. 1970 1 19 195723.5 33,731 8 2.4 1 9 7 o 7 119.189 8 2.2 4 151639.2 38,767 119.229 7 2.9 +270-3 86r 1 "618-32. 4 33.557 1197289- 1970 3 27 034253,2 - H r 3,2 1970 33.549 119,315 10 2.4 33,626 119.650 . 4J 8 2.3 1880- 0 1 — 220445,5 -38-, 510" 1970 6 1? " 119'. 477' T - 1.9 1927 5,8 33.633 119.539 ,45 5 2.2 1970 7 2 - 125.456-,.0 33». 1.9 3- 1970 7 16 119,242 4 1 , 5. 030547.0 33,359 119.720 ♦ 1970 3 11 101645.0 38,783 8 4-970 — 8 13- 119,726 12 2.4 -07-4755,5 33.783. 11 -9+ 707- 47- 2,5 1970 9 17 0843 ,5 38.348 1 971 1 24 119.700 9 2.1 114250,7 33.396 119,267 4 ♦ IV / 1 2 12 ■“141020.3' -38717-3- ■ 1971 2 19 230142.3 33,660 -H 9V615 -I “1 v T 1971 119,831 6 2.3 6 25 120224.3 38.747 119,548 .4 6 2,0 1771“ 7“ 2 030476. 0 33.724 1197413 7tf 1971 7 11 1259 ,3 33,220 8 2.0 S l liL 119.138 .79 5 2.6 t o 137+A V -j/il 7 38,738 119+587- -.58- - 5- 2,9 1971 8 20 003251.8 33,793 119.157 6 3,8 l?7i 3 2L 132$ 1.4 33.701 119,552 .28 5 2.5 — 7 r-~ 0 3 1 0 1 2 t7 38,908 117, 683" 7 3 7 1971 9 23 3 2.3 054510,5 38.638 119.462 .32 5 ' 2,3 127.1 9 7 7 ; -L5-4213.3 38.6.92 1971 9 23 119-A 51- -A2. ~ 5_ 2.7 1971 154339,4 38.693 119.457 5 2.7 9 23 155747.1 33.699 119,465 .43 5 2.4 1974-— 9 27 1 01 q ■=; p ,, aS^-ARa 1971 -11-9 . 466- - 6- 2 . 4- 10 25 0911 2,3 33.715 119.283 7 2.1 1971 11 3 . 4-0-7 t _ 180643 0 39.000 119. 22; 1.8 1 / / I 1 4- JL J IuOjjj♦ 1 2.4 1972 1 9 162745.5 33.687 1191273 5 1.9 1972 3 1 054754, 1 38,729 119.992 1277“ '4 10 212631.0 R 2.6 1972 38.813 “TT97702 8 2.1 4 14 151413.0 33,926 119.708 6 » 4222 -4-29 f .s s i p ? p 33.611 -U.5V510 1972 5 1 092342,9 17- 2.1 33,743 119,604 4 * 1972 5 3 035731.6 38,636 119,517 .36 5 1.9 IV/1 j 1 j j j » t t 0007 2 38 *7?- — 17 97453 - 6- 2 . 8 ' 1972 5 15 133243,2 33.650 1972 5 15 734119.9 119.441 6 ♦ 38.527 119,506 .2- 2.1 1972 6 13 040318,1 38,967 119.584 4 2,6 1972 7 5 164349,4 38.861 119.601 .26 5 2,2 1 f / ^ / *’ 1 1 x8 8 -4-5 .“0 38,-5-44 1-1-9-.-480 t 45- -27- 2,0 1972 7 18 201323.2 33,932 119.002 8 1,7 1972 7 18 2101 ,8 33.612 119.509 6 3,0 1972 ' 7 24 , 5 190354 "38”, 987"““119T144 y 1 . 7 ' 1972 7 30 102536,7 33,307 119,293 9 2,7 1 9 7 7 _2____1___-.115043 .2 33.320 119,-438 R * .L . 1972 ‘ 9 15 221833,1 33.632 112,525 9 2 , 2 1972 ? 17 050760.0 38,667 ] Q7P O T7 1 1 77PQ C\ 119.475 4 2 . 3 •38,823 119+081- -6- 2.6 ?97 7 9 30 024546.5 38.746 119.554 1,9 1972 9 30 031332.5 33,750 8 fc-9??... 119.559 2 2.4 18--t2 ~ 0 8 8 -4 ± 8 t 0 - -32,-3 39 71-8t 1-41 -8- * 1973 1 3 1010 9.2 33,433 119.409 9 2.3 1223 J-17_ 125141.4 38,745. 119-.602 _AO_ 6_ 2.6 1973 1 22 154155.6 33.304 119.364 .37 7 2.7 1973 092643,1 38.345 119,169 .27 6 2,4 122

A B t-973----3“-13- -161032.5 38-. -359-- 1 19t T42--rBT- - 7 - 1973 4 13 095347.0 33.724 119.838 ♦ 70 6 1973 6 4 233242,3 33.406 119.097 .47 6 1973 'A V “034647.0 33",T34 T 17.37 6----7Z T TT. 1973 8 23 212922,8 38.654 1973 _40_ 16 091147 , A 33-. 494- 1973 10 r)C*5 • J 135021,6 33,969 119.753 ,50 1973 10 n cr 190352,4 33.880 119,762 .58 4473-TO- no -425459-45 38.335 -119-4782 ■ ,3 6- 1973 12 14 1255 7.1 38.293 119.402 ,21 1974 2 26 135220.1 38.867 119.040 0.10 t?T4— 3-&J- -73-2611.9- -3Bt743- -7 19 .650- 4t 65~ 1974 4 11 204546.8 38.683 119,443 ,30 1974_ 4 11 2108 4.1 33.699 119.431 .57 1974 "TTT TOT? i 6~. i 38.271 119,099' ,TT 1974 4 27 131422.9 38.059 119,054 0,17 D 13,45 1924- -4 - 27 --4-45255.4 38.050 119.077 0.29 B- 9^,2 1974 5 6 35057.3 38.126 119.072 2,23 D 39,42 1974 5 21- 22025,5 33,026 119.049 0.13 D 3 , 73 1974 5 27 91948.3 38.730 119,047 0,33 D 3 6 ,9 2 1974 6 21 02346.7 38.197 119.466 0,00 C 2,75 4374 6 26 -2324-23.9 -78772? 119,277- 2,42 H 53,25 1974 7 6 061040.4 33,760 119,646 .47 1974 7 14 105033.5 33,047 119.512 0,13 C 5.50 1-374 3 1 1 -0G23227 4- -33t554 -119,422- .43 1374 3 13 144420.7 33,652 119,130 .32 10 3,7 974 8 J7A1P.9 38, _ 33 Q - “ 35. 06 r.O. -? i:i t s:S 3 23 38.674 119.096 0.10 7 2.1 H 15,62 g_28 >.256_Q..55- J- - 1 . 8 - B 2.9 2 3 23 221950.3 38.242 119.210 0,1? 14 2.6 D 1.79 9 9 125956.0 33,435 119.336 0,39 11 2.7 D 1,46 •9~ f 125957.1 38,42? TT9-r-260— t48 11 -2 .3 -B 9 15 52516.4 38,126 119,006 0,05 6 1.4 D 0,42 9 27 7 453.3 33.713 119.001 0.08 10 2,3 D 14,82 -10-14- T4TT31T8 38.665 119 ,317----743 -371--- 10 27 154514,3 38.186 119.196 0.09 5 2.0 D 16,6S .1.3—1.1 2059. ..2.3 -3 S -5 2 0 - -115» 055—U.59 10 4.3 -B-77 .4 2 1974 U 30 155820.8 33,774 119,337 .25 6 2.3 1974 12 19 11 2 3.9 38,108 119.316 0,28 5 1,6 B 5.03 .+375.3- 7 - 754317,6 -37 r 3 64- 773723-3----rl?- 10 2.2------1975 12 024643.6 38.831 119,540 .35 9 2.1 3 24 090641,7 33.230 119,337 ,27 12. ffl 4 2 173419.5 38.578 119,050 0,24 24 2 7 7 ' IT 10,43 1975 5 H 235510.5 38.608 119.730 0.00 0 2.3 497-5 5 —5- 004032-^-1- -387400- -44? ,730-0-.00 0 2,0 - 1975 5 5 004114.3 33.608 119.730 0.00 0 2.6 1975 5 5 011918.4 38.608 119.730 0,00 0 2.0 T975 § 5 ' 013013.6- -3-8t408- -149,730-0.00 0 -3,2 - .1975 crJ 0134 1.1 38.608 119.730 0,00 0 2,0 1975 •J 014312.4 38.608 119.730 0.00 0 2.4 Trrrr 015540 VO -33T608- 4177730-0700- "371----- 1975 0203 .9 33,608 119.730 0,00 2.4 II5_Z5 cr e 1975 •J ._! 061027.7 38,603 119,730 0.00 0 2,0 cr cr 1975 j J 062954.2 38.608 119,730 0,00 0 3,8 c tr 197-5 J —-06332-2-. 5 -4 3 5 ^ 4 0 3 --4 4 -9 ,2 5 0 0.-00- - 0 C* cr 2 ,0 J j 065152.4 38»608 119,730 0.00 0 2.8 1975 cr cr 1975 .j 065243,4 38,608 119,730 0.00 0 n•— t 7 1 1775- J 5 0 6 j ? T 7 7 ? ■' 3a. 60a 119.730 0.00 ~xt 2 ; o ...- cr •Jcr 075624.0 38.603 119.730 0.00 0 1975 ur 2,2 1975 u Zi 33 1603_-119.730 0.00 0 cr cr. 2 , 1 - - 1975 .j 095634.0 38,603 119,730 0.00 0 2.2 1975 z 5 210045.6 38.608 119.730 0,00 0 n n 7975 5 5 - ■ 21032?» 7 ■ j u T uVO -i: 19,730 0,00 0 o o UEMLtMJW H A I L J.UJ ...II L n c n B n m n B n

123

p £ F G '1973 5 1 093543.3 38.608 119.737 0 7 0 7 " 3 7 7 1975 5 e 53414.3 38.053 119.054 0,29 2 .8 D 11.74 17755 -5T0" 1623 4.7 '3F75U8'" TT77730~0,W O' "2V2 1975 6 e 932 5.2 38.185 119,261 0,18 17 2 . 0 D 10,43 1925 .7- 49- .334___..45 9 2 .7 1975 7 24 j j j J 38,314 119,537 0.06 6 2.9 D 15.48 1975 30 055231.1 33.421 119.326 .18 7 2,0 1975 A l l -1-23637.7- -3&T-414- 44-9,332— .-41- 6 -2 .0 - 1975 14 1751 3,4 38.795 119.692 .59 6 2 .6 1975 1 195618,3 38.204 119.200 0,19 26 2 ,4 D 4,50 tTT^- -9 -1 5 - -45615.7 -38-rfr68— mT0-7^-t>r20 47- - 2 , 2 - 4 1 - - 0.51 1975 14' 4 21243,2 38.316 119.136 0.20 23 2.4 C 13.44 1 9 75 .10 .149,863...... 24 5 3.1, 1975 11 7" 075127.7 33.639 119.692 .46 8 2.5 1975 11 18 115147.4 38.357 119,280 0,37 17 3 .2 D 13,79 1975 12 '31 4i25&42r^ 3 8 t 825— 11-7,481 - ,45 10 1.3 197 A 9 102457.5 38,769 119,578 0,46 12 1,9 10 ,00 1976 24 64439.4 38.023 119.118 0.0, 10 2.4 c 10,05 1776 4~20~ '22350 7;V 3 8 7 9 3 2 1 .7 .. 10 .00 1976 4 24 21414.4 38.021 119.156 0,07 8 . 2,3 D 1.32 1975 4-26- -7144442- 38r402— 149.026 0,00 4 1,9 C 3.89 1976 4 9 P 122339* j 38.893 119.660 0.48 6 1.9 10 ,00 197 A 5 20 83515,3 38,707 119.387 0.29 7 1.6 10 .00 1976 -5-24- 105411,9 30 .712— 419 *380- 0,33 7 1.7 10 .00 1976 6 1 23 423.2 38,161 119.645 0.26 7 1,8 D 15.27 1976 6 3 1019 4.3 38.043 119.162 0.00 4 2.0 c 11.42 t m — 6—r 111548.8 38,301— 149-7397-0466- 46" ~ 2 l 7 ~ 7 0,25 1976 7 6 104039.7 38.412 119.357 0.14 20 2.5 c 10,72 •1976 _7 40 5 5 1 2 8 .5 38.456, .4 1 5 ,3 7 2 .0,19 16 2.8 C 8.55 197 6 10 19 65716.1 33.322 119,492 0,15 7 2,7 D 6,40 4976 10 22 165052.3 38.544 119,661 0.08 14 3.5 C 8,39 >1-9-74 -12- - 4- -1-33-944-.-2— 33-r767— 1-19,704 0722 12- 2,5 c 5,15 197A >2 4 133937,0 38.917 119.941 0.12 3 3.9 c 2,75 •1977 2 5 223642.7 33.006 11?.053,0,35 4 0 , 1.9 D 17.15 20- 2 .2 19 ,90 “ 624°6.!4 33 .*"438” "149! 235" 0! 12 15 4,9 r 6,71 631 5.1 38.486 119,290 0,20 16 3.0 C 11,36 -433647,7— 33^276— 149,163 -0,1-6 9 1.8 D 5,53 231707.4 38.936 119.063 0,31 2,4 10,00 2015 1.4 38.130 119.263 0,22 13 2,2 c 17,90 57102-,-l— 38t 865— H-7T0-3-3-0-. 32 4-4- 1 .6 - 2 0 .9 0 23254.6 33.358 119,396 0,30 12 1 .7 D 10.15 105133.0 38.299 119,371 0.29 15 2.0 r. 10.93 32917.1 38.049 “ 119,007 0.05 6 1,5 c 12,33 92548.6 38.613 119.392 0,23 5 0,0 10 .00 4 2 3 7 5 0 .9 -49-W -77-449.624 0,35 9 1.1 io ,00 18 112647,7 38,631 119.567 0.24 8 l.S 10 .00 1014 6.9 38.313 119.140 0,21 11 2,7 r . 10,59 18tS23T4— 367787— 1477490 t>", 06 7 0,0 ~ t 1,45 2 543,9' 38.696 119.592 0.03 7 1.9 c 9.32 182734,0_ 38a 700,_449,597- 0.10 8 4.4- c 8.55 7 726.8 33.332 119.704 0.13 15 1.9 r: 9.26 3 121.1 38,377 119.384 0.22 12 1.9 c 12,62 1 3 2 2 3 ,9 — 38-1-66-1-— 4 1 9 .4 5 2 0,23 21 2,6 C 3.67 1411 1,4 38.743 119,426 0.16 13 1.4 C 12,84 111540,9 33,731 119,783 0,08 14 1.7 C 13.52 1120 5 7 0 — 5 8 t 775— 147,774 0.17' 14 1.9 C 14.11 112553.4 38,794 119,790 0.14 21 2.9 B 7.05 45231,9 3 3 ,796 119.795 0.14 19 4,6 B 10,01 “4595072 387798‘■'119,784 0,05 9 0.0 C 11.76 I • • 1 h— ' P7* ‘IH * r 1 ■| 11-4 |»— I—-1—I •— r|_fk r i—l—i i—f=* f— r— r— t -* ft—*■ pTt T^7*- i—c itl*. .—:. . » - 4 -»-* f. -O -O >OSD}0 -0 o-jo •O'O ^ *T^ *—- t“. ^ r-—W f* ' r—. pp n— I— h1Kl-*PHI^h ' -J NJ 'J ‘'•J SJ --J vj vj *-..j I'd -of-o-0'0'4'0'0't> --0-0 M3 <1 -p'-o -O -jO --0 CO C£ CO ix» jx> CO CO C O COCOpii 03 C^COCO CO OOCO j# CO CO ij COo5|i^5 CO cfjCOCQ f j o COC O p S -jS vJSjvISivJvji.jvJ 0^06 6 1 CO 03 <33 133 COiCO CO Cp CO 03 pO 0.! 03133 CO 0| CO 03 CD 03 03 p3 03 aji 03 03 CO CO » ^k I—f*"*| *• -‘*" » ^ ^—*- i—fc •I -* t r - §**»— ! 1 l> * fc O P O o p - O - 4 -O-OM3 < ,-0 fO -0 P 1V3 <1 -jo -O'-Oj-O -O -3 -O-O-jo <>■ O |o -O s,j• o -O sb 'O 'oj'O N 3'C'O S3 V v 3 ’V3N3-O -0 4 ■ O-O fO -0 • ii-o O n3 -o fj ro M h - f*-* j |\ji\ j i—ri—^ ^ (-.r— . > -t* -#■ -*> £> -£► ~t> f— o n d n j q s n _t* _i>. ■o co 133co Co |0 vi ' J v! -j ' jvjo-t.nr.ncnij\ cn ,1 CJ3-C. - > .C* ^.i-C. JS..C1- .C. .b- _c._c.pi ji -fcf-c -t. -c. c.-fc. .fa .fa. .4 -c. .c. * j>

• ro ro 1—*■ *- r-. r~. -* ro i-f i-* ijo r o w LH -i ► O O ~*o- o*.» of a e n ro r j ro ro ro ro r- 3 ro ro 1 o ro rofco ro r-J r-o ro r o ro ro SSWWVlWCh I . H M u i o o W h-^o oC'Cncot -iCNcn .ncno; -c.wr.jww cncfi ,n ror-j w k w c n w _n cn o 4_n c.i r.ncn w w ro ro w 1 o ro ro w ro k ro no i4* >-* o o r-. oicncni.n fa CD L C O O ' c n w cn^> W -C. C 3 I—. I-. C -t»-t> ww w r o r - . ci cn fc- Cr- -vie* C O O fa-vj 03r-. o W O j-j -r» o co co if-1 cr"0 Vi C.J c Jro J> • J <3_t> C. . v V O W O M O 1 w r o cn -£»■ Cii fa-t. cntocni-. ro rocnci cnro -t i-*Mcn c n i i . n w cn W C>'Ocn' -J0'!-'N3r-*c. r-'t-.J.vJO' O fa '.!&'03 ^--o CO 0) CN^> -t» * r* L W W - :> cn~fc. C. -fc.ro.' w w c.j cn r 3W-fc.fJ0>r-.p»cnc. -fc. Cl .n cn c i cn fa . =. cn OWCM)J |vJH vicoiv. c n n o o 0 - 0 >J'OC'l>Wl(.ilWW 03o^v jcno-i,nvivj j h s ♦ ♦ * * roi-. iS v w 133 --o c "O v<.nnoo3 w c O' ro r 3 rov i o vi c i h * no • fa cn h 4 > 0 ) S | o vi o (-* r-.cn v i c n c c o j > o o v i j - i o vi w i-* on w -t> ~ o co 4-ro--o qo cnrofo co t>)o ro rp vj -> co w c - o O' 1 n- cn -t. oocn'0C3rj'0 ocnoicnfa ir-vi W W W W W W W W W W _____3JWC. W W W w w jJ w o i w w w w w WWCJC.JC.IW W W : o w c. 3 . 1 j w w 00 Op 03 03 D 03 CO j3 0303/3303 a 0 3 0 3 0 COCO 33 03 0 ! I33COCO 130 03 c.i w c w w w w c i w w w w w w w c w w c >j w o i w w t o w CO 133 C 30303 ( 0 030303 CO a C O C D C p C0C0|O3 '33 Cp 00 133 93 COCOJ33 03 O CO CO CD 133 CO 23 030103 CO U3 03 ■o-'Oioco viitncoco Vlvl-'JCDVI vl 03 O 103 vl ‘ O CO vj CO 133 C 3 CO 03 'J 03 Vi 33 VI v VJ co SJ VIVI 03 VI cpco VljOOS VlfvICO vlcoco V VIVI >-“ C ' C O C O O h --o no t-.cn ■ h^-cd -o i - - M r-ocn -to ro o O ' J VICO CO ©3 00 wro j-OMiioro-ooco ■< •o cn O' v vj o o c i c n c o o C.OOj'O O ' P O O 'P CO--0(-OCi"4lOi-‘ifc>0O' IIO I-'O o O0)i.^OSI LIp-WW O ’O --91331—. -jo OH - . f-. K -c. -. •>! ro cn 1 -C. w ■; >cn w c. v cnvj ro cr O'Mr-.WCJCO to

jr-. a o cn cn I-.C0W cn wfa <3in03i-.cnfa cpo -oo cn -o cn v -iicr- cr"6 r n oooopoopoooopooooobopoooooooooopoooooooooo4,poooooooc|ooj>ooooaoi> 0 0 0 O Mi r-.nOr-.nOi~.! v of no ro to cn > vo vo fa V 03 o V cn CP cn fa ^ iw w V V 133 ^ V V ^ oi> 1^ w Q olfa V w co^ o > fa cs w fa V w iri cn. fa W ^ 0003 V w v i cn --o O' 1-. lo I > ro no no no w tt fa Wf a - o f a r o ^ O' V'On-‘Ci'c.nroa3fa 0 0 v v wc o o o o Or - . ^ -o O'cm-* -o o o m c o M w <3 o - facovj co O o ’cipocnrocno^cnKifaMof. oc n o wen H^nnoonoonnnqnriooirtewoonDowtdwnowoooonndoodnonnrtoowonbestiJnowoohrjciwown I - - r—. *—. r—* r-1 ■—' i—. i—* r-. t-> +— — *— >-. ^ ^ _ > 0 Q S^SSi^S2Sas^a£SS!3GSS£3K^5S3§Sg®SSS§Ki:5SS83as:stJSgr:gSCa353SI2S.

ro 125

P F G

S8 :n ? i:- H 1978 12 13 354 8.1 38.810 119.77? 0,07 11 1,7 C 10,23 4973- 4-2- 4 -4— 1—4 , 6 -38-^9-84-419.332 0.11 10 1,9 C 8,12 197? 1 a >23227.6 38,800 119,773 0,13 7 1,5 r 6.60 3197? 2 2 182925.4 38.790 119,79? 0.13 22 3,4 B 6.76 17? 7 9 - 4 -H )5 6 2 2 . 9 -30.454— i 19-.-421 0.34 6 3.3 - 13,30 9 79 d 24 91344,0 38,731 119,796 0,05 10 . 1.4 C 12,59 497? 5 2 131851.2 33.53? 119.501 0.13 12 2.8 C 7.54 i t m 3 " I B ------9 5 2 1 5 ,6 •33 ,579— 419.-547 0T26— 3" "1.?- ~D 8,40 11979 5 1? 20 826.7 33.662 119.815 0,15 10 2,2 D 2-14 157.9. 6 . 3 _ 1354 1,5 -3S-.773 419..782 0,19.16 2,0 D 3,10 XL97? 6 23 172746.6 38.261 119.361 0.17 7 3.4 10.00 1197? 6 23 185130,8 38.275 119,358 0.15 12 2.1 D 3.96 — 6 -2 ? -----24740 . 9 -38.284— 419-.-331 0,10 -7 2.0 -D 1,74 197? 7 25 02630.0 38.534 119.801 0.10 10 1.9 Ii 13.98 197? 7 25 002632.0 38,632 119,782 ,11 10 , 1.9 1 7 7 7 T 75— 1714 7,7 "387753— rr?7327~070j"T7 ' 173' ~B 7,23 197? 3 13 22 7 3.4 38.791 119.551 0.06 10 2,1 I 5,47 L22S__8. 1.5____5 2 0 5 3 .4 - -38.663 —119..440 0,03 -5 1.5 -D 1.48 197? 3 18 14713.8 33.428 119,359 0.02 6 2.0 D 1 ,33 1979 8 18 24536.5 38.431 119.357 0.06 7 2.0 I 1.54 17-77 8 1-7- - 5 43 -5.7 -38-r4-34— 119,357 0.11 -7 2.4 e 16,67 1979 8 18 213325.6 38.069 119.032 0,20 8 2,3 c. 14.68 1779 ft 30 01724.0 38,077 119,068 0.04 7 2,0 r 9.65 1 7 7 7 “ TO 7....205441,2 "387233— FI? ,352 0712 12' 4.9 "~C 3,44 177? 10 7 21 5 7.9 33.215 119,352 0.23 10 3.0 c 8.14 1 7 7 ? 1.0 7 . 2 1 . 95.9 .7 ■38.244-119,358 0,10 11 3.9 c 8.45 1979 10 07 211000,1 38,246 119,351 .15 11 3.8 4,60 19 79 ’ o 7 212052.7 38,243 119.359 0,10 10 4.4 r . 8,0? 4-7?? 1-0 7— 243055,7- -38-,-240 -119.360 0.12 11 2,3 r . 9,24 4979 io 7 214117,5 33,237 119-331 0.23 11 1,9 I 8,87 197? 10 7 214523,4 38.213 119.335 0,11 10 4,1 r: 3,57 C9 7 T 10— 7— 22 4 ? 4,3— 1? .341 0713 If 3.4 r . 9,68 497? 10 233244,4 33,208 119.337 0,15 10 3.7 c 9,03 I??? 10 -23.44 8 ,7 38x215 _ 419.3310.12 11 3,3 C 3.33 1979 10 7 234517,3 33,219 119,339 0,09 9 3.0 C 8.6? 1979 10 S 02354,3 38.242 119.365 0,17 11 5»6 I 8,52 4979-10- 8— - 04744♦ 5— 38-r226— H 9 ,358 0.13 11 4,0 £ 8,55 1979 10 3 12946.8 38.235 119.353 0,15 12 2.8 n 10,14 4979 10 049,3 38,236 119.365 0.20 12 2,8 i 9,36 1777" TO 3 2~"T53,3 ""387244 41973737)713 TO 3.0 £ 9,01 1979 10 8 2 231.0 33.219 119.310 0.42 3 3,3 I 27,61 L9X9- 10- 8------3 5 4 2 3 , 7 -TS-kOOS— 119.331- 0.12 1-0- 4.4 £ 9,14 197? 10 8 92356.0 33.205 119.329 0.11 12 3.0 c 9,08 1979 10 S 93836.0 38.221 119,333 0,10 10 3.7 C 8.26 47-79- 40- 1214 1 ,6 --5-&T-2-5?— 419,356 0,11 10 4,0 € 8,29 497? 10 9 22 010.1 38,222 119,340 0,10 10 4.2 C 7.58 497? 10 9 23 730.9 33.245 119,364 0.12 10 2.1 c 9.48 ItT99-tit-- 7 — 733356,0 -3-8T226— tl9 .34 f-0 .il 9 2.0 c 9,16 197? 10 10 62126,2 38.213 119.339 0.16 11 1.7 c 10,05 1979 10 10 63130,7 33.225 119,344 0,.17 9 1,8 I 4,66 4977" TO 10 12 4 6 5 5 7 7 " T §_____ 7 2 4 9 119.361: 0.16 10 1.7 c 9,48 197? 10 10 125518,7 38,243 119,363 0,28 14 2.8 c 7,05 477-7-1-0-4-0- -44-1-72-2.0- -38.216 . 149-,343 O f 9 1-1- 2.0 I 7,95 197? 10 10 1419 1,1 38.230 119.332 0,19 11 1.9 r. 3,16 197? 10 10 144131.4 2,9 1779 ro~ 38,224 119,333 0,13 11 c 3,21 Itr -1926 6,9 •38.245— r t f 366 0.1 Z ~ T T 2,9 c 9 »56 1979 1.0 10 194934,9 38.235 119,353 0.14 10 2.0 c 9.65 197.9T0 U ..21522.6— 3ST236 ___149.357 0.18 12 2.1 c 10.41 1979 10 il 52525.6 33,620 119.277 0.11 14 2.5 B 13.64 1979 10 11 95351.8 38,246 119,366 0.14 12 1.9 C 9.21 1979 if - ii— 151110.0— 59r217— Hr9.335 0.12 11 1.9 c 8 .88 1979 10 11 153137.2 38.235 119.357 0.17 11 2.3 c 10,30 '197? 10 li 203545.1 33,232 119,338 0,11 9 1,6 c 9-57 IilJ»WJUl,.J.LL..LI.UI. INI I- 1-- I iMILKJU I - 1II LJ.J-11 II 1.1 II III .111 I ll-IIMIIIII......

126

3 D E G >177?'1CT V. '31412.1— 337247' — 1 7 7 .3 6 7 ” 0.11 12 1-3 'C 8,60 1979 10 12 44447.? 38.24? 119.371 0.11 10 2.6 D 10.54 1925_.1.0—12. __11?.340 0.07 3 2.1 D 2,02 1 9 7? 10 13 Jj j i 33» 119.342 0.11 9 1*? C 11,07 >1979 10 13 142012.5 38.246 119.366 0.15 11 2.8 C 11,11 -1 0 -1 4 - -336- 5-. 8- -38 .-2-21- — 1 4 ? .3 3 5 0.13 9 1*9 D 9,23 19 7??..10—1-4— 145355.J— Z 2 . J 2 Z I 119.34 2-0-^0-9—-6 -2 ♦ Q -0 -4 -0 .5 4 1979 10 14 145400.1 38.250 119.339 ,15 7 2.0 1979 10 16 12348.9 ““ ” “ 119.357 0.13 9 2.3 C 11.28 43-30- 940Th ■ 38-. 2 4 9- 119.364 0,09— 7—3-r2- -Q— 9-r98 197? 10 24 83124.1 38.246 119,36? 0.15 10: 2.0 C 13,02 197? 10 24 83548.5 38.244 119.372 0,13 10 1.9 C 11,40 vm- 10--54- 0 50 4 6'>5- ■38-7-246—ITi 119 ,3'?2- 0-.17> tfr-fr -e-±0r«9 1979 10 25 143055.8 38.247 119.367 0.1? 11 2.0 C 1L.11 1979 10 27 121348.7 38.248 119.346 0.13 10 1.8 D 9 <94 1779“ TI— T 1'3'42“ 378' 38,23/ 119.339' O rr3 7 ”~2 .“0” “CTTO .5 0 197? 11 7 211135.0 38.248 119.370 0.14 9, 2.3 C 11.05 197.9 JJ— 20_ 721 7 5 8 , 4-. .Za+252. 119.373 0.15 -71 2.-0- - C - 1 5 . L 0 1979 ’ l l 25 84335.8 38.249 119.363 0.07 7 3,0 C 16,61 197? 11 25 951 4.9 38.243 119.363 0,02 7 2.4 C 16.57 1979 11— 2 7 1 1439' 3 v 2' 3 8 v 4 40- 119 ,061 0 .1 0 ' ♦ £~ -e~t4T28 19 79 12 13 104434.8 3 8 .2 6 3 119.344 0.20 10 2,8 C 11.91 1.97-2- 17 13 _11352B.»JL 38.258 119.344 0,11 10 2,8. _C_11 .0 9 1979 12 14 011746.5 38.571 119.613 0.43 / 2.2 i960 2 7 93735.3 38.042 119,040 0.18 3 2.1 C 15,54 4790- ■ 3 - 4 3 - 1 3 2 0 2 0 .-5- 38 .304- 119.785-0 r0 2 - 6--2-rO- - € - 4 1 .2 7 1980 3 22 141154.8 38.789 119.787 0,01 3 3 , 5 C ..i ♦ 10 1980 5___5 192625.5 38.433 119,073 0.19 7 2 . 3 C 12.31 rteo- 7 01 -1-50765T9- --38V288- 119.314'"' .1-3“ -TTTrtr 1930 7 3 205255.5 38.767 119.300 0,04 2.7 C 19.82 1980 8 11 195130.9 38.790 119,686 0,03 3 1,6 C 14,74 ITBO”TUTTS” "2 7 4 7 2 0 7 7 ” 3 8 7 7 7 7 “ T T 7 7 7 7 2 - 8' “171” 1980 12 08 070620.9 38.690 8 1,7 9,10 i?.,oa. 077307.7- -IfuAaEL _U.9_.45Q__ .19- a _l-.5- __ 11.80 12 03 165649.2 38,692 119,471 .17 10 3.7 15.60 12 03 172353,9 38.702 119.452 . 12 11 2.8 13.10 •i n n o n -7 _ *7 n / nn 11Q A 7 *1 .4-7..___7 ... 12—06" l’/iT£/T7 Ju \ O/U 11 / l *T / *1 Jm . I 12 08 181838.7 38.691 119,457 ,15 8 2,1 14.70 12 09 203440.8 38,690 119.478 ,40 9 2.2 ITc W 130923.9 38.689 119,445 .1? 8 1.2 11 .'50 1980 12 10 130953,8 38,671 119.448 ,33 8 2,1 15.30 1950 -12- 1-55-1-50-T-2- 33.-862 --447w53-0^22- 5- -1-.7 -D— 3,-38 1931 2 18 13543.6 38.812 119.796 0.24 10 10,00 1931 3 23 173035.3 38.27? 119,415 0.24 9 2,6 10.00 rrar "7 17 32030,3 33 ;71?- TT7T61S6T .“20"' IT -277” 1931 9 1? 15 515,0 38,782 119.531 0,05 6 1,9 C 3.96 1981 22_27_ 2_Z1 402.1 38.747 J2L9.5 8.1-0,05-27 Q.S- __ 6.40 1961 11 27 172334.4 58,743 119.577 0.07 6 0,8 3,60 1981 11 28 51520.5 58.742 119.580 0.03 6 1.0 3.40 1952— 1-25- 142746.3- 33-,306- ■119,784-0-^ 3 4- -2-.-0- -G— 7,2 2 1982 1 41834.3 38.630 119.787 0,23 9 2.2 C 1,68 1982 5 9 191916.7 38,310 119.057 0.12 14 2.9 A 8.67 5-54- 103031,-7" 4fr74?3- 119,340 O -.'l-l- "IT -2t 4 -443.-7-7 1982 5 13 103953.7 38,401 119.333 0,12 13 B 13,76 . y.v-i-vv v.-__A X / t .L .V . J . ..V . / ♦ 6 10 - t r o - 38.786 119,638 0.23 8 1 . 7 c 0,85

1982 6 10 2125 2.4 3 8 .7 9 8 119.621 0.11 10 40. ♦ 7/ c 4.61 1 007 77 163ft 4, ft 3ft "?96 119.778 0,31- -IQ. n _L7 Oft 1982 3 17 1417 7.7 38.665 119.540 0,14 10 2.7 B 14.93 1982 3 27 133620,9 38.800 119,774 0,30 7 1,6 c 13,95 r? 8 2 “ y ~ 4“ i j 752.1 38.827 119.766 “0 .2 ? ) 1 .?' L — l'T t a 1982 9 9 85736.4 38.632 119.480 0.24 6 1.7 c 0.71 r« 19R2. —3—_23_ ..-Q32.-LHA- 3 a n o a _ 119. 39-1-.Q u a.. -U- .2 ^ a _ -XL 7.^93. 1982 10 8 102937.5 38.263 119.126 0,24 8 2.2 D 14,47 1982 10 9 112043.1 38.262 119,102 0.12 15 T 1 B 10,65 4702- -H— 4- 9 3 6 3 4-,0- 30,938- -H 9 . 2 2 5 --OvO-7- - - 9- 4 V 7 - -B— 9 r89- >-982 11 6 17"027.8 38.837 119.238 0.09 Ii 18,24 127

APPENDIX A: PART 2 Focal Mechanism and. Stress Axis Data Refer to Table 5 and. Figure 6 for additional information.

All mechanisms illustrated here are on equatorial equal-area stereonets except for number 8, which is on a lower hemisphere equal area stereonet. Composite mechanisms were selected on the basis of spatial and temporal similarities, but the results should be viewed with caution, as in most cases., ' C' quality events were used and the depth may vary as much as 100%. These events were not relocated for this study. For numbers 5 and 12, the style of faulting, whether strike-slip or normal, could not be determined from these results. The extreme possibilities are illustrated here. Because the data on depth may be faulty, the conformity of this region with the findings of Vetter and Ryall (1983) on a change in focal mechanisms with depth cannot be confirmed. It is not ruled out, however, and as it stands, the data support their findings. The data for the quakes used in compiling the composite diagrams is shown at the bottom of each page. The data is listed as follows: Year Date Time Lat i tude Long i tude no. stations magn i tude qua 1i ty depth p

128

0

©South end Smith Valley - iO/ll/79 Depthi 13.66 km

© Bridgeport Swarm Compoiite < 10 km depth Time Mag. 1979 10/10 38°13.'*3' 119°20.6l * 9 0 5.50 S T iT 1.93 1979 10/10 38 I k . 89* 119 21.66' 10 0 9.82 1216 1.89 1979 10/10 38° l k . 58* 119°2I.76' Ik C 7.01 1255 2.82 1979 10/7 3 8 ° l2 .k 8 ' 119°20.21' 10 c 9-50 2332 3.68 129

1978 10/11 251 38°31.75' 119„30-03 * 1979 5/02 1318 38°35.28* H 9°29.92' 1979 5/18 9521 38°35.27* 119 30.01• RRH

1 3 0

APPENDIX B: Holocene Scarps in the northern Sierra Nevada Great Basin Boundary Zone near the Sweetwater Mountains: Location, height, displacement and slope data.

Location is given using township and range lines where available; if not, nearest road junctions are listed.

Parent Material refers to the type of material which has been displaced. Scarps in coarser material such as bouldery deposits tended to have higher slopes. In most cases, parent material was unindurated.

Apparent Height refers to the height of the scarp before the regional slope in which the scarp appears has been measured. In other studies (Bryant, 1983, 1984; Clark, 1967) this is the only given value, butas is shown below, the apparent height can often be much higher than the actual displacement.

Displacement refers to the calculated amount of actual^off- set during- the event which caused the scarp to form. This has been calculated using the diagram shown in Wallace (1978), and takes into account the regional slope in which the scarp occurs. The slope of the fault plane is arbitrarily placed at 70°.

Slope is the highest value recorded during the measurement of the scarp profile, using the Brunton Compass. 131

Antelope Valley Fault Zone Apparent Displace- I.D. no. or Location Parent Material Height ment Slope Source

T9N R22E sect. 12 NEi NWi Coarse gravelly alluvial 5.5m 2.3m 24° 1-28-01 (s of Slinkard Creek) fan material

T9N R21E sect. 23 NEi NWi Coarse gravelly alluvial 5.0m 3.0m 22° 1-15-01 (w of Topaz) fan material

T9N R22E sect. 36 SWi SEi Coarse cobbly alluvial 4.5m 3.3m 34° 1-47-01 (2 km N of Coleville) fan

T9N R21E sect. 26 SEi SEi Sandy to cobbly alluvial 7.0m 33° 1-49-01 (near landslide) fan

T9N R22E section 26 Alluvial fan 6.7m 32° Bryant, 1983

T8N R22E sect. 12 NEI Bouldery colluvium 5.0m 4.5m 37° 1-25-01 (n of Meadowcliff) 132

3 2H 1 3 2 1 I___ 1__ _ _ L METERS

Antelope Valley Fault Zone, cont. 1 A n n a r e n t OiSDlace- 1.0. no. or Height ment Slope Source Location Parent Material . 5.0m 32u Bryant, 1983 T8N R22E section 12 Alluvial fan 4.5m 23° Bryant, 1983 T8N R22E section 1 Alluvial fan 4.2m 2.6m 25° 1-20-01 T8N R23E sect. 19 SEi NEi Coarse sands and (n of Walker) grus 5.0m 3.0m 25° 1-21-01 T8N R23E sect. 19 SEi NEi Coarse sands and (n of Walker) grus 4.0m 3.0m 25° 1-34-01 T8N R23E sect. 29 SWi El Bouldery fan and (Mill Creek) fluvial sediments 3.5m 3.0m 23° 1-34-02 T8N R23E sect. 29 Swi El Bouldery fan and (Mill Creek) fluvial sediments L

Location Parent Material Height ment Slope Source

T8N R22E sect. 23 Pleistocene alluvium 4.0m 14° Bryant, 1983

T8N R22E sect. 14 Pleistocene alluvium 9.0m 20° Bryant, 1983

T8N R22E sect. 14 Pleistocene alluvium 4.5m 2.0m 20° 1-18-01

T8N R22E sect. 23 Pleistocene alluvium 3.0m 1.7m 19° 1-19-01

Sonora Basin

T6N R23E sect. 20 NEi SEi Tioga outwash 4.7m 3.0m 25° 1-36-01

T6N R23E sect. 20 NEi SEi Tioga outwash 4.5m 3.0m 22° 1-36-02

T6N R23E sect. 32 Tioga moraine 6.5m 35° Bryant, 1983 T6N R23E sect. 32 Tioga Moraine 6.5m 32° Bryant, 1983 Apparent Displace I.D. no. or Location Parent Material Heiqht ment Slope Source

Smith Valley Fault Zone

T10N R24E sect. 30 SWi Colluvium 11.0m 7.5m 33° 1-59-01 (s of Wellington)

T10N R24E sect. 30 SWi SEi Alluvial fan 5.5m 4.5m 24° 1-60-01 (s of Wellington)

T11N R23E sect. 34 NWi SEi Alluvium 9.0m 5.7m 24° 1-32-01 (n of Hoye Canyon)

T U N R23E sect. 27 NWi NEi Alluvium 13.0m 7.5m 28° 1-07-01 (Wedertz Canyon area)

T12N R23E sect. 21 Alluvial fan 10m 3.5m 28° 1-06-01 (Nevada Hot Springs)

T12N R23E sect. 21 Alluvial fan 7.0m 3.3m 36° 1-05-01

T9N R24E sect. 5 NWi Finegrained alluvial 3.0m 2.5m 19° 1-60-02 (Valley floor near Desert fan Creek mouth) 135

1- 62-01

METERS

ADoarent Displace- I.D. no. or Location Parent Material Height ment Slope Source

Bridqeport Valiev Reqion

T4N R25E sect. 5 Valley floor alluvium 1.5m 10° Bryant, 1984 T4N R24E sect. 3 Tahoe moraine 10m 20° Bryant, 1984 T4N R24E sect. 3 Tioga outwash 4.0m 22° Bryant, 1984 T4N R24E sect. 10 Tenaya(?) moraine 8.0m 22° Bryant, 1984

Sweetwater Valley Area

T8N R25E sect. 31 NWi SWi Alluvial fan 3.3m 2.5m 23° 1-62-01

North Sweetwater Valley, Alluvial fan 5.0m 27° 1-41-02 near Risue Cyn Road

North Sweetwater Valley, Alluvial fan 3.5m 18° 1-41-01 near Rickey Mine Road