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FAULT ORIENTATIONS OF THE SIERRA NEVADA FRONTAL FAULT

ZONE IN THE VICINITY OF LONE PINE, CALIFORNIA:

IMPLICATIONS FOR BASIN AND RANGE EXTENSION

Brian Gadbois Undergraduate Thesis

Draft #4: July 14, 2015

Advisor: Dr. Phil Armstrong

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ABSTRACT

The eastern boundary of the Sierra Nevada is defined by a system of east-dipping normal

faults known as the Sierra Nevada Frontal Fault Zone (SNFFZ). It generally is assumed that

these faults dip about 60˚ E. Recent work on the northern section of the SNFFZ near Bishop

shows normal faults that dip 35˚-46˚ - much less than the assumed typical dips. Pleistocene to

Holocene extension rates are 0.2-0.3 mm/yr., but these estimates are based on a 60˚ dip. If fault

dip is shallower than 60˚, computed long-term horizontal extension rates will be significantly

greater than initially assumed because the horizontal component of slip will be as much as three

times greater than expected. My new work shows that normal faults west of Lone Pine dip 35˚

E. This study uses detailed fault mapping of surface exposures west of Lone Pine at Tuttle Creek

across ~300 m of elevation change to further constrain fault dip. Fracture data were collected

from footwall structures in granitic rock along the main fault in order to evaluate relations

between fractures and fault orientations. Normal fault and footwall fractures at Tuttle Creek dip

~34˚ E. Estimated long-term extension rates based on a measured dip of 34˚ are an average of

0.74 mm/yr., which is two and a half times greater than those based on assumed 60˚ dips.

PURPOSE AND HYPOTHESIS

The Sierra Nevada Frontal Fault Zone (SNFFZ) includes a series of east-dipping normal

faults located at the eastern front of the Sierra Nevada Mountains. Various studies have been

conducted in Owens Valley and the surrounding Basin and Range region in order to gain an

understanding of how faults in the area accommodate stress. Unruh et al. (2003) concludes that

normal faults along the SNFFZ presently accommodate northwest microplate translation rather

than Sierra Nevada uplift or Basin and Range extension. According to Mohr-Coulomb failure

criterion, optimal orientation for normal faults is 65-70˚ (Byerlee, 1978). Therefore, it generally

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is assumed that SNFFZ normal faults also dip ~60˚. Le et al. (2007) combined alluvial fan

surface ages derived from cosmogenic radionuclide dating with measured vertical fan offsets to

calculate vertical slip rates of the SNFFZ using assumed fault dips of 60˚. Recent work by

Phillips and Majkowski (2011) shows normal faults near Bishop dip 35-46˚. Shagam (2012) and

Mottle (2015) also show sections of the SNFFZ near Independence and Bairs/George Creeks that

dip 22˚ to 34˚. If these shallow dips are correct, then horizontal extension rates based on 60˚ dips

should be recalculated because the horizontal component of slip could increase as much as three

fold. I hypothesize that the normal faults along the SNFFZ near Lone Pine dip at shallower

angles than generally assumed.

GEOLOGIC BACKGROUND

Owens Valley is located just east of the Sierra Nevada Mountains and forms the

western boundary of the Basin and Range province (Figure 1). The valley is bound by normal

faults that make up the SNFFZ to the west and normal faults that make up the Inyo Mountains

fault zone and White Mountains fault zone to the east (Le et al., 2007). During the Paleozoic, the

area that is now Owens Valley was a marginal marine environment dominated by thick

sequences of sedimentary rock. Sometime in the late Paleozoic the continental margin was

transformed into a subduction zone when the Farallon plate collided with the North American

plate. Arc volcanism was prevalent throughout the Mesozoic culminating in the formation of the

Sierra Nevada batholith (Saleeby, 1999). Chase and Wallace (1986) hypothesize that cooling of

this magmatic arc resulted in its elastic strengthening, which prevented isostatic equilibrium

during erosion in the Cenozoic. They further hypothesize that rapid uplift was initiated after

Basin and Range extension broke the elastic plate holding the range in place.

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Figure 1. Google Earth image of the Basin and Range Provence and its bounding fault zones in the western United States. The Sierra Nevada Frontal Fault Zone (SNFFZ) is the western margin of the Basin and Range. Owens Valley

lies immediately east of the SNFFZ. The Wasatch fault zone bounds the Basin and Range on its eastern margin. The

Basin and Range is named for its horst and graben topography. Generally, downthrown graben basins are bound on

either side by normal range-front faults. Range-front normal faults of the Basin and Range are assumed to dip at

angles greater than 45˚.

Faulting during the Miocene was initiated along north-south striking faults in response

to east-west extension. About 8 Ma, extension shifted from east-west to northeast-southwest.

This shift was accommodated by the reactivation of preexisting north-south faults (Phillips and

Majkowski, 2011). Jones et al. (2003) hypothesize that about 3-4 Ma, a delamination event

removed the batholithic root beneath the Sierra Nevada resulting in increased uplift of the range

and a relatively rapid increase to extension rates immediately east of the Sierras. Modern Owens

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Valley most likely originates from late Pliocene tectonism in response to increasing extension

rates resulting from the delamination event (Unruh et al., 2003). Today, northeast-southwest

extension continues and is accommodated by reactivated Miocene faults (Phillips and

Majkowski, 2011).

The Sierra Nevada and portions of the Central Valley that make up the Sierra Nevada

microplate are moving northwest ~10-14 mm/yr, and rotating counterclockwise relative to the

stable North American plate resulting along dextral, normal, and oblique faulting (Argus and

Gordon, 1991). Many studies were conducted in these areas in an attempt to interpret how stress

is accommodated along the range front and in Owens Valley. Unruh et al. (2003) focuses on

active Quaternary faults, mainly of the SNFFZ, and their orientations on the eastern margin of

the Sierran microplate. Left-stepping trends observed in normal faults of the SNFFZ are

consistent with accommodation of Sierran - North American motion. The eastern California

shear zone mainly accommodates 11±1 mm/yr of right-lateral shear on the Owens Valley -

White Mountain and Fish Lake Valley fault zones (Dixon et al., 2000).

A majority of this study is focused on adding to or confirming the work of Le et al.

(2007), therefore it is important to understand their work on the SNFFZ between Oak and Lubkin

Creeks (Figure 2). The SNFFZ is predominantly east-dipping north-northwest-striking normal

faults, offsetting Quaternary sediment or granitic bedrock in the footwall. Vertical slip rates

calculated by Le et al. (2007) are the first to be based on numerical ages derived from

cosmogenic radionuclide dating of offset surfaces along the SNFFZ. Abandonment ages of fan

surfaces range from 125ka to 20ka. Assuming a starting fault dip of 60˚, these ages coupled with

vertical fault scarp offset measurements yield vertical slip rates of 0.2-0.3 mm/yr and horizontal

extension rates of 0.1-0.2 mm/yr (Le et al., 2007). Recent studies by Phillips and Majkowski

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(2011) and Shagam (2012) show faults of the SNFFZ near Bishop and Independence dip

shallower than the assumed ~60˚.

Figure 3 shows a hypothetical example that demonstrates the effect of fault dip on

horizontal extension rates. Assuming a normal fault dips 60˚ and has a hypothetical vertical slip

rate of 1.0 mm/yr, the horizontal component of slip is calculated as 0.6 mm/yr (Figure 3a).

Changing the fault dip to 30˚ while keeping all previous parameters the same yields a calculated

horizontal component of 1.7 mm/yr (Figure 3b), a 300% increase. If these shallow dips are

correct, then the horizontal component of slip could increase as much as three fold.

Figure 3. Hypothetical example of how fault dip effects horizontal extension rates. Vertical slip rates can be

determined by dating surfaces offset by faults. Horizontal extension rates can be calculated using trigonometric functions. (a) Assuming a normal fault dips 60˚ and has a hypothetical vertical slip rate of 1.0 mm/yr, the horizontal

component of slip is calculated as 0.6 mm/yr. (b) Changing the fault dip to 30˚ while keeping all previous parameters

the same yields a calculated horizontal slip rate of 1.7 mm/yr, a 300% increase.

STUDY AREA

Tuttle Creek is located in Owens Valley directly west of Lone Pine and south of Whitney

Portal in eastern California (Figure 4). The creek cuts a deep channel through an extensive

alluvial fan that was deposited by catastrophic outburst flooding (Blair, 2001). The study area is

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approximately 900 m north-south and 700 m east-west. A strike length of approximately 300 m

of an east-dipping normal fault are exposed on the north canyon wall (Figure 5). The fault cuts

granitic bedrock on the footwall and alluvium covered granitic bedrock on the hanging wall.

Joints approximately 2-8 m long and sub-parallel to fault strike are exposed in bedrock on the

north canyon wall.

The main field units along the SNFFZ include, as defined by Le et al. (2007), Mesozoic

granitic bedrock, Quaternary alluvial fan surfaces subdivided by relative age (Qf1, Qf2, Qf3),

present-day river channels (Qf4), and glacial outburst deposits (Qm). The locations of these

units were confirmed in the field (Figure 4). Qf1is the oldest alluvial fan surface deposited in

approximately 30-100 m mounds. It is partially varnished and contains weathered fragments of

granitic rock, underlain by rounded to sub rounded clasts of cobbles and pebbles intermixed in a

coarse grus sand matrix. Mean surface exposure age of an offset Qf1 surface south of Bairs

Creek is 123.7±16.6 ka (Le et al., 2007). Qf2 is subdivided into two surfaces. Qf2a contains

sub-developed bar and swale topography. It is slightly varnished, moderately vegetated and

underlain by unconsolidated fan deposits composed primarily of weathered coarse angular to

subangular grus. Qf2a is most distinguishable from Qf2b by its lack of boulders. Qf2b also

contains well-developed bar and swale topography but is less weathered than Qf2a surfaces. It is

sparsely overlain by granitic boulders while boulder concentration increases closer to drainages

and decreases distance away. Mean Qf2a surface exposure age north of Symmes Creek is

60.9±6.6 ka (Le et al., 2007).

Qf3 deposits are dominant along the range front and are subdivided based on their

various stages of desert varnish and boulders densities (Le et al., 2007). Qf3a is densely

vegetated with weak desert varnish. Boulders are rare on this surface, which is comprised of a

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grus matrix of fine grained granite clasts. Mean surface exposure age for Qf3a north of

Shepherd Creek is 25.8±7.5 ka (Le et al., 2007). Qf3b has subtle bar and swale topography and

is distinguishable by its 1-2.5 m deep channels. Subangular to sub rounded granitic boulders

averaging about 1-3 m high are underlain by granitic pebbles in a coarse grained sand matrix.

Qf3c has well preserved bar and swale topography. The surface is unvarnished and contains

boulder lined channels consistent with glacial out-burst flooding. Norht of Independence Creek,

mean surface age of Symmes Creek’s Qf3c modern channel is 4.4±1.1 ka (Le et al., 2007). Qf4

is the youngest alluvial surface. It represents active and recently abandoned channels cutting all

other surfaces. Recent debris flow surfaces and densely vegetated boulder levee bars are also

included. Mean surface age of Shepherd Creek’s modern Qf4 channel is 4.1±1.0 ka (Le et al.,

2007). Qm refers to Quaternary glacial moraine deposits predominantly located at Onion Creek

west of Independence, in northern Owens Valley.

Tuttle Creek canyon is dominated by Qf4-filled channels capped by Qf3 deposits (Figure

5). The north canyon wall is comprised of channel cut Qf3 deposits, which show offset due to

normal faulting. The south wall is mainly made up of densely vegetated Qf3 deposits.

Le et al. (2007) identified three fault scarps in the Tuttle Creek area. The longest of these

is the normal fault exposed across both sides of the canyon. The fault offsets Qf3a deposits on

the north bank of Tuttle Creek resulting in a subdued fault scarp ~1.5 m high (waypoint 203 in

Figure 4). This scarp can be traced 120 m north to the mouth of a narrow, grus-filled drainage

(waypoint 204 in Figure 4). The fault is located approximately at the center of the drainage and

can be traced up and over the canyon wall and down a similar drainage on the other side. Two

other scarps are located on the south wall east of the main fault. Scarps in the south wall are

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difficult to locate due to the loose structure of Qf3 sediments. Data presented in this study are

focused on the north wall of Tuttle Creek where the fault mostly cuts Kg.

Figure 5. Northwest view of Tuttle Creek north canyon wall. Dashed yellow line indicates SNFFZ normal fault

trace. Relative motion shown by “U” representing upward motion of the footwall and “D” representing

downward motion of the hanging wall. Qf3a is a densely vegetated grus composed of granitic clasts. Qf3b is

sub rounded coarse granitic boulders underlain by fine grus.

METHODS

A basic geologic map was compiled on a 1:10000 scale color Google Earth basemap

image using field units defined by Le et al. (2007) (Figure 4). Field units and contacts were

confirmed in the field. The geologic map encompasses the length of the fault surface exposure

up and over the northern wall of Tuttle Creek canyon. It also includes the locations of minor

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fault scarps to the north and to the west. Because Tuttle Creek was not a major site in their

study, Le et al. (2007) did not include specific geologic information of the area such as

subdivided contact locations of Qf2 and Qf3 surfaces. However, field locations of faults and

contacts between Qf2 and Qf3 agree with those of the Quaternary Fault Database and bulk

geologic mapping of Le et al. (2007).

A Topcon GB-1000 differential GPS (D-GPS) receiver was used to collect x,y,z data

every ~30 meters along the fault surface exposure, resulting in approximately 40 data points.

Two receivers are used in this process. One receiver acts as a base station, making accurate

location measurements that are used to correct locations taken by a second receiver (rover). Base

station calibration requires the station to be powered on and stationary for minimum of 30-50

minutes. During this time the base station locates itself, which will allow corrections to be made

to data collected by the rover. The rover is used to collect survey points along the exposure at

the contact between scarp and footwall. Multiple data points are collected over a predetermined

data acquiring interval of 5 seconds. GPS data were recorded for 30 seconds at each station to

produce a mean final x,y,z reading at that location. GPS data were post-processed using TopCon

Tools software.

Shagam (2011) evaluated Garmin handheld GPS unit data to see if it could be used in lieu

of D-GPS equipment to conduct surveys. He concluded that whereas latitude and longitude data

were within reasonable margin of error, elevation data were not. This study also compares

Garmin handheld GPS data to TopCon D-GPS data in UTM WGS84. To find elevation from

Garmin data, UTM northing and easting data from Garmin were plotted on Google Earth.

Elevation at each location was recorded manually based on Google Earth elevation data. These

elevations from Google Earth are used in x,y,z comparisons. Garmin GPS x,y,z data and D-GPS

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x,y,z data are plotted in Figure 6. R2 values close to 1 show stronger correlation between the

data sets.

X,y,z data were input to an Excel program written by Fred Phillips (New Mexico Tech)

in order fit a 3-D surface to the data assuming the fault is planar. X and y data were plotted

against a user defined strike-parallel line in Figure 7a. This line is modified by entering positive

and negative azimuth values, which change the orientation of the line. Figure 7b shows

correlation between distance along dip direction and elevation. Distance along dip direction is

calculated as the x distance between each data point perpendicular to the strike-parallel line.

Information from these two plots was then used to infer a 2-D right triangle from which

trigonometric equations were derived. Fault dip (ϴ) is calculated using:

𝜃 = tan−1 (𝑒𝑙𝑒𝑣𝑎𝑡𝑖𝑜𝑛

𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑙𝑜𝑛𝑔 𝑑𝑖𝑝 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 )

Footwall fracture orientations just north of Tuttle Creek and along the range front at the

canyon mouth were collected to evaluate relations between fracture and fault orientations (Figure

4, Locations A-E). Orientation data were collected using a Brunton compass. Orientations were

then analyzed in Stereonet8 (Rick Allmendinger, Cornell University) and compared to measured

fault dip.

RESULTS AND INTERPRETATIONS

Garmin handheld GPS data were collected and their accuracy compared against TopCon

D-GPS data to test the feasibility of using handheld GPS rather than D-GPS equipment. GPS

waypoints were recorded within ~1 m of the rover during the D-GPS survey. Extra waypoints

that were used to note fracture locations were removed from the data set before comparisons

took place. D-GPS data were recorded in Lat-Long and converted to UTM WGS 84 using a

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spreadsheet. Both data sets were placed in the same table so that every GPS point correlated

with its D-GPS counterpart. Comparisons were made by subtracting Garmin data from TopCon

data to get delta values. Positive delta values indicate Garmin data that are greater than TopCon,

and negative delta values indicate Garmin data that are less than TopCon. The results are shown

graphically in Figure 6.

Figure 6. Garmin vs. TopCon x,y,z data comparisons. Comparisons are made by subtracting Garmin data from TopCon data to

get delta values. These delta values are plotted against corresponding TopCon data in plots a, b, and c. One to one ratios,

expressed as data points centered on the zero line shown in red, indicate zero difference between Garmin and TopCon data. Plot a compares northing data. Plot b compares easting data. Plot c compares elevation data. The three data points circled in red are

considered outliers due to mechanical error and were removed from the data set during analysis. “Average Garmin Difference”

is defined as the average delta value or average difference between the two data sets.

Garmin northings average 1.5 m greater than those recorded by TopCon (Figure 6a) and

Garmin eastings average 0.9 m greater than those recorded by TopCon (Figure 6b). These delta

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values are within error considering that handheld data were collected within a meter of the rover.

Google Earth elevation, based on Garmin locations average 19 m greater than those recorded by

TopCon (Figure 6c). These values are not within reasonable margin of error; however, if Garmin

waypoints 217, 218, 219, (circled in red), are considered outliers and removed from the dataset,

then Garmin elevation data become evenly scattered around the 20 m line. The three outliers are

probably related to poor location recordings due to signal interference from the presence of trees.

Elevation accuracy is most important to these surveys because it has the greatest effect on

calculating ϴ angle in three point problems. Handheld GPS would be acceptable for these types

of surveys because elevation data can be corrected using Google Earth elevations recorded from

Garmin locations.

Forty TopCon differential GPS data were collected across ~500 m of fault exposure at

Tuttle Creek from which 3D fault surface is derived. “TUTBASE1” indicates base station

location in Figure 4 and was removed from the data set before post-processing. Phillips’ Excel

sheet treats x, y, and z data as individual components in order to construct multiple three point

trigonometric problems to which a 3-D plane can be fit (Figure 7). Two graphs are output by the

program, which assumes a planar fault when performing calculations. Figure 7a graphically

displays x and y data and a user defined strike line. Figure 7b shows correlation between

elevation and distance along dip direction in the form of a regression line. Fault dip is defined

as the inverse tangent of the slope of the regression line. The strike line is anchored at the first

data point (4046324.287, 394503.946). Optimal fault strike at Tuttle Creek is N12˚W which

yields the regression line y=-0.6869x+2007.9 and a best fit dip of 35˚ E. This dip is much less

than the assumed dip of 60˚ used in similar studies along the SNFFZ (Phillips and Majkowski,

2011; Shagam, 2011; Mottle, 2015).

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Figure 7. Fault model developed by Fred Phillips. Assumes the fault is planar. (a) X and y data are plotted as orange

circles. User defined best-fit strike line, shown in bold, provides two points of equal elevation when coupled with

calculated distance along dip direction, shown with dashes. (b) Distance along dip direction is plotted against

elevation data at the same time strike line is being fit. Strike line in Plot A is considered best-fit when R2≈1.0. Waypoint elevation defines a third point and dimension from which dip can be derived. (c) Two dimensional

representation of a right triangle provides the framework for application of trigonometric functions.

Dip is defined as 𝛳 = 𝑡𝑎𝑛 −1(𝑒𝑙𝑒𝑣𝑎𝑡𝑖𝑜𝑛

𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑙𝑜𝑛𝑔 𝑑𝑖𝑝 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛).

Shear Fractures

Fracture data were collected from footwall structures, such as Riedel shears, joints, and

deformation zones, sub parallel to the main fault in order to evaluate relations between fracture

and fault orientations (Figure 8). All measured fracture orientations were entered into Stereonet8

(Rick Allmendinger, Cornell University), which was used to calculate a best fit orientation of

N33˚W, 35˚E (Figure 8e). Best fit orientation data from the D-GPS study is in agreement with

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Figure 8. Fracture orientation data. A): Southwestern view of the north wall of Tuttle Canyon. Black lines indicate

surface fault exposures and fault scarp locations. Letters in white circles correspond with fracture locations. B), C),

D): Examples of fracture locales with 2 m human for scale. Photographs are oriented west (left edge) to east (right

edge). E): Stereonet analysis of fracture orientations collected at Tuttle Creek show best fit orientation of N33W, 35E.

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Figure 9. Overview of footwall fractures along the range front fault. Black line indicates contact between Kg

(Cretaceous granitic bedrock) and Qf3a (Quaternary alluvium). A): Location D looking due south toward Tuttle Creek

Canyon. Prominent set of fractures dipping ~32˚NE. B): Location E looking due north. Dip angle of fractures is

variant and may be influenced by external processes.

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Figure 10. Example of hairline fractures and shocked granitic clasts located a few meters below location C. 2 m human

for scale. Note that fractures cut the aplite dike in the center of the photo, but they do not offset it.

fracture orientation data. These data further support low angle fault dip because footwall

structures form sub parallel to the main fault. Variation in dip angle of fracture sets at each

location is discussed below.

Location A, roughly defined as the top third of the fault trace, is dominated by en echelon

meter-scale joints and half meter-scale deformation zones (Figure 8b). Joints ~2-8 m long dip

subparallel to the fault at ~30˚-38˚. Thin hairline joints often occur in tandem with these larger

joints. Hairline joints form parallel to larger joints and do not appear to deform or offset the

surrounding rock. Stereonet analysis of 17 fracture orientation data at and surrounding Location

A yields an average dip of 32˚NE.

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Location B is dominated by meter-scale Riedel shears (Figure 8c). These Riedel shears

occur in conjugate sets. Shallow dipping R-shears are synthetic to motion on the fault and dip

slightly acute in relation to the fault. Steep dipping R’-shears are antithetic to motion on the fault

and dip up to 75˚NE at Location B. Average dip of R-shears is 25˚NE. Average dip of R’-

shears is 67˚NE. Very few hairline joints were observed in association with larger Riedel shears.

Joints and fractures at Location B do not show evidence of offset.

Individual large scale fracture characteristics described thus far are observed “in sequence”

at Location C (Figure 8d). Parallel meter-scale joints are observed in the upper few meters of

granitic rock, similar to Location A. Blocky portions below these joints result from R- and R’-

shears dipping 35˚ and 83˚ degrees, respectfully. Below these features, innumerable centimeter-

scale hairline fractures and shattered granitic clasts make up the main shear zone of the fault.

Granitic clasts range in size from coarse gravel to fine sand. Macroscopic analysis of hairline

fractures at Location C yields an average dip of 30˚NE. Corrugations on the underside of the

blocky section, at the transition between solid and sheared rock near the center of the outcrop,

may show evidence of slickenlines; however, these features are too subdued for accurate

classification to be completed.

Along the range front, at locations D and E, dominant bedrock fracture sets occur in steep

and shallow conjugate sets (Figure 9). According to the USGS Quaternary Fault and Fold

Database, the SNFFZ is located at the contact between Qf3a and Kg. Tens of meters above this

contact (toward the top of both images in Figure 9), shallow fractures dip ~33˚NE and steep

fractures dip ~65˚NE. Dominate fracture sets become increasingly steeper the closer they get to

the fault trace. Shallow fractures dip ~45˚ and steep fractures dip ~80˚. External processes such

as chemical weathering and exfoliation may influence the shape of fractures which may interfere

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with the accuracy of these readings. Additional study is needed to determine possible reasons for

variations in fracture dip at Locations D and E. Centimeter-scale hairline fractures and shattered

granitic clasts are observable ~200 m NW of Location D where the range front fault intersects

Qf2b mounds and Kg.

Other fracture characteristics that occur in multiple locations along the main fault include

dilation fractures and shear fractures across aplite dikes. Dilation fractures along the main fault

occur in conjugate sets. Steep fracture sets dip 53˚-80˚ NE. Shallow fracture sets dip 19˚-45˚

NE. These dilation fractures are often located atop prominent shear zones along the fault, which

suggests that they may have formed as a result of volumetric changes in the rock due to shear

stress along the fault zone. One example of dilation fractures can be seen behind the human

scale in Figure 8a. Most large-scale fractures in close proximity to the main shear zone can be

characterized as having a 15 cm zone of shattered and deformed granitic clasts and an 8 cm zone

of parallel hairline joints (Figure 10). Shear forces decrease further away from the main shear

fracture, thus zoning occurs. It is important to note that hairline fractures and large-scale joints

do not offset intersecting aplite dikes.

Based on general trends of footwall fractures in granitic bedrock observed at Tuttle

Creek, common features associated with faulting along the SNFFZ include hairline fractures

associated with meter-scale joints, acute synthetic R-shears paired with conjugate synthetic R’-

shears, and centimeter-scale shattered granitic clasts, all of which form parallel or subparallel to

the main fault. Centimeter-scale hairline fractures and shattered granitic clasts, indicative of

shear along the main fault, can be used as proxies to determine fault dip in areas where SNFFZ

faults are poorly exposed. Additional study is needed to refine this method of identifying

SNFFZ faults.

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DISCUSSION

Shagam (2012) and Mottle (2015) conducted similar studies at Independence Creek and

Bairs Creek yielding fault dips of 34˚ E and 23˚ E, respectively, which are in agreement with

dips determined by Phillips and Majkowski (2011). The purpose of these studies was to evaluate

fault dip because current long-term slip rate estimates are calculated based on an assumed 60˚

fault dip. Table 1 is a compilation of long-term extension rates calculated by Le et al. (2007),

Shagam (2012), and Mottle (2015). Using our average measured dip of 31˚, I recalculated

extension rates of Le et al. (2007). Le et al. (2007) calculated Pleistocene extension rates of 0.2

mm/yr. Based on our measured dip of 31˚, Pleistocene extension can be revised to 0.5 mm/yr –

two and a half times greater than assumed. Holocene extension rates, calculated to be 0.9 mm/yr

(Le et al., 2007), can be revised to 2.7 mm/yr, which is three times greater than assumed.

Additionally, Rood et al. (2011) reports Holocene extension rates of 0.3-0.7 mm/yr for SNFFZ

faults between Lake Tahoe and Mono Lake based on assumed 60˚ dip. Similar revisions would

be required if normal faults along the central SNFFZ are dipping shallower than measured in this

study. Although comparisons to geodetic data were not made in this study, Phillips and

Majkowski (2011) conclude that if actual dips of SNFFZ faults are less than 30˚, horizontal

extension rates would better agree with geodetic rates associated with Owens Valley.

Phillips and Majkowski (2011) estimated the displacement rate across Northern Owens

Valley by calculating the difference between 6 average station velocities in the southern Sierra

Nevada block and 5 average station velocities in the White/Inyo Mountains to get the

displacement vector 4.1±0.8 mm/yr. toward 305˚. They decomposed their displacement vector

into two components, fault parallel displacement along the Owens Valley Fault Zone and fault

perpendicular displacement or extension across Owens Valley. If Phillips and Majkowski’s

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extensional component of 1.5±0.3 mm/yr. were to be accommodated solely along SNFFZ faults,

then the reevaluated extension rate in this study (2.7 mm/yr.) would be considered an

overestimation. However, when considering long term Pleistocene to Holocene extension rates

in this study and others, Dixon el al. (2000) and Phillips and Majkowski (2011)’s rate of 1.5

mm/yr. falls neatly in the range of 0.3-2.7 mm/yr.

Notes: Numeric comparisons between extension rates based on an assumed 60˚ fault and those based on a measured

dip of 31˚. Measured dip is average for an area between Independence Creek and Tuttle Creek. Independence and Shepherd Creek data are from Shagam (2011). Bairs Creek data are from Mottle (2014). Revised rates of extension

are based on trigonometric analysis of planar faults that have measured dips of 31˚. Revised late Pleistocene extension

rates are 2.5 times greater than those based on assumed 60˚ dip. Revised Holocene extension rates are 3 times greater

than those based on 60˚ dip.

Regional extension rates across the Basin and Range Provence should also be considered

when reevaluating extension rates of individual faults or fault zones within the Basin and Range.

One important component to consider is extension rates of the Wasatch Fault zone (WFZ) in

central Utah, which defines the eastern margin of the Basin and Range. When modelling

Holocene extension rates, Malsevisi et al. (2003) considered typical normal fault dips of 60˚ as

well as low angle normal fault dips of 30˚ based on increasing evidence that the Wasatch Fault is

24

a listric fault. Corresponding extension rates of 1.6 mm/yr and 4.7 mm/yr differ by 3.1 mm/yr

for 60˚ and 30˚ dipping faults respectively. Extension rates based on moderately dipping (<45˚)

normal faults for the WFZ are a better fit to geodetic data for the region (Malsevisi et al., 2003).

The mechanical feasibility of low-angle normal faulting is controversial. In extensional

settings such as the Basin and Range, it is common for active high-angle normal faults to be

rotated to inactive low-angle normal faults via domino-style fault block rotation (Proffett, 1977)

Is it possible that the SNFFZ has experienced similar rotation? Normal faults surveyed by

Phillips and Majkowski (2011), Shagam (2012), Mottle (2015), and faults surveyed in this study

are located between Owens Valley and the Sierra Nevada block, which is defined as the western

margin of the Basin and Range. Domino-style rotation is more likely to affect subsidiary faults

within the hanging wall. Since the SNFFZ does not include subsidiary faults, then it is unlikely

that domino-style rotation has occurred. Regardless of whether or not rotation has occurred,

seismic data presented by Phillips and Majkowski (2011) shows that moderate- and low-angle

normal faults near Bishop remain active today. Additional studies, such as gravity surveys, may

help evaluate the fault dip in the subsurface.

CONCLUSIONS

Detailed fault mapping of surface exposures west of Lone Pine yields new evidence that

the SNFFZ dips more shallow than assumed. From this we reason that:

1.) Normal faults of the SNFFZ in the vicinity of Lone Pine, CA dip 35˚ E – much

shallower than assumed. Footwall fractures are subparallel to the normal fault at

Tuttle Creek and dip similarly at 35˚ E.

2.) Late Pleistocene to Holocene extension rates should be reevaluated to account for

shallow dip. My new data are consistent with those of Phillips and Majkowski

25

(2011), Shagam (2012), and Mottle (2015). Reported slip rates of 0.3-0.9 mm/yr can

be revised to 0.5-2.7 mm/yr using average dip of 31˚ from the aforementioned

studies.

3.) Handheld GPS units are effective proxies for D-GPS units when conducting surveys.

Errors in elevation data can be corrected using Google Earth elevations from Garmin

locations.

4.) Footwall fractures that form subparallel to range bounding normal faults of the

SNFFZ can be used to infer fault location in areas where faults are poorly explosed.

Additional studies would need to be conducted to explore open questions regarding fault

dip in the subsurface, dip of the SNFFZ between right-stepping segments, and whether or not

range bounding low-angle normal fault geometries can be applied to the Basin and Range

Provence as a whole.

26

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