Central Washington University
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All Master's Theses
Master's Theses
Fall 2015
Geologic Mapping in the Black Mountain Area,
Northern Eastern California Shear Zone: Testing a
Kinematic and Geometric Fault Slip Transfer
Model
Kevin M. DeLano
Central Washington University, [email protected]
Follow this and additional works at: http://digitalcommons.cwu.edu/etd
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Recommended Citation
DeLano, Kevin M., "Geologic Mapping in the Black Mountain Area, Northern Eastern California Shear Zone: Testing a Kinematic and
Geometric Fault Slip Transfer Model" (2015). All Master's Theses. 277.
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GEOLOGIC MAPPING IN THE BLACK MOUNTAIN AREA, NORTHERN
EASTERN CALIFORNIA SHEAR ZONE: TESTING A KINEMATIC AND
GEOMETRIC FAULT SLIP TRANSFER MODEL
__________________________________
A Thesis
Presented to
The Graduate Faculty
Central Washington University
___________________________________
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Geology
___________________________________
by
Kevin Michael DeLano
November 2015
CENTRAL WASHINGTON UNIVERSITY
Graduate Studies
We hereby approve the thesis of
Kevin Michael DeLano
Candidate for the degree of Master of Science
APPROVED FOR THE GRADUATE FACULTY
______________
_________________________________________
Dr. Jeffrey Lee, Committee Chair
______________
_________________________________________
Dr. Anne Egger, Committee Member
______________
_________________________________________
Dr. Christopher Mattinson, Committee Member
______________
_________________________________________
Dean of Graduate Studies
ii
ABSTRACT
GEOLOGIC MAPPING IN THE BLACK MOUNTAIN AREA, NORTHERN
EASTERN CALIFORNIA SHEAR ZONE: TESTING A KINEMATIC AND
GEOMETRIC FAULT SLIP TRANSFER MODEL
by
Kevin Michael DeLano
November 2015
New geologic mapping, structural, kinematic, and 40Ar/39Ar geochronology
studies in the Black Mountain area, northern eastern California shear zone, are used to
test a kinematic fault slip transfer model for the Owens Valley fault-Mina deflection
transition. In the Black Mountain area, range bounding ~NNW- to ~NS-striking and
lesser NW- to NE-striking normal faults cut Mesozoic, Miocene (22.42 ± 0.05 Ma),
Pliocene (3.53 ± 0.06 to 3.29 ± 0.02 Ma), and early-middle Pleistocene (1936 ± 12.7 to
766 ± 3.1 ka) rocks. Palinspastically restored cross-sections show that offset Pliocene
markers record 1.5 +0.7/-0.6 km of ~ENE-WSW extension since ~3.29 Ma, indicating an
extension rate of ≥0.5 ± 0.2 mm/yr. We combine our new rates with slip rates on nearby
fault zones to refine a kinematic fault slip transfer model for the Owens Valley faultMina deflection transition and address the discrepancy between geodetic strain rates and
geologic slip rates across the region.
iii
ACKNOWLEDGMENTS
I dedicate this work to my grandparents, Robert and Carmen DeLano, Evan and
Joan Reynolds, and step-grandmother, Lydia Keerd, for supporting my education and
telling me to go explore.
Fieldwork was primarily funded by a USGS-EDMAP grant awarded to Dr.
Jeffrey Lee. Additional support for fieldwork, geochronology, and tuition was awarded to
K. DeLano from Central Washington University, the Geological Society of America, the
Northern California Geological Society, the West Seattle Rock Club (Northwest
Federation of Mineralogical Societies), and White Mountain Research Center (University
of California). I thank the Seattle Rock Club for their generosity and the White Mountain
Research Center team for their flexible logistical support, especially during
thunderstorms.
First, endless thanks are due to Dr. Jeffrey Lee, for his guidance, endless patience,
and record breaking turn-around time on edits. I am grateful for my field assistants, Peter
Dubyoski and Tucker Lance, for their excellent company, dedication, and for breaking
and carrying all my geochronology samples. I thank Dr. Anne Egger and Dr. Chris
Mattinson for serving on my committee and providing critical feedback. I thank Dr.
Andrew Calvert and staff at the USGS station in Menlo Park, CA, for their help with the
40
Ar/39Ar geochronology sample preparation and analysis. I would like to thank Zach
Lifton for his knowledge of geodetic strain rates and geologic slip rates across the
northern ECSZ. I am grateful to Wendy Bohrson, Peter Dubyoski, Eric Kirby, Audrey
iv
Huerta, and Bruce Pauly for our conversations on various aspects of this project. Moriah
Kauer, Linda Shepard, and Craig Scrivner, thank you for logistical and technical support.
Eliya Hogan, thank you for inspiring me to undertake a geologic mapping MS
project. I thank the Washington whitewater community for giving me a place to go play. I
thank my friends for all the adventure, beer, and moral support. Finally, I thank my
family for everything, especially for first taking me to Mono Lake.
v
TABLE OF CONTENTS
Chapter
Page
I
INTRODUCTION ........................................................................................ 1
II
GEOLOGIC SETTING ................................................................................ 6
Owens Valley Fault and White Mountains Fault Zone ........................... 7
Normal Faulting Across the Volcanic Tableland .................................... 9
Sierra Nevada Frontal Fault Zone ......................................................... 10
Geology of the Black Mountain Area ................................................... 10
Kinematic Fault Slip Transfer Models Across the Northern
Eastern California Shear Zone-Mina Deflection Transition ................. 11
III
GEOLOGIC ROCK UNITS AND AGES IN
THE BLACK MOUNTAIN AREA ........................................................... 14
40
IV
Ar/39Ar Geochronology ...................................................................... 23
FAULTS ACROSS THE BLACK MOUNTAIN AREA ........................... 27
Geometry of Key Miocene-Quaternary Rocks ..................................... 27
Fault Geometry...................................................................................... 30
Extension Direction............................................................................... 32
Magnitude of Extension ........................................................................ 34
Extension Rates ..................................................................................... 37
V
DISCUSSION ............................................................................................. 39
Timing and Rate of Volcanism and Extension
Across the Black Mountain area ........................................................... 39
Recalculated Extension Rates across the Volcanic Tableland .............. 41
A Refined Model of Fault Slip Transfer: Owens Valley Fault
to Mina Deflection ................................................................................ 41
Discrepancy Between Geodetic Strain Rates and Geologic
Slip Rates .............................................................................................. 47
VI
CONCLUSIONS ........................................................................................ 50
REFERENCES ........................................................................................... 51
vi
TABLE OF CONTENTS (continued)
APPENDIXES .............................................................................................................. 61
Appendix A—40Ar/39Ar Geochronology .............................................. 61
Appendix B—Stereonet 8 ..................................................................... 75
vii
LIST OF TABLES
Table
Page
1
40
Ar/39Ar Geochronology for the Black Mountain Area ............................ 24
2
Extension Rates Across the Black Mountain Area ..................................... 35
3
Documented vs. Predicted Slip Rates in the Refined Fault Slip
Transfer Kinematic Model (see Fig. 13) ..................................................... 44
4
Geologic Slip and Extension Rates on Faults Along a N53E Transect
Across the Northern ECSZ ......................................................................... 48
A1
40
Ar/39Ar Geochronology Data for Plagioclase in Basalt Groundmass
Concentrates, Sample BMF14-023, Unit Pbp............................................. 64
A2
40
Ar/39Ar Geochronology Data for Plagioclase in Basalt Groundmass
Concentrates, Sample BMF14-103, Unit Pbc ............................................. 66
A3
40
Ar/39Ar Geochronology Data for Plagioclase in Basalt Groundmass
Concentrates, Sample BMF14-002A, Unit Pbc .......................................... 68
A4
40
Ar/39Ar Geochronology Data for Plagioclase in Basalt Groundmass
Concentrates, Sample BMF14-109, Unit Pbb............................................. 70
A5
40
Ar/39Ar Geochronology Data for Sanidines in Rhyolite Tuff Unit Qtc,
Sample BMF14-007A ................................................................................. 72
A6
40
Ar/39Ar Geochronology Data for Sanidines in Rhyolite Tuff Unit Mrt,
Sample BMF14-032A ................................................................................. 74
viii
LIST OF FIGURES
Figure
1
Page
Simplified tectonic map of the western U.S. Cordillera showing the
modern plate boundaries and tectonic provinces .......................................... 2
2
Shaded relief map of the Walker Lane belt and northern part of the
eastern California shear zone showing the major Quaternary faults ............ 2
3
Shaded relief map of the southern Mina deflection and northwestern
margin of the eastern California shear zone, showing
major Quaternary faults. ............................................................................... 3
4
Nagorsen-Rinke et al.’s (2013) simplified fault slip kinematic model
for the northwestern ECSZ and southwestern Mina deflection. ................... 5
5
Field photograph showing a range bounding, ~NS-striking, west-dipping
normal fault (dashed black line; solid ball on the hanging wall)
juxtaposing a hanging wall of Holocene deposits in the southeast arm
of Adobe Valley upon a footwall underlain by map units Mzg, Mrt,
Map, Pbc, Pbp, Pvc, and Qtc, which define the Benton Range (Fig. 3;
Plate 1). ....................................................................................................... 12
6
Field photograph showing the angular unconformity (black line, dotted
where buried) across which the Quaternary Benton Stream tuff
(unit Qbst) was deposited upon Miocene to Pliocene Benton
stream gravel deposit (unit MPbsg). ........................................................... 18
7
Field photograph showing moderately (~25° to 30°) east-dipping
Pliocene boring basalt flow (unit Pbb) unconformably overlying
Mesozoic granite, undifferentiated (unit Mzg). .......................................... 19
8
Photomicrographs of distinctive mineralogical features of three
widespread basalt flows in Black Mountain area. ...................................... 21
9
40
Ar/39Ar geochronology age spectra for plagioclase from basalt flow
groundmass. ................................................................................................ 25
10
40
Ar/39Ar geochronology age spectra probability distribution plots
for sanidine from rhyolite tuff deposits ...................................................... 26
11
Equal area stereonet plots of flow foliation and bedding attitudes
collected in each structural domain (a, Domain 1; b, Domain 2;
c, Domain 3) of the Black Mountain area ................................................... 29
ix
LIST OF FIGURES (continued)
Figure
Page
12
Equal area stereonet plot of fault plane, joint, and fault striation
attitudes in unit Mzg within the Black Mountain area................................ 34
13
Refined kinematic fault slip model (cf. Fig. 4) for the northwestern
ECSZ and southwestern Mina deflection ................................................... 43
14
Discrepancy between geodetic strain and geologic slip rates in the
northern ECSZ. ........................................................................................... 49
A1
40
Ar/39Ar geochronology age spectrum and isochron analysis for
basalt unit Pbp, sample BMF14-023........................................................... 63
A2
40
A3
40
A4
40
A5
40
A6
40
Ar/39Ar geochronology age spectrum and isochron analysis for
basalt unit Pbc, sample BMF14-103 ........................................................... 65
Ar/39Ar geochronology age spectrum and isochron analysis for
basalt unit Pbc, sample BMF14-002A ........................................................ 67
Ar/39Ar geochronology age spectrum and isochron analysis for
basalt unit Pbb, sample BMF14-109........................................................... 69
Ar/39Ar geochronology age spectrum and isochron analysis for
rhyolite tuff unit Qtc, sample BMF14-007A .............................................. 70
Ar/39Ar geochronology age spectrum and isochron analysis for
rhyolite tuff unit Mrt, sample BMF14-032A .............................................. 73
x
LIST OF PLATES
Plate
Page
1
Geologic map of the Black Mountain area, 1:24000 scale ......................... 15
2
Stratigraphic column of Mesozoic igneous basement, Miocene, Pliocene,
and early to middle Pleistocene volcanic and sedimentary rocks exposed
in the Black Mountain area ......................................................................... 16
3
Present-day and palinspastically restored geologic cross-sections ............. 28
4
Fault Map of the Black Mountain area, Mono County, CA. ...................... 31
5
Present-day and palinspastically restored geologic cross-sections for
Z-Z’ with faults rotated 10° shallower and 10° steeper .............................. 36
xi
CHAPTER I
INTRODUCTION
Dextral slip between the Pacific and North American plates is partitioned between
the San Andreas Fault system (~80% of the plate motion) and the Walker Lane belt
(WLB)-eastern California shear zone (ECSZ) (~20% of the plate motion) (e.g. Dixon et
al., 1995, 2000) (Figs. 1 and 2). Fault slip across the WLB-ECSZ is accommodated along
a dominant set of NNW- to NW-striking dextral faults, which strike sub-parallel to
Pacific-North America (PA-NA) plate motion, NS- to NE-striking normal faults, and
ENE-striking sinistral faults (Figs. 1 and 2). Numerous investigations along the northern
ECSZ, Mina deflection, and central WLB have centered on documenting fault slip rates
over geologic and geodetic time scales.
Along a N53E-trending transect across the northern ECSZ at latitude ~37.5°N,
approximately perpendicular to motion of the Sierra Nevada block (SN) relative to NA, a
regional and a local discrepancy exists between geodetic strain rate and geodetic slip rate
studies (Fig. 3) (e.g. Frankel et al., 2011; Foy et al., 2012; Lifton et al., 2013). Along the
transect, the sum of the most recent late Pleistocene to Holocene geologic fault slip rates
on the normal Lone Mountain (Lifton et al., 2015), normal Clayton (Foy et al., 2012),
normal Emigrant Peak (Schroeder, pers. comm., 2015), dextral Fish Lake Valley (Frankel
et al., 2011), and dextral White Mountains fault zones (Lifton et al., 2012), recalculated
parallel to N37W SN-NA plate motion, is 5.8 +1.3/-0.6 mm/yr (Lifton and Lee, pers.
comm., 2015), which is ~55% of the modeled geodetic SN-NA, N37W, plate motion
velocity of 10.6 ± 0.5 mm/yr (see Fig. 1 in Lifton et al., 2013).
1
2
3
To resolve part of the discrepancy, Nagorsen-Rinke et al. (2013) proposed a fault
slip transfer kinematic model whereby dextral shear along the NNW-striking dextral
Owens Valley fault (OVF) is partitioned into two components (Fig. 4). The first
component is a northeast step onto the NNW-striking dextral White Mountains fault zone
(WMFZ), which accommodates a minimum dextral slip rate of 1.9 +0.5/-04 mm/yr since
38.4 ± 9.0 ka (Lifton et al., 2012; Lifton and Lee, pers. comm., 2015) (Fig. 4). The
second component is a northward transfer of slip onto ENE-striking sinistral faults in the
Adobe Hills area of the Mina deflection via ~NS- to NW-striking normal faults in the
Volcanic Tableland, ~NNW- to ~NS-striking normal faults in the Black Mountain area,
and NW-striking dextral-normal oblique faults in the River Spring area (Figs. 3 and 4).
Across ~NNW- to ~NS-striking normal faults in the Black Mountain area, NagorsenRinke et al. (2013) predicted an ~ENE-WSW horizontal extension rate of 0.6 mm/yr
since the Pliocene.
To test the predicted extension rate across the Black Mountain area and thereby
this kinematic model, we completed new geologic mapping, structural and kinematic
studies, and 40Ar/39Ar geochronology in the Black Mountain area. Combining the
documented magnitudes of horizontal extension with the age of Miocene to Pleistocene
volcanic rock units allows us to calculate a Pliocene extension rate across the Black
Mountain area. To address the discrepancy between geodetic and geologic slip rates
along a transect across the northern ECSZ, we add existing and refined slip rates on the
Volcanic Tableland (Pinter, 1995; this study) and Round Valley segment of the Sierra
Nevada frontal fault zone (Berry, 1997) to a geologic slip rate sum (Fig. 3).
4
5
CHAPTER II
GEOLOGIC SETTING
The northern ECSZ is a ~100 to 200 km wide domain of NW dextral shear and
EW-extension that extends from the ENE-striking Garlock fault ~280 km north to the
Mina deflection (Fig. 1). Across the northern ECSZ, subparallel NNW- to NW-striking
dextral faults are connected by NE-striking normal faults; together these faults
accommodate PA-NA dextral shear and EW-extension (Fig. 2) (e.g. Reheis and Dixon,
1996; Lee et al., 2001b; Unruh et al., 2003; Wesnousky, 2005; Faulds and Henry, 2008).
The changes in relative motion between the PA-NA plates since ~12 Ma and a purported
Pliocene increase in gravitational potential energy beneath the southern Sierra Nevada
have been proposed as the forces driving spatial and temporal changes in the distribution
of NW dextral shear and EW-extension across the northern ECSZ, as well as pulses of
faulting (e.g. Atwater and Stock, 1998; Unruh et al., 2003; Jones et al., 2004; Zandt et al.,
2004; Faulds et al., 2005; Faulds and Henry, 2008; Lee et al., 2009b; Saleeby et al., 2012;
Nagorsen-Rinke et al., 2013).
From ~12 to 8 Ma, the rate of motion of the Pacific plate relative to a stable North
America increased from ~33 mm/yr to ~52 mm/yr at an azimuth of ~N60W. Around 8
Ma, the azimuth of displacement between the PA-NA plates rotated to ~N37W, but the
rate did not change (Atwater and Stock, 1998).
At ~8 Ma in the northern ECSZ, coincident with the rotation in plate motion
azimuth, intracontinental plate boundary driven dextral shear shifted westward from the
Las Vegas shear zone to the NW-striking dextral Fish Lake Valley-Death Valley fault
zone (FLV-DVFZ) (Fig. 2) (Faulds and Henry, 2008). In the Pliocene, PA-NA dextral
6
shear and EW-extension in the northern ECSZ shifted westward again, from the FLVDVFZ to fault zones in the Owens Valley, including the OVF, WMFZ, and the Sierra
Nevada frontal fault zone (SNFFZ) (see Fig. 1A in Jones et al., 2004). Coeval with the
westward shift is a geologically documented Pliocene pulse of volcanism and
deformation throughout the southwestern Mina deflection and northern ECSZ (Larsen,
1979; Sternlof, 1988; Manley et al., 2000; Lee et al., 2001a, 2009b; Stockli et al., 2003;
Casteel, 2005; Tincher and Stockli, 2009; Nagorsen-Rinke et al., 2013; Warren, 2014).
The Pliocene increase in gravitational potential energy beneath the southern
Sierra Nevada, a predicted consequence of theorized delamination of the lithospheric root
beneath the Sierra Nevada, is the hypothesized force driving the Pliocene pulse of
deformation and volcanism in the northern ECSZ, extending from the Garlock fault ~280
km north to the Mina deflection and across ~50 km wide zone extending east from the
Sierra Nevada mountains (Fig. 1) (Malservisi et al., 2001; Wakabayashi and Sawyer,
2001; Jones et al., 2004; Zandt et al., 2004; Lee et al., 2009a, b; Saleeby et al., 2012;
Nagorsen-Rinke et al., 2013; Warren, 2014).
Owens Valley Fault and White Mountains Fault Zone
Owens Valley, a ~175 km long linear graben, is bounded on the west by the
Sierra Nevada and on the east by the White-Inyo Mountains (Fig. 3). Exposed within the
Owens Valley, are the Owens Valley fault (OVF) and the White Mountains fault zone
(WMFZ), which together form one of the major ~NNW- to NW-striking dextral fault
systems that accommodates dextral shear across the northern ECSZ (Figs. 2 and 3). The
OVF is a ~100-110 km long, NNW-striking, dextral fault system that extends from
Owens Lake to a few kilometers south of Bishop (Figs. 2 and 3) (Beanland and Clark,
7
1982; Lee et al., 2001b, 2009b; Stockli et al., 2003; Bacon and Pezzopane, 2007). Since
~83 Ma, the OVF, or its precursor structure, has accommodated ~60-65 km of dextral
slip, likely during two distinct deformation episodes in the late Cretaceous-early
Paleogene and the late Neogene (Kylander-Clark et al., 2005).
The regional Pliocene westward shift in regional dextral shear reactivated the
OVF and WMFZ as a complex system of dextral, normal, and oblique faults (Stockli et
al., 2003; Glazner et al., 2005). Since reactivation, the OVF-WMFZ system has likely
accommodated ~6-9 km of dextral displacement (Stockli, 1999; Lee et al., 2001b). Two
studies estimated Holocene and late Pleistocene slip rates along the northern part of the
OVF. Approximately 45 km south of Big Pine (Fig. 3), Lee et al. (2001b) calculated a
Holocene dextral slip rate of 1.8 ± 0.3 to 3.6 ± 0.2 mm/yr. About 1 km south of Big Pine
(Fig. 3), Kirby et al. (2008) calculated a late Pleistocene (55-80 ka) dextral slip rate of
~2.8-4.5 mm/yr. The most recent large magnitude earthquake on the OVF occurred in
1872. The earthquake, with an estimated moment magnitude of 7.5-7.7, ruptured 100 ±
10 km of the OVF. Dextral offset averaged 6 ± 2 m with a maximum of 10 m (Beanland
and Clark, 1982).
The OVF terminates south of Bishop where slip is partitioned northeastward onto
the WMFZ, a ~70 km long, NNW-striking, dextral-normal oblique fault system (Figs. 3
and 4) (Schroeder et al., 2003; Stockli et al., 2003; Kirby et al., 2006; Sheehan, 2007;
Lifton et al., 2012; Nagorsen-Rinke et al., 2013). Slip along the WMFZ initiated at 12
Ma, which has since accommodated ~8 km of west-down dip-slip displacement, and
uplifted the modern White Mountains horst (Stockli, 1999, 2003). Since the Pliocene, the
WMFZ has accommodated normal-dextral oblique slip (Lienkaemper et al., 1987; Smith
8
and Priestly, 2000; Stockli et al., 2003). Estimated Pleistocene to Holocene dextral slip
rates on the WMFZ vary. Kirby et al. (2006) calculated a minimum dextral slip rate of
0.37 to 0.57 mm/yr since 60-90 ka. A more recent study (Lifton et al., 2012; Lifton and
Lee, pers. comm., 2015) documented minimum late Pleistocene to Holocene slip rates on
the WMFZ : (a) 1.1 ± 0.1 mm/yr since the eruption of the Bishop Tuff (zircon U/Pb age
of 766.6 ± 3.1; Chamberlain et al., 2014), (b) 1.9 +0.45/-0.4 mm/yr since 70-115 ka, (c)
1.9 +0.5/-04 mm/yr since 38.4 ± 9.0 ka, and (d) 1.8 +2.8/-0.7 mm/yr since 6.2 ± 3.8 ka.
To test and refine the Nagorsen-Rinke et al. (2013) kinematic model and address the
regional and local discrepancies between geologic and geodetic slip rates across the
northern ECSZ, we use the WMFZ slip rate of ≥1.9 +0.5/-04 mm/yr since the late
Pleistocene to Holocene.
Normal Faulting Across the Volcanic Tableland
The Volcanic Tableland is the geomorphic surface of the Bishop Tuff, a rhyolitic
pyroclastic flow, which ejected south and east into the Owens Valley during the eruption
of Long Valley caldera ~766 ka (Gilbert, 1938; Bailey et al., 1976; Chamberlain et al.,
2014). The surface is warped into broad, NW-trending anticlines and synclines (Bateman,
1965). From south to north, the Volcanic Tableland is cut by NS-striking normal faults
that curve into zones of NW- to NNW-striking, left-stepping, en echelon normal faults
across the central and eastern Volcanic Tableland (Pinter, 1995). Bateman (1965) was the
first to suggest that dextral shear contributed to the formation of NW-trending anticlinal
and synclinal warps and en echelon normal fault pattern. For normal faults across the
southern part of the Volcanic Tableland, Pinter (1995) calculated ~0.4 mm/yr of
horizontal EW-extension since eruption of the Bishop Tuff. Nagorsen-Rinke et al. (2013)
9
postulated that these faults accommodated the kinematic transfer of a component of NW
dextral shear from the OVF to the Mina deflection (Figs. 3 and 4).
The epicenter of the 1986 Ms 6.2 Chalfant Valley earthquake sequence was in the
northeastern Volcanic Tableland (Fig. 3), indicating these faults are seismically active.
Surface rupture propagated to the southeast, through the Chalfant Valley, and onto an
~11 km long segment of the WMFZ (Fig. 3) (Lienkaemper et al., 1987). Smith and
Priestly (2000) used aftershock hypocenters to interpret a NW-striking, SW-dipping
strike-slip fault in Chalfant Valley and suggested this structure transfers dextral slip from
the WMFZ northwest to the Volcanic Tableland.
Sierra Nevada Frontal Fault Zone
The Sierra Nevada frontal fault zone contributes to the accumulation of geodetic
strain across the northern ECSZ, but is not included in previous sums of geologic slip
rates at this latitude, ~37.5°N (Fig. 3) (Lifton et al., 2013; Lifton and Lee, pers. comm.,
2015). The Round Valley normal fault (RVF), a segment of the SNFFZ that bounds the
west side of Round Valley, strikes ~NS to NW-SE (Berry, 1997) and dips 60-70° eastnortheast (Fig. 3) (Bateman, 1965). Along the Round Valley fault, Berry (1997)
documented a minimum latest Pleistocene to Holocene (15-25 ka) vertical slip rate of
~0.4-0.5 mm/yr, which indicates a minimum horizontal ~EW extension rate of ~0.1-0.3
mm/yr.
Geology of the Black Mountain Area
The Black Mountain area, located northeast of Long Valley caldera (Fig. 3), was
first mapped as part of the Geologic Map of the 1:62,500 Glass Mountain Quadrangle
(Krauskopf and Bateman, 1977). The area exposes Mesozoic igneous basement
10
unconformably overlain by Miocene rhyolite tuffs, andesite lava flows, andesite lava
flow breccias, Miocene to Pliocene sedimentary deposits, Pliocene basalt fields, and
Quaternary rhyolite tuffs, sedimentary deposits, and river terrace deposits (Plates 1 and 2)
(Krauskopf and Bateman, 1977; this work). Range bounding ~NS- to NNW-striking,
west-dipping normal faults, with ~50-1330 m of throw and secondary ~NW- to NEstriking, west-dipping normal faults, with ~10-160 m of throw, cut all units, except most
middle Pleistocene to Holocene sedimentary features and deposits (Fig. 5, Plates 3 and 4)
(this study; see Faults across the Black Mountain Area section below). Formation of
coarse-grained river terraces in the field area have been linked to southward drainage of
Pleistocene Lake Russell (Reheis et al., 2002). Nevin (1961) estimated 20% EWextension across the NS-striking normal faults in the Black Mountain area. Based on this
estimate, and assuming that the basalt flows cut and offset by these normal faults were
the same age as the Pliocene basalt flows in the Adobe Hills, Nagorsen-Rinke et al.
(2013) predicted a Pliocene ~ENE-WSW extension rate of ~0.6 mm/yr across the Black
Mountain area (Fig. 4).
Kinematic Fault Slip Transfer Models Across the
Northern Eastern California Shear Zone-Mina Deflection Transition
Recent investigations have documented Pliocene and Quaternary slip rates on
faults in the northwestern ECSZ and southwestern Mina deflection to develop, refine, and
spatially expand fault slip transfer kinematic models for this region. Investigations along
the eastern and central ENE-striking Coaldale fault yielded a sinistral slip rate of ≥0.4
mm/yr since ~3 Ma (Bradley, 2005; Tincher and Stockli, 2009). Across the NE-striking,
northwest dipping Queen Valley normal fault, Lee et al. (2009a) documented ~0.3 mm/yr
11
of horizontal extension since the late Pleistocene. Using these slip rates, as well as
published dextral slip rates along the WMFZ (Kirby et al., 2006), Lee et al. (2009a)
proposed a kinematic model across the Queen Valley area whereby NW dextral slip on
the WMFZ is partitioned northward into two components: (a) 0.35 mm/yr of slip is
partitioned onto the ENE-striking sinistral Coaldale fault via the NE-striking normal
Queen Valley normal fault and (b) 0.5 mm/yr of slip is partitioned into the southwestern
Mina deflection via the NW-striking oblique dextral-thrust Coyote Springs fault (Fig. 3).
12
A combination of geologic mapping, structural studies, and geochronology in the
Adobe Hills, southwestern Mina deflection by Nagorsen-Rinke et al. (2013) (Fig. 3)
showed a minimum of ~0.4-0.5 mm/yr of sinistral slip since the Pliocene on ENEstriking sinistral faults. Because this slip could not be kinematically linked to the WMFZ,
Nagorsen-Rinke et al. (2013) proposed a kinematic model whereby a component of NW
dextral shear on the OVF was transferred north to ENE-striking sinistral faults in the
Adobe Hills area via NS- to NNW-striking normal faults in the Volcanic Tableland, ~NSto NNW-striking normal faults in the Black Mountain area, and NW-striking dextral
faults in the River Spring area (Fig. 4).
Warren's (2014) geologic mapping, structural studies, and geochronology across
the River Spring area showed a minimum Pliocene net dextral slip rate of 0.4 ± 0.1
mm/yr across a zone of NW-striking dextral faults and a minimum net sinistral slip rate
of 0.2-0.5 mm/yr on ENE-striking faults. In the River Spring area, NW-striking dextral
faults, dominant in the southeastern part of the field area, mutually cross-cut ENEstriking sinistral faults that are dominant in the northern part of the field area. Warren
(2014) interpreted this fault transition as the boundary between the northern ECSZ to the
south and the southwestern Mina deflection to the north. Building on the kinematic
models from Lee et al. (2009a) and Nagorsen-Rinke et al. (2013), Warren (2014)
predicted that ~NS- to NNW-striking normal faults in the Black Mountain area and the
NW-striking dextral-thrust oblique Coyote Springs fault in Queen Valley transferred fault
slip onto the NW dextral faults exposed in the River Spring area.
13
CHAPTER III
GEOLOGIC ROCK UNITS AND AGES IN THE BLACK MOUNTAIN AREA
Based on field mapping a ~149 km2 area at 1:12,000 scale across the Black
Mountain area (Plate 1), we separate geologic units in the area into pre-Miocene,
Miocene, Pliocene, and Quaternary phases (Plate 2). The Mesozoic plutonic basement is
nonconformably overlain by Miocene rhyolite tuffs, andesite lava flows, andesite lava
flow breccia, and stream deposits. Pliocene basalt and andesite flows, cinder cones, and
stream deposits are in angular unconformity with underlying Miocene rocks and deposits,
and are in angular unconformity with overlying Quaternary units, including rhyolite tuffs,
alluvial, fluvial, eolian, colluvial, lacustrine, playa, and landslide deposits and features
(Plates 1 and 2).
Mesozoic units
The plutonic basement is comprised of Triassic granodiorite and Jurassic granite
associated with the Sierra Nevada batholith (Krauskopf and Bateman, 1977), here
collectively referred to as undifferentiated Mesozoic granite, unit Mzg (Plates 1 and 2).
Triassic diorite and gabbro dikes, unit Trdg, intrude unit Mzg in the western map area
(Plate 1) and Jurassic aplite dikes, unit Jap, intrude unit Mzg in the eastern map area
(Plate 1), though they are widespread elsewhere (Krauskopf and Bateman, 1977; this
study).
Miocene rocks
The Miocene rhyolite tuff, unit Mrt (22.417 ± 0.048 Ma; this study), is
widespread across the northeastern map area where it nonconformably overlies granite
(Plates 1 and 2). The Miocene rhyolite tuff is reddish tan and white, moderately welded,
14
and contains a moderately phyric assemblage of quartz + plagioclase + sanidine ± biotite
phenocrysts with distinctive devitrified yellow pumice in an aphanitic glassy matrix
(Plate 2). The Miocene purple andesite flow, unit Map, outcrops locally in the
northeastern map area where it overlies unit Mrt (Plates 1 and 2). The Miocene purple
andesite flow weathers to a distinctive reddish purple to brown knobby, crumbly outcrop
and contains a moderately phyric assemblage of distinctively altered plagioclase +
clinopyroxene + hornblende + olivine + orthopyroxene phenocrysts (Plate 2). The
Miocene andesite lava flow breccia, unit Mab, outcrops locally in the northeastern map
area where it overlies unit Map (Plates 1 and 2). The Miocene andesite lava flow breccia
contains gravel to boulder sized, poorly sorted, angular clasts in an aphanitic brown
matrix (Plate 2).
Miocene-Pliocene rocks
Two fluvial deposits of Miocene-Pliocene age are documented in the Black
Mountain area (Plates 1 and 2). The Miocene-Pliocene Benton rounded gravel and cobble
deposit, unit MPbsg, is exposed northwest of the Benton Hot Springs (Plate 1). Unit
MPbsg is a ≥ 3 m thick cross-bedded gravel deposit with cobble interbeds and lenses
(Fig. 6, Plates 1 and 2). The base of unit MPbsg is buried. The Miocene-Pliocene rounded
gravel and cobble stream deposit, unit MPg, outcrops locally in the northeastern map area
where it nonconformably overlies unit Mzg and is capped by a Pliocene basalt flow
(Plates 1 and 2).
17
Pliocene rocks
Pliocene basalt and andesite flows nonconformably overlie undifferentiated
Mesozoic granite, and overlie Miocene volcanic rocks and Miocene-Pliocene fluvial
deposits in angular unconformity (Fig. 7, Plates 1 and 2). Inconsistent stratigraphic
relationships between flows and overlapping 40Ar/39Ar ages across the Adobe Hills,
Black Mountain, Huntoon Springs, and River Spring areas suggest that these Pliocene
lava flows interfinger (Nagorsen-Rinke et al., 2013; Hogan, 2014; Warren, 2014; this
study).
18
The Pliocene boring basalt flow, unit Pbb (3.53 ± 0.06 Ma; this study), is
widespread in the western map area and outcrops locally in the northeast map area (Plate
1). Pliocene boring basalt contains a weakly phyric assemblage of fine grained
clinopyroxene + olivine + plagioclase phenocrysts (Plate 2). The lack of coarse grain
phenocrysts distinguishes unit Pbb (Figs. 8a, b, Plate 2).
The Pliocene hornblende andesite, unit Pah, outcrops as a lava flow in the
northeastern map area and also appears as feeder dikes that cross-cut exposed Mzg along
the crest of the Benton Range in the southeastern map area, north and south of the
Pliocene basalt field (Plate 1). Pliocene hornblende andesite contains a weakly phyric
19
coarse grained assemblage of plagioclase + biotite + hornblende ± clinopyroxene
phenocrysts (Plate 2). Unit Pah is the same as a widespread andesite flow mapped north
of the Black Mountain area, in the River Spring, Huntoon Springs, and Adobe Hills areas
(Fig. 3) (Nagorsen-Rinke et al., 2013; Hogan, 2014; Warren, 2014). 40Ar/39Ar
geochronology on unit Pah in the Huntoon Springs area yielded emplacement ages of
3.46 ± 0.01 and 3.47 ± 0.02 Ma (Plate 2) (Hogan, 2014).
The Pliocene glomerocrystic olivine basalt flows, unit Pbc (3.40 ± 0.01 Ma and
3.41 ± 0.02 Ma; this study), are widespread lava flows dominating the southeastern map
area where they unconformably overlay unit Mzg (Plate 1). Pliocene glomerocrystic
basalt contains a moderately phyric assemblage of olivine + clinopyroxene + plagioclase
+ glomerocrysts of olivine + pyroxene phenocrysts (Figs. 8c, d, Plate 2). The abundance
of olivine and glomerocrysts distinguish unit Pbc (Figs. 8c, d, Plate 2).
The Pliocene olivine basalt flows, unit Pbo, outcrop sparsely in the southwestern
map area and contain a weakly phyric assemblage of olivine ± plagioclase phenocrysts
(Plates 1 and 2). 40Ar/39Ar geochronology on unit Pbo in the Adobe Hills and River
Spring areas yielded emplacement ages of 3.28 ± 0.03 and 3.39 ± 0.03 Ma, respectively
(Plate 2) (Nagorsen-Rinke et al., 2013; Warren, 2014).
The Pliocene pyroxene basalt flows, unit Pbp (3.29 ± 0.02 Ma; this study), is
widespread in the northeastern map area and outcrop locally in the southeastern and
western map areas (Plate 1). Pliocene pyroxene basalt contains a weakly phyric
assemblage of clinopyroxene + orthopyroxene + olivine + plagioclase ± rare
glomerocrysts of olivine + pyroxene phenocrysts (Figs. 8e, f, Plate 2). The abundance of
pyroxene phenocrysts distinguishes unit Pbp (Figs. 8e, f).
20
21
Where mineral assemblage could not be observed, lavas were mapped as
undifferentiated Pliocene basalt flows, unit Pb, because outcrops exhibited the same flow
morphology, color, and dip as nearby accessible Pliocene basalts. Pliocene cinder cones,
unit Pvc, overlie Pliocene basalt flows in the southern map area and are defined by a
cone-shaped exposure and the abundance of weathered red pebble to gravel sized basalt
cinder and pebble to cobble sized scoria (Plates 1 and 2). These cinder cones developed
on top of Pliocene basalt lavas. Warren (2014) reported an 40Ar/39Ar emplacement age of
2.94 ± 0.06 Ma for unit Pvc in the River Spring area (Fig. 3, Plate 2).
Quaternary rocks
Quaternary tuffs nonconformably overlie unit Mzg and overlie Miocene-Pliocene
rocks in angular unconformity. The oldest datable Quaternary deposit, the Benton Stream
tuff, unit Qbst, outcrops northwest of the Benton Hot Springs, where it overlies unit
MPbsg in angular unconformity (Fig. 6, Plate 1). The Benton Stream tuff is a ~10 m
thick, white rhyolite tuff containing a weakly phyric assemblage of biotite + hornblende
+ quartz + sanidine phenocrysts in an aphanitic glassy matrix. Unit Qbst contains ~0.20.4 m thick lenses of rounded poorly sorted gravel and pebbles, suggesting multiple
eruptive events depositing tuff into a fluvial environment (Plate 2). 40Ar/39Ar
geochronology on a sample from the base of unit Qbst is in progress.
The Tuff of Taylor Canyon, unit Qtc (1936.12 ± 12.65 ka; this study), drapes the
eastern flanks of topographic ranges where it overlies unit Qbst and, presumably early
Pleistocene, fluvial terraces, unit Qot, in Watterson Meadow (Plates 1 and 2). Unit Qtc
overlies Pliocene basalts in angular unconformity and unit Mzg in nonconformity. The
Tuff of Taylor Canyon is a widespread, 10-15 m thick, poorly consolidated, white
22
rhyolite tuff containing a weakly phyric assemblage of quartz + plagioclase + sanidine ±
biotite phenocrysts in an aphanitic glassy matrix. Based on the emplacement age, we
correlate unit Qtc to ~1.9 Ma explosive eruptions from the Glass Mountain volcano,
which is located along the northeast margin of Long Valley caldera ~5 km west of the
map area (Fig. 3) (Krauskopf and Bateman, 1977; Metz and Mahood, 1985).
The Bishop Tuff, unit Qbt (766.6 ± 3.1 ka; Chamberlain et al., 2014) overlies unit
Qtc near the Benton Hot Springs and nonconformably overlies unit Mzg on the eastern
flank of Blind Springs Hill (Plate 1) (Crowder et al., 1972). The Bishop tuff is a pink,
poorly consolidated, rhyolite tuff.
While at first glance the four rhyolite tuffs in the Black Mountain area appear
similar, their compositions are distinguishable. Compared to unit Qbst, unit Qtc is more
poorly consolidated and is richer in angular gravel to cobble lithic fragments and
obsidian. Unit Mrt is moderately welded and has a unique abundance of quartz and
devitrified yellow pumice. Qbt is distinctively pink.
40
Ar/39Ar Geochronology
Seven 40Ar/39Ar geochronology samples were collected from datable basalt and
tuff units to constrain the emplacement age of these units, timing of faulting, and to
calculate extension rates. Under the supervision of Dr. Andrew Calvert (USGS), four
basalt and three rhyolite tuff samples were prepared and dated. To yield flow
emplacement ages, plagioclase phenocrysts separated from groundmass concentrates of
samples of dense basalt flow interiors were separated, irradiated, and analyzed following
methods described in Appendix A of Nagorsen-Rinke et al. (2013).
23
To yield tuff eruption ages (Dalrymple and Duffield, 1988), sanidine phenocrysts
from the interior of rhyolite tuff deposits were separated, irradiated, and analyzed with a
continuous CO2 laser system and mass spectrometer following methods in Dalrymple
(1989). 40Ar/39Ar age spectra for basalt samples and probability density plots for rhyolite
tuff samples are shown in Figures 9 and 10, respectively. Additionally, analytical data
and interpreted ages are reported in Tables A1 to A6 in Appendix A and are summarized
in Table 1, respectively. Appendix A provides a complete description of analytical
techniques for sanidine minerals separated from rhyolite tuffs.
24
25
26
CHAPTER IV
FAULTS ACROSS THE BLACK MOUNTAIN AREA
Faults are best exposed and were mapped in detail where they cut and offset
Miocene-Quaternary rocks. Some faults are longer than they are mapped. Faults were not
mapped extensively in the undifferentiated Mesozoic plutonic basement because granites
lack offset markers and distinguishing fault planes, which lack fault striations, from joints
is impossible (Plate 1). Some faults were not mapped because it was not possible to
follow them where buried under late Pleistocene to Holocene active alluvial fans, fluvial,
eolian, colluvial, playa, lacustrine, and landslide deposits and features (Plates 1 and 3).
Geometry of Key Miocene-Quaternary Rocks
Angular unconformities separate Miocene, Pliocene, and Quaternary rock
sequences. Older rocks dip more steeply and exhibit greater offsets across normal faults
than younger rocks. Contact patterns in the northeast map area (Plate 1) and one of two
flow foliations (n = 2; Fig 11c) measured in Miocene rocks (Plate 1) suggest these units
are dipping east-northeast more steeply, >30°, than the overlying Pliocene units and
therefore record a larger magnitude of vertical offset across normal faults compared to
Pliocene units (Plates 1 and 3). Flow foliations measured in Pliocene basalts and
andesites show these units are mostly dipping 15-30° northeast-southeast (Fig. 11, Plate
1). In Black Canyon, in the western map area, stacked Pliocene basalt flows are dipping
east-southeast to the same degree (Plate 1). We observe no differential offset among
Pliocene basalts flows, but do between the Pliocene basalt flows and the overlying
Quaternary volcanic tuffs, which are dipping 5-8° east-northeast.
27
29
Fault Geometry
Deformation in the Black Mountain area is dominated by normal faulting as
evidenced by linear to moderately curved range fronts, exhumation of older units in
mountain ranges, and vertically offset Mesozoic granitic basement, Miocene to Pliocene
volcanic rocks, Quaternary tuffs, incised river terraces, and three incised alluvial fans
across faults (Plates 1 and 4).
We use fault strike, length, and amount of throw, measured from cross-sections,
to separate normal faults into two faulting styles. The boundary between faulting styles is
gradational. The first fault geometry, range-bounding faults, is characterized by ~NS- to
NNW-striking, moderately to steeply (45° to 60°) west-dipping normal faults that are
straight to curvilinear, 1.5 to 9 km in length, and show ~50-1330 m of throw since the
Pliocene. Range bounding normal faults typically bound the western flanks of NStrending topographically high standing ranges (Fig. 5, Plates 3 and 4). The other fault
geometry, small offset faults, is characterized by NW- to NE-striking, steeply (50° to
60°) southwest- to northwest-dipping, straight to curvilinear normal faults ranging from
200 m to 2.5 km in length with ~10-50 m of throw since the Pliocene (Plates 3 and 4). A
few small offset faults show ~50-160 m of throw since the Pliocene (Plate 3).
We use fault orientation and average dip of flow foliation and bedding to separate
the Black Mountain area into three domains (Fig. 11, Plates 1 and 4, see Appendix B for
stereonet methodology). Domain 1, across the southern third of the map area, is
characterized by ~NS- to NNW-striking range bounding and ~NW- to NE-striking small
offset normal faults and east-northeast dipping Pliocene to Quaternary units (Fig. 11a,
Plates 1 and 4). Both fault geometries cut and offset all units except late Pleistocene to
30
Holocene alluvial fans, fluvial, eolian, colluvial, playa, and lacustrine deposits and
features (Plates 1 and 3).
Domain 2, across the central map area, is characterized by ~NS-striking range
bounding and small offset normal faults and east dipping Pliocene to Quaternary units
(Fig. 11b, Plates 1 and 4). Both fault geometries cut and offset all Mesozoic to midPleistocene units and, presumably late Pleistocene age, incised alluvial fans near Benton
Hot Springs (Plates 1, 3, and 4). Faults are buried under late Pleistocene to Holocene
active alluvial fans, fluvial, eolian, colluvial, playa, and lacustrine deposits and features
(Plates 1 and 3).
Domain 3, across the northern map area, is characterized by ~NS-striking range
bounding and ~NW- to ~NE-striking small offset normal faults and east-northeast
dipping Miocene to Pliocene units (Fig. 11c, Plates 1 and 4). Both fault geometries cut
and offset all units, except late Pleistocene to Holocene alluvial fans, fluvial, eolian,
colluvial, playa, and lacustrine deposits and features (Plates 1 and 3).
Extension Direction
We use tilts of Miocene to Quaternary volcanic rocks and fluvial deposits and, to
a lesser degree fault strikes and dips, to define the direction of Pliocene-Quaternary
extension across each domain within the Black Mountain area. In Domain 1, Pbc and Pbp
flow foliation attitudes (n = 36) yield an average strike and dip of N30W, 15° NE (Fig.
11a). These data, in conjunction with Domain 1’s fault geometry defined by dominant
~NS- to NNW-striking normal faults, connected by lesser NW- to NE-striking normal
faults, suggest a NE-SW (~60-240°) extension direction (Fig. 11a, Plates 1 and 4, Table
2).
32
In Domain 2, Pbb and Pbp flow foliation attitudes (n = 15) and MPbsg and Qbst
bedding (n = 2) yield an average strike and dip of N5W, 13°NE (Fig. 11b). These data, in
conjunction with Domain 2’s fault geometry defined by ~NS-striking range bounding and
small offset normal faults, suggest an approximately EW (~85-265°) extension direction
(Fig. 11b, Plates 1 and 4, Table 2).
In Domain 3, Map, Pah, Pbb, and Pbp flow foliation attitudes (n = 25) yield an
average strike and dip of N18W, 16° NE (Fig. 11c). These data, in conjunction with
Domain 3’s fault geometry defined by ~NS- to NNW-striking range bounding and ~NWto NE-striking small offset normal faults, suggest an ENE-WSW (~72-252°) extension
direction (Fig. 11c, Plates 1, 3, and 4, Table 2).
Throughout the field area, unit Mzg preserves fault planes and closely spaced
(~10-20 m) joints which are subparallel to fault planes. A stereonet plot of fault planes (n
= 9) and joint (n = 1) attitudes in unit Mzg yields an average strike and dip of N31W, 58°
SW, suggesting an NE-SW (59-239°) extension direction across the entire Black
Mountain area (Fig. 12). These data, measured mostly on small offset faults throughout
the field area, suggest a NE-SW extension direction across the Black Mountain area that
is parallel to the extension direction in Domain 1 (Fig. 11a) and subparallel to the
extension direction in Domain 3 (Fig. 11c). The extension direction calculated for
Domain 3 and the extension direction based on fault and joint attitudes measured in unit
Mzg are different because the dataset measured in unit Mzg was collected mostly on
small offset faults and is not representative of dominant range bounding fault geometries
found in Domain 3.
33
Magnitude of Extension
We estimate the magnitude of extension by palinspastically restoring the downdip displacement of the unconformity between Pliocene basalts and older units, which
defines an ideal offset marker, along each normal fault across the three geologic crosssections (Plate 3). On each of the present-day geologic cross-sections, tie points were
selected where the Pliocene-Mesozoic basement unconformity intersects with the
outermost faults on the cross-section. To restore Pliocene offset, from west to east the
unconformity was moved up-dip (Plate 3). To determine how much the PlioceneMesozoic granite unconformity has been tilted, we measured the angle of a straight line
connecting the two tie points on each restored cross-section (Table 2). The restored crosssection was then rotated counter-clockwise by the tilt of the unconformity, returning the
unconformity to its assumed original horizontality when Pliocene basalt flows were
emplaced (Plate 3, Table 2).
34
The restored unconformity on the EW-trending Y-Y’’ cross-section, from the
central part of the field area (Domain 2), yields a shallow east tilt of ~13°, whereas the
restored unconformities on the NE-trending cross-sections X-X’’ and Z-Z’, from the
southern and northern parts of the field area (Domains 1 and 3), respectively, yield
slightly steeper northeast tilts of ~16° and ~19°, respectively. The difference in horizontal
distance between tie points on the present-day geologic cross-sections vs. on the
palinspastically restored cross-sections yields the magnitude of horizontal extension
parallel to the trend of the cross-section (Plate 3). For cross-sections, X-X’’, Y-Y’’, and
Z-Z’, the magnitude of horizontal extension is 1.2, 1.1, and 1.5 km, respectively (Plate 3,
Table 2), which indicates 18%, 10%, and 20% extension, respectively (Table 2).
The most significant source of error in our horizontal extension calculations are
fault dips, which were calculated using three point problems or assigned the same dip as
adjacent faults. To estimate the error associated with the magnitude of extension we
palinspastically restored section Z-Z’, the cross-section that yielded the maximum
horizontal extension, using an error in fault dip of ± 10° (Plate 5). These restorations
indicate a range in horizontal extension magnitude range from 2.2 km (faults rotated 10°
35
shallower) to 0.9 km (faults rotated 10° steeper) across Z-Z’ (Plate 5). Thus, 1.5 + 0.7/ 0.6 km (an estimated error of 40 to 46%) is our preferred maximum horizontal extension
magnitude across the Black Mountain area (Plate 5, Table 2). Another source of error,
likely considerably less than the ~46% error associated with fault dip, is the estimated dip
of Pliocene basalt map units projected across the map area in the present-day geologic
cross-sections. Thus, as a conservative estimate of uncertainty, we apply a 46% error to
our estimated extension magnitudes for sections X-X’’ and Y-Y’’, which yields
horizontal extension magnitudes of 1.2 ± 0.6 and 1.1 ± 0.5 km, respectively (Table 2).
Extension Rates
To obtain a rate of horizontal extension across each structural domain of the
Black Mountain area, we divided the magnitude of horizontal extension by the age of the
youngest Pliocene basalt offset along each cross-section (Plate 3, Table 2). Across
Domains 1, 2, and 3, the minimum horizontal extension rates are 0.4 ± 0.2 mm/yr, 0.3 ±
0.1 mm/yr, and 0.5 ± 0.2 mm/yr, respectively (Table 2). Since section Z-Z’, across
Domain 3, yields the largest magnitude of horizontal extension, our preferred minimum
Pliocene extension rate is 0.5 ± 0.2 mm/yr, oriented ENE-WSW (~72-252°), across the
Black Mountain area (Table 2). The ≥0.5 ± 0.2 mm/yr ENE-WSW extension rate across
the northern Black Mountain area since ~3.29 Ma is the same, within error, to that
predicted in the Nagorsen-Rinke et al. (2013) kinematic model (Fig. 4).
Extension rates across Domains 1, 2, and 3 agree, within error. The extension rate
along section Z-Z’ is ~0.1 mm/yr higher than the extension rate across section X-X’’,
probably because section Z-Z’ extends farther southwest than section X-X’’, capturing
faults that are beyond the southwest end of section X-X’’. The 0.3± 0.1 mm/yr EW37
extension rate across the central Black Mountain area (section Y-Y‖) is smaller than rates
along the other two sections because this section crosses the tip of the Blind Springs hill
fault (Fig.3, Plate 1), whereas the other sections do not. Fault tips record a smaller
magnitude of offset compared to the center of a fault (Dawers et al., 1993)
38
CHAPTER V
DISCUSSION
Timing and Rate of Volcanism and Extension Across the Black Mountain Area
In the Black Mountain area, we document three deformation events, a preMiocene, a Miocene and a Pliocene-Quaternary. Prior to ~22.4 Ma, a pre-Miocene
deformation event brought the Mesozoic plutonic basement to the surface. Although our
observations are limited, Miocene rocks appear to exhibit greater tilts and show larger
vertical offset across normal faults than Pliocene rocks, suggesting a Miocene episode of
extension and volcanism in the Black Mountain area. The Miocene deformation and
volcanism event is consistent with a documented pulse of Miocene volcanism and
extension in the southern WLB, Mina deflection, and northern ECSZ (Oldow et al., 1994;
Dilles and Gans, 1995; Stockli et al., 2003; Tincher and Stockli, 2009; Rood et al., 2011;
Nagorsen-Rinke et al., 2013; Warren, 2014). Restored cross-sections show that Pliocene
basalts flowed over sub-horizontal ground surface of Mesozoic plutonic basement and
tilted Miocene rocks (Plate 3). The angular unconformity between the Pliocene basalts
and underlying Miocene rocks suggests a period of quiescence and erosion following the
Miocene deformation episode.
In the Black Mountain area, Pliocene lavas were erupted over a short period of
time, ~204 ka, between ~3.53 and ~3.29 Ma, similar to the short period (≤500 ka) of
basalt volcanism during the Pliocene in the Adobe Hills, Huntoon Springs, and River
Spring areas (Nagorsen-Rinke et al., 2013; Hogan, 2014; Warren, 2014). 40Ar/39Ar
emplacement ages for Pliocene basalt and andesite lavas in the Adobe Hills (NagorsenRinke et al., 2013), Black Mountain area (this study), Huntoon Springs (Hogan, 2014),
39
and River Spring (Warren, 2014) areas range from as old as 3.53 ± 0.06 Ma (unit Pbb) in
the Black Mountain area (this study) to as young as 3.13 ± 0.02 Ma (unit Pbc) in the
Adobe Hills, suggesting relatively rapid, over a ~400 ka period, and widespread
(minimum 350 km2) eruption of lavas across this region (Fig. 3).
Pliocene rocks are tilted and vertically offset across normal faults, indicating a
post ~3.29 Ma episode of Pliocene extension and volcanism in the Black Mountain area.
In the Black Mountain area, the absence of differential offset among Pliocene basalts and
the observation that Quaternary tuffs record less tilt and vertical offset across normal
faults than the basalts indicate that the Pliocene episode of extension began after eruption
of the basalt lavas, ~3.29 Ma, but before eruption of the Tuff of Taylor Canyon at ~1.9
Ma. The timing of Pliocene extension in the Black Mountain area is coeval with the onset
of Pliocene sinistral and dextral fault slip in the Adobe Hills and River Spring areas
(Nagorsen-Rinke et al., 2013; Warren, 2014); extension across the Queen Valley fault
(Stockli et al., 2003), the Eastern Inyo fault zone (Lee et al., 2009b), and the
southwestern Inyo Mountains (Casteel, 2005); and sinistral slip along the Coaldale fault
(Bradley, 2005; Tincher and Stockli, 2009) suggesting a regional Pliocene deformation
event across the northern ECSZ and Mina deflection.
The differential offset between the Tuff of Taylor Canyon (unit Qtc) and the
Bishop Tuff (unit Qbt), whereby the older Qtc records a larger magnitude of vertical
offset compared to the younger Qbt, indicates that extension continued at least into the
middle Pleistocene. Rare normal fault scarps cutting incised alluvial fans west of the
Benton Hot Springs, which overlie unit Qbt, show that extension continued after the
eruption of unit Qbt at ~766 ka (Plate 1).
40
The lack of fault scarps in Holocene deposits, but exposure of old, eroded scarps
in presumably late Pleistocene incised alluvial fans near the Benton Hot Springs (Plate 1)
may indicate that (a) a long (>10,000 yrs) earthquake recurrence interval on faults in the
Black Mountain area, and/or (b) earthquakes were small in magnitude, and thus
associated with small surface ruptures which were easily erased by erosion.
Recalculated Extension Rates across the Volcanic Tableland
Assuming 60° ± 10° dips on the normal faults that cut the Volcanic Tableland,
Pinter (1995) documented 290 +131 m/-107 m of horizontal EW-extension across the
southern part of the central and western Volcanic Tableland. This extension estimate did
not include the largest fault scarp in the southern Volcanic Tableland, the ~150 m tall,
west-dipping, ~NS-striking normal fault scarp in Fish Slough, located in the southeastern
part of the Volcanic Tableland (Fig. 3). Using the ~150 m height of the fault scarp and
assuming 60° ± 10° fault dip, we calculate ~87 m +39 m/-32 m of horizontal EWextension across the Fish Slough fault. Adding this magnitude of extension to Pinter’s
(1995) extension estimate yields a total horizontal EW-extension magnitude of 377 m
+170 m/-139 m across the southern Volcanic Tableland. Dividing this magnitude by the
age of the Bishop Tuff, 766.6 ± 3.1 ka (Chamberlain et al., 2014), yields a horizontal
EW-extension rate of 0.5 ± 0.2 mm/yr across the southern Volcanic Tableland (Fig. 3).
A Refined Model of Fault Slip Transfer: Owens Valley Fault to Mina Deflection
Our new Pliocene extension rates across the Black Mountain area, recalculated
extension rate across the Volcanic Tableland (Pinter, 1995; this study), combined with
recent fault slip rate studies along the dextral WMFZ (Lifton et al., 2012; Lifton, 2013),
slip rates along dextral and sinistral faults in the River Spring area (Warren, 2014), and
41
published sinistral slip rates across the Adobe Hills (Nagorsen-Rinke et al., 2013) allow
us to refine the Nagorsen-Rinke et al. (2013) and Warren (2014) kinematic models (cf.
Figs. 4 and 13). While fault slip rates in the kinematic model range from Pliocene to late
Pleistocene, we assume that slip rates have remained constant since the Pliocene. A
kinematic model for the transfer of dextral slip from the OVF to the Mina deflection has
to account for observed fault slip and horizontal extension rates on the northern OVF
(Kirby et al., 2008) and southern WMFZ (Lifton et al., 2012; Lifton and Lee, pers.
comm., 2015), and the southern Volcanic Tableland (Pinter, 1995; this study), the Black
Mountain (this study), River Spring (Warren, 2014), and Adobe Hills areas (NagorsenRinke et al., 2013) (Fig. 13a, Table 3).
To model fault slip transfer from the OVF to sinistral faults in the southwestern
Mina deflection, we partition 2.8 mm/yr of dextral slip along the OVF (Kirby et al.,
2008) northward into two orthogonal components that are the same, within error, as
documented slip vectors: (1) 0.5 mm/yr of ENE-WSW extension across the Volcanic
Tableland and (2) 2.1 mm/yr of NNW dextral shear along the WMFZ (Fig. 13b, Table 3).
Deconvolving 2.8 mm/yr of slip on the OVF into these two orthogonal components
leaves ~0.6 mm/yr of NW dextral slip unaccounted for (Fig. 13b, Table 3). The ―missing‖
0.6 mm/yr of NW dextral shear may be accommodated along: (a) broad folds developed
in the Volcanic Tableland as a consequence of dextral shear (Bateman, 1965), (b) dextral
shear transfer between the Sierra Nevada and Volcanic Tableland into Long Valley
caldera, (c) undocumented faults, and/or (d) along the WMFZ since slip estimates along
this fault are a minimum. If slip rates on the northern OVF are as high as 4.5 mm/yr
42
43
44
(Kirby et al., 2008), than the magnitude of missing slip is considerably larger at ~2.3
mm/yr.
We deconvolve the documented ~0.5 mm/yr EW-extension rate across the
Volcanic Tableland into an ENE-WSW extension vector across the Black Mountain area,
yielding a predicted extension rate of ~0.5 mm/yr, the same as we document (Fig. 13b,
Table3). We deconvolve ~0.5 mm/yr of ENE-WSW extension across the Black Mountain
area into orthogonal vectors parallel to observed slip rates in the River Spring area (Fig.
13b, Table 3). Our predicted NW dextral slip rate for the River Spring area is ~0.2 mm/yr
less than is documented, probably because our fault slip transfer map is oversimplified
(Fig. 13b, Table 3). We hypothesize the Coyote Springs fault (Figs. 3 and 13) is feeding
the missing dextral slip onto NW-striking faults in the eastern River Spring area. We
predict 0.4-0.5 mm/yr of SW-extension, which Warren (2014) did not observe (Fig. 13b,
Table 3b). We speculate that the predicted extension occurred along NW-striking,
southwest-dipping normal faults that opened Adobe Valley and are now buried under fill.
We deconvolve ~0.5 mm/yr of ENE-WSW extension across the Black Mountain
area into orthogonal vectors parallel to observed slip rates in the Adobe Hills area (Fig.
13b, Table 3). We predict 0.4-0.5 mm/yr of sinistral slip on ENE-striking sinistral faults,
which agrees with documented slip rates (Fig. 13b, Table 3) (Nagorsen-Rinke et al.,
2013). Our model also predicts 0.2 mm/yr of NW-extension, which Nagorsen-Rinke et al.
(2013) did not observe, but suggested was accommodated on normal faults northwest of
the Adobe Hills. The general agreement between observed and predicted slip rates in the
Volcanic Tableland-Black Mountain-River Spring-Adobe Hills fault system suggests that
our refined kinematic model is geologically reasonable.
45
The second component of fault slip transfer from OVF is a right-step to the
northeast onto the WMFZ. Transfer of dextral slip from the WMFZ northward into the
Queen Valley area, including the Queen Valley normal fault and the Coyote Springs
dextral-thrust oblique fault remains poorly understood. In Lee et al.'s (2009a) kinematic
model describing this transfer, they assumed that the dextral slip rate along the northern
part of the WMFZ was the same as that documented along the southern part; this
assumption may not be valid.
Based on study of aftershock hypocenters from the Ms 6.2 Chalfant Valley
earthquake sequence, Smith and Priestly (2000) hypothesized that a NW-striking,
southwest-dipping strike-slip fault in the Chalfant Valley transfers dextral slip from the
WMFZ northwest into the Volcanic Tableland (Fig. 3). We speculate that this and
similarly oriented faults, now buried under Hammil Valley, likely transfer slip
northwestward from the WMFZ into the Volcanic Tableland-Black Mountain-River
Spring-Adobe Hills fault system. As a consequence, the dextral slip rate along the
northern WMFZ is probably less than that along southern part, but is unknown. Lacking
slip rates on the northern WMFZ to constrain the amount of slip bleeding off the WMFZ,
we cannot adequately constrain this part of the kinematic model (Fig. 13).
46
Discrepancy Between Geodetic Strain Rates and Geologic Slip Rates
Across the northern ECSZ, the modeled geodetic strain rate, 10.6 ± 0.5 mm/yr, is
about twice the sum of the late Pleistocene geologic slip rates (Lifton et al., 2013).
Geologic slip not included in this comparison, such as extension across the Volcanic
Tableland and SNFFZ, may help resolve this discrepancy between geologic and geodetic
slip rates (Frankel et al., 2011; Nagorsen-Rinke et al., 2013)
To address the regional discrepancy between geodetic and geologic slip rates, we
first recalculate Lifton et al.'s (2013) modeled geodetic strain rate of 10.6 ± 0.5 mm/yr
across the northern ECSZ, which was calculated parallel to N37W (motion of the Sierra
Nevada relative to Ely, NV (SN-NA), to a vector parallel to the present-day PA-NA plate
motion azimuth, N47W (Dixon et al., 2000); this yields a geodetic strain rate of 10.4 ±
0.5 mm/yr. Both vectors capture NW dextral shear across the WLB-ECSZ, however the
PA-NA vector captures ~EW extension across the entire Basin and Range, whereas the
SN-NA vector uses Ely, NV, in the central Basin and Range, as stable North America
(Fig. 1). Second, we sum geologic slip rates, calculated parallel to N47W (Fig. 14, Table
4), on all the faults with calculated slip rates crossed by the N53E transect (Figs. 3 and
14). This sum yields a geologic dextral shear rate of 6.4 +1.7/-1.0 mm/yr (Table 4), which
is ~60% the geodetic strain rate across the northern ECSZ (Fig. 14b).
Second, we address the local discrepancy between geologic slip rates and geodetic
strain rates between the crests of the Sierra Nevada and White Mountains, across the
RVF segment of the SNFFZ, Volcanic Tableland, and WMFZ, by summing geologic slip
rates calculated parallel to N47W (Fig. 14, Table 4). This sum yields a minimum NW
dextral shear rate of 2.1 +0.7/-0.6 mm/yr, about half of the geodetic strain rate of ~3.9
47
mm/yr. We suggest that summed geologic slip rates are less than the geodetic strain rate
across the northern Owens Valley because: (a) transient strain accumulation is not
documented in geologic slip rates (e.g. Oskin and Iriondo, 2004), (b) elastic strain
accumulation across the FLVFZ and/or San Andreas fault system results in a larger
geodetic velocity, (c) assuming the dextral slip rate along the northern OVF is closer to
4.5 mm/yr (Kirby et al., 2008), then transfer of ~2 mm/yr of this onto undocumented
structures across northern Owens (Fig. 3), and/or (d) magmatic activity in Long Valley
superimposed on top of the tectonic geodetic signal.
48
49
CHAPTER VI
CONCLUSIONS
Our new geologic mapping, structural, and geochronology studies in the Black
Mountain area indicate three deformation episodes, pre-Miocene, Miocene, and PlioceneQuaternary. The dominant Pliocene-Quaternary episode is characterized by rapid and
widespread basalt lava eruption, subsequent ENE-WSW horizontal extension, and
episodic deposition of Quaternary rhyolite tuffs. Palinspastic restoration of crosssections, combined with 40Ar/39Ar geochronology, yields ≥0.5 ± 0.2 mm/yr of ENEWSW Pliocene horizontal extension across the Black Mountain area. Combining this rate
with published fault slip rates across faults in the northern ECSZ and southwestern Mina
deflection allow us to test and refine the Nagorsen-Rinke et al. (2013) kinematic fault slip
model across this region. In our kinematic model, 2.8 mm/yr of NW dextral slip on the
OVF is partitioned northeastward onto the WMFZ and northward onto the Volcanic
Tableland-Black Mountain-River Spring fault zone and into the Adobe Hills area of the
Mina deflection. However, at least 0.6 mm/yr of NW dextral slip on the OVF remains
unaccounted for in this kinematic model. We add in geologic extension rates across the
Volcanic Tableland and Sierra Nevada frontal fault to help resolve, in part, both the
regional discrepancy between geodetic strain rates and geologic slip rates across the
entire northern ECSZ and the local discrepancy between the crests of the Sierra Nevada
and White Mountains across the Volcanic Tableland.
50
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APPENDIXES
Appendix A – 40Ar/39Ar Geochronology
40
Ar/39Ar geochronology samples from the Black Mountain area were analyzed at
the US Geological Survey, Menlo Park, CA under the supervision of Dr. Andrew Calvert.
Plagioclase minerals in groundmass from basalt flows were prepared and analyzed
following procedures outlined in Nagorsen-Rinke et al. (2013) supplemental file 1.
Sanidine crystals from rhyolite tuffs were prepared and analyzed following procedures
outlined by Dr. Andrew Calvert below (Appendix A – Supplemental File 1). Age
spectrum and isochrones from plagioclase analyzed from basalt samples are shown in
Figures A1 to A4 and Tables A1 to A4. Age spectrum for sanidine from rhyolite tuff
samples are shown in figures A5 and A6 and Tables A5 to A6.
Supplemental File 1– 40Ar/39Ar Analytical techniques for sanidine crystals,
USGS Menlo Park
40
Ar/39Ar geochronology was performed using single fragments of sanidine
crystals segregated by crushing, sieving, heavy liquid, and magnetic techniques. Samples
were carefully handpicked under a binocular microscope. For irradiation, 25 mg separates
were packaged in Al foil and placed in a cylindrical quartz vial, together with fluence
monitors of known age and K-glass and fluorite to measure interfering isotopes from K
and Ca. The quartz vials were wrapped in 0.5 mm-thick Cd foil to shield samples from
thermal neutrons during irradiation. The samples were irradiated for one hour in the
central thimble of the U.S. Geological Survey TRIGA reactor in Denver, Colorado
(Dalrymple et al., 1981).The reactor vessel was rotated continuously during irradiation to
avoid lateral neutron flux gradients and oscillated vertically to minimize vertical
61
gradients. Reactor constants determined for these irradiations were indistinguishable
from recent irradiations, and a weighted mean of constants obtained over the past five
years yields 40Ar/39ArK = 0.0010±0.0004, 39Ar/37ArCa = 0.00071±0.00005, and
36
Ar/37ArCa = 0.000281±0.000006. TCR-2 sanidine from the Taylor Creek Rhyolite
(Dalrymple and Duffield, 1988) was used as a fluence monitor with an age of 27.87 Ma.
Results were recalculated to Fish Canyon sanidine = 28.294 Ma (Renne et al., 2011)
using RTCs/FCs = 1.00881±0.00046, determined in Menlo Park. TCR-2 is a secondary
standard calibrated against the primary intralaboratory standard, SB-3, that has an age of
162.9 ± 0.9 Ma. Fluence monitors and unknowns were analyzed using a continuous CO2
laser system and mass spectrometer described by (Lanphere and Dalrymple, 2000). Gas
was purified continuously during extraction using two SAES ST-172 getters operated at
4A and 0A.
Mass spectrometer discrimination and system blanks are important factors in the
precision and accuracy of 40Ar/39Ar age determinations of Pleistocene samples because of
low radiogenic yields. Discrimination is monitored by analyzing splits of atmospheric Ar
from a reservoir attached to the extraction line and for these samples D1amu = 1.010795 ±
0.000169. Typical system blanks including mass spectrometer backgrounds were 1.5x1018
mol of m/z 36, 9x10-17 mol of m/z 37, 3x10-18 mol of m/z 39 and 1.5x10-16 mol of m/z
40, where m/z is mass/charge ratio.
Apparent ages are calculated from single crystal fusion data using atmospheric
ratios and plotted as probability density functions (Deino and Potts). For isochron plots,
data are not corrected using an atmospheric ratio. Ages and isotopic ratios reported below
are 1.
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Appendix B – Stereonet 8
We used Stereonet 8 by Richard Allmendinger (Figs. 11 and 12) to plot attitudes,
including flow foliation, bedding, granite joints, fault, and slickenline attitudes, on
stereonets. Stereonet 8 uses structural geology algorithms and spherical projections
described in Allmendinger et al. (2012) and Cardozo and Allmendinger (2013).
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