molly partridge thesis proposal...molly partridge thesis proposal project title volcano-tectonic...
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Molly Partridge Thesis Proposal Project Title Volcano-tectonic history of the northern Warner Range, northeastern California Project Advisor Dr. Anne Egger Introduction The term “ancestral Cascades” has been used to describe Oligocene to Miocene arc
volcanism related to subduction along the western North American margin (Fig. 1) (du
Bray et al., 2009). However, the extent and evolution of the ancestral Cascades is poorly
constrained, because the volcanic rocks of this age have often been cut by faults, eroded,
or covered by younger volcanism. Two different sets of faulting are present throughout
the northwestern Basin and Range, including a set of northwest trending normal faults
and a set of north-northeast trending fractures (Egger and Miller, 2011; Scarberry et al.,
2010). Late Miocene to Pliocene rocks cover much of the northern edge of the Ancestral
Cascades (Fig. 1) (Reed et al., 2005), which means that, unless the younger volcanics
have been faulted, it is difficult to obtain information about the underlying Oligocene and
Miocene units. As a result, many questions remain about the Ancestral Cascades,
including: what was the extent of the ancestral Cascades during the Oligocene? How did
the ancestral Cascade arc evolve from the Oligocene to the Miocene? And what are the
relationships between the faulting and volcanism?
The Warner Range in northeastern California (Fig. 2) provides an opportunity to add
detail to our knowledge about the extent and evolution of the ancestral Cascades. Arc-
related Oligocene- and Miocene-age volcanic rocks have been mapped in the Warner
Range (Fig. 1 and 2) (Colgan et al., 2011; Egger and Miller, 2011). Within the Warner
Range, the Oligocene volcanic rocks are made up of mostly basaltic and andesitic flows
with some interbedded ash-flow tuffs, dated at 27.5-24 Ma (Fig. 2) (Colgan et al., 2011).
These volcanic rocks are locally sourced: Egger and Miller (2011) mapped two
Oligocene volcanic vents within the central Warner Range (Fig. 2). Miocene arc-volcanic
rocks (also locally sourced) have been dated at 16-14 Ma, are found in the southern
Warner Range and are also made up of mostly basaltic to andesitic lava flows and some
tuffs (Fig. 2) (Colgan et al., 2011). The units in the Warner Range have been tilted and
offset by the N-trending Surprise Valley normal fault (Fig. 2) (Egger and Miller, 2011),
exposing the relationships between these units that are obscured elsewhere. In addition,
small-offset NW-trending faults variably cut these volcanic units, and their origin is
poorly understood.
The focus of this project is the northern Warner Range (Figure 3), which has not yet been
mapped at the same scale as the rest of the range. Specifically, I will address the
following questions in my research:
• Are there additional Oligocene and/or Miocene volcanic centers in the northern
Warner Range like those mapped in the central and southern parts of the range?
How does the presence or absence of these volcanic centers affect the current
interpretation of the extent of the Ancestral Cascades?
• What are the cross-cutting relationships between the different volcanic units and
the faults that could help place age constraints on the units and faulting in the
Warner Range?
I propose to conduct detailed geologic mapping, petrographic analysis, and geochemical
analysis in the northern Warner Range in northeastern California to better understand
these relationships. My work will bring us closer to answering the big questions about the
Ancestral Cascades in the northwestern Basin and Range by providing new data that is
currently unknown in the Northern Warner Range.
Background The northwestern Basin and Range extends from northeastern California to southeastern
Oregon, south of the High Lava Plains (Fig. 1). Magmatism in the northwestern Basin
and Range has occurred primarily in three episodes: 1) Oligocene arc-volcanism (27-22
Ma) related to subduction (Colgan et al., 2011; Scarberry et al., 2010); 2) Middle
Miocene arc-volcanism (Colgan et al., 2011) and flood basalts associated with the
Yellowstone hotspot (Camp and Ross, 2004); 3) Late Miocene to Pliocene extension-
related volcanism (ca. 8-3 Ma), consisting of volcanic units that filled in topographic
lows (Carmichael et al., 2006; McKee et al., 1983) (Fig.2).
Oligocene arc-volcanic rocks mapped thus far within the Warner Range include the Lake
City Basalts, the Cedar Pass Complex, the Hays Volcano, and Ash-Flow Tuffs (Fig. 2)
(Colgan et al., 2011; Egger and Miller, 2011). These rocks are depleted in Nb, Ta, Ti, and
Zr when compared to typical magmas from a subduction setting (Fig. 5) (Colgan et al.,
2011). The 87Sr/86Sri ratios in these Oligocene volcanic rocks are similar to mafic rocks of
the southern modern Cascades (Colgan et al., 2011).
Mid-Miocene volcanic rocks in the Warner Range were originally believed to be part of
the Steens Basalts and they have been mapped as such in the past (Camp and Ross, 2004).
Geochemical analysis has shown that, when compared to the Steens Basalts, mid-
Miocene rocks in the Warner Range contain much less Nb, Ta, and Ti for a given MgO
content (Fig. 6) (Colgan et al., 2011). Recent field mapping has further distinguished
these rocks from the Steens Basalts, as these mid-Miocene basalts are geologically and
geochemically more similar to the ancestral Cascades, like the Oligocene basalts in the
same area (Colgan et al., 2011; Egger and Miller, 2011). They were previously believed
to be one continuous unit but recognition of volcanic vents throughout the Warner Range
and detailed geologic mapping has shown that they are not continuous (Carmichael et al.,
2006; Egger and Miller, 2011).
An episode of late Miocene to Pliocene extension-related volcanism centered on the
Modoc Plateau (Fig. 1) also extends into the Warner Range. The Modoc Plateau lies west
of the Warner Range and east of the modern Cascades (Fig. 1) so it marks a transitional
area between arc-volcanism and extension (McKee et al., 1983). These mafic flows have
been classified as low-K, high-alumina olivine tholeiites (Hart et al., 1984) and dated to
~2.5 to 8 Ma (Carmichael et al., 2006; McKee et al., 1983). Within the Warner Range,
these basalts have been mapped as filling in the topographic lows around the Oligocene
and Middle-Miocene volcanic edifices (Egger and Miller, 2011).
The northwestern Basin and Range also exhibits two sets of faults and fractures that are
apparent in the Warner Range. There are north-south oriented Basin-and-Range-style
normal faults that account for most of the extension in the area, though they are not very
numerous (Egger and Miller, 2011). These include the Surprise Valley Fault, which
bounds the Warner Range, and the Hays Canyon Fault, located across Surprise Valley to
the east of the Warner Range (Fig. 2) (Egger and Miller, 2011). Previous work has shown
that these faults likely developed after 14 Ma because they cut rocks that have been dated
at 14.1 ± 0.4 Ma in the southern Warner Range (Egger and Miller, 2011). These faults are
prominent throughout California and Nevada but they die out to the north in southern
Oregon (Fig. 1) (Jordan et al., 2004; Lerch et al., 2008; Scarberry et al., 2010).
Purpose of study The Warner Range is one of the few locations where all of these units can be observed in
cross-section because exhumation has occurred along the Surprise Valley Fault (Fig. 2)
(Egger and Miller, 2011). Previous mapping within the central and southern parts of the
range has provided detail that led to current interpretations of the region, including at
least two Oligocene volcanic vent locations that have been identified in the central
Warner Range (Fig. 2) (Egger and Miller, 2011). However, the northern Warner Range
has not been mapped in detail and questions remain about the relationships between
volcanic units and faulting throughout the area.
Colgan et al (2011) proposed a slab tear separating the ancestral Cascades in California-
Nevada from similar-age rocks in Washington-Oregon (Fig. 1), but little work has been
done to define the northern extent of the ancestral Cascades, despite the presence of
Oligocene volcanic rocks beyond the proposed slab tear (Fig. 1). In addition, it is unclear
how the ancestral Cascade arc changed and migrated from the late Oligocene to the
Miocene. No volcanic rocks were erupted within the central Warner Range from 24 to 16
Ma (Colgan et al., 2011) but volcanism during that period in the Abert Rim area (Fig. 1)
(Scarberry et al., 2010) suggests this was a local occurrence rather than a cessation of arc
volcanic activity entirely. My proposed field area is located in an area where arc-
volcanism was active during the Miocene to the south and flood basalts erupted to the
north (Fig. 2) and may contain a record of the transition between volcanic processes.
Another factor that potentially influenced the distribution of Oligocene and younger
volcanic vents and deposits is faulting. My field area includes the Fandango Valley, a
normal fault-bound valley that cuts obliquely across the Warner Range and parallels a
pervasive, NW-trending structural fabric (Fig. 2 and 3). There are also NNE-trending
Basin-and-Range-style normal faults around my field area, the most prominent of which
is the Surprise Valley Fault (Fig. 2). Scarberry, et al. (2010) studied the ages of these
NNE-trending structures and, by using dates of volcanic units along with cross-cutting
relationships with the faults, determined that the faulting propagated from southeast to
northwest during the late Miocene. This study focused on the Abert Rim Fault, located
north of the Surprise Valley in southern Oregon (Fig. 1). Egger and Miller (2011) believe
motion along the Surprise Valley Fault propagated in the same fashion, beginning in the
southern Warner Range about 14 Ma and reaching the northern Warner Range after 7 Ma.
Fission-track and (U-Th)/He thermochronology of apatite in a sample of granite along the
Surprise Valley Fault shows that there were likely two episodes of slip along the fault
during the late Miocene and Pliocene (Colgan et al., 2008). The first episode of slip lasted
from 14 to 8 Ma (Colgan et al., 2008). It accommodated about 1/3 of the total slip along
the Surprise Valley Fault, but its timing is poorly constrained (Egger and Miller, 2011).
Most of the remaining slip and exhumation of the Warner Range can be accounted for by
the second extensional period (Egger and Miller, 2011). Further mapping in the area will
provide better age constraints for all of the faults and fractures present.
Methods I will spend about 2 months during the summer of 2012 performing detailed geologic
mapping within the Fort Bidwell quadrangle (Fig. 3) and collecting samples for
geochemical and petrographic analysis. I will map the Fort Bidwell quadrangle at a
1:24,000 scale (Fig. 3). Mapping will be done on a topographic base with
orthophotoquads and assembled in ArcGIS. I will also create cross-sections (Fig. 3) to
better interpret the relationship between volcanic units and faulting throughout the
Warner Range.
While mapping, there are several things I will look for, including cross-cutting
relationships between the volcanic units and faults and between the NW-trending
structures and NNE-trending faults. These relationships will help place constraints on the
timing of faulting and volcanic flows. I will also pay attention to the orientation of dikes:
if I find dikes parallel to faults and fractures in the area and perpendicular to extension, it
is likely they formed during extension. In contrast, radiating dikes may lead me to
previously unmapped volcanic vents. Some radiating dikes can be seen in Figure 2,
oriented toward vents. I think it is likely that I will find multiple cross-cutting
relationships while in the field, though I may not be able to determine their significance
until I complete geochemical analysis of samples. Discovery of one or more previously
unmapped volcanic vent is also likely.
Geochemical and petrographic analysis will help distinguish units. 24 samples will be
made into thin sections and 20 samples will be sent for geochemical analysis. I will likely
not be absolutely certain of which unit each sample I bring back is a part of so I will
study the hand samples, comparing them to characteristics described in previous studies
to determine which samples should be sent for geochemical analysis and be made into
thin sections. Basalt is common in the Oligocene, mid-Miocene, and late-Miocene to
Pliocene units but previous work has shown samples from all three periods are
geochemically distinct (Fig. 4) (Colgan et al., 2011). Oligocene and Miocene samples
range from basalt to andesite but the Miocene lavas are generally more alkalic (Fig. 5)
and depleted in Nb, Ta, and Ti, for a given MgO content when compared to the Steens
Basalt (Fig. 6) (Colgan et al., 2011). Pliocene volcanics can be distinguished because
they are much less enriched in rare earth elements than the Oligocene and Miocene
volcanic rocks and are distinctively low in potassium (Fig. 5) (Colgan et al., 2011).
In thin section, Oligocene basalts from the Warner Range have olivine and plagioclase
that are both commonly altered; these basalts contain very small amounts of hornblende
when compared to the younger rocks in the area (Fig. 7) (Colgan et al., 2011). In contrast,
Miocene basalt commonly has unaltered plagioclase, pyroxene, and olivine phenocrysts
(Fig. 8) (Colgan et al., 2011). The Pliocene tholeiitic basalts are the easiest to distinguish
in the field because they are light to medium gray, nonporphyritic, and have diktytaxitic
texture. In thin section, the olivine crystals are darker green than those in the Oligocene
and Middle-Miocene basalts, and occasionally display prominent ophitic texture with
large clinopyroxene and orthopyroxene crystals encompassing multiple smaller
plagioclase crystals (Fig. 9).
By combining my analyses with information from previous studies, I will be able to
distinguish samples from each volcanic episode. Upon completion of geochemical and
petrographic analysis I will be able to address my specific research questions and
increase understanding of geologic relationships within the Warner Range. If I find more
Oligocene volcanic vents in my field area, I will extend the northern limit of known
Ancestral Cascade volcanoes. Cross-cutting relationships between the volcanic units and
the different fault sets will help me place better age constraints on both volcanism and
extension. The earlier period of extension most likely did not last from 14 to 7 Ma, but
there is not currently enough data to further constrain the slip (Egger and Miller, 2011). I
may be able to find a cross-cutting relationship between the fault and one of the volcanic
units that could place more accurate age constraints on the extension. It is also possible
for more age constraints to be placed on the later episode of extension that began around
3 Ma, or I may find evidence that the second episode of faulting occurred while Pliocene
basalts were erupting in the Warner Range. Dikes parallel to each other and extension
could help me determine age relationships between the younger volcanism and extension
throughout the range; radial dikes could lead back to a volcanic vent and suggest that
volcanism occurred during a period without active extension.
Implications Colgan et al. (2011) proposed a slab tear in the subducting Farallon Plate that led to the
Ancestral Cascades forming much farther inland than the Modern Cascades (Fig. 1). For
this model to work, the Ancestral Cascades could not extend farther north than they are
currently mapped. If I find more Oligocene volcanic vents within the Fort Bidwell
quadrangle, the slab-tear model may need to be reevaluated.
The evolution of the Ancestral Cascade arc from the Oligocene through the Miocene is
not well known. Previous mapping has not shown any Early-Miocene arc-volcanism in
the Warner Range but it could be possible for me to recognize an Early-Miocene volcanic
vent within the northern Warner Range, suggesting that volcanism didn’t stop throughout
the entire Warner Range during the Early-Miocene and may have been present in a larger
area than previously thought.
Schedule Spring 2012 • Take a volcanology course
• Select a field assistant • Research and become familiar with field area, noting land
ownership and roads • Compile existing data into an ArcGIS database • Create base maps of Fort Bidwell quadrangle and
surrounding area Summer 2012 • Spend about 2 months field mapping
• Send samples to WSU for geochemical analysis; cut and send thin section billets to Spectrum Petrographics
Fall 2012 • Transfer data from field maps to ArcGIS • Begin petrographic analysis
Winter 2013 • Study the results from the geochemical analysis • Add data from petrographic and geochemical analysis • Draw cross-sections of map area • Begin writing thesis.
Spring 2013 • Complete thesis and present at the GSA Cordilleran Section
Budget Category Cost Per Diem (field assistant and myself) at $40/day for 60 days
$2,400
Mileage (my vehicle, 1500 miles round-trip to field area plus field miles, $0.51/mile)
$765
Lodging (1 night/week at Sunrise Motel in Cedarville, CA for shower, laundry, etc. at $65/night)
$390
Sample bags, markers, field supplies $92 Thin sections (24 at $16) $384 Map printing costs $100 Geochemical analysis (20 at $120) $2,400 Total $6,531 Budget Justification I will have thin sections made for petrographic analysis. I will send 24 billets to Spectrum
Petrographics, Inc. and they will cost $16 per thin section (price found at:
http://www.petrography.com/). I plan to send 20 samples to the GeoAnalytical Lab at
Washington State University for x-ray fluorescence analysis and it will cost $120 per
sample (price found at: http://www.sees.wsu.edu/Geolab/service/price.html).
Geochemical and petrographic analyses have been done on samples from other studies.
By doing these analyses on my samples, I will be able to compare my results with data
from previous studies. I will drive my own vehicle to the field area. This is 1500 miles
round-trip and I plan on driving while in the field. At reimbursement of $0.51 per mile,
this will cost $765. Sample bags, markers, and field supplies will cost about $92. Dr.
Anne Egger has been awarded a grant from EDMAP for my research project. This grant
will fund my field work, the geochemical analysis of samples, and the cost of making
thin sections. I applied for a grant from the Geological Society of America (GSA) and
received an additional $1000 that will contribute to additional sample analysis.
Figures
Figure 1. Modern and Ancestral Cascade geochronology. Data from du Bray et al. (2009) and Colgan et al. (2011). Points show the three main periods of volcanism. (I still need to add the Late-Miocene to Pliocene volcanic units to the map)
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Figure 2. Northern and central Warner Range. This figure shows the detailed geologic mapping that has previously been done in the Warner Range (Egger and Miller, 2011). The Fort Bidwell quadrangle is shown within the bold box (I haven’t added the box in illustrator yet).
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Figure 3. Fort Bidwell quadrangle research area. This figure shows the roads throughout my research area, my tentative cross-section lines, and geochemistry that has been done in the area.
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Figure 4. Total alkali-silica classification diagram for Warner Range volcanic rocks (Colgan et al., 2011). Oregon and Washington Cascades from du Bray et al. (2006); inferred California and Nevada arc from du Bray et al. (2009).
Figure 5. Plots of primitive-mantle-normalized rare earth element data from Warner Range volcanic rocks (Colgan et al., 2011).
Figure 6. From Colgan, et al. (2011). Geochemical data from mid-Miocene Warner Range volcanic rocks (Colgan et al., 2011) plotted with analyses from the Steens Basalt (Johnson et al., 1998).
Colgan et al.
746 Geosphere, June 2011
with a subduction source for the Miocene vol-
canic rocks, and we also interpret them to be part
of the arc formed by subduction of the Farallon
plate beneath northern California, which was
ongoing in the Miocene (e.g., Atwater and
Stock, 1998). Neither the Oligocene nor the
Miocene lavas show obvious evidence of crustal
contamination based on their similar 87Sr/86Sri
over the entire range of SiO2 (Fig. 12A). This
does not rule out some component of crustal
contamination, however, since the isotopic sig-
nature of the accreted crust that underlies the
region would be minor and hard to trace. The
Miocene lavas have more radiogenic 87Sr/86Sri
(Fig. 12A), smaller negative Nb anomalies (Fig.
12D; lower Sr/P, Fig. 12C), and slightly higher
Ce/Pb (Fig. 12B) than the Oligocene lavas. Sr
isotope ratios are not negatively correlated with
Ce/Pb (Fig. 12B), and Ce/Pb does not corre-
late at all with SiO2 content (Fig. 12F), so we
attribute these patterns to the Miocene lavas
having a different mantle source than the Oligo-
cene suite. The Miocene source may include a
smaller !uid component derived from the sub-
ducting slab, but one with more radiogenic Sr.
The more radiogenic Sr is accompanied by less
radiogenic Nd, so this component cannot simply
be seawater. The shift to higher Ce/Pb from the
Oligocene to the Miocene mantle sources is also
inconsistent with a larger sedimentary compo-
nent in the Miocene source.
Miocene lavas in the Warner Range and
nearby to the south have a distinctly different
composition and eruptive style from volcanic
rocks of overlapping age erupted in the Basin
and Range Province to the east, notably the
Steens Basalt, with which they have some-
times been included on regional maps (e.g.,
Camp and Ross, 2004; Brueseke et al., 2007;
Camp and Hanan, 2008). Miocene lavas in
northeastern California erupted to form a chain
of shield volcanoes, while the Steens Basalt
Figure 10. Plots of chondrite and primitive-mantle–normalized (Sun and McDonough, 1989) rare earth element (REE) data from Warner
Range volcanic rocks. Oligocene and Miocene lavas are depleted in Nb, Ta, Zr, and Ti, and enriched in the light REEs, distinctly different
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Figure 14
Rock/Primitive Mantle
Figure 7. Photomicrograph of sample WR07AE04 from the Oligocene arc-volcanic rocks. PPL—plain polarized light; SwT—swallowtail texture; Pl—plagioclase; Ol—olivine.
Figure 8. Photomicrograph of sample WR07AE116 from the Mid-Miocene arc-volcanic rocks. XPL—cross-polarized light; Ox—oxides; Cpx—clinopyroxene.
Figure 9. Photomicrograph of sample WR07AE111 from the Late-Miocene to Pliocene low potassium, high-alumina olivine tholeiites. PPL—plain-polarized light; Pl—plagioclase; Ol—olivine; Ox—oxides. References Camp, V. E., and Ross, M. E., 2004, Mantle dynamics and genesis of mafic magmatism
in the intermontane Pacific Northwest: Journal of Geophysical Research, v. 109, no. B8.
Carmichael, I. S. E., Lange, R. A., Hall, C. M., and Renne, P. R., 2006, Faulted and tilted Pliocene olivine-‐tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range: Geological Society of America Bulletin, v. 118, no. 9-‐10, p. 1196-‐1211.
Colgan, J. P., Egger, A. E., John, D. A., Cousens, B., Fleck, R. J., and Henry, C. D., 2011, Oligocene and Miocene arc volcanism in northeastern California: Evidence for post-‐Eocene segmentation of the subducting Farallon plate: Geosphere, v. 7, no. 3, p. 733-‐755.
Colgan, J. P., Shuster, D. L., and Reiners, P. W., 2008, Two-‐phase Neogene extension in the northwestern Basin and Range recorded in a single thermochronology sample: Geology, v. 36, no. 8, p. 631-‐634.
du Bray, E. A., John, D. A., Putirka, K., and Cousens, B. L., 2009, Geochemical database for igneous rocks of the ancestral Cascades Arc; southern segment, California and Nevada: U. S. Geological Survey Data Series, v. DS-‐0439.
Egger, A. E., and Miller, E. L., 2011, Evolution of the northwestern margin of the Basin and Range: The geology and extensional history of the Warner Range and environs, northeastern California: Geosphere, v. 7, no. 3, p. 756-‐773.
Hart, W. K., Aronson, J. L., and Mertzman, S. A., 1984, Areal distribution and age of low-‐K, high-‐alumina olivine tholeiite magmatism in the northwestern Great Basin: Geological Society of America Bulletin, v. 95, no. 2, p. 186-‐195.
Johnson, J. A., Hawkesworth, C. J., Hooper, P. R., and Ben Binger, G., 1998, Major-‐ and trace-‐element analyses of Steens Basalt, southeastern Oregon: Open-‐File Report -‐ U. S. Geological Survey, v. OF 98-‐0482, p. 30.
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