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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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.  

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