Thesis Proposal .... Fission-track and (U-Th)/He thermochronology of apatite in a
sample of granite along the. Surprise Valley ..... sample: Geology, v. 36, no. 8, p.
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. Arcrelated 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 extensionrelated 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, midMiocene 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 CaliforniaNevada 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 arcvolcanism 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
• • • • •
Summer 2012
• •
Fall 2012
• • • • • • •
Winter 2013
Spring 2013
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 Spend about 2 months field mapping Send samples to WSU for geochemical analysis; cut and send thin section billets to Spectrum Petrographics Transfer data from field maps to ArcGIS Begin petrographic analysis Study the results from the geochemical analysis Add data from petrographic and geochemical analysis Draw cross-sections of map area Begin writing thesis. Complete thesis and present at the GSA Cordilleran Section
Budget Category Per Diem (field assistant and myself) at $40/day for 60 days Mileage (my vehicle, 1500 miles round-trip to field area plus field miles, $0.51/mile) Lodging (1 night/week at Sunrise Motel in Cedarville, CA for shower, laundry, etc. at $65/night) Sample bags, markers, field supplies Thin sections (24 at $16) Map printing costs Geochemical analysis (20 at $120) Total
Cost $2,400 $765 $390 $92 $384 $100 $2,400 $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
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