Geology

Our work on the geology of the region consists of geologic mapping, logging the core drilled for geothermal exploration, geochemical analysis of volcanic rocks, and geochronology and thermochronology.

Geologic mapping

Geologic maps are one of the basic datasets for understanding the geologic history of a region, and learning how to both create and interpret geologic maps is a fundamental component in the education of every geologist. In 2004, we began bringing students into the field to map the geology of the Warner Range - Surprise Valley region at a 1:24,000 scale, more detailed than previously existed for this area. Our initial efforts focused on the region surrounding the high-resolution seismic reflection line.

The first step in beginning to map in a new region is to examine what work has already been done there, and to spend some time familiarizing yourself with the rocks. Most of the previous work in the Warner Range has been done on a smaller scale, so we hoped to define more units within the units already mapped by other workers.

	All of the rocks exposed in the Warner Range are younger that about 35 million years, and they are all sedimentary and volcanic rocks. The youngest volcanic rocks in the region are about 4 million years old. We worked to be able to distinguish units of different ages and depositional environments in order to both create a more detailed map and to determine the geologic history of the region. We were also able to examine older rocks by looking at the rocks brought up to the surface through core drilling, described below.
Student Konstanze Stuebner examining a volcaniclastic debris flow near Payne Peak. Photo courtesy Anne Egger
		These students are sitting on the ridge approximately at the location of the white circle. They have measured the orientation of the basalt they are sitting on, which is dipping about 8 degrees to the west, and are now looking east, the way the white arrow is pointing. From their vantage point, they can see where to draw in the boundaries of a landslide (Qls) and the river alluvium (Qal). Mapping will continue during the summer of 2006.
Students Hari Mix and Steve Davis mapping structures on the east side of the Surprise Valley. Photo courtesy Anne Egger

These basalt units probably correlate to the strong reflectors on the east side of the seismic profile. If so, they allow us to draw strong connections between the orientation of rocks at the surface and the way they continue below the surface.

Core Log

During the summer of 2005, a core was drilled in the Lake City geothermal field. Dick Benoit was kind enough to give us access to that core, which we logged and sampled, and on which we have begun to conduct geochronological and geochemical analyses.

Section of core from depth of 2837' consists of pebbly volcanic mudstone. Photo courtesy Joe Colgan	Section of core from depth of 3973' consists of arkosic sandstone and conglomerate. Photo courtesy Joe Colgan	Section of core from depth of 4017' consists of well-rounded cobble conglomerate with granitic clasts. Photo courtesy Joe Colgan

The rocks within the core consist of volcanic, volcaniclastic, and sedimentary rocks down to a depth of 4717'. The core crosses the range-bounding fault at a depth of 1260', near which is a zone of intense alteration and brecciation.

Geochemical analysis

Analyzing the geochemistry of volcanic rocks can give us an indication of the possible source of the magma. Because the Warner Range is on the border between the Basin and Range and the Modoc Plateau, there are multiple possible sources:

• Subduction-related magmatism (like Mt. Shasta)
• Extension-related magmatism (like the rest of the Basin and Range)
• Hot spot- or plateau-related magmatism (like the Columbia Plateau basalts)

Each of these potential sources has a different geochemical signature of major and trace elements. This work is still in process.

Significant work on the young basalts in the region has been done by Ian Carmichael and others at the University of California at Berkeley.

Geochronology and Thermochronology

By determining the age of volcanic rocks and the timing of uplift within the Warner Range - Surprise Valley region, we can assess the relationship between magamtism and extension. The age of volcanic rocks is generally determined through ⁴⁰Ar/³⁹Ar analysis either of whole-rock basalts or of individual minerals such as hornblende. Again, significant work on the youngest basalts has been done by Ian Carmichael and others at the University of California at Berkeley. Our geochronological work is ongoing.

In addition, by using techniques such as apatite fission track analysis and U-Th/He analysis, we can determine when rocks passed through temperatures below 100° C, most likely related to rapid motion along the Surprise Valley fault bringing those rocks closer to the surface. Preliminary work indicates that this motion is as young as 3 Ma in the Warner Range, but much more work remains to be done in this area.

Example of U-Th/He data for the Warner Range, showing a 3 Ma cooling age near the base of the range. Though more ages are needed to constrain uplift and exposure, these indicate that major motion along the fault is quite recent. Cross-section courtesy Joe Colgan.

Finally, we can also use geochronological techniques to date granite cobbles within the conglomerate at the base of the core. While this does not tell us how old the conglomerate itself is, we can use this data to find a potential source area for the granite cobbles and thus the conglomerate. Preliminary results are sufficient to rule out regions, such as the Klamaths, but are not yet specific enough to pinpoint the source area.