What: This project will document the kinematics and thermal evolution of middle and lower crustal rocks exposed during exhumation of the Ruby Mountains-East Humboldt Range metamorphic core complex (RM-EHR) in northeastern Nevada. Although the RM-EHR is widely recognized as a classic metamorphic core complex, debates persist about the timing and kinematics of crustal extension. Students will conduct detailed geologic mapping and collect structural, sedimentologic, petrologic, and low-temperature thermochronologic data to better characterize the tectonic evolution of the RM-EHR core complex. These results will be used to test competing models for the structural development of the complex, with implications for our understanding of mid-to-lower-crustal deformation more generally.
When: June 29-July 26 (tentative)
Where: The East Humboldt Range, Northeastern Nevada (~2.5 weeks) and Washington and Lee University, Lexington, VA (~1.5 weeks).
Who: Six students and two project leaders: Jeffrey Rahl, Washington and Lee University (tectonics, structural geology, thermochronology), and Allen McGrew, University of Dayton (tectonics, structural geology, petrology)
Project Overview and Goals: The Ruby Mountains (RM), East Humboldt Range (EHR), and Wood Hills (WH) of northeastern Nevada are widely-recognized as an exemplar of the metamorphic core complexes occupying the interior of the western U.S. Cordillera from Sonora, Mexico to British Columbia Canada. Of the classic core complexes in the northeastern Great Basin, the RM-EHR-WH is located farthest into the hinterland of the Mesozoic Sevier orogenic belt and exposes the deepest-seated rocks (including Nevada’s only Archean exposures) and the most extensive outcrops of migmatites and peraluminous granites. As such, it exposes critical relationships for understanding the large-scale structure and tectonic evolution of the crust from Late Mesozoic to Recent time.
The central objective of this proposal is to document the timing and kinematic evolution of the earliest stages of exhumation of the WH-EHR crustal section. We will employ three complementary approaches: 1) low-temperature thermochronology (including zircon fission-track dating) of lower plate units in the EHR, WH, and Pequop Mountains to document exhumation-related cooling; 2) sedimentologic and stratigraphic analyses to document the early unroofing history of the metamorphic footwall; and 3) microstructural and quartz crystallographic fabric work to characterize the kinematics of mylonitic deformation within the EHR and WH. Collectively, these data will enable us to address the following questions:
- When did extension begin and first expose the metamorphic footwall at the surface?
- Did the locus of extension progressively migrate to the NW with time, or was cooling more uniform regionally?
- Do the kinematics of deformation vary within the mylonitic shear zone? How do quartz crystallographic preferred orientations (CPOs) vary with respect to crustal position? Can the quartz textures be used to measure variations in the vorticty of the mylonitic deformation as a function of structural position
Potential Student Projects: The interdisciplinary nature of the project will provide opportunities for students with a range of interests, skills, and institutional support. All projects will be integrated with targeted field mapping and observations.
The timing and pattern of exhumation of the core complex (1-2 students). Low-temperature thermochronology represents a key approach to documenting the timing and pattern of exhumation during crustal extension. In this project, a student will collect samples from a NW-SE transect stretching from the EHR to the southeast through the WH and Pequop Mountains. Zircon fission-track analyses will be obtained to document cooling through temperatures of 240-250°C in order to characterize the timing of exhumation throughout the core complex and to test models predicting a westward progression of exhumation.
Sedimentology of the Clover Creek Canyon section (2 students). Sediments of the ~1000 m thick Clover Creek Canyon section are exposed on the eastern flank of the EHR. This section preserves the oldest sedimentary record of unroofing of the footwall, but the age and provenance of this unit is not yet well-understood. Students will measure the exposed parts of the section to document the sedimentary facies, provenance, and other aspects of the deposits. Some samples may be targeted for detrital zircon U-Pb geochronology.
Mylonitic deformation in the EHR and Wood Hills (3-4 students). Quartz crystallographic-preferred orientation patterns preserve information on the geometry, symmetry, and temperature of crystal-plastic deformation. Our reconnaissance CPO work documents variations in quartz fabrics possibly related to sample proximity to the detachment and/or varying degrees of pure versus simple shear within the EHR shear zone. Students will collect samples systematically from one or more transects to document variations in the microstructure and quartz CPO patterns in the EHR shear zone. East of the EHR, the Wood Hills represent the up-dip continuation of the deformed mylonites exposed in the EHR core complex and provide an ideal opportunity to characterize CPO variations in an extensional system near the brittle-plastic transition zone. These data will provide insights into the nature of deformation at deep crustal levels during exhumation of the EHR core complex.
High-temperature deformation microstructures (1 student). Petrographic observations reveal recrystallization along shear bands in microboudinaged hornblende and other phases in the EHR, implying extensional deformation active at high-grade conditions. One student could perform a detailed mapping and petrographic-based analysis documenting this and other microstructural relationships to give insights into this early episode of extensional deformation.
Working Conditions: Students will spend about ~2.5 weeks in Nevada and ~1.5 weeks at Washington and Lee University in Lexington, VA. During the field-work component, we will be based at a well-established National Forest Service campground. Students MUST be comfortable with the outdoors. Students are expected to bring appropriate clothes for field work, including good boots and several layers of clothes. Camping equipment, including tents and cooking equipment, will be provided.
Daily work will involve strenuous hiking over steep terrain and at significant elevation. There is possible interaction with dangerous insects or animals, including rattlesnakes, ticks, and spiders. During the field work phase of the project, cell phone service will be spotty, and we will have limited recharging capability. The nearest town with supplies will be about 20 minutes away; the closest hospital facility is about an hour away.
Recommended Courses/Prerequisites: We seek students with a strong interest in tectonics and comfort with outdoor field work. We look for students who have completed coursework in most (or all) of the following areas: structural geology/tectonics; mineralogy; petrology; sedimentary geology/stratigraphy. Experience at a field camp or in a field geology course is also recommended. Work from this project must carry over into the senior year (2014-15) as a required senior thesis (or equivalent) and the supporting letter from the on campus advisor must indicate how a thesis requirement figures into the senior year course load.
Questions about the project or its logistics can be directed to Jeffrey Rahl ([email protected])