Earth and Planetary Sciences ETDs


Jason Ray

Publication Date



A newly discovered northeast-trending subvertical shear zone, up to 3 kilometers wide, forms the backbone of the Virgin Mountains in southern Nevada. This zone, named for excellent exposure in Cabin Canyon, contains a well-developed mylonitic foliation that records a complex history of movement and strain within the Virgin Mountains. The overall movement history is interpreted to involve a combination of northwest shortening deformation and dextral, northwest-side-up, oblique shear. The mylonites within the zone are comprised of amphibolites, granodiorites, a variety of meta-sedimentary rocks, and leuco-pegmatite dikes. The oldest rocks involved in shearing are similar to ~1.7 Ga of the Grand Canyon, but mylonitization is inferred to be ~1.4 Ga based on relatively low temperature microstructures and similarity to other partitioned dextral/transpressive shear zones in northwestern Arizona. Mapping and microstructural analysis show that the dominant northeast-striking foliation (S2) developed under relatively high temperatures within the upper amphibolite facies, likely at 1.7 Ga. At a later time, likely 1.4 Ga, these rocks underwent further shortening and oblique dextral shear, which boudinaged the amphibolites and original pegmatites causing dramatic grain size reduction (mylonitization). This transpressional deformation created sub-horizontal lineations and shear sense indicators such as delta and sigma porphyroclasts, shear bands, C-S fabrics, and bookshelf sliding. Pegmatites were intruded at various stages during mylonitization as shown by field relationships of folded pegmatites that have been cut by later pegmatite veins. These pegmatites were folded into tight and isoclinal folds via inhomogeneous shortening at a low metamorphic grade (<500 degrees Celsius). Ultra-mylonites within the shear zone, which may represent high strain zones, have also been folded following shearing indicating continued transpression or a younger deformational event.

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Crossey, Laura




Cabin Canyon, Virgin Mountains

Document Type