Date of Award
Geology and Geophysics
Kathleen E. Johnson
New, detailed field work in the Silurian hills, southeast of Death Valley, reveals a pattern of a complex Cenozoic brittle fault overprint of rocks containing evidence of at least three episodes of Mesozoic thermal-ductile deformation. The Cenozoic brittle fault overprint of rocks containing evidence of at least three episodes of Mesozoic thermal-ductile formation. The Cenozoic faulting consists of five distinct sets of structures ordered by cross cutting relationships. The oldest, BF1, are fragments of reverse faults trending NW and dipping 45-60 SW. BF2 consist of NW trending, en echelon, sinistral strike-slip faults. BF3 is a conjugate set of N-NE trending high-angle normal faults with 100's m offset. BF4 is a complex array of conjugate strike-slip and normal faults. NNW trending dextral and NNE trending sinistral faults offset and are offset by N and NW trending conjugate sets of high-angle normal faults. The younges set, BF5, are low-angle normal faults with no consistent basal surface or transport direction. These are interpreted to be right lateral, strike-slip extension (dextral transtension, BF4 & BF5) preceded by right lateral, vertical axis rotation, and strike-slip shortening (dextral transrotation and transpression, BF2 & BF3), all related to the interaction of the Garlock fault and northwest trending dextral shear. 30° of clockwise vertical axis rotation is inferred to have affected the Silurian Hills since the Middle Miocene as the result of BF2 & BF3 deformation.
The Mesozoic deformations are; 1, a greenschist facies, low-strain but tightly folded N vergent, crystalline basement through Pahrump Group section; 2, a greenschist facies, high-strain, 100's m thick, mylonite zone with stretching lineations trending NE-SW; and 3, an upper amphibolite facies (high T, low P), high-strain E-W trending, S plunging Pahrump Group section, syntectonically intruded by multiple igneous phases U-Pb dated from 177 Ma to 97 Ma. The two Pahrump Group sections are interpreted to form as the result of NW directed backthrusting in the hinterland of the Sevier orogeny and were juxtaposed by NNW normal, extensional transport along the mylonite zone after crustal thickening. K-Ar biotite cooling ages suggest the mylonite zone developed during the Late Cretaceous to Eocene time.
Finally, the protolith of the tectonostratigraphic Riggs Formation to be two formations of the Pahrump Group, the Crystal Spring and Beck Spring Formations.
Comstock, John E., "Vertical Axis Rotation in the Silurian Hills: A Cenozoic Overprint on the Mesozoic U.S. Cordilleran Magmatic Arc" (1997). University of New Orleans Theses and Dissertations. 2241.