Evidence for regional extensional faulting at Grey Rocks Ridge, Eastern Klamath Mountains, California
Graduation Date
2008
Document Type
Thesis
Program
Other
Program
Thesis (M.S.)--Humboldt State University, Environmental Systems: Geology, 2008
Committee Chair Name
Susan M. Cashman
Committee Chair Affiliation
HSU Faculty or Staff
Keywords
Mineralogy, Klamath Mountains, Mylonite, Geochemistry, Eastern Klamath Belt, X-ray fluorescence, Copley Greenstone, Detachment fault, Xrf, Petrography, Metasedimentary, Horse Heaven Meadows, Dike, Grey rocks, Trinity terrane, Panther Rock, Greenschist facies, Devonian, Bragdon Formation, Fault, Pluton, Klippe, Metavolcanic, Outlier, Humboldt State University -- Theses -- Geology, Redding terrane, Greenstone, Structural geology
Abstract
Geologic mapping of the Grey Rocks Ridge area identified three klippes of Devonian Copley Greenstone separated from the underlying Trinity terrane by an undulating extensional fault and preserved by oblique down-dropping on NW-NE striking faults. The Grey Rocks klippe and the Horse Heaven Meadows klippe dip shallowly (5 to 10 degrees) to the N-NE and have convex boundaries. The basal fault is associated with regional extensional faulting that extends across the Eastern Klamath Mountains. Geochemical analysis links the greenstone pillows of the Grey Rocks and Panther Rock klippes to the Copley Greenstone of the Redding terrane. The Horse Heaven Meadows klippe is comprised mainly of fractured slate with minor amounts of volcanic breccias and tuffs, forming a localized lens within the Copley Greenstone. A similar metasedimentary and metavolcanic lens is located in the southern section of the Grey Rocks klippe. The three klippes form the hanging wall of a regional extensional fault that juxtaposes the three klippes over the underlying Trinity terrane. The undeformed greenstone pillows of Grey Rocks and Panther Rock klippes and the fractured metavolcanic and metasedimentary units of the Horse Heaven Meadows klippe clearly contrast with the mylonite and fault breccia at the contact and with the serpentinized mafic/ultramafic complex (Trinity terrane) of the footwall. Geochemical values of fault zone mylonite indicate elevated values of Al2O3 and depleted values of MgO when compared to ultramafic compositional values, supporting a mixing of upper and lower plate components. Fault outcrop characteristics include mylonite, foliated fault breccia zones, and parallel foliation observed in proximal greenstone and ultramafic rocks. Quartz veins run both parallel and perpendicular to foliation, suggesting that injection of hydrothermal fluids accompanied faulting and accommodated extensional deformation of more coherent rock. In thin section, fault proximal rocks in the upper and lower plate exhibit shear microstructures including foliation, s-c fabrics, planar mineral alignment, sigma clasts, pull-apart clasts, and perpendicular to foliation quartz veining. Felsic dikes intrude the fault breccia at the base of the Horse Heaven Meadows klippe and intrude parallel to the fault zone at the base of the Grey Rocks klippe. Emplacement of the parallel dike is interpreted to be syntectonic with the extensional faulting event based on shear and extensional microstructures such as anastomosing foliation and perpendicular to foliation quartz veining.
Recommended Citation
Fudge, Emily C., "Evidence for regional extensional faulting at Grey Rocks Ridge, Eastern Klamath Mountains, California" (2008). Cal Poly Humboldt theses and projects. 1018.
https://digitalcommons.humboldt.edu/etd/1018
https://scholarworks.calstate.edu/concern/theses/9306t190v