Evidence for regional extensional faulting at Grey Rocks Ridge, Eastern Klamath Mountains, California

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.