EVIDENCE FOR REGIONAL EXTENSIONAL FAULTING AT GREY ROCKS RIDGE, EASTERN KLAMATH BELT, EASTERN KLAMATH MOUNTAINS, SHASTA COUNTY, CALIFORNIA

Geologic mapping of the Grey Rocks Ridge area identified a klippe of greenstone pillow basalts and slate overlying a shallowly-dipping fault that extends across the Trinity ultramafic complex in the Eastern Klamath Mountains. The northern section of the klippe is comprised of fractured slate that depositionally overlies the greenstone pillow basalts that form the southern section of the klippe. Foliation in the fractured slate is most prominent near fault contacts where a northwest/southeast strike dominates, (dip direction varies). In the northwest section of the klippe, a 50-foot section of bedded slate and fine-grained siltstone strikes north/south and dips 65° to the east. The discordance between the bedding dip and the underlying fault contact definitively refutes interpretation of the basal contact as a depositional contact. The undeformed greenstone pillow basalts and fractured slate forming the hanging wall, clearly contrast with the sheared, deformed rock at the contact with serpentinized mafic/ultramafic rocks of the footwall.

The undulating basal fault of the klippe dips shallowly (5 to 10°) to the north-northeast. Characteristics used to identify the fault vary at different locations surrounding the klippe. Characteristics observed include thick, layered sections of sheared rock, quartz veins, parallel foliation observed in greenstone and ultramafic rocks, and foliated fault breccia zones. Quartz veins are observed solely near (< two hundred feet) contact zones locally creating en echelon veins in the slate. In shear zones, quartz veins run parallel to foliation, suggesting that injection of hydrothermal fluids accompanied faulting. Footwall foliation in the ultramafic rocks parallels foliation in nearby hanging wall greenstone, striking northwest and dipping shallowly to the northeast, suggesting a shear zone. The foliated fault breccia zones are comprised of a serpentinite matrix with fine-sand to boulder- sized mafic and ultramafic clasts. Characteristics of this fault contact support prior work hypothesizing a Tertiary regional extensional faulting event. Petrographic analyses to determine the identity of Grey Rocks sedimentary and greenstone units relative to other Eastern Klamath Mountain units are in progress.