A contact previously considered to be part of the Sacramento Mountains detachment fault (SDF), exposed in the Sacramento Mountains metamorphic core complex, is reinterpreted as an unconformity between Tertiary rhyolite of Eagle Peak and cataclastically deformed crystalline lower-plate rocks. This reinterpretation is based on outcrop-scale topographic relief and the absence of deformation along the base of the rhyolite, even where underlying rocks are severely deformed and altered. The rhyolite is dated at about 14.3 Ma.
Lower-plate rocks of the SDF comprise gneiss intruded by granodiorite, tonalite, and leucogranite. Mylonitic fabrics are variably developed in lower-plate rocks and are cut by brittle shear zones that contain ultracataclasite, and by high- and low-angle faults. Upper-plate rocks include gneiss; nonmylonitic granodiorite, tonalite, and granite; and middle Miocene sedimentary and volcanic rocks. On the south side of Eagle Peak, a thick ultracataclasite zone dips about 30° west, contains subhorizontal hematite-stained slickenlines, and is interpreted to be part of the SDF system. This ultracataclasite zone is unconformably overlain by more gently dipping, undeformed, 14.3 Ma rhyolite of Eagle Peak. The unconformity records unroofing of a domed SDF after a period of 8 to 9 m.y. of progressive unloading of the lower plate in an evolving extensional shear zone. Extension on part of the SDF was accommodated by a system of high-angle, oblique-slip faults.