A group of faults surrounding the Trinity terrane in the core of the Klamath Mountains province of northern California displays a commonality in physical features and in age that sets them apart from other faults in the Klamath Mountains. The faults dip shallowly outward from the central Trinity terrane and feature younger-on-older, detachment-fault geometry. Several exhibit shallowly south-southwest– or south-southeast–plunging slickenside striations. Distinctive muscovite-bearing dacite, muscovite-bearing rhyolite, and plagioclase porphyry dikes are common along these faults, as are epithermal mineralization and associated small, but economic gold and/or copper massive-sulfide deposits. Two of the faults crosscut the Triassic Siskiyou thrust. In addition, the Cecilville fault cuts the Late Jurassic Soap Creek Ridge fault, so extensional faulting postdates the widely developed Nevadan deformational event in the Klamath Mountains. A 126 ± 3 Ma K- Ar whole-rock age obtained on a muscovite-bearing rhyolite dike intruding the fault zone at Menzel Gulch, on the southern margin of the Trinity terrane, gives a minimum age of Early Cretaceous for this fault. Evidence for Tertiary extensional faulting on the La Grange fault, previously reported by Schweickert and Irwin, records a later episode of extension on the southern margin of the Trinity terrane.