The amphibolite ''sole'' that occurs along the basal thrust of the Josephine ophiolite (Madstone Cabin thrust) is distinct from those described for ophiolites such as the Bay of Islands and Oman. Antigorite-bearing serpentinite mylonites occur directly above the amphibolite, and the presence of antigorite + brucite indicates formation between 300¿ and 400¿C. The antigorite mylonites also contain relict lizardite/chrysotile, suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite. Further evidence for prethrusting low-temperature serpentinization comes from a lizardite/chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the Josephine ophiolite. The prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite. The amphibolite is chemically diverse and has been divided into two groups. Group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas. Group 2 amphibolite is metavolcanic (contains some quartzite) and has magmatic affinities to mid-ocean ridge basalt and within-plate basalt, an association that is distinctive of older basement rocks elsewhere in the Western Klamath terrane. We suggest the amphibolite was derived from the footwall of the Madstone Cabin thrust, including country rocks for the Chetco complex (Group 2) and possibly older intrusive phases of the Chetco complex itself (Group 1). Previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the Chetco complex, that intrusion of the Chetco complex overlapped in time with deformation of the amphibolite, and that during and after emplacement the Josephine ophiolite and overlying flysch were intruded by abundant arc-related dikes and plutons. Emplacement of the Josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an active magmatic arc. ¿ American Geophysical Union 1996