Detailed geologic mapping and structural analysis reveal previously unrecognized structural relationships between two major faults in the western Transverse Ranges: Specifically that the Pine Mountain and western Big Pine faults are a single continuous north-dipping reverse fault. This new evidence contradicts an old hypothesis that the Pine Mountain fault is offset along the Big Pine fault by left-lateral strike-slip displacement. The western Big Pine- Pine Mountain fault dips from 80 to 45 degrees north and places Early Eocene rocks on the north over a tight syncline of Late Eocene through Pliocene rocks on the south. Faults and fold axes in the Coast Ranges north of the fault zone consistently strike northwest and do not correlate with the west-striking faults and folds south of the fault zone. Large contrasts in paleomagnetic and stratigraphic data across the western Big Pine- Pine Mountain fault indicate that this fault is a major tectonic boundary separating the rotated western Transverse Ranges on the south, from non-rotated terranes on the north. Differences in paleomagnetic declinations measured from Eocene and Early Miocene volcanic and sedimentary rocks, show that this boundary fault zone has accommodated approximately 60 to 70 degrees of differential rotation since Late Miocene time.