Relationship of Big Pine, San Guillermo, and Ozena Faults, Northwestern Ventura County, California: ABSTRACT

Abstract

The Big Pine, San Guillermo, and Ozena faults are interpreted to have experienced significant strike-slip movement. The San Guillermo and Ozena faults are northwest-trending, southwest-dipping, high-angle reverse faults that once formed a continuous trace. This trace has been offset left-laterally about 8½ mi. by post-middle Pliocene movement on the ENE-trending, essentially vertical Big Pine fault (a possibility suggested by Hill and Dibblee, 1953). Prior to being offset, the continuous San Guillermo-Ozena fault experienced at least 3½ miles of right (?)-lateral movement in late Tertiary time. The case for the San Guillermo-Ozena fault correlation is based on the following criteria. (a) Both the San Guillermo and Ozena faults have the same general trend, are high-angle reverse faults, and juxtapose the same general stratigraphic sequences near their junctions with the Big Pine fault: SSW-dipping lower Eocene marine strata on the hanging wall against Miocene-Pliocene continental strata on the footwall. (b) Two additional correlative geologic features, a unique fault and a facies contact, intersect the Big Pine fault, and their offsets are compatible with the San Guillermo-Ozena fault correlation. Southwest of each fault in its respective locale are (1) a paralleling auxiliary fault containing a rhyolite intrusive found only in and around the auxiliary fault and (2) a facies contact within the Eocene section where the Juncal Formation is overlain by the Matilija (?) Formation. (c) Projected upward, the facies contact and the San Guillermo and Ozena faults intersect, forming a line which pierces the upward projection of the Big Pine fault in each locale. A calculated net slip of 8.5 miles in a direction that pitches 6° southwest along the Big Pine fault has raised relatively the south block, yielding a 4,700-foot dip-slip component. This explains minor stratigraphic differences around each fault. (d) Subsurface data show at least 3½ miles of lateral displacement along the Ozena fault and require the existence of an offset portion. The San Guillermo fault is the only logical correlative. End_of_Article - Last_Page 1088------------