The Cowhole Mountains, near Baker, California, are an east-tilted remnant of the Jurassic magmatic arc that extends from western Nevada through southeastern California into Arizona and Sonora, Mexico. In Middle Jurassic time, two northeast-erly trending grahens bounded by syndepositional faults accumulated 550 m and 700 m of predominantly eolian sandstone (Aztec Sandstone). Because east dip in one of the grabens increases from 30° at the base of the sandstone section to 65° at its top, we infer that structure to have developed during tilting to the west on east-dipping, listric normal faults. Westward tilting continued to accumulate during extrusion of an overlying Middle Jurassic volcanic series (Cowhole volcanics). Felsic volcanic rocks that buried the grabens comprise ignimbrites, volcaniclastic rocks, and flow breccias, emplaced in a proximal but extra-caldera setting. Plutons and sills were intruded at shallow levels, possibly during both graben filling and extrusion of volcanics. Local magmatism of significant volume ended with intrusion of a dike swarm that may be correlative with the Independence dike swarm (148 Ma). After the range tilted 90° toward the east, small glide blocks and landslides detached from high portions of the eroding range and descended into the southern graben. Our model suggests that two sets of high-angle normal faults, oriented nearly at right angles to each other, were active during the same Middle Jurassic interval. A modern analogue may be provided by the Central American arc graben, in which similar transtensional structures have developed perpendicular to the regional graben where it is intersected obliquely by the Chiapas shear zone.

Abstract Geologic relationships observable at this site are displayed Paleozoic sedimentary and Mesozoic sedimentary and rocks preserved beneath a cap of Jurassic rhyolite on a herein referred to as Rhyolite Ridge. Rhyolite Ridge liesat northeastern corner of the Cowhole Mountains approximately 13 mi (21 km)south of Baker, California, comprising portions sections 12 and 13 of T12N, R9E and 7 and 18 of T12N, R1OE, San Bernardino Principle Point. Rhyolite Ridge (not labeled) be found in the southwest corner of the Seventeenmile Quadrangle, 7 1/2-minute Topographic Series of the U.S. Geologi cal Survey. The ridge may be reached by taking the paved baker Road south from the four-way stop in central Baker or turning south from either of the central Baker off-ramps 1–15. Figure 1 indicates mileage between points mentioned in following text.