Reconnaissance geologic map of the San Gorgonio Pass area, Southern California: Supplement 1 from "The San Andreas Fault Zone in San Gorgonio Pass, California" (Thesis)

In San Gorgonio Pass, 70 miles east of Los Angeles, a complex network of faults separates two of the highest mountain ranges of Southern California. The San Andreas fault, which forms a part of the network, exhibits several unusual features in this area. Among these are the absence of rift topography, absence of lateral stream offsets, an abrupt change in trend of the fault trace, seismic evidence for the predominance of thrusting over strike-slip movements, and a lack of great earthquakes in historic time. Crystalline rocks of Mesozoic and earlier age crop out over most of the map area. North of the pass, the San Gorgonio igneous-metamorphic complex comprises an old metamorphic terrane of intermediate to basic composition and probable igneous parentage, and Mesozoic(?) plutonic rocks of quartz monzonitic composition. These plutonic rocks have intimately intruded and in large part reconstituted the older metamorphic rocks. The resulting migmatitic gneiss is the most widespread rock of the area, and includes flaser gneiss, green-schist, and piedmontite-bearing gneiss as distinctive varietal types. Rocks of the San Jacinto Mountains south of San Gorgonio Pass are distinctly different from those to the north, and comprise texturally uniform granodioritic and tonalitic rocks that contain sparse inclusions and septa of metasedimentary rocks. Nearly all of the sedimentary rocks in the pass area are of alluvial-fan or flood-plain origin, and they reflect a Quaternary and late Tertiary history of recurrent deformation and deposition. The upper Miocene(?) Coachella fanglomerate is the oldest exposed sedimentary rock, and is overlain with marked angular unconformity by all younger units. Lower Pliocene(?) incursion of tropical marine waters into the Salton trough is represented by a thin stringer of Imperial formation which is conformably underlain and overlain by continental strata of the Hathaway and Painted Hill formations, respectively. All of these rocks are overlain with marked angular unconformity by Quaternary Cabezon fanglomerate, which probably is correlative with upper beds of the Pliocene-Pleistocene San Timoteo(?) formation in the western part of the map area. Other Quaternary deposits, each showing complex structural relationships to adjacent rocks, are the deformed gravels of Whitewater River, Heights fanglomerate, and Burnt Canyon breccia. Recent alluvium covers the floor of the pass. Flows and dikes of olivine basalt occur in the Coachella fanglomerate and Painted Hill formation. Lithology of clasts in the sedimentary rocks indicates derivation predominantly from rocks of the San Gorgonio igneous-metamorphic complex to the north. Quaternary alluvial fans of Heights and Cabezon fanglomerate, which once buried a former rugged topography, are now being dissected along the north side of the pass. Surfaces of low relief and associated stream terraces resulting from this dissection are the Beaumont plain, Banning Bench, and Pine Bench surfaces. Upstream divergence of the Banning Bench surface from present stream levels is attributed in part to tilting. Farther east, the older Cabezon surface shows many effects of warping. This surface probably is correlative with an area of low relief at altitudes of 6500 to 8000 feet near Raywood Flat, and suggests Quaternary arching of the mountain range along a north-south axis. Within San Gorgonio Pass, alluvial fans derived from areas to the north dominate those derived from the steeper San Jacinto scarp to the south. This unequal development of fans is attributed to greater flood-producing rainfall and larger drainage area on the north, together with more easily erodable rock in this area. Most of the faults that show Recent movement are well delineated by springs and vegetative contrasts. Other springs are caused by exposure of unconformities, and by superposition of streams onto the rugged pre-Cabezon topography. The San Andreas fault is a continuous linear feature for a distance of more than 400 miles northwest from San Gorgonio Pass, but within the pass it curves abruptly southward and butts into the east-trending Banning fault at an angle of 45 [degrees] . Recent strike-slip movement on this part of the fault probably amounts to less than one mile, and post-Mesozoic displacement probably has not exceeded a few tens of miles. The Banning fault, a major break that delineates the north side of the pass, extends for a distance of more than 50 miles eastward from a point near Redlands through the pass into the Coachella Valley. Within the pass, it is a steeply north-dipping reverse fault except for a zone of low-angle thrusting between Millard and Whitewater Canyons. At least 5000 feet of vertical displacement has taken place on this fault since San Timoteo time, and a right lateral offset of 5 miles is suggested. Recent displacement is limited to the segment of the fault east of Millard Canyon. Pre-Pliocene lateral displacement may have been great, but is not demanded by evidence in this area. The Mission Creek fault branches from the San Andreas fault north of Banning, and is a major north-dipping fracture that is continuous for at least 40 miles to the southeast. The Pinto Mountain fault diverges from the Mission Creek fault at a low angle, and probably is continuous for more than 50 miles to the east; in this interval it forms the southern boundary of the northwest-trending fault system of the Mojave Desert. The Mill Creek fault branches from the San Andreas fault north of San Bernardino, and has guided erosion along deep linear valleys in the high mountains; this fault apparently dies out eastward. Within the San Bernardino Mountains all of the faults north of the Banning fault separate crystalline rocks of the same family; these rocks are similar in their migmatitic structural features, remnants of amphibolite, intrusion by quartz monzonite, and high content of titanium minerals. Thus post-Mesozoic lateral displacements of hundreds of miles along these faults seem to be precluded. Although lateral displacements of a few tens of miles are possible, no observed evidence appears to demand such movements. Late Tertiary and Quaternary vertical movements are suggested by the physiography of the mountains. No Recent movements have occurred on parts of the Mission Creek and Mill Creek faults. Recent movements on both the Banning and San Andreas faults probably were caused by a stress system involving a generally north-south maximum principal stress, with an east-west least principal stress of only slightly lesser magnitude than the vertical stress. In the vicinity of San Gorgonio Pass, an older east-west line of weakness causes the east-west stress effectively to become the intermediate stress, so that thrust faulting predominates over strike-slip faulting in this one local area. San Gorgonio Pass is bounded by a reverse or thrust fault on the north, and indirect evidence suggests a similar fault on the south. Quaternary and late Tertiary displacement on these faults, rather than erosion, is primarily responsible for the present physiography of the pass. Local conversion of San Andreas-type lateral strain into vertical displacements on the bounding faults is a reasonable explanation of both the pass itself and the unusually high peaks adjacent to it