Department of Earth Sciences

Our Geological Environment

From its location near the active boundary of the Pacific plate with the North American plate, UC Riverside gives its faculty and students ready access to a variety of geological, geochemical, and geophysical problems. The campus stands on batholithic rocks at the northern limit of the Peninsular Ranges where they approach two elements of the Transverse Ranges. The latter are separated from each other by the San Andreas fault. The Peninsular Ranges abut the San Bernardino Mountains to the east across the San Andreas fault; the San Gabriel Mountains are encroaching from the north over an active thrust system.

These three mountain ranges near the Riverside campus preserve different segments of a zoned suite of Mesozoic granitoid plutons. Their motions have grouped together a Pre-Cambrian anorthosite complex, Pre-Mesozoic cratonal metasediments, a Mesozoic oceanic greenschist series, and Mesozoic sedimentary and volcanic rocks. The pre-plutonic marbles are an important source of cement and lime products. The skarns at the Crestmore Quarry, four miles north of the campus, are world-famous for their diversity of minerals.

The active margins of these ranges and their intramontane basins are the sites of late Cenozoic sediment accumulation, active seismicity, landsliding, hot spring activity and urban development. They are within the world's best developed regional seismic monitoring network and the focus of "state of the art" earthquake prediction and engineering hazard mitigation.

The geomorphology bears a strong imprint from active faulting and landslides. Spectacular ground rupture from the 7.5 Ms Landers earthquake provides an ideal area for studying complex surface rupture. Wrightwood, on the east margin of the San Gabriel Mountains, is now a famous example of landslide and mudflow activity.

Riverside has grown up beside the Santa Ana River, which has the largest drainage area of any river reaching the southern California coast. The well-developed Santa Ana flood plain begins at the San Andreas fault and extends west as the latest stage of infilling of the Los Angeles Basin, between the western Transverse and Peninsular Ranges. The late Cenozoic fill of the Los Angeles Basin is a fine example of a marine to non-marine transition and the site of many producing oil fields. Since the residential and commercial development in this semi-arid basin requires water and generates toxic waste, groundwater management is a serious consideration.

The Coachella Valley and Salton Trough, southeast of Riverside between the eastern Transverse and Peninsular Ranges, are a tectonic extension of oceanic spreading in the Gulf of California, but isolated from it by the Colorado River delta. The trough overlies a leaky transform fault system that has generated active pull-apart basins, Quaternary volcanism, and hydrothermal convection systems. Several areas are already developed for geothermal energy. A locally very-high geothermal gradient produces active metamorphism, reaching lower amphibolite facies within 2500 m of the surface. The Salton Sea geothermal field has active deposition of ore minerals, and its high-salinity fluids may be a mineable resource. Economic interest along the margins of the trough has been enhanced by the nearby discovery of bulk- mineable disseminated gold deposits, such as at Mesquite and Modoc.

To the northeast, across the Transverse Ranges, lie the basins and ranges of the Mojave Desert. The ranges expose Precambrian-Triassic sequences of miogeoclinal and marine carbonate platform sequences now complicated by late Paleozoic, Mesozoic, and Tertiary tectonics. These ranges have been the site of commercial production of nearly 20 metals, notably gold, silver, iron and tungsten. Precious metals development is continuing at Randsburg, Castle Mountain and Colosseum. Thirty non-metallic mineral commodities have also been produced, with modern production centered on cement, lime, rare-earth elements, cinder, borates, hectorite and zeolites. The Cenozoic Mojave basins contain a rich record of Oligocene to Recent deposition with enclosed vertebrate fossils that provide the local reference section for the Clarendonian and the type section of the Barstovian North American Land Mammal Ages. The modern basins include many fine examples of playas, pediments and alluvial fans. There are also widespread Tertiary to Holocene volcanic rocks throughout this desert region.