Geology topics

Research about western California

Source-linked reports with geographic coverage including western California.

2 recordsLinked to original sources

USGS2025

Plate tectonics and volcanism in western California

Introduction Western California is home to a variety of volcanic rocks. The locations, ages, and chemical compositions of these volcanic rocks help tell part of the fascinating story of California’s plate tectonic evolution over the past 40 million years. These volcanic rocks are a product of multiple tectonic processes, including subduction of divergent and transform plate boundaries beneath continental North America, opening of a slab window, creation and migration of a tectonic triple junction, and the birth and growth of the San Andreas Fault. This fact sheet explains these tectonic processes and discusses their role in shaping the volcanic history of western California over the past 40 million years. By studying the volcanic rock record in western California, geologists are able to piece together how regional volcanism and plate tectonics are linked in space and time. Recognizing this linkage helps scientists to understand possible future volcanism in the region, potential hazards associated with this volcanism, and the impacts these hazards may have on population and infrastructure. The U.S. Geological Survey California Volcano Observatory (CalVO) closely monitors the parts of western California with the greatest potential for volcanism.

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USGS1980

Folding and faulting of strain-hardening sedimentary rocks

The question of whether single- or multi-layers of sedimentary rocks will fault or fold when subjected to layer-parallel shortening is investigated by means of the theory of elastic-plastic, strain-hardening materials, which should closely describe the properties of sedimentary rocks at high levels in the Earth's crust. The most attractive feature of the theory is that folding and faulting, intimately related in nature, are different responses of the same idealized material to different conditions. When single-layers of sedimentary rock behave much as strain-hardening materials they are unlikely to fold, rather they tend to fault, because contrasts in elasticity and strength properties of sedimentary rocks are low. Amplifications of folds in such materials are negligible whether contacts between layer and media are bonded or free to slip for single layers of dolomite, limestone, sandstone, or siltstone in media of shale. Multilayers of these same rocks fault rather than fold if contacts are bonded, but they fold readily if contacts between layers are frictionless, or have low yield strengths, for example due to high pore-water pressure. Faults may accompany the folds, occurring where compression is increased in cores of folds. Where there is predominant reverse faulting in sedimentary sequences, there probably were few structural units.

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