Petrology and Structural Geology of the Pre-Cenozoic Crystalline Rocks of the Mt. Eden Area, Riverside County, California

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geology, petrology, California, pre-Cenozoic crystalline rocks, metasedimentary rocks, faults


Earth Sciences | Environmental Sciences | Geology | Tectonics and Structure


The Mt. Edna area, located southwest of Beaumont, California, consists of Pre-Cenozoic crystalline rocks and Tertiary and Quaternary sedimentary rocks. The sediments lie unconformably upon the crystalline rocks, which form the basement complex. The sediments were not studied in this investigation.

The Pre-Cenozoic crystalline rocks consist of Paleozoic (?) metasedimentary rocks, which belong to the sillimanite-almandine-muscovite subfacies of the almandine-amphibolite facies. The rocks are the regionally metamorphosed products of argillaceous sandstones and silt-stones, shales, sandstones, mudstones, and marble. These metamorphic rocks include quartz and biotite-quartz schists with various accessory minerals, quartzites, calc-silicate schists, gneisses, and hornfels, and marble. That their parental sedimentary rocks were pelitic assemblages, is indicated by the presence of sillimanite.

The metasedimentary rocks are intruded by leucogranodiorite, which represents a phase of the sequential emplacement of the Southern California Batholith. The composition of this igneous rock suggests that it represents one of the later stages of the batholith's emplacement. The intrusion of the leucogranodiorite was primarily along the foliation planes of the pre-existing structure. Contact metamorphism was extremely minor.

Pegmatite dikes cross-cut all the rock types of the area, indicating a slightly later time of emplacement than the leucogranodiorite.

The structure of the area is a homocline that strikes northwest and dips steeply to the northeast. The attitude of this homocline, which is defined by the foliation which is essentially parallel to the bedding, is consistent with that of the region, and suggests that it might be a part of a large fold, created during the period of regional metamorphism.

Minute folding in the schists is present as evidence of deformation resulting from the intrusion of the igneous bodies.

Joint systems and the recently active Claremont fault, San Jacinto fault zone, and minor faults in the area point to deformation which may be the result of similar forces which have been active upon the region since Pre-Cenozoic times.

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