Geologic Map of the Lake Mathews 7.5' Quadrangle, Riverside County, California

This data set maps and describes the geology of the Lake Mathews 7.5' quadrangle, Riverside County, California. Created using Environmental Systems Research Institute's ARC/INFO software, the data base consists of the following items: (1) a map coverage containing geologic contacts and units, (2) a coverage containing structural data, (3) a coverage containing geologic unit annotation and leaders, and (4) attribute tables for geologic units (polygons), contacts (arcs), and site-specific data (points). In addition, the data set includes the following graphic and text products: (1) a postscript graphic plot-file containing the geologic map, topography, cultural data, a Correlation of Map Units (CMU) diagram, a Description of Map Units (DMU), and a key for point and line symbols, and (2) PDF files of the Readme (including the metadata file as an appendix), and the graphic produced by the Postscript plot file.
All but the southeast corner of the Lake Mathews quadrangle is in the Perris block, a relatively stable, rectangular-in-plan area located between the Elsinore and San Jacinto fault zones in the northern Peninsular Ranges Province. In the southwest corner of the quadrangle, a small triangular-shaped area that is part of the Santa Ana Mountains structural block, is separated from the Perris block by a short segment of the Elsinore fault zone. The active Elsinore fault zone, a major component of the San Andreas fault system, consists of a series of en echelon northwest-striking right lateral faults located in a graben-like structure.
There is limited relief within the quadrangle because of the presence of two prominent erosion surfaces. The lower Perris surface (about 1,500 feet elevation) has low relief and dominates the physiography of the northern half of the quadrangle. This surface is discontinuously covered by coarse-grained, clastic, non-marine sedimentary rocks of the middle Miocene-age Lake Mathews Formation. A higher Gavilan-Lakeview surface (about 2,100 feet elevation) occurs in the eastern part of the quadrangle, and is locally covered by small exposures of fluvial conglomerate that contain exotic clasts of red rhyolite.
The Lake Mathews quadrangle is underlain almost entirely by Cretaceous and older basement rocks. Two different types of metamorphic rocks are exposed in the quadrangle. In the northeast is a northwest trending exposure of amphibolite grade biotite-bearing schist of probable Mesozoic age. This schist separates massive textured granitic rocks to the west from foliated and layered granitic rocks to the east. The large expanse of metamorphic rock between Temescal Wash and Lake Mathews is low metamorphic grade, typically siliceous, but highly variable in composition.
Cretaceous plutonic rocks in the quadrangle are part of the composite Peninsular Ranges batholith, and represent a wide variety of mafic to intermediate composition granitic rocks. Most are massive-textured with the exception of the crudely foliated biotite-hornblende tonalite of the Val Verde pluton in the northeast corner of the quadrangle. The Cajalco pluton, which consists of biotite monzogranite, granodiorite and lesser amounts of biotite-hornblende granodiorite, by far, accounts for most of the granitic rocks in the quadrangle. It is a shallow level pluton emplaced by magmatic stoping into largely intermediate composition volcanic and volcanoclastic rocks and metamorphic rocks in its western and southern extent and into gabbroic rocks in its northern extent. The pluton appears to be tilted up to the northeast with the texture of the rock changing from subporphyritic rock containing beta-quartz-appearing phenocrysts in the southwestern part of the pluton to coarser-grained hypautomorphic texture rock in the eastern part. Located in the upper part of the pluton and in overlying wall rock in the shallow western part of the pluton is widespread metasomatic tourmaline rock. Locally parts of the pluton have been completely replaced by tourmaline but more commonly tourmaline occurs in discrete thin zones, generally along joints. Some of the larger masses of tourmaline rock, locally termed tourmaline 'blowouts', contain cassiterite and sulfides. One large mass of cassiterite-bearing tourmaline rock supported a tin mining and smelting operation.
In the southeast corner of the quadrangle is the northwest part of the Gavilan ring complex. This shallow plutonic complex centered southeast of the quadrangle is predominantly tonalitic composition, characterized by the presence of hypersthene, which is rarely found in Peninsular Ranges batholithic rocks of intermediate composition.
Most of the southern part of the quadrangle is underlain by siliceous volcanic and volcanoclastic rocks considered to be coeval with the batholith and which are considered to represent the supra-part of the batholithic magmatism. These rocks generally range in composition from rhyolite to andesite, but latite is probably the predominate composition.
Paleocene continental rocks of the Silverado Formation occur within the Elsinore fault zone and nearby on the adjacent Perris block. Clay-rich parts of the Silverado Formation have been mined for industrial clay. Near Arlington Mountain, in the northwest part of the quadrangle, are two very small occurrences of conglomerate that consist of exotic welded-tuff clasts and a few exotic bedded quartzite clasts. Extensive Quaternary alluvial deposits are found along the south side of Lake Mathews and in the Temescal Valley along the Elsinore fault zone.