LAKEVIEW 7.5' QUADRANGLE: PROTOTYPE QUADRANGLE, NATIONAL GEOLOGIC MAP DATABASE PROJECT

The Lakeview 7.5’ quadrangle, Riverside County, southern California, was selected as a prototype quadrangle for development of a logic tree for the description of plutonic rocks for the National Geologic Map Database Project and to test the feasibility of detailed rock unit attribution in a geologic map database. It was selected as a prototype because it contains a limited number of bedrock units, several of which have multiple, detailed geochemical, isotopic, and physical properties databases. Geologic units in the quadrangle were previously attributed using an early attribution scheme (USGS Open-file Reports 97-859, 97-860, 97-861). Plutonic rocks of the Peninsular Ranges batholith in the quadrangle have been the subject of intensive analytical investigations. In particular, nearly all aspects of the 100 Ma Lakeview Mountains pluton, which is the most extensive batholithic rock in the quadrangle, have been comprehensively studied. This pluton is composed of compositionally heterogeneous tonalite that crystallized at about 5 Kb pressure. Extreme mineralogic heterogeneity in the unit is produced by ubiquitous schlieren that range in composition from essentially biotite-hornblende rock to quartz-plagioclase rock. Data are available to portray systematic variation in attributes such as major and trace elemental chemistry, Sr and Rb isotopes, density, magnetic susceptibility, and gravity. These extensive databases allow various multivariate types of analyses combining various attributes leading to further insights into the genesis of the pluton.

Crossing the northeast part of the quadrangle, the San Jacinto Fault Zone forms a deep and narrow pull-apart basin at a right step in the fault zone. Inversion of gravity data gives an approximation of the thickness of sedimentary fill within the basin, about 3,000 m. North of the San Jacinto Fault Zone, a variety of lacustrine and fluvial Pliocene and Miocene continental sedimentary rocks of the San Timoteo Badlands have been attributed using the current science language under development by the Science Language Technical Team of the North American Data Model Steering Committee.