Geologic map of the Miocene volcanic rocks in the Eastern Santa Susana Mountains: Supplement 2 from "Miocene volcanic activity in the Los Angeles Basin and vicinity" (Thesis)

The Los Angeles basin is an elongate topographic depression on the southwestern coast of California. It marks the site of a former marine basin in which a maximum of about 30,000 feet of sediments accumulated during Tertiary times. Volcanic activity in this basin began during early middle Miocene (Relizian) time and ended during late Miocene (Mohnian) time. The products of this eruptive episode consist primarily of flows and tuff-breccias of olivine basalt, basaltic andesite, and hypersthene-augite andesite. Dacite end rhyolite were extruded locally in the northeastern part of the basin, but they constitute only a small fraction of the total volume of lavas and pyroclastic rocks of the region. Petrographically, the rocks are typical of the basalt-andesite-rhyolite association of the circum-Pacific volcanic belt. An investigation was undertaken to determine, as closely as possible, the timing of the volcanism and the composition of the volcanic rocks in different parts of the basin, and to study criteria whereby correlations of the volcanic portions of the stratigraphic section might be achieved. To this end, field mapping and collecting were done in those parts of the basin in which the effusive rocks crop out, and data from wells which penetrated these rocks in the subsurface were compiled. Thin-section studies, wet chemical analyses, and spectrochemical analyses of selected specimens were made during the course of the study. The results indicate that Tertiary volcanic activity in the Los Angeles basin was restricted wholly to the Miocene epoch. The first phases of this activity began simultaneously in the northwestern and southeastern parts of the area during Relizian time. Pyroclastic olivine basalt was thrown into the sea near the eastern end of the Santa Monica Mountains, northwest of Los Angeles, and olivine-bearing basaltic andesite was extruded subaerially near Sunland, 12 miles to the north, while flows and breccias of hypersthene-augite andesite accumulated in the vicinity of the San Joaquin Hills, 40 miles to the southeast. During the Relizian-Luisian transition interval (early to late middle Miocene), submarine flows of basalt and basaltic andesite were expelled near the city of Inglewood, southwest of Los Angeles. In the early part of the Luisian stage, peperites of hypersthene-augite andesite were injected into marine sediments accumulating in the vicinity of the Palos Verdes Hills, and at the same time, volcanic activity broke out on a grand scale in the northeastern part of the basin. In rapid succession, flows and breccias ranging in composition from olivine basalt and hypersthene-augite andesite to rhyolite were deposited on and near the Miocene shore, in what is now the vicinity of Glendora and Pomona. Flows of olivine basalt were laid down on the sea bottom in the east central part of the basin at this time, and were followed by deposition of pyroclastic rocks of similar composition. Near El Modeno, at the eastern margin of the basin, activity persisted until the end of Luisian time. Flows of olivine basalt, palagonite tuff, and breccias of basaltic andesite accumulated there. The dying phases of activity are represented by the Whittier diabase, an intrusive body of basaltic and dioritic composition that was emplaced in the Whittier fault zone, southeast of Los Angeles, during middle or late Mohnian time. Less well confirmed evidence of supposed Mohnian volcanism is represented by basaltic tuff-breccia beneath the Dominguez oil field, north of the Palos Verdes Hills. In most of the areas, dikes and other small intrusive bodies were injected into the country rocks after surface manifestations of igneous activity had ceased. These intrusive bodies are less well dated than the lavas, but most are thought to be of approximately the same age as the associated flows. In the San Joaquin Hills, however, an augite andesite exposed near the coast is at least as young as Luisian, and possibly is even younger, whereas extrusive activity in this area took place during late Relizian time. Between 400 and 800 square miles of lava are believed to underlie the Los Angeles basin proper. This represents only a small fraction of a much larger volcanic province that includes the western Santa Monica Mountains and Ventura basin to the northwest, and the continental shelf off the coast of California to the southwest. Miocene volcanic rocks are known to underlie large parts of all these areas. Throughout the Los Angeles basin and surrounding region, beds of rhyolite tuff of various ages are interbedded with the Tertiary sedimentary rocks. Several of these tuffs were studied in detail, and it is concluded that most of them represent wind-borne products of volcanic activity in areas to the northwest (southern and central Coast Ranges) and/or southwest (present floor of the Pacific Ocean). It was found that no one rock type in the area is restricted in time to a particular part of the Miocene epoch. As an example, olivine basalt of Relizian age occurs in the eastern Santa Monica Mountains, and identical rocks of Luisian age occur in the east central and northeastern parts of the basin. Similarly, hypersthene-augite andesite of Relizian age is exposed at the southeastern end of the basin, and Luisian lavas of like lithology are exposed in the northeastern part. A detailed study of the rocks reveals that the few distinctions that can be made on the basis of petrography are functions of geographic, rather than stratigraphic, differences. Rocks of andesitic composition are restricted primarily to the eastern and southern parts of the basin, and basalts predominate in the northwestern part. Variation diagrams based on wet chemical analyses show a distinct contrast in chemical composition between the flows and breccias of the northeastern part of the basin and those exposed along the coast. The latter are richer than the former in FeO and MgO, and poorer in Al2O3. These differences are believed to be the result of assimilation of pre-Tertiary "basement" rocks beneath the floor of the basin by magma in the process of ascent. The coastal lavas occur entirely within an area underlain by the Catalina metamorphic facies, a chloritic schist accompanied by saussuritic gabbro and serpentine. The lavas in the northeastern part of the basin are underlain by "basement" rocks of granodiorite and quartz diorite. A series of calculations indicates that magmatic differentiation of the volcanic rocks in the Glendora area probably took place by crystal fractionation of early-formed ferromagnesian minerals and calcic plagioclase, accompanied by assimilation of granodiorite. Differences in composition arising in this manner cut across time boundaries, and thus hinder stratigraphic correlation of the rocks. Quantitative spectrochemical analyses indicate numerous variations in the trace-element composition of the rocks. The sources of these variations include contamination of core samples, minor differences between neighboring flows of like composition, and large differences between various members of a single differentiation series. Differences in composition resulting from deuteric alteration of the rocks were found to be surprisingly small. If all of these variations are considered and the trace-element analyses of the rocks are compared, the only distinctions that can be made are geographic rather than stratigraphic. It is concluded that further attempts to establish compositional criteria for the purpose of correlation would be unproductive and unprofitable. Volcanic activity in the region coincided with profound tectonic unrest. The Los Angeles basin lies in the area of intersection of two major tectonic belts: the east-trending Transverse Ranges and Murray Fracture Zone, and the northwest-trending Coast Ranges and Peninsular Ranges. It is thought that deformation within, and at the base of the earth's crust in this area was responsible for the localization of volcanic activity. The transverse Volcanic Province of southern Mexico is similarly located at the intersection of the east-trending Clarion Fracture Zone and the northwest-trending circum-Pacific orogenic belt. Tertiary subsidence of the Los Angeles basin was relatively slow until middle Miocence time. During the Relizian stage, rapid settling began concomitantly with the outbreak of volcanism