Chapter 8: Whole-rock K-Ar ages and geochemical data from middle Cenozoic volcanic rocks, southern California: A test of correlations across the San Andreas fault

Abstract

Potassium-argon determinations (n = 19) and whole-rock trace-element analyses (n = 9) on volcanic rocks from the Plush Ranch, Vasquez, and Diligencia Formations located along the San Andreas fault system in southern California support earlier correlations that suggest palinspastic proximity of the transtensional basins in which the lavas erupted. Volcanic rocks of the Plush Ranch and Vasquez Formations, now located southwest of the San Andreas fault, were extruded contemporaneously at about 23.1 to 26.5 Ma and 23.6 to 25.6 Ma, respectively, whereas those of the Diligencia Formation, now located northeast of the fault, erupted about 20.6 to 23.6 Ma. The rocks yield trace-element and isotopic ratios that define a petrologic suite unique in southern California. These subalkaline and calc-alkaline rocks range from medium-potassium basalt to high-potassium dacite. Vasquez samples are generally higher in SÍO2 than are those from either the Plush Ranch or Diligencia Formations. Similar rare-earth-element patterns exhibit moderate light-REE and flat heavy-REE enrichment. Spider diagrams for selected incompatible major and trace elements have relatively tight and featureless patterns. Initial whole-rock 87 Sr/ 86 Sr ratios range from 0.7048 to 0.7062 and plagioclase 5 ls O ranges from 6.2 to 7.7; both values correlate positively with SÍO2. These data indicate that the volcanic rocks were derived from similar magmas, probably incorporating different amounts of crustal component. Our data indicate that the volcanic rocks formed at only slightly different times, had similar petrogenetic histories, and have been separated along the San Andreas fault system. We cannot distinguish, however, either the magnitude or timing of movement on the various strands of the system in southern California that resulted in their disruption.

Tectonics of Ridge Basin region, Southern California

“…Enough data are now in hand to state that the formations correlate, and parts of them are ~16 m.y. in age (Hendrix and Ingersoll, 1987;Frizzell and Weigand, 1993;Hendrix, 1993;Ehlert, this volume, Chapter 4). These correlations make the youngest piercing points that show the maximum displacement on the Canton-San Gabriel fault system.…”

“…1, 2) (Crowell, 1952a(Crowell, , 1960(Crowell, , 1962Carman, 1954Carman, , 1964. Soledad Basin units (primarily upper Oligocene-lower Miocene Vasquez Formation and middle Miocene Mint Canyon Formation) (Hendrix, 1993;Frizzell and Weigand, 1993;Ehlert, this volume, Chapter 4) are correlated with those in the eastern Lockwood Valley area (mainly upper Oligocene-lower Miocene Plush Ranch Formation and middle Miocene Caliente Formation) (Cole and Stanley, 1995). Studies of the Mint Canyon and Caliente Formations show that they were deposited along the same dispersal and depositional system (Ehlig and Ehlert, 1972;Ehlig et al, 1975;Ehlert, 1982, and this volume, Chapter 4).…”

Paleotectonics of a complex Miocene half graben formed above a detachment fault: The Diligencia basin, Orocopia Mountains, southern California

“…Volcanic eruption and intrusion were shortlived, based on the thinness of volcanic strata and interbedded sediment (Spittler and Arthur, 1982;Squires and Advocate, 1982); reliable K-Ar ages range from 23.6 ± 0.5-21.3 ± 0.6 Ma (Frizzell and Weigand, 1993). Thin lenses of nonmarine limestone containing freshwater ostracodes are locally interbedded with basaltic fl ow rock; spring-tufa deposits and pillow basalts testify to the interaction of magma and water (Spittler and Arthur, 1982;Squires and Advocate, 1982).…”

Overview of rocks bordering Ridge Basin, southern California

“…Beds in the Plush Ranch Formation of the eastern Cuyama Basin have an age between ca. 27 and 21 Ma (Carman, 1954(Carman, , 1964Ballance et al, 1983;Yeats, 1987b;Frizzell and Weigand, 1993;Cole and Stanley, 1995). The Vasquez and Plush Ranch Formations, therefore, belong to both the Oligocene and lower Miocene series.…”