EXTINCT VOLCANIC CENTERS OF THE WESTERN SANTA MONICA MOUNTAINS AND THE OXNARD PLAIN, ONSHORE CALIFORNIA: A FIELD AND PHOTOGRAPHIC SURVEY (1992-2022)

Between 30 to 17 Ma, the California margin underwent a transition from convergent to transform tectonics causing the formation of the western Santa Monica Mountains and the Oxnard Plain. These tectonic events involved subduction, plate capture, rotation, crustal thinning and extension, rifting, and magmatism. These processes influenced the formation of middle Miocene rocks in this area.

By 20 Ma, the Conejo Valley and Oxnard Plain were submerged beneath an ancient shallow sea. In this shallow sea, the sands, gravels, and assorted clastics of the Topanga Formation were among the first sediments to be deposited during the initial phase of tectonic rifting. At 17 Ma, decompression melting formed magmas that rose through metamorphosed, underplated oceanic sediments and continental crust, triggering the eruption of the submarine and later subaerial Conejo Volcanics. Sedimentary rocks were melted by magmatic intrusions that now form the present-day physiographic feature known as the Santa Monica Mountains. Once submerged under this ancient shallow sea, the volcanic centers grew upward through the oceanwater column, eventually breaching sea-level to form lava flows and pillow basalts. The volcanic centers were formed in part by caldera collapse.

Today, the Topanga Formation unconformably underlies the Conejo Volcanics in the Santa Monica Mountains. Within these mountains, evidence of Conejo Volcanic intrusions includes blocks of the Topanga Formation that occur as xenoliths in small magma chambers. Other intrusive features such as dikes and sills are found throughout the Topanga Formation. Tectonic activity followed by these eruptions may have caused turbidity flows in the area. Over 100 intrusions have been mapped throughout Ventura and Los Angeles Counties.

We examined several Conejo Volcanic centers situated in a field that was formed by an episode of volcanic eruptions that produced multiple breccia and lava flows during the middle Miocene. We separated the rocks of the Conejo Volcanic field into 14 areas based on volcanic structure, geomorphic features, volcanic rock composition, and recently mapped faults and mass wasting deposits. We concluded that there were multiple episodes of volcanic eruptions over a 4 million year period, ranging from 17.4 to 13.4 Ma, based on published radiometric age dates.