Regional Geology

Current Structural Setting

The current structural setting has the San Andreas Fault between the Pacific (San Diego) and North American (San Francisco) Plates. There are currently no subduction zones located in offshore Southern California. So don’t worry, the volcanic activity that was produced in the past is not currently active.

Note: There is a divergent plate boundary to the south in the Gulf of Mexico. A transform plate boundary (The San Andreas Fault) extends over most of the State of California. Farther north, there is a convergent plate boundary with subduction off the coast and andesite volcanoes inland.

Above Figure #1a: This figure shows the current plate tectonics of the west coast of North America.

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Structural Setting and Timing of the Calavera Hills Volcanic Plug

Structural Setting

The features found at Calavera Hills were formed in very different structural setting. Extensive subduction occurred on the western edge of North America from Early Mesozoic to Early Miocene time (approximately 200 to 19 million years ago MYA)(Figure 1b). Oceanic crust equaling the size of the entire Pacific Basin was subducted underneath the North American plate during this time. A transition from subduction (and plate convergence) to primarily a transform boundary occurred during Miocene time (from approximately 19 to 5 MYA). This transform boundary eventually became the San Andreas Fault (Atwater 1998). During the transition a spreading center was obliquely subducted along the convergent boundary. As the warm, buoyant spreading center was subducted the overlying continental crust was uplifted, fractured, and parts were significantly rotated. Some of rotational events were fairly large; one of them is believed to have formed the Transverse Ranges to the north of Los Angeles (Fritsche, 2001). This series of events left zones of weakness in the continental crust where magma could ascend to the surface to become lava. The volcanic plugs found in Northern San Diego County are thought to be a product of this tectonic transition from subduction to lateral motion.

Timing

The volcanic plug found at Calavera Hills is mapped as Miocene in age, however none of the plugs found in Northern San Diego County have been radiometrically dated. Relative dating observations have been used to determine the approximate age of the plug found at Calavera Hills. The volcanic rocks found here cut the Eocene Santiago Formation (Tan 1996). This makes them younger than Eocene with a likely window of activity between late Oligocene and Miocene. This timing correlates very well with other similar regional volcanic activity that occurred along our coastline (to the north and south of us) during the same timeframe. One example of this is the Conejo Volcanics found on the western side of the Santa Monica Mountains just north of Los Angeles (Weigand 2002). In addition, there is a great deal of volcanic ash found locally in sedimentary rocks of this age. It has not been determined which of the volcanic plugs (or all) located in this area produced the ash (Abbott 1999).

Above Figure #1b: This figure shows the process of subduction as it relates to a convergent plate boundary.

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