San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Geologic History of San Diego County

How and where do these igneous rocks form?

Geologists still debate the exact mechanism(s) by which these diverse rock types were formed but all agree that subduction of ancient oceanic crust was the overall mechanism. During subduction, cold oceanic crust (light blue) is thrust deep into the earth’s mantle (dark blue) and heated by its ambient conditions to the point of partial melting. These melts or magmas then rise upward to create the different rock types so prominent in the western zone. Some rocks are created by partial melting of the down-going ocean crust, others from partial melting of the underlying mantle, and yet others by various combinations of these two sources.Illustration of subduction of ancient oceanic crust

What is of special interest, however, is where this subduction occurred. Recent studies have suggested that the entire western zone of the batholith is an exotic terrain, i.e., one that did not form where it is presently found but instead drifted in as part of one of earth’s tectonic plates. Paleomagnetism is the study of earth’s ancient magnetic fields as recorded in the iron-bearing minerals found in igneous rocks. As newly crystallized minerals such as biotite, magnetite, and hornblende cool below a certain temperature (referred to as their Curie Point) the nature and direction of the earth’s magnetic field at the time of crystallization is locked into the structure of these minerals. If the crustal plate that holds the rock has moved since its crystallization, the iron-bearing minerals still retain information such as the paleolatitude at which the sample formed. This data can then be used to reconstruct plate motions and unravel the enormous jigsaw puzzles that are today’s continents.

The paleomagnetic data from the igneous rocks in the western zone of the batholith indicate that this enormous package of rocks, running from Riverside County southward through Baja California, formed at least a thousand kilometers south of its current position and must have been rafted into place by plate movements.

Western PRB Motion 1
Western PRB Motion 2

The growing western zone arc moved more or less northwestward and roughly parallel to the present coastline along strike-slip faults similar to the infamous San Andreas fault. This would have been a somewhat complicated geologic setting, with subduction west of the arc and strike-slip motion to the east. Left illustration.

This 100 km thick tectonic plate that carried the developing western zone (and its overlying active volcanoes) ground into the older Jurassic arc that lay just offshore of the continent at more or less the present latitude. In doing so, the collision compressed and deformed the Jurassic plutons and created the gneissic textures in the granitic rocks now exposed along the core of our Laguna Mountains. Right illustration.

As the western arc migrated towards its eventual collision with the Jurassic arc, it was growing larger and larger as gabbroic, and later granitic magmas, were being created at depth in the subduction system. Some of these magmas rose to form large active volcanoes whose remnants can still be found just inland from the coast and protruding from its cover of younger sedimentary rocks. These rocks, the Santiago Peak Volcanics, underlie familiar peaks such as Del Cerro, Cowles Mountain, Black Mountain, and Mount Miguel. These rocks are chemically similar to the granitic plutons found throughout the western zone of the batholith. It has been argued that they represent the volcanic cover that formed the roof of the batholith. If so, then the entire western zone is tilted so that shallower volcanic rocks are exposed near the coast and their deeper plutonic equivalents exposed further inland.

Cowles Mountain, overlooking the community of San Carlos
Cowles Mountain, overlooking the community of San Carlos, is composed of remnants of ancient volcanoes that are part of the older western batholith. Left photo.
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