Geologic History of San Diego County

The oldest rocks in San Diego County are a series of scattered exposures of metamorphosed sedimentary rocks called the Julian Schist. Named after the historic community in east San Diego County, these rocks represent the sediments that accumulated in the shallow seas off the coast of North America nearly 200 million years ago. This thick blanket of sediments extended miles off-shore and wrapped eastward around the then edge of the continent into what is now central Mexico. The sediments have since undergone extensive uplift and erosion so that they now exist only as the scattered exposures depicted in the figure below.

The story of the sediments begins in the shallow seas adjacent to the North American continent. There, muds and sands derived from the Precambrian rocks that now underlie western North America and northern Mexico began to accumulate in layers. As sea level in the ancient Pacific Ocean rose and fell, alternating layers of mud and sand began to form in deeper waters. These sediments accumulated into greater and greater thicknesses until the weight of the overlying material compressed the base of the sediment pile. Remnant ocean waters were squeezed out, grains were compacted, and sedimentary rocks such as mudstone and sandstone were created.

Generalized geologic map of San Diego County. Click on image for a larger size map view.

Outcrops of Julian Schist along Sunrise Highway (S-1). The banding represents the different rock layers in the original sedimentary rock. Click on photo to see a larger image view.
Also within this eastern metasedimentary package are exposures of amphibolite (a dark metamorphic rock composed of hornblende and plagioclase) that were basaltic lavas before metamorphism. They have an unusual chemistry, one that indicates formation in an ancient oceanic spreading center. This suggests that the ocean basin along the western edge of the continent was pulled apart in a manner similar to the rifting of Baja California from mainland Mexico or the separation of the Japanese arc from Asia. Both the marbles and the amphibolites described above hint at a long and complex history for these ancient rocks that formed the earliest vestiges of what is now San Diego County. Ancient Gneisses: The first arc Exposed along the winding stretches of Sunrise Highway (S-1) in the central part of San Diego County is a north-northwest trending belt of granitic rocks called the Cuyamaca-Laguna Mountain Shear Zone (CLMSZ). Mostly described as gneisses, these rocks have the same minerals found in normal granites such as quartz, feldspar, mica, and hornblende. What makes these rocks different is that the darker minerals, biotite mica and hornblende, have been recrystallized into tiny rod-like aggregates by large-scale tectonic forces.
Close view of the alignment of dark minerals in the Jurassic granite gneiss. Click on photo for a larger image view.

This realignment occurred at depths of about 8 miles (13 kilometers) and at temperatures approaching 600°C (1100°F). This information, gleaned from many decades of laboratory studies on the behavior of minerals under varying pressures (depth) and temperatures, indicates that these rocks have undergone over 9 miles of uplift to reach their present elevation of nearly 6000 feet above sea level.

These granitic gneisses also contain very small amounts of the mineral zircon which crystallized from the original granite magma. Although zircon has a rather simple chemical formula (ZrSiO4), it commonly contains trace amounts of the radioactive element uranium (U). This element decays at a known rate to a series of secondary elements such as lead (Pb) whose abundance can be measured and then used to calculate the age at which the original granite crystallized.

Several measurements of the secondary elements, made from samples collected along Sunrise Highway, indicate that most of the original granites were solidified during the Jurassic Period between about 160 and 170 million years ago. Geologists interpret this to indicate that subduction of an oceanic crustal plate had created a volcanic arc and its underlying plutonic basement adjacent to the western edge of the North American continent. Remnants of this volcanic-plutonic arc can also be found in the western Sierra Nevada foothills and in smaller mountain ranges scattered throughout the Mojave Desert. The recrystallized and aligned minerals in the rocks in central San Diego County suggests that these granites contain the record of another, more recent geologic event that transformed them from normal granites into granitic gneisses.

Weathered outcrop of Jurassic gneiss with several dark, wispy fragments of Julian Schist. Enclosure of these fragments by the gneiss indicates that the schist fragment must be older than the original granite. Click on photo for a larger image view.	Typical outcrop of Jurassic gneiss along Kitchen Creek in the Laguna Mountains.

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