Geologic History of San Diego County

Another consequence of this collision is that the two tectonic plates, one carrying the western zone arc and the other hosting the North American continent, were both topped with relatively low-density crustal rocks. When the plates ground into one another, neither could be forced below the other. The motion of the northeast-directed Pacific plate was impeded so that eventually, in another 5 or 10 million years, the tectonic plate that was responsible for the western arc broke away and continued its drive under both of the two preexisting arcs and the North American continent. This new motion would create the third magmatic arc that forms the eastern zone of the Peninsular Ranges Batholith.

The new, shallower subduction plane begins to create the La Posta-type melts that will form the large plutons of the eastern zone. The cross section faces southward as if looking into Baja California.

By about 100 million years ago, the subducting oceanic plate had broken away from the western arc and magmatism in this zone had all but ceased. The western “faucet” had been turned off and a new magmatic “tap” was about to be turned on. An increase in the spreading rate back in the central Pacific basin began to push the oceanic plate under the newly accreted western batholith but now at a much shallower angle. This change shifted magmatism eastward towards the boundary between the continent and the Jurassic arc.

For reasons not fully understood, the melting rate of the rapidly subducting plate increased dramatically. Unlike the western zone with its diversity of rock types and the several tens of millions of years for its creation, large bodies of homogeneous melt ballooned into existence within a few million years. These new dynamic conditions resulted in the rapid rise of enormous bodies of melt at the approximate juncture between the newly accreted western zone and the then western edge of the North American continent.

The present-day situation has the La Posta-type plutons east of both the Jurassic granites and the western zone of the batholith. The small yellow segments shown in the figure represent pieces of the older sedimentary materials engulfed by the rising melt (south-facing cross section).

The melts rose quickly, before much crystallization could occur, and came to rest at moderate depths in the crust. Here, they cooled in place to form a series of plutons with similar ages, compositions, and internal structures. Between about 97 and 93 million years, the bodies crystallized inward to form a string of concentrically zoned plutons that extend from Riverside County southward through Baja California. The largest of these, the La Posta pluton, occupies much of eastern San Diego County and covers an estimated 1700 kms2. Its counterparts, to the north in Riverside County and to the south in Baja California, range up to 1200 km2 in exposed area.

Distribution of 90-100 million year old granitic plutons (pink) of the eastern zone of the Peninsular Ranges batholith. Blue approximates the western zone of the batholith.	Typical outcrops of the La Posta pluton along Old Highway 8.

The rise of these melts into crustal rocks that were already warmed to some extent by the magmatism associated with the western zone of the batholith caused great changes. As the melts rose, the crustal rocks above them were quickly forced upwards into young mountain ranges that may have rivaled the Sierra Nevada of today. The static magma chambers cooled slowly and crystallized inward to create the internal zoning observed in these plutons. In doing so, they released their latent heat of crystallization to the older overlying metasedimentary assemblage.

These ancient sediments, now micaceous schists, marbles, and quartzites, were uplifted, faulted, and heated to extreme temperatures. Experimental studies indicate that initial magmatic temperatures may have been close to 1000° C (1830° F), sufficient indeed to begin melting in mica-laden schists. Muscovite, quartz, and feldspar fused together to form water-rich granite magmas which were less dense than the surrounding schists.

Trace elements such as beryllium, boron, lithium, and phosphorus were selectively concentrated into the newly formed melts. They rose buoyantly and worked their way upwards to crystallize into the myriad of pegmatite dikes whose gem minerals have made southern California world famous. Pink and green tourmaline, aquamarine, golden topaz, and blood-red almandine garnet are but a few of the gems found in these earliest igneous rocks of the eastern zone. The pegmatites occur as somewhat irregular, light-colored bands or dikes cutting through the darker colored metasedimentary rocks from Jacumba northward to Riverside. Even today, small mines near Mesa Grande, Pala, and Warner Springs produce museum quality specimens.

Two separate pegmatite dikes slice through older rocks near Sacatone Overlook in eastern San Diego County.	Whitish streaks on Tule Mountain, near Jacumba, are pegmatite dikes which have been explored for gem minerals.

But gems were not the only consequence of the rise of the La Posta-type magmas into the overlying metasedimentary rocks. Hot, water-based fluids derived from the metamorphic breakdown of the micaceous minerals in the schists diffused their way into the sandstones, now quartzites, interbedded with the mica schists. These fluids selectively dissolved bits of placer gold between the quartz grains and carried the precious metal upwards to cooler environments where the fluids would crystallize into the gold-bearing quartz dikes for which Julian became so famous.

Examples of gem tourmaline from San Diego County.

Near the paleosurface, deeply circulating groundwater became heated by the magmas at depth and rose back to the surface creating hot springs, geysers, and a Yellowstone-like landscape. Metals such as copper, lead, and zinc were leached from the surrounding rocks by the circulating hot meteoric waters and concentrated into sulfide ore deposits. Most of this is gone, the result of 100 million years of erosion. All that remains of that hostile but beautiful landscape are a series of thin, clay-filled fractures barely visible in scattered road cuts along I-8.

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