THE GEOLOGY AND TECTONIC SETTING OF SAN DIEGO BAY, AND THAT OF THE PENINSULAR RANGES AND SALTON TROUGH, SOUTHERN CALIFORNIA
Dr. Monte Marshall
Professor Emeritus of Geology and Geophysics, San Diego State University
SAN DIEGO BAYWelcome to one of the most beautiful bays on the west coast--and the first one discovered in Alta California by the Portuguese navigator, who was working for Spain, Juan Cabrillo, in 1542. As we sail around the harbor you will see that the bay is surrounded by 400-feet-high mesas to the northeast; much citified sea level plains to the southeast; a long, narrow, sandy strand to the southwest; and the 400-feet-high, flat-topped Point Loma peninsula to the northwest (Fig. 1). To the north is the flat delta of the San Diego River, where the airport is built and where the river used to flow into the bay. It was here the Spaniards landed because of the easy access to fresh water. Now, the only entrance to the almost closed bay is a 2000 foot channel between Pt. Loma and the naval base on North Island (no longer an island).
Almost all the rocks you see exposed around the harbor are marine clastic rocks, like conglomerates, sandstones, and mudstones. The mesas to the northeast are capped by mid-Pleistocene terrace deposits (0.5-1 Ma), which were deposited on Plio-Pleistocene sandstones (about 2 Ma). Low-lying areas, like downtown San Diego, cities to the south, and the city of Coronado and its strand to the south sit on a 10 Ka marine terrace deposit. Point Loma peninsula is also capped for much of its length by one of the same mid-Pleistocene terraces you see on the mesas directly east across the bay. Only in the steep cliffs of Pt. Loma, especially at its southern tip, can you see a thick section of late Cretaceous (about 75 Ma) conglomerates and mudstones beneath the terrace deposits. Given that you are in California, you probably wouldn't be surprised to learn that you are now surrounded by active faults! Or that this bay is the result of slip on some of them. San Diego Bay is simply the surface expression of a north-south-trending, nested graben. The graben is bounded on its east side by the strands of the predominantly dip-slip, down-to-the-west, La Nacion fault zone and on its west side by strands of the down-to-the-east Point Loma fault zone (Fig. 2). Oblique slip strands of the Rose Canyon fault zone run up its center. The deepest part of this graben lies at the south end of the bay where metamorphic/granitic basement was encountered at 6000 feet in a wildcat oil well. These faults are probably all less than about 1 Ma and are a part of the San Andreas Fault System that extends west some 200 Km from the San Andreas fault zone into the continental borderland (Fig. 3). Detailed gravity measurements by the author and his students suggest that the graben is filled with about 4000 feet of sedimentary rocks beneath downtown San Diego (Fig. 4). Given its north-northwest trend this 12 mile wide by 20 mile long zone of crustal extension is probably due to transtension in the Rose Canyon fault zone. More about this later.
THE PENINSULAR RANGESDepending on how clear the air is, you can see our mountains, the Peninsular Ranges, that begin only about 15 miles east of the bay and extend east for another 65 miles to the desert and the Salton Trough. The story of the eastern half of these mountains begins with that of their metamorphic and granitic basement. During the Paleozoic (?), sedimentary rocks, such as carbonates, sandstones, and mudstones were deposited on a Proterozoic continental shelf and slope at the then western edge of North America. These rocks were later folded, metamorphosed, and intruded by Cretaceous granitic magmas. But let's step back a bit to better understand the plate tectonics of southern California. For most of the last several hundred million years, the west coast of North America experienced subduction of a series of known, like the Kula and Farallon, and unknown oceanic plates. Diving easterly to northeasterly these plates generated the magmas that rose and cooled about 100 Ma to form the eastern half of the Peninsular Ranges Batholith. But subduction also brought to western shores pieces of continental crust and island arcs that formed far out in the Pacific Ocean, i.e., the allochthonous terranes that currently make up much of the west coast of North America, especially Alaska. Paleomagnetic, geochemical, geophysical, and structural studies all suggest that the western half of the Peninsular Ranges was a 120 Ma island arc formed some 1000 Km to the south and was then translated north and sutured to our coast!
THE SALTON TROUGHTo show you how geologically active Southern California has been and still is, we should discuss the geology of the great desert valley just to the east of our mountains. The Imperial Valley, or Salton Trough, contains a brackish lake, the Salton Sea, which lies almost 300 feet below sea level. Sea water from the Gulf of California is prevented from flooding the below-sea-level portion of the valley by the great pile of sand that is the Colorado River delta. But, the reason that much of the valley is below sea level is that the crust in this area is being actively stretched to the point that the southern end of the Salton Sea is underlain by a spreading center-just like the East African rift valleys and one of the few continental spreading centers/rift zones in the world (Fig. 5)!
As mentioned above, the west coasts of North and South America were zones of subduction for many hundreds of millions of years. And, except for Alta and Baja California, subduction still rules along these coasts. Just prior to 30 Ma the Farallon plate was subducting beneath most of western North and South America and just to the west of it, separated by a spreading center, lay the Pacific plate. At about 30 Ma an eastward-extending promontory of the Pacific plate collided with the North American plate near Los Angeles, forcing the trailing edge of the Farallon plate and the spreading center down into the subduction zone (Fig. 6). Thus, direct contact between the Pacific and North American plates was initiated and the plate boundary became a transform or strike-slip fault. More and more of the trailing edge of the Farallon plate and spreading center was subducted north and south of LA, forming two triple junctions, one that migrated north and one that moved south. The length of the transform boundary between the triple junctions, which at that time was located offshore/underwater, grew. The northwest-southeast trending Salton Trough--Gulf of California area had already been experiencing extension and even the invasion of shallow seawater since Miocene time (15 Ma). But when the southern triple junction reached the Cabo San Lucas/Mazatlan area, at about 5 Ma, a series of spreading centers alternating with transform faults began to tear Baja California away from Mexico and form the deep water Gulf of California (Fig. 7). The northernmost transform fault, beginning at the southeast shore of the present Salton Sea and connecting with older faults, extended all the way to Cape Mendocino and out into the ocean to join the Gorda and Juan de Fuca spreading centers (Figs. 5, 7). Thus at about 5 million years ago the modern San Andreas transform fault system was born. With increasing distance west of the San Andreas the continental crust is more and more a part of the Pacific plate and shares its northwesterly absolute motion. Of the about 50 mm per year of relative motion between the two giant plates, 30 mm/y occurs on the San Andreas, 12 mm/y on the San Jacinto, 5 mm/y on the Elsinore, 1-2 mm/y on the Rose Canyon right beneath you, and the remained on the off-shore faults (Fig. 3). This process of translating and rifting Southern California away from North America has extended over the last 5 Ma and has produced a very complicated geology in Southern California--which, at the least, involves many northwest-trending right lateral and northeast-trending left lateral strike slip faults, transpressional mountain ranges, and transtensional basins or rhombochasms.
SOME FINAL THOUGHTSDuring this several hour cruise your eyes have seen just the tip of the geology and tectonics that surround you. Now with your mind's eye you have seen the large mountains and basins that the inexorable motions of crustal plates, millimeter by millimeter, have created. The faulting, rifting, and translation of crustal blocks that have formed San Diego Bay differ from those that formed the Salton Trough and Sea in scale and rate only. Unlike Kansas City, the land forms you see today in San Diego were very different a million years ago-and will be very different a million years from now. SO, please hurry back to our beautiful region! References
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