The seafloor remains among the most inaccessible regions of our planet. Even though at least 70% of Earth's surface is composed of oceanic crustal rocks, only a small portion of the global seafloor has been mapped in detail. Seafloor geologic processes are intimately connected to the global plate tectonic system, and understanding seafloor geologic features was critical in developing modern plate tectonic theory.

The Southern California Continental Borderland is a broad area offshore southern California extending approximately from Santa Barbara to the Mexican border and seaward from shore approximately 180 km.

The borderland owes its origins to complex tectonic processes associated first with an ocean-continent convergent boundary, then later development of the transform boundary separating the Pacific and North American plates.

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The image above shows a small portion of the Southern California Continental Borderland approximately 180 km seaward of San Diego. The image was derived from detailed geophysical surveys and processed using Geographic Information System (GIS) software. The view is looking northward toward San Clemente Island. Notice there are several submerged platforms (Tanner and Cortes Banks) and deep basins (East Cortes and San Nicholas Basins). The basins are up to 1.8 km deep.

The image below is another virtual view of the borderland looking at the submarine edifice surrounding San Clemente Island. Notice that the exposed portion of the island is only a small part of the actual island platform. Also note an apparent submarine fan or submarine debris flow associated with the submarine canyon on the northwest end of San Clemente Island.

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Cortes and Tanner Banks are isolated platforms bound by faults. They are not presently exposed above sea-level, but rise to within 10 meters of the sea surface. Both of these platforms were exposed above sea-level during the last major glaciation, and their flat tops suggest wave erosion as sea-level rose at the end of the last ice age. Not also the Tanner Submarine Canyon leading to the East Cortes Basin.

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Finally, observe the ragged scarp near the base of Tanner Bank. This scarp marks the initiation point of a large submarine landslide/debris flow. Material from this slide extended outward over most of San Nicholas Basin. What factors might have triggered this event?

The Southern California Continental Borderland is a remarkable geologic province associated with the present-day transform boundary separating the Pacific and North American plates. Its origins are best understood in the context of the Theory of Plate Tectonics.

Southern California Borderland Fly By (3211.0K)

1. Consider the sea-level video from the Ch.14 Virtual Vista. How much of Cortes and Tanner Banks do you think was exposed during the sea-level lowstand (125 m below present sea-level) of the last glacial maximum?

2. Do you think Cortes and Tanner Banks will be exposed as islands again? How much of a decline in sea-level would be required to expose the platform tops?