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Geology of California's Imperial
Valley
A Monograph by Eugene Singer
THE GULF OF CALIFORNIA
SEVEN MILLION YEARS AGO, in Pliocene time, the Baja
California peninsula did not exist, nor did the Gulf of
California, known in Mexico as the Sea of Cortez. Their
mutual evolution and geologic history not only make a
fascinating story, but one that has had a profound effect
upon the Salton Valley.
The Gulf of California and the Salton Trough are components of a single geologic structure that averages less than 100 miles in width yet is more than 1,000 miles long. In this context, the Salton Trough is considered the landward extension of the Gulf.
This point becomes clear when one considers that the Salton Valley exists only because the Colorado River delta is a natural earthen dam that completely excludes the waters of the Gulf. If it was not for that dam, the Valley would be submerged as far north as Indio.
So, any study of
the valley's physical history and environment must include
the Gulf. More so than usual, in fact, since this area is
one of earth's most dynamic and changing regions, and the
long term future of the valley has many speculations of
wonder.
Geology of
the Gulf
Briefly stated, the Gulf of California is an elongated depressed block along a zone of major, roughly parallel faults. It is a complex fault zone in which total relief, i.e., the elevation difference between the sea floor of the Gulf relative to the mountains of Baja California to the west and the Sierra Madre of mainland Mexico to the east is greater than 10,000 feet.
The San Andreas fault is considered a major element of this fault zone. The fault presently disappears beneath the alluvium of the Imperial Valley, but is thought to continue southeastward into the Gulf, merging into the East Pacific Rise along the Gulf's axis.
The general structure of the Gulf is very well observed in this satellite composite photo. It is linear in direction, with the landward portion, the Salton Valley comprising a triangular area in the north. Clearly visible is the Colorado River delta, the Mexicali Valley and the Imperial Valley represented by dark tan color, the Salton Sea's conspicuous gourd-shaped area and the Coachella Valley at the apex. One may also interpolate the East Pacific Rise as the axis of the Gulf translating into the San Andreas Fault Zone beneath the delta.
The northern one fourth of the Gulf is shallow; at no place greater then 600 feet deep. Here, the Colorado River delta has contributed greatly toward filling the north end with fine sand and silt, making the sea floor much shallower than it is in the south. The sea bottom of the Gulf grades upward to the north, gradually and eventually mergeing into the marshlands of the delta. This character of the Gulf is observed at the beaches of San Felipe, 120 miles south of Mexicali.
The basement rocks under the north end of the Gulf are covered by as much as 25,000 to 30,000 feet of these deltaic sediments. This enormous volume of silt suggests the amount of erosional debris taken from the Colorado Plateau to the Gulf by the Colorado River over several million years.
Further south, the east side of the Gulf floor slopes gently from the mainland Mexico coast to an irregular escarpment near the center of the Gulf. Between the 25th and 26th parallels of latitude, one section of this escarpment is a great submarine cliff nearly 6,000 feet high.
Much of the south half is occupied by a remarkable depression in the sea floor, extending 250 miles southeastward from a point east of tiny Isla Tortuga. It widens into enormous proportions in places. The enclosing contour of this depression is 5,400 feet below sea level. This great submerged basin is occupied by three separate basins. The deepest basin, 10,740 feet, lies in the center of the Gulf between the 25th and 26th parallels.
Between 30 and 40 islands varying in size are found in the Gulf. Some islands, while very small in area, tower to surprising elevations with great ocean deeps close offshore. Many islands are elongated, with shorelines generally parallel to the axis of the Gulf, suggesting their origin as uplifted blocks along faults.
Isla Angel de la Guarda, the second largest island, is unique. It is at the 29th parallel, in the upper half of the Gulf. This 45-mile long island lies about ten miles offshore from Baja California, yet is separated from it by a long, narrow and deep submarine trough which averages 4,000 feet deep. The deepest portion is more than 5,000 feet below sea level. (This island is conspicuous in the satellite photograph at the top of this page.) Considering that the island lies only a few miles offshore from the peninsula proper, the extreme depth of the intervening basin suggests significant vertical displacement along faults on the sea bottom.
The several broad, deep basins in the Gulf are probably wedge-shaped blocks, or grabens, bounded by faults. The most convincing evidence of this is that the deep basins are true closed depressions, with elongated steep sides, rather than open-ended submarine valleys.
Volcanic
activity in the Gulf area was intense in Miocene time, about
25 million years ago. Extensive basalt flows are found in
the southern part of Baja California. Volcanoes were active
through the Pleistocene, and have continued active to the
present. Isla Tortuga is a very young volcano in the Gulf.
Las Tres Virgenes and Isla Coronada have been active in
historic times, as well.
A Hypothesis as to the Origin of the Gulf of California
The Baja peninsula, including most of southwestern California is an elongated slab of the North American continent that has been sheared from the mainland and moved to its present position.
The postulated movement of the peninsula may be retraced backward in time and in direction. Looking back to the Cretaceous, the peninsula is brought into a likely former position with the mainland. In this arrangement, the Sierra Madre del Sur appears to continue without break from mainland Mexico into Baja California.
In Pliocene time, about seven million years ago, a zone of separation developed on the East Pacific Rise. The future Baja California peninsula and a piece of future California were sheared from mainland Mexico along a lateral fault, possibly the ancestral San Andreas fault which was then, as now, oriented northwest-southeast. During this early period of development, movement was right lateral, with the sheared-off slab moving northwest, but always in close contact with the mainland.[1]
The northwest movement seems to have been repeated slippage along the principal members of the San Andreas fault system -- the Elsinore fault, the San Jacinto fault, and the main San Andreas strand itself.
About four million years ago, the San Andreas fault proceeded to play a key role in the next phase, the opening of the mouth of the Gulf of California. While the Baja California peninsula continued to move to the northwest as a whole, its southern end began to rotate westward, opening a seaway between the new peninsula and the mainland.
This movement is thought to have been caused by torsional stress. That is, while the primary movement continued to be northwest, the northern end of the Baja California peninsula became locked against the mainland, causing the southern end to rotate westward, creating the seaway that was to become the Gulf of California.
As the Gulf continued to open, Baja California moved out to sea while the Peninsular Ranges of California remained attached to the mainland block. Movement along the San Andreas fault system was vigorous, and the San Jacinto fault became the most active member. At this time, the San Jacinto Mountains and the Santa Rosa Mountains were sharply compressed and uplifted.
Like a massive lever, the rotation caused intense compression forces at the north end, near the fulcrum. The squeezing effect helped to raise the Transverse Ranges of the Los Angeles basin (the Santa Monica, San Gabriel and San Bernardino Mountains), orienting them east-west, as opposed to the northwest-southeast direction of other major ranges of central and southern California.
Coincident with these forces, there were stresses that formed the offshore basins, such as the Santa Barbara Channel and the underwater grabens off the southern California coastline.
It thus appears
that the East Pacific Rise extends well into the Gulf of
California and sea floor spreading is widening the Gulf.
Tensional thinning at the spreading center between the North
American plate and the Pacific plate is further supported by
the existence of a large intrusion of magma beneath the
sediments of the Salton Sea. The result is regional
metamorphism of the sedimentary fill beneath the Salton Sea,
resulting in high heat flow and the geothermal activity and
volcanism in the Imperial Valley.
Return
to the beginning of this chapter.
[1] The San Andreas in California illustrates this condition. While the western block moves steadily to the northwest, it is still in intimate contact with the mainland block. There is no separation evident.