Geology of California's Imperial
A Monograph by Eugene Singer
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
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
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
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
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
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.
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
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.
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.
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
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
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
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.
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.
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
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
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.
these forces, there were stresses that formed the offshore
basins, such as the Santa Barbara Channel and the underwater
grabens off the southern California
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.
to the beginning of this chapter.
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.