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                Ch. 4  Classification of Coasts                   

Ch. 6  Southern California Bight


Chapter 5


California Continental Borderland



California’s Continental Borderland and the Southern California Bight refer to the same geographical area of coastal California and the Pacific Ocean.   The Southern California Bight, which will be covered in Chapter 6, is the curved indentation of the west coast of North America south of Point Conception down to the Mexico border.   The Bight includes the Pacific Ocean out to the Patton Escarpment roughly 100 miles offshore of southern California.  

The California Continental Borderland represents a unique tectonic region that formed along with the San Andreas Fault during the past 27 million years. Specifically, the Borderland is the unusual and active ocean floor offshore of southern California.   So, Southern California Bight is an oceanographic term, and California Continental Borderland is a geologic term.   To understand the origin of the Borderland, you must have a working knowledge of plate tectonics theory, which is provided below.


Plate Boundaries

There are three basic types of plate boundaries, formed as the edges of lithospheric plates move relative to each other.   Plate movements are due to convection of the asthenosphere (beneath the lithosphere), and gravity, which pull elevated portions of plates downward.   Below are brief descriptions of each type of boundary.

1. divergent boundary - plates move away from each other, forming a rift valley where new crust is created due to volcanic activity along with ridges that run parallel to the rift valley.   Ridges are uplifted due to the high heat flow beneath a divergent boundary, as illustrated by the diagram below, from the plate tectonics web site by the Moorland School, United Kingdom..

2. transform boundary - plates move horizontally past each other transferring stress into the surrounding crust, with lots of earthquakes but little to no volcanic activity occurring.   In general, transform boundaries exist to connect one divergent boundary to another divergent boundary.   This is illustrated in the diagram below, by Dr. Bruce Railsback, University of Georgia.

3. convergent boundaries - plates move toward each other, producing uplift of the crust, tall mountains and very high-magnitude earthquakes.   Subduction boundaries form where ocean crust is forced beneath another plate, and collision boundaries form where two continental plates collide with each other.   Both diagrams are from the Plate Tectonics web site of Moorland School, United Kingdom.



Origin of the California Continental Borderland

The California Continental Borderland has developed in association with the San Andreas Fault, and it is still being shaped due to movement along this transform plate boundary between the Pacific and North American plates.   Although the San Andreas Fault is 50 to 100 miles inland from the coast line, it exerts a profound influence on southern California’s continental margin, making it very active and unique from anywhere else on Earth.   The diagram below illustrates the relationship of the Pacific and North American plates, and the plate-tectonics role the San Andreas Fault plays by connecting the East Pacific Rise to the Juan de Fuca Ridge.   (from Irwin, 1990)

The following is a brief history of the development of the California Continental Borderland based largely on the work of Tanya Atwater, UC Santa Barbara.   A link to one of her Quicktime animations showing the evolution of tectonic activity in the eastern Pacific Ocean basin is below.

1. Around 120 million years ago, subduction was occurring along the coast of much of California, with powerful earthquakes and explosive volcanic activity common.   A modern analogy would be coastal Oregon and Washington today, with subduction occurring offshore in the Cascadia trench, volcanism in the Cascade volcanoes, and the occasional powerful earthquake.   Here, continental shelves are narrow due to steep shorelines and earthquake activity.   

2. Subduction activity along the coast of southern California started diminishing approximately 27 million years ago when the East Pacific Rise began subducting beneath the central coast of California.   At this point, transform motion developed to connect the separated components of the East Pacific Rise still existing to the north and to the south.   The proto-San Andreas Fault formed about 18 million years ago, as a series of offset faults west of the present San Andreas Fault.   The plate boundary was flexed outward, away from the continent.   Shearing along strands of the proto-San Andreas Fault was accompanied by extension of the ocean crust, forming faults that would later determine major features of the California Continental Borderland.   Roughly 17 million years ago, a large block of crust began rotating clockwise as the Pacific Plate continued to move toward the northwest past the North American Plate.   Other smaller crustal blocks rotated, subsided, and/or uplifted until about 4 million years ago, when the San Andreas Fault formed in its present location.

3. The San Andreas Fault now connects the East Pacific Rise in the northern corner of the Gulf of California to the Juan de Fuca Ridge and Cascadia subduction zone off the northern California coast.   Stress due to the grinding of the Pacific and North American plates past each other is transferred to offshore faults of the Borderland, making it tectonically active today.  

The brief history of the California Continental Borderland above leaves out many details.   To learn more, click on this link to see the paper on the tectonic history of southern California by Tanya Atwater which is cited at the end of this chapter.   Or, click on the web link below to view one of Dr. Atwater's animations showing the evolution of the San Andreas Fault.


The diagram below is from a professional paper by W. P. Irwin, with the USGS, giving you another perspective to understand the formation and evolution of the San Andreas Fault.


Description of the California Continental Borderland

The California Continental Borderland refers to the ocean floor and offshore islands from the southern-California shoreline out to the Patton Escarpment.   Beyond the Escarpment, the ocean floor is more typical of a deep ocean basin, with a relatively flat ocean bottom and a few seamounts.   

1. The Borderland is characterized by a series of generally northwest-trending ridges, banks, islands, and basins.   The ridges are not divergent plate boundaries such as the Mid-Atlantic Ridge and the East Pacific Rise.   Instead, they are products of compression and uplift along faults or folds in the crust.   Similarly, banks are elevated linear areas such as the Pilgrim Banks, or locations of underwater mountains (seamounts) that are just below present ocean level.   Cortez Bank is an example of this type of bank.   The southern Channel Islands (San Nicolas, Santa Catalina, and San Clemente islands) are elongate, with their long edges oriented toward the northwest.   

2. Seismic profiling of the ocean floor and underlying sediment and rock indicates that active faults are present along at least one side of each island, with folded rock and sediment and lesser faults present along other sides of islands.   Similarly, the isolated basins of the Borderland are bounded by active faults and zones of intensely folded and crumpled sediment and rock.   View a Powerpoint presentation, given by Dr. Robert Francis, which addresses active faults and other structures within the Inner Borderland area adjacent to Los Angeles and Long Beach harbors, Long Beach and Orange County.  

The diagrams below indicate that the California Continental Borderland is a seismically active region, with each dot representing an earthquake epicenter.


3. The generally northwest trend of the basins, ridges, banks and islands of the Borderland is roughly parallel to the orientation of the San Andreas Fault, suggesting that their formation and evolution are directly related to movement along the transform plate boundary.   This is apparent on the modified Monterey Bay Aquarium Research Institute map below.  



4. Sedimentation in the California Continental Borderland is a combination of terrigenous and biogenous processes.

a. Basins of the Borderland range in depth from 3,000 to 6,000 feet in depth.   They receive terrigenous and biogenous sediment in several ways.   

(1) Finer-grained terrigenous sediment (clay and silt) is carried out over the Bight/Borderland by offshore winds, or introduced into the coastal ocean by rivers during and after rain events.   Surface ocean currents can disperse these tiny particles throughout the Bight/Borderland.   Gravity pulls the tiny grains to the ocean floor over a period of weeks to months.

(2) Coarser-grained terrigenous sediment (sand and gravel) is carried into basins by turbidity currents descending through submarine canyons from shallow, nearshore water out onto the basin floor.   (Turbidity currents form when a major storm or an earthquake disturb shelf sediment, mixing it with water.   This mixture is denser than the surrounding water, so gravity pulls it down slope to greater depth.)   Typically, the sand and gravel form a submarine fan on the periphery of a basin.  

(3) Landslides from steep coastal cliffs and slopes can also send coarse sediment into basins where the shelf is narrow, such as along Palos Verdes Peninsula on the mainland coast, or along many of the Channel Islands' coasts.   Local rip currents can transport the sand, gravel and cobbles to the nearby shelf edge where they cascade downward to the edges of the basins.   The northwestern end of Catalina Island, shown below, has steep cliffs that drop off to a very narrow shelf, and then into the Catalina Basin.



(4) Larger biogenous sediment typically originates from nearshore coastal water, and is carried into the basins, along with sand and gravel, by turbidity currents.   It is usually mixed with terrigenous sediment on submarine fans.   Carcasses of whales that die as they swim across the Southern California Bight can settle on the deep basin floors, randomly contributing their large skeletons to the basin sediment.   (These carcasses, until picked clean by scavengers, can form isolated and unique ecological zones on an otherwise barren basin floor.)

(5) Smaller biogenous sediment is usually in the form of fecal pellets excreted by swimming (nektonic) marine animals.   The tiniest, clay-sized pellets originate from filter-feeding zooplankton.   Such fecal pellets are very abundant on the deep-basin floors.

The diagram below shows the Borderland basins.   The large basins further from the mainland California coast contain very different sediment than the Santa Monica and San Pedro basins.   Explain why this would be the case, and compare and contrast the sediment that exists within the smaller nearshore basins to the larger offshore basins.


b. The shallow ocean floor of the Borderland, the continental shelf and narrow island shelves, receives a wide range of terrigenous and biogenous sediment.

(1) Along the mainland coast of California, the primary sediment is sand weathered and eroded from inland mountains and coastal cliffs.   This terrigenous sediment is transported by longshore currents, helping to form the beautiful beaches that southern California is famous for.   Rip currents can carry this sediment offshore, but eventually it is returned to the beach by small waves.   With time, most beach sediment's fate is to descend into a submarine canyon and then out onto basin floors.   At every beach there is a small biogenous sediment component, mainly fragmented shells of molluscs.

(2) Borderland islands receive little rainfall, and their overall surface areas are small in comparison to the mainland, so little terrigenous sediment is carried into the coastal zones of the islands.   So, their beaches tend to be short and narrow, with a larger component of biogenous sediment than beaches of the mainland.   This condition extends into the nearshore coastal-ocean floor as well.

c. Ridges and banks of the California Continental Borderland are separated from islands and the mainland by numerous basins which trap most terrigenous sediment.   Therefore, ridges and banks are covered primarily by biogenous sediment of all sizes and shapes.



Effects of the Continental Borderland on Coastal Southern California

The varied topographic features of the Continental Borderland have a significant effect on waves and currents offshore of southern California.

1. Borderland features influence currents in the Southern California Bight, especially with regard to eddy currents, which mix water from the surface down to the ocean floor on the bottom of Borderland basins.

2. Borderland islands impact  wind waves by absorbing waves that directly strike the islands, forming a wave shadow behind the islands.   The islands also diffract wind waves, causing destructive interference of interacting swell and reducing energy and height of swell that eventually breaks along the coast of southern California.   Note the significant decrease in wave height behind the Channel Islands on the Scripps wave map below.


3. Since the Continental Borderland is seismically active, the potential for tsunami generation here is very real.

a. Several active faults, including the Palos Verdes and Cabrillo faults, cut across the San Pedro Bay shelf just off shore of Long Beach.   Seismic profiles indicate that there is an element of vertical movement along theses faults, which could potentially produce a tsunami should a long stretch of either fault rupture with significant vertical movement (up or down).

Below is a United States Geological Survey image derived from multi-beam sonar data.   The locations of the Palos Verdes and Cabrillo faults are shown, based on work by Dr. Robert Francis (CSU Long Beach Department of Geological Sciences).



Below is a seismic profile of the Palos Verdes Fault (vertical discontinuity near middle of profile), clearly showing the west (left) side of the fault uplifted about 12 feet higher than the east (right) side of the fault.

Since the Palos Verdes Fault is so close to shore, there would be little if any warning time for coastal inhabitants in case a tsunami was generated due to movement along this oblique-slip fault.

b. Tectonic activity within the Borderland can produce earthquakes that might, in turn, generate mass wasting along the steep island coasts and adjacent ocean floor.   This could result in the formation of tsunami aimed directly at coastal southern California.   The most significant local tsunami threat for the Long Beach and Orange County coast is from Santa Catalina Island.   A powerful earthquake on the island could produce a sizeable slide from the east-facing island slopes, or trigger an underwater slide from the narrow continental shelf.   Either prospect could send a destructive tsunami racing 25 miles across the San Pedro Channel, with little time to warn and evacuate coastal residents and recreationists.  

The USGS diagrams below provide a three-dimensional perspective of the Borderland near Long Beach and Catalina Island.   Note the narrow continental shelves that drop off steeply into the San Pedro Basin.




Below is a photograph of the northeastern side of Catalina Island, which faces the southern-California mainland.   Note the steep slopes with the potential for large-scale mass wasting.   If a tsunami originated from this coast, and traveled at an average speed of 450 miles per hour, how long would it take to strike the Long Beach coast?   (Determine distance from first diagram above, then use the equation time = distance/rate.)



Below is a three-dimensional multi-beam sonar image, from Normark, et al., 2004, that clearly shows the deposit from a major submarine slide and debris flow that occurred 7,500 years ago along the continental slope off the Palos Verdes Peninsula coast.   Based on the size of the deposit and how far it traveled across the ocean floor, it is estimated that it may have caused a tsunami whose original wave height was from 25 to 160 feet high.   If a similar tsunami occurred today from the same location, would Long Beach be affected?   (Hint: review the Papua New Guinea tsunami event.)




The California Continental Borderland's history is a tale of complex plate-tectonics activity and long-term sedimentation.   The product of these processes is one of the most unusual and tectonically active ocean-floor regions on the planet.   The features of the Borderland affect ocean waves and current circulation, and add tremendous variety to the coastal ecosystems of southern California.



Atwater, T., 1998, Plate Tectonic History of Southern California with emphasis on the Western Transverse Ranges and Santa Rosa Island, in Weigand, P.W., ed., Contributions to the Geology of the Northern Channel Islands, Southern California: American Association of Petroleum Geologists, Pacific Section, MP 45, p. 1-8.

Berelson, W.M., 1991, The Flushing of Two Deep-Sea Basins, Southern California Borderland, Limnol.Oceanogr., 36(6), 1150-1166.

Fisher, M.A., Normark, W.R., Langenheim, V.E., Calvert, A.J., and Sliter, R., 2004, Marine Geology and Earthquake Hazards of the San Pedro Shelf, Southern California, U.S. Geological Survey Professional Paper 1687.

Francis, R.D., 2009, Inner Borderland research on Palos Verdes Fault and San Pedro Basin, verbal communication.

Gorsline, D.S., De Diego, T., and Nava-Sanchez, E.H., 2000, Seismically Triggered Turbidites in Small Margin Basins: Alfonso Basin, Western Gulf of California and Santa Monica Basin, California Borderland, Sedimentary Geology, v. 135, issues 1-4, pp 21-35. 

Irwin, W.P., 1990, Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault System, California: U.S. Geological Survey Professional Paper 1515.

Normark, W.R., McGann, M., and Sliter, R., 2004, Age of Palos Verdes Submarine Debris Avalanche, Southern California, Marine Geology 203, Issue 3-4,pp 247-249

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