14.8: Detailed Figure Descriptions

Figure 14.1 Location of the Transverse Ranges Province of California

This province is situated south of the Coast Ranges Province, southwest of the Mojave Province, and to the north of the Peninsular Ranges Province. Province is oriented east-west and includes the offshore Channel Islands of San Miguel, Santa Barbara, Santa Cruz, Santa Rosa and Anacapa. The shaded relief base map reveals an east west ridge of mountains that is continuous across this province. South of this ridge of mountains is a flatter geologic basin with isolated uplifted mountain ranges.

Figure 14.2.1 Geology of the Transverse Ranges Province of California

This map highlights map units found within the Transverse Ranges Province according to large ranges of geologic age and lithology. The San Andreas fault is a prominent feature, trending northwest and forming the southern boundary of the western portion of this province, including the lozenge-shaped San Bernadino Mountains. As the San Andreas Fault trends northwestward, it crosses to the northern boundary of the San Gabriel Mountains. The southern boundary of the San Gabriel Mountains is the San Gabriel Fault. Other major fault systems in this region include a number of east-west striking faults which are truncated by the southern San Gabriel Fault on the east. These faults extend offshore to the west.

Precambrian and Mesozoic metamorphic and igneous intrusive rocks occur in the eastern region, corresponding to the San Bernadino and San Gabriel Mountains. Patches of Paleozoic sedimentary rocks occur in the north-central San Bernadino Mountains, and patches of Paleozoic sedimentary rocks are found in the western San Gabriel Mountains. The western portion of the Transverse Ranges Province, including the Channel Islands, is dominated by younger (Neogene and Paleogene) sedimentary rocks and volcanics. The southern edges of the province and patches within the western part of the province include Quaternary surficial deposits.

Figure 14.3.3. Simplified cross-section through the central Ventura Basin

A cross section from north to south across the Ventura basin reveals a series of folds and faults that are typical of this region. In this example, a large syncline made of Miocene through Quaternary aged sedimentary rocks underlies the Ventura Basin. The Monterey Formation is the base layer in this synclinal structure. To the north, the San Cayetano Thrust fault pushes older rocks in the Santa Paula Range southward over this subsurface synclinal formation, causing oil bearing units in the footwall to be slightly overturned toward the south. To the south, the south-dipping Oak Ridge reverse fault pushes Upper Eocene through Pleistocene rocks northward over the syncline. Upper Eocene through Oligocene oil-bearing layers in the in the hanging wall of the Oak Ridge fault are anticlinally folded beneath an oil field. These units include oil sands, oil shales and tar sands.

Figure 14.3.9 Major structural blocks of the Los Angeles Basin

A map of the Los Angeles area southward from the San Gabriel Mountains shows major structural blocks. At the north is the northwestern block, which includes the San Fernando Valley, the Santa Monica Mountains and the Verdugo mountains and is bounded on the south by the east-west striking Santa Monica and Raymond Hill fault system. The northeastern block is located south of the San Gabriel Mountains and includes the San Jose Hills, and the San Gabriel Valley. It includes the northwest-trending Elysian Park anticline and the Repetto Hills and is bounded to the south by the northwest striking Whittier Fault. The Central Block is located to the southwest. Bounded to the northeast by the Whittier fault, this block includes most of Los Angeles and involves several northwest trending anticlines. Its southwestern edge is the northwest striking Newport-Inglewood Fault and related faults. Across this system to the southwest is the Southwestern Block. This area is bounded on the north by the Santa Monica Fault, and includes Santa Monica, The Baldwin and Dominguez Hills areas, Long Beach, and the Palos Verdes Peninsula. Notable anticlinal and synclinal structures trend northwest-southeast in this region.

Figure 14.4.1 Evolution of the San Andreas Fault system

A time series shows steps in the evolution of the San Andreas fault system.

1. At 30 Ma, the Farallon Plate is being subducted beneath the North American Plate. The Pacific Plate is offshore and is separated from the Farallon Plate via a divergent plate boundary.
2. At 20 Ma, the Farallon Plate has been subducted and broken into two smaller plates (the Juan de Fuca Plate to the north and the Cocos plate to the south) which continue to be subducted beneath North America. These plates are separated by a right lateral transform boundary which has developed between the Pacific Plate and the North American Plate. The northern triple junction between the Juan de Fuca Plate, the Pacific Plate and the North American pPate is called the Mendocino Triple Junction. The southern triple junction between the Cocos Plate, the Pacific Plate and the North American Plate is called the Rivera Triple Junction
3. At 10 Ma, the distance between the Mendocino and Rivera Triple Junctions has increased as the right lateral transform boundary between the Pacific and North American Plates has lengthened. Subduction continues to the north of the Mendocino Triple Junction and to the South of the Rivera Triple junction.
4. At 5 Ma, the distance between the Mendocino and Rivera Triple junctions has continued to increase as the right lateral transform boundary between the Pacific and North American Plates has lengthened. Subduction continues to the north of the Mendocino Triple Junction and to the South of the Rivera Triple junction.
5. At Present, the modern San Andreas fault system has developed between the Mendocino and Rivera Triple Junctions. Subduction of the Juan de Fuca Plate occurs north of the Mendocino Triple Junction, while sea floor spreading has developed to the south of the Rivera Triple Junction, in the Gulf of California.

Figure 14.4.2 Sequential diagrams showing the rotation of the Western Transverse Ranges

Four panels outline the gradual changes in the configuration of the Transverse Ranges from 20 Ma to present.

At 20 Ma, the western margin of North America consists of three different types of plate boundaries. In the north, the Juan de Fuca Plate is subducted beneath North America. At the Mendocino Triple Junction, the boundary becomes a transform style where the Pacific Plate slides northwestward along the North American Plate. This boundary changes to one of divergence at the Rivera Triple Junction where the Pacific Plate and Cocos Plate diverge. To the South, the Cocos Plate is subducted beneath North America. From the Mendocino Triple Junction southward along this boundary, the crustal blocks which will become the area (block) around San Francisco, Santa Barbara area (block) and the Channel Islands block are aligned parallel to the coastline. The future Santa Barbara area and San Diego area are at roughly the same latitude, inland from the Rivera Triple Junction, and the block that will become the future Channel Islands is south of this.

By 12 Ma, the area is now mostly the transform boundary between the North American Plate and the Pacific Plate. The elongate San Francisco block is separated from the North American Plate by several large faults that parallel plate motion. To the south, the Santa Barbara block has begun to rotate in a clockwise fashion and is approximately 45 degrees from its original position. Basins have opened to the north and south of this rotated block. The Channel Islands block remains parallel to the coast and is now bounded by a basin on its east and southern edges. The San Diego block remains inland.

At 4 Ma, the Pacific Plate continues to move to the northwest relative to North American Plate. A transform boundary that is parallel to this motion has developed inland from the San Francisco, Santa Barbara blocks, and meets a divergent boundary just north of the San Diego block. This divergent boundary is under the future Gulf of California, separating the future Baja Peninsula from the rest of North American Plate. This San Francisco block and Channel Islands blocks remain roughly parallel to the plate motion, but the Santa Barbara block has rotated almost 90 degrees clockwise relative to its initial position. A triangular basin has developed north of this block, and a large linear basin has developed south of it, separating the Channel Islands block from the San Diego block.

0 Ma shows the present configuration of the southern California Plate boundary. The modern San Andreas fault system is developed, taking a large bend to the east in the vicinity of Los Angeles, after which it meets that rifting associated with the submerged East Pacific Rise, which continues under the Gulf of California. The San Francisco block has moved to it’s present location, and the Santa Barbara block has fully rotated into it’s east-west orientation. The offshore Channel Islands block is completely submerged and remains parallel to the plate motion. Basins that opened to the north of the Santa Barbara block are now partially submerged, and those to the south are completely submerged offshore of San Diego and the modern Baja Peninsula.

Figure 14.4.3 Diagram of a restraining bend geometry similar to that of the Transverse Ranges Province

A restraining bend is formed when a right lateral transform boundary bends to the right. In this example, which is a simplification of the “Big Bend” that the San Andreas system takes in the Transverse Ranges area, a right lateral strike-slip fault is oriented roughly north-south. The fault bends to the right and then returns to its original orientation. In the region where it has bent to the right (east), the boundary is now no longer parallel to the plate motion. Because it is almost perpendicular, the plate motion is resolved into a compressional vector and a shear vector. Most of the motion is compressional however, so a series of reverse faults form in this section. These faults lead to the formation of mountain ranges that are oriented in an east-west sense, like the strike of the faults.

Figure 14.4.4 Current GPS (global positioning system) permanent station network in Southern California

A map of the Transverse Ranges including the offshore Channel Islands and the Salton Sea shows the locations of hundreds of GPS stations across this region with vectors representing the rate of movement relative to North America. The movement detected by the stations to the west of the San Andreas boundary indicates that they are uniformly moving to the northwest (with the Pacific Plate) at rates of approximately 40 mm/yr. Stations located inland from the San Andreas boundary show no movement; the abrupt transition is consistent with the location of the San Andreas fault system.

Figure 14.5.1 The 1971 San Fernando earthquake, also known as the Sylmar earthquake — Los Angeles County, Southern California

Instrumental intensity information surrounding the epicenter of the Sylmar earthquake is mapped. Intensity generally decreases with distance from the quake. Maximum intensity levels of XI or extreme in the area between Northridge and the San Gabriel Mountains were measured. South of Northridge, intensity levels of VII (very strong) were reached. Beyond this, almost the entire Los Angeles region, including Palmdale, Malibu and the areas just north of Long Beach, experienced intensity levels of VI (strong).

The Instrumental Intensity scale that is included relates Instrumental Intensity to Peak Velocity, Peak Acceleration, Potential Damage, and Perceived Shaking. This scale is listed tabulated here.

Instrumental Intensity Scale

Perceived Shaking	Not Felt	Weak	Light	Moderate	Strong	Very Strong	Severe	Violent	Extreme
Potential Damage	none	none	none	Very Light	Light	Moderate	Moderate/Heavy	Heavy	Very Heavy
Peak Acceleration (% g)	<0.17	0.17-1.4	1.4-3.9	3.9-9.2	9.2-18	18-34	34-65	66-124	>124
Peak Velocity (cm/s)	<0.1	0.1-1.1	1.1-3.4	3.4-8.1	8.1-16	16-31	31-60	60-116	>116
Instrumental Intensity	1	II-III	IV	V	VI	VII	VIII	IX	X+

Figure 14.5.2 The ShakeMap computed for the 1994 Northridge earthquake

Instrumental intensity information surrounding the epicenter of the Northridge earthquake is mapped. Intensity generally decreases with distance from the quake. Maximum intensity levels of XI or extreme in the area between Santa Clarita and Northridge were measured. South of Northridge, intensity levels of VII (very strong) were reached as far south as south Los Angeles and into Oxnard. Beyond this, almost the entire Los Angeles region, including Palmdale, Malibu and the areas just north of Long Beach, experienced intensity levels of VI (strong).

he Instrumental Intensity scale that is included relates Instrumental Intensity to Peak Velocity, Peak Acceleration, Potential Damage, and Perceived Shaking. This scale is listed tabulated here.

Instrumental Intensity Scale

Perceived Shaking	Not Felt	Weak	Light	Moderate	Strong	Very Strong	Severe	Violent	Extreme
Potential Damage	none	none	none	Very Light	Light	Moderate	Moderate/Heavy	Heavy	Very Heavy
Peak Acceleration (% g)	<0.17	0.17-1.4	1.4-3.9	3.9-9.2	9.2-18	18-34	34-65	66-124	>124
Peak Velocity (cm/s)	<0.1	0.1-1.1	1.1-3.4	3.4-8.1	8.1-16	16-31	31-60	60-116	>116
Instrumental Intensity	1	II-III	IV	V	VI	VII	VIII	IX	X+