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 14.1.1 Digital elevation model of the Transverse Ranges Province

This image shows a shaded relief map of Southern California highlighting the Los Angeles region, illustrating how basins and surrounding mountain ranges reflect active tectonics and faulting, with a white oval marking the area of focus. The label “Los Angeles” appears near the center. The Pacific Ocean is shown to the left, clearly separating land and water. Within the outlined area, low, smooth regions represent sediment-filled basins, while raised, textured areas represent mountain ranges, including the Transverse and Peninsular Ranges, which trend in different directions. The contrast between flat basins and uplifted mountains reflects ongoing crustal movement, where deformation along faults creates both elevation and space for sediment accumulation.

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 Bernardino 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 Bernardino and San Gabriel Mountains. Patches of Paleozoic sedimentary rocks occur in the north-central San Bernardino 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 14.2.2: Foliated Mendenhall Gneiss

This image shows a boulder of foliated Mendenhall gneiss, about 33 cm (13 inches) long, resting on soil with small rocks and nearby vegetation, illustrating how foliation and folded mineral layers record metamorphic processes and tectonic deformation deep within the Earth. The rock displays alternating black and white mineral layers arranged in wavy, parallel bands that curve across the surface and wrap around the edges. This layered pattern, called foliation, is part of the rock’s internal structure. The banding and folding indicate the rock formed under high heat and pressure and was later deformed by tectonic forces, such as compression during mountain building.

Figure 14.2.5: Evolved Magma

This image shows how magma evolves during cooling through fractional crystallization, where crystal formation and settling change composition and produce more felsic, evolved magma over time. Arrows indicate progression from left to right, and a bottom label reads “crystallization and cooling temperature.” In the left panel, the magma is labeled “parental magma” and described as “homogeneous magma,” with no visible crystals. In the middle panel, “crystals of high temperature minerals form,” shown as small shapes distributed throughout the magma. In the right panel, labeled “evolved magma,” text states “evolved magma is more felsic than parental magma.” Crystals are concentrated at the bottom, with arrows pointing downward, and a caption reads “early-formed crystals form cumulate at bottom of magma chamber.” Across the diagram, crystal position, arrows, and labels show that as magma cools, early-formed minerals crystallize and settle out, leaving the remaining melt more silica-rich.

Figure 14.3.2 Principal late Tertiary to Quaternary depositional basins and upland areas

The map labels three basins: Ventura Basin (to the east), Soledad Basin (to the west), and Los Angeles Basin (to the south). Mountain ranges labeled include the Topatopa Mountains (far northwest), Santa Susana Mountains (northwest), San Gabriel Mountains (northeast), Verdugo Mountains (west), Santa Monica Mountains (southwest), and the Simi Hills (between the Santa Susana and Santa Monica mountains). Additional labeled features include Oak Ridge (between the Topatopa, Santa Susana and Santa Monica mountains), and the San Fernando Valley (middle of the map).

A labeled fault, the San Andreas Fault, appears near the upper right edge. Rivers and water bodies shown include the Santa Clara River, the Los Angeles River, Lake Piru, and Castaic Lake.

Cities and towns labeled on the map include Fillmore, Acton, Burbank, Pasadena, Los Angeles, Santa Monica, and Malibu. County labels include Los Angeles Co and Ventura Co. Major highways shown include Interstate 5, Interstate 405, Interstate 210, Interstate 10, and U.S. Route 101.

The coastline is shown along the lower edge of the map, with the offshore area labeled Continental Borderland. Map reference elements include a north arrow, scale bars for kilometers and miles, and latitude and longitude labels along the border.

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 and that include oil sands, oil shales, and tar sands. 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, beneath an oil field directly to the south of the Santa Paula Ridge.

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 to the north of South Mountain.

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.

Inset Figure 14.4.1 Location map of the San Bernardino Mountains

This is a topographic map showing the San Bernardino Mountains which range in elevation from 218 m to 3504 m. Basins sampled include 7 near the central basin, 7 further south, and 4 further northeast.

Inset Figure 14.4.2 A north-south transect through the San Bernardino Mountains

This is a topographic map showing the long term denudation and cosmogenic nuclide denudation. The table below lists these from north to south.

Denudation rates

Feature	Average long-term denudation rate (mm/ka)	Average cosmogenic nuclide denudation rate (mm/ka)
Northern margin of plateau	70 ± 40	95 ± 10
Plateau	50 ± 30	100 ± 20
Southern margin of plateau	180 ± 90	670 ± 170
Ridge near San Andreas Fault Zone	1200 ± 400	1500 ± 290

Inset Figure 14.4.3 Denudation rates for the basins

The graph is a scatter plot showing average basin denudation rate (mm·ka⁻¹) versus average basin hillslope gradient (degrees). At low gradients (about 8–22°), denudation rates are low, generally below 200 mm·ka⁻¹. As hillslope gradients increase above ~25–30°, denudation rates rise sharply to values between about 1000 and 3000 mm·ka⁻¹, with larger uncertainty. The x-axis is divided into sub-threshold topography at lower gradients and threshold topography at higher gradients. The table below is of the data on the graph.

Denudation Rates

Avg. basin hillslope gradient (°)	Avg. basin denudation rate (mm·ka⁻¹)	Approx. uncertainty (± mm·ka⁻¹)
9	50	30
10	70	40
14	90	40
16	120	40
18	110	40
19	100	30
21	150	40
24	230	50
26	520	70
27	230	50
28	650	80
29	300	60
30	150	40
31	1200	200
31	1600	300
31	2200	500
36	2700	500
37	1100	200
37	1250	200
38	1200	200

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+

Figure 14.5.3: Northridge Event

This image is a cross-section beneath the San Fernando Valley comparing the 1971 San Fernando and 1994 Northridge earthquakes, illustrating how fault geometry, main shock locations, and aftershock distributions are used to map buried thrust faults and understand where rupture begins and stops at depth. This diagram showcases how main shock locations and aftershock patterns provide critical evidence for mapping hidden faults, helping explain how large earthquakes occur on buried thrust systems beneath the Los Angeles region.

The profile is oriented with north to the right, and surface labels include San Fernando Valley, Santa Susana Mountains, and Santa Clara Valley. The horizontal axis is labeled “Horizontal Distance from 1994 Main Shock (kilometers)”, and the vertical axis is labeled “Depth (kilometers)”, increasing downward from near 0 at the surface to about 20 kilometers depth. Two main earthquakes are marked with star symbols:

• A red star labeled “1994 Main Shock” located deeper on the left side, near the base of the pink fault plane
• A blue star labeled “1971 Main Shock” located on the right side at a slightly shallower depth, near the base of the teal fault plane

Colored lines represent modeled fault planes:

• A pink line shows the 1994 Northridge fault, dipping downward toward the south and not reaching the surface
• A teal line shows the 1971 San Fernando fault, dipping in the opposite direction toward the north

Hundreds of small plotted points represent aftershocks:

• Red circles cluster around the pink fault plane, marking aftershocks associated with the 1994 Northridge earthquake
• Blue circles cluster around the teal fault plane, marking aftershocks associated with the 1971 San Fernando earthquake

These aftershock clusters form elongated patterns that trace the shape and extent of each fault surface at depth. The two fault planes approach each other but remain separate. The geometry and distribution of aftershocks suggest that the Northridge rupture terminated about 5 kilometers below the surface, likely where it encountered structural barriers related to the 1971 fault system. The placement of the star symbols at the lower ends of each fault plane indicates the likely starting points, or hypocenters, of rupture, while the spread of dots shows how energy propagated along the faults.

Figure 14.6.2 Wilmington Oil Field within the Los Angeles Basin

The image is a map of the Los Angeles–Long Beach coastal region showing the locations of several oil fields in relation to cities, the coastline, and offshore areas. The Torrance Oil Field is located near the coast west of Long Beach, inland from the Pacific Ocean and south of central Los Angeles. The Dominguez Oil Field lies north of Long Beach, inland and south of Los Angeles. The Long Beach Oil Field is situated within and just north of the city of Long Beach. The Wilmington Oil Field occupies a large area south of Long Beach, extending from inland areas near the city southward to the coastline and continuing offshore. Southeast of Long Beach, near the coast, is the Seal Beach Oil Field. Farther offshore to the southeast, beyond the coastline, is the Belmont Offshore Oil Field, located entirely in ocean waters.