11.9: Detailed Figure Descriptions

Figure 11.1 Location of the Coast Ranges Province of California

The Coast Ranges province spans 400 miles, from Humboldt south to Santa Barbara. The province has two distinct regions. The Northern Coast Ranges run parallel to the coast from the Oregon border south to the northern San Francisco Bay Area. The Southern Coast Ranges run parallel along the coast from the eastern San Francisco Bay Area south to Santa Barbara.

Figure 11.1.1 Geology of the Coast Ranges

This geologic map of the California Coast Ranges depicts various rock units, faults, and geologic time periods. A legend on the right explains the different colors representing distinct rock types and ages. A small inset map in the bottom left shows California with a black rectangle highlighting the Coast Ranges region.

• The main map displays the Coast Ranges with different colors scattered throughout, corresponding to geologic units, indicating rock types and ages.
• Fault lines occur throughout the region and are represented by dashed lines for subsurface faults and solid lines for surface faults.
• The map extends from northern to southern California, showing major geological features and formations.

The legend categorizes the geologic units by age and rock type:

Cenozoic (youngest rocks)

• Quaternary surficial deposits
  • Along the immediate coastline of California
  • In river valleys and floodplains, including:
    • Salinas Valley
    • Russian River valley
    • Santa Clara Valley
  • In coastal lowlands and alluvial basins
  • Especially common near San Francisco Bay, Monterey Bay, and southern coastal plains
• Quaternary and Neogene non-marine deposits
  • Interior valleys of the Coast Ranges
  • Foothills east of major coastal mountains
  • Parts of:
    • Salinas Valley margins
    • Southern Coast Ranges near Santa Barbara and Ventura
• Cenozoic volcanic rocks
  • Northern Coast Ranges, especially:
    • Clear Lake volcanic field
    • North of San Francisco Bay
  • Small, scattered patches elsewhere in the northern map area
• Neogene and Paleogene sedimentary rocks
  • Widespread throughout the central and southern Coast Ranges
  • Large belts:
    • West of the San Andreas Fault
    • Between coastal areas and interior valleys
  • Prominent in:
    • Santa Cruz Mountains
    • Central Coast near Big Sur
    • Southern Coast Ranges north of Los Angeles Basin

Mesozoic

• Cretaceous sedimentary rocks
  • Scattered belts mainly in the central Coast Ranges
  • Often adjacent to Franciscan Complex rocks
  • Common in fault-bounded slivers near major faults
• Franciscan Complex (sedimentary and metamorphic rocks)
  • Extensive belt along the western side of the Coast Ranges
  • Dominant unit in:
    • Northern Coast Ranges
    • West of the San Andreas Fault
  • Continues discontinuously from northern California southward
• Granitic igneous rocks (mainly Mesozoic age)
  • Limited and isolated bodies within the Coast Ranges
  • Most visible in:
    • Southern Coast Ranges
    • Near Santa Barbara and Ventura areas
  • Much less extensive than in the Sierra Nevada (shown only in inset context)
• Serpentinite and other ultramafic rocks
  • Narrow, linear bodies scattered throughout the Coast Ranges
  • Commonly:
    • Along major fault zones
    • Near the San Andreas Fault system
  • Especially visible in central California
• Mesozoic metamorphic and sedimentary rocks
  • Smaller belts interspersed among Franciscan and Cretaceous units
  • Mostly in the central and southern Coast Ranges
  • Often fault-bounded

Paleozoic

• Paleozoic sedimentary and metamorphic rocks
  • Very limited exposures
  • Mostly in the southern Coast Ranges
  • Appear as small, isolated patches

Precambian (oldest rocks)

• Pre-Cambrian rocks
  • Extremely rare and very small exposures
  • Isolated locations in the southernmost Coast Ranges

Figure 11.2.1 Cross section of the Mesozoic active margin of California

This diagram illustrates a cross-section of the Coast Range Ophiolite and Franciscan Complex during the Mesozoic, highlighting major geologic features, faulting, and the relationship between tectonic units. It shows the subducting oceanic slab, various belts of the Franciscan Complex, and the overlying Great Valley Group.

Key Features:

• Subducting Oceanic Slab: Located at the base of the diagram, this region represents the oceanic plate sinking beneath North America.
• Franciscan Complex Belts (just above the Oceanic Slab):
  • Coastal Belt (lowest layer): A highly deformed zone above the subducting slab, consisting of mélange (a chaotic mix of rock types).
  • Central Belt Mélange (next layer): Another deformed rock unit, distinguished by its blocky, jumbled texture.
  • Eastern Belt (highest layer in this complex): Subdivided into the Pickett Peak and Yolla Bolly terranes, these rocks were deeply buried and metamorphosed before being uplifted.
• Coast Range Ophiolite & Tehama-Colusa Mélange (next layer): A section of ancient oceanic crust that was scraped off the subducting slab and now forms part of the Coast Ranges.
• Great Valley Group (highest layerr): Overlies the Coast Range Ophiolite and represents sediments deposited in the ancient forearc basin.
• Coast Range Fault: A significant fault separating the Great Valley Group from the Franciscan Complex, marking a major tectonic boundary.

This cross-section helps illustrate how subduction and accretion have shaped the Coast Ranges over millions of years, with different belts representing materials scraped from the oceanic plate and stacked onto North America.

Figure 11.2.2 Location of the Coast Range Ophiolite

This figure shows the location of the Coast Range ophiolite. The Coast Range ophiolite is located in a narrow north-south trending band along the eastern edge of the northern Coast Ranges, where the Coast Ranges meet the Central Valley (it is not present where the Coast Ranges directly abut the Klamath Mountains). The Great Valley Group parallels this band to the east. There are fragments of Coast Range Ophiolite in some areas west of the main band, as well as fragments in both the eastern and western portions of the southern Coast Ranges.

The map divides California into different geologic or tectonic origins.

• Accreted terranes (western and northwestern California)
  • In the northwestern corner, large areas are labeled “Accreted terranes.”
  • These represent crustal fragments that formed elsewhere and were later attached to North America.
  • The Klamath terranes appear in the far north, shown with a distinct line pattern.
  • These units dominate the geology near the Oregon border and the far northern Coast Ranges.
• Franciscan accretionary complex
  • This complex forms a long, narrow belt along much of coastal California and is subdivided into three belts: Eastern belt, Central belt, and Coastal belt
  • These belts run roughly northwest–southeast, parallel to the coastline, and represent rocks scraped off a subducting oceanic plate during long-term subduction along the continental margin.
• Coast Range ophiolite
  • The Coast Range ophiolite occurs in narrow, discontinuous slices.
  • These rocks represent ancient oceanic crust and upper mantle that were emplaced onto the continent.
  • They commonly lie between the Franciscan Complex and the Great Valley Group.
• Great Valley Group
  • The Great Valley Group forms a long, continuous band through central California.
  • This unit represents thick marine sedimentary rocks deposited in a forearc basin.
  • The map explicitly labels this region as the “Forearc basin.”
  • It lies between the Franciscan rocks to the west and the Sierra Nevada to the east.
• Sierra Nevada
  • The Sierra Nevada is shown on the eastern side of the map with a light, patterned fill.
  • This region represents a large continental magmatic arc, formed by long-lived subduction-related volcanism and plutonism.
  • It forms the eastern boundary of the Great Valley basin.
• Salinia block
  • The Salinia terrane appears as a long, narrow block along the central California coast, shown with a cross pattern.
  • It is offset from the Sierra Nevada and interpreted as a displaced fragment of continental crust.
  • The block is bounded by major faults and contrasts sharply with surrounding accretionary rocks.
• Modoc Plateau
  • In the northeastern corner, the Modoc Plateau is shown with a dark patterned fill.
  • This region represents volcanic and extensional geology, distinct from the accretionary and subduction-related rocks to the west.
• Fault systems
  • The San Andreas Fault (SAF) is prominently marked as a thick black line running northwest–southeast through coastal and central California.
  • It separates the Pacific Plate (to the west) from the North American Plate (to the east).
  • Other fault-related labels appear, including SF (San Francisco area) and additional structural abbreviations marking fault-bounded regions.

Figure 11.2.4 Ophiolite sequence

Marine sediment and rocks are underlain by pillow basalts in relatively narrow bands. Below, sheeted dikes and sills (diabase) are underlain by a deep layer of gabbro. Just below this, we reach the earth's crust, with gabbro and ultramafic cumulates. Below this lie the ultramafic rocks of the mantle. This entire sequence can reach up to 6 miles in depth.

Figure 11.3.1 The Great Valley Sequence

The Farallon Plate subducts beneath the North American Plate, forming key geologic features of California: the Coast Range, Great Valley, and Sierra Nevada.

Key Features and Processes

• Mid-Ocean Ridge: Located on the left, where new oceanic crust forms as the Pacific Plate and Farallon Plate move apart.
• Subduction Zone: The Farallon Plate descends beneath the North American Plate. As it sinks, water is released, causing melting in the mantle.
• Geologic Provinces:
  • Coast Range: Formed from uplifted ocean sediments scraped off the subducting plate in an accretionary wedge.
  • Great Valley: A low-lying region, the forearc basin, filled with sediments eroded from nearby mountains.
  • Sierra Nevada: A chain of volcanoes, or volcanic arc, formed from rising magma produced by melting of the subducting plate. Cooled magma beneath the volcanic arc form granite rocks, which later uplift to form the Sierra Nevada batholith.

Tectonic Motion

• The Pacific Plate and Farallon Plate move apart at the Mid-Ocean Ridge.
• The Farallon Plate subducts beneath North America, driving volcanic activity and mountain-building processes.

Figure 11.4.2 Cross section of the West Coast of North America at about 100 Ma

As the ocean crust and lithosphere thrust under the North American plate, pillow basalts, chert, and limestone-capped seamounts were scraped off and mixed with graywacke sandstone and shale shed from the continent, to create the Franciscan. In addition, as the lithosphere continued to move beneath the North American plate, serpentinite formed at the upper mantle. All of these elements pushed up against the Sierra Arc, which has a core of granite.

Accessibility Guidance

The process illustrated in this diagram is described within the text. However, the process is difficult to understand without a diagram. Some users may therefore benefit from the tactile image, Continental Subduction Zone, which includes an accretionary wedge.

Figure 11.5.1 The tectonic setting of the west coast of North America in the Mesozoic

The Farallon plate sat between the continental North American plate to the east, and the Pacific plate to the west. The Farallon and Pacific plates are labeled "oceanic lithosphere" on the map, while the North American plate is labeled "continental lithosphere." The boundary between the Pacific and Farallon plates was divergent (a spreading center). The boundary between the Farallon and the North American place was convergent (a subduction zone).

Figure 11.5.3  Map of the San Andreas Fault showing locked and creeping sections

The San Andreas Fault runs along the coastline north of San Francisco and moves inland just north of Monterey Bay. It then proceeds in a southeast direction, terminating at the Salton Sea. As discussed in the text, the northern and southern sections of the fault are locked. Between the locked segments, from San Juan Bautista to Parkfield, the fault is creeping. Key landmarks shown along the fault are San Francisco in the northern segment and Los Angeles in the southern segment. The San Andreas Fault Observatory at Depth (SAFOD), a USGS project, is located in the creeping zone, near Parkfield.

Figure 11.5.4 UC Berkeley Memorial Stadium and the Hayward Fault (Aerial and LiDAR Views)

• This figure shows two views of the UC Berkeley Memorial Stadium and the surrounding neighborhood. One view is an aerial photograph, and the other is a LiDAR image that highlights differences in ground elevation. In both images, the path of the Hayward Fault is indicated, with a solid line marking the portion of the fault whose position is well-established and a dashed line marking where the fault trace is inferred.

The stadium was constructed directly across the Hayward Fault. The fault runs lengthwise through the seating bowl and divides the stadium into two structural halves, with each half located on opposite sides of the fault. Because the Hayward Fault here experiences primarily aseismic creep, the two sides of the ground slowly slide past one another over time. As a result, the eastern and western halves of the stadium are gradually shifted out of alignment. This ongoing movement causes cracks, foundation separation, and deformation in stadium walls and seating sections.

In the surrounding neighborhood, the same slow fault movement also displaces streets, retaining walls, and building foundations. The LiDAR image highlights subtle offset landforms that are not easily visible in regular photographs.

Figure 11.5.7 Schematic map of Northern California

This schematic map of northern California illustrates several volcanic fields, major faults, and the changing position of the Mendocino Triple Junction over the past 12 million years.

The volcanic fields are shown as separate geographic clusters, each labeled with its name and its approximate age range. From southeast to northwest, the volcanic fields shown are:

• Quien Sabe Volcanic Field (~12 Ma),
• Berkeley Hills Volcanic Field (~10 Ma)
• Sonoma Volcanic Field (~7-10 Ma)
• Clear Lake Volcanic Field (4-8 Ma)

The map also shows several major right-lateral strike-slip faults that are part of the San Andreas Fault system. The volcanic fields get progressively younger in the direction of the migrating Mendocino Triple Junction, which is marked at several past positions spaced along the coastline.

The displacement of volcanic units along the fault system indicates that parts of these volcanic fields have been offset by tens to hundreds of kilometers over time. The figure illustrates the link between plate boundary evolution, fault motion, and volcanic activity in the Coast Ranges during the Cenozoic.

Figure 11.6.1  Satellite image of San Francisco Bay Area

This satellite image of the San Francisco Bay Area highlights the region's major faults, mountain ranges, and paleoshorelines—past positions of the coastline after the Last Glacial Maximum (LGM). The image shows how significantly sea level has risen over the past 18,000 years and contextualizes California’s dynamic coastline and tectonic setting.

Geographic Features

• The San Francisco Bay Area is shown in high-resolution satellite imagery, including the cities around the San Francisco Bay, Diablo Range to the northeast, and Santa Cruz Mountains to the southwest.
• The Pacific Ocean lies to the west, and the San Francisco Bay is central in the image.
• Farallon Islands are labeled offshore to the west, and were accessible on foot between 11,500 and 14,000 years ago
• The San Andreas Fault runs offshore parallel to the coast.
• The Hayward Fault runs through the East Bay hills near Oakland and Berkeley.

Paleoshorelines

• The map includes labeled shoreline positions representing times when sea levels were lower due to glacial ice storage:
  • ~18,000 years ago (farthest west)
  • ~14,000 years ago
  • ~11,500 years ago
  • ~10,500 years ago
  • ~8,000 years ago (closest to the modern shoreline)
• These zones are represented as nested, semi-transparent colored bands extending progressively farther offshore with increasing age.