11.9: Detailed Figure Descriptions

Figure 11.1.1 Geologic Map of the Coast Ranges

The image is a detailed geologic map of the California Coast Ranges, depicting 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 corresponding to geologic units, indicating rock types and ages.
• Fault lines 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, listing:

• Cenozoic (youngest rocks): Quaternary surficial deposits, Quaternary and Neogene non-marine deposits, and Cenozoic volcanic rocks.
• Mesozoic: Cretaceous sedimentary rocks, Franciscan Complex sedimentary and metamorphic rocks, granitic igneous rocks, and Mesozoic metamorphic and sedimentary rocks.
• Paleozoic: Paleozoic sedimentary and metamorphic rocks.
• Precambrian (oldest rocks): Pre-Cambrian rocks.
• Other notable rock types: Serpentinite and ultramafic rocks.

The right side of the image includes a timeline-style label marking the different geologic eras (Cenozoic, Mesozoic, Paleozoic, Precambrian), aligning with the rock units.

Figure 11.2.2

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.

Figure 11. Tectonic Cross-Section of California's Mesozoic Active Margin

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 gray-shaded region represents the oceanic plate sinking beneath North America.
• Coast Range Ophiolite & Tehama-Colusa Mélange (Pink): A section of ancient oceanic crust that was scraped off the subducting slab and now forms part of the Coast Ranges.
• Franciscan Complex Belts:
  • Coastal Belt (Orange): A highly deformed zone above the subducting slab, consisting of mélange (a chaotic mix of rock types).
  • Central Belt Mélange (Yellow): Another deformed rock unit, distinguished by its blocky, jumbled texture.
  • Eastern Belt (Blue): Subdivided into the Pickett Peak and Yolla Bolly terranes, these rocks were deeply buried and metamorphosed before being uplifted.
• Great Valley Group (Green): 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 Map of the Franciscan Complex and Coast Range Ophiolite

This generalized geologic map of California highlights the locations of the Franciscan Complex, Coast Range Ophiolite, and surrounding geologic provinces. It highlights major fault systems, tectonic belts, and accreted terranes. The map spans from Eureka (41°N) to the Santa Barbara (34°N).

Key Geologic Features:

• Franciscan Accretionary Complex:
  • Eastern Belt (Blue): Highly metamorphosed rocks, primarily from subducted oceanic crust.
  • Central Belt (Yellow): A mélange of various rock types formed through intense deformation.
  • Coastal Belt (Orange): The least metamorphosed section, containing younger accreted materials.
• Coast Range Ophiolite (Pink): Represents remnants of ancient oceanic crust and mantle, now uplifted and exposed along the Coast Ranges.
• Great Valley Group (Green): Thick sedimentary deposits that accumulated in the forearc basin east of the Franciscan Complex.
• Major Fault Systems (heavy black lines):
  • San Andreas Fault (SAF): A major transform fault running through central and coastal California.
  • Sur-Nacimiento Fault (SNF): A secondary transform fault paralleling the San Andreas separating the Salinian block (Salinia from Cuesta Ridge, Point Sal, and Santa Barabara).

Figure 11.3.1 Ancient Subduction Zone 

This diagram illustrates the subduction of the Farallon Plate beneath the North American Plate, forming key geologic features of California. It shows the Mid-Ocean Ridge, accretionary wedge, forearc basin, and volcanic arc, depicting the processes that shaped 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 is descending beneath the North American Plate. As it sinks, water is released, causing melting in the mantle (blue droplets in the diagram).
• Geologic Provinces:
  • Coast Range (Accretionary Wedge): Formed from uplifted ocean sediments scraped off the subducting plate.
  • Great Valley (Forearc Basin): A low-lying region filled with sediments eroded from nearby mountains.
  • Sierra Nevada (Volcanic Arc): A chain of volcanoes formed from rising magma (red plume) produced by melting of the subducting plate.
    • Granite Rocks: Cooled magma beneath the volcanic arc, which later uplifted to form the Sierra Nevada batholith.

Tectonic Motion indicated by arrows:

• 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.

11.6.1 Shoreline Change Since the Last Glacial Maximum

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:

• Land Areas: 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.
• Water Bodies: 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, which were once accessible on foot during lower sea levels.
• Red lines mark fault zones:
  • San Andreas Fault: Runs offshore parallel to the coast.
  • 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 (dark purple, 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.