10.7: Detailed Figure Descriptions

Figure 10.1 Province Map

The Klamath Mountains province is a mountainous province near the northwest corner of California. The province continues into Oregon though this is not shown on this California map. The Province is bounded by the Cascade Range to the east and the Coast Range to the west. It also shares a small border with the Great Valley to the southeast.

Figure 10.1.1 The process of accretion at a subduction zone

This diagram illustrates the accretion of an island arc. The relative motion between the continental crust and oceanic crust is convergent.

At time 1 (A) in the diagram, an island arc sits offshore of a continent and two subduction zones are present. These are labeled "subducting slabs" in the diagram. Oceanic crust between the island arc and the continent subducts beneath the continent, while crust on the other side of the island arc subducts beneath the island arc. This process forms two parallel subduction zones. Ocean sediments have accumulated on top of the oceanic crust.

At time 2 (B), the island arc has moved closer to the continent and an accretionary wedge composed of oceanic sediments has developed along the edge of the continent.

At time 3 (C), the island arc has reached the continent and accreted to it. The two subduction zones have merged into one and the former accretionary wedge sits between the main continent and the accreted arc, both of which are now part of the continent.

Figure 10.1.2 Terrane map of the Klamath mountains

The geology of the Klamath Mountains and the Coast Ranges in Oregon and northern California. Geology of the surrounding area is simply labeled as “Mesozoic and Tertiary sedimentary rock, postdates accretion of Klamath Terranes," which is included for geographic context. From west to east, the terranes of the Klamath mountains are F (pale green): Mesozoic rock of Coast Ranges; mostly Franciscan Fm.”; Wk (teal): “Western Klamath Terrane, mostly Jurassic”; WPz (bright green): “Western Paleozoic and Triassic Terrane”; CM (orange): “Central Metamorphic Terrane (Devonian)”; and EK (brick red): “Eastern Klamath Terrane (Early Paleozoic to Jurassic)”. Other rocks on this map include: C (tan): “Condrey Mountain Schist (Mesozoic),” which is surrounded by WPz and is located near the Oregon/California border; Jurassic Plutons (Jp; pink), which are scattered throughout all terranes except F and Kh: “Cretaceous Hornbrook Formation”, which exists along the eastern edge of the Klamath Mountains, near the Oregon/California border. Redwoods National and State Parks and Oregon Caves National Monument are shown for reference as well as the cities of Eureka, Crescent City, Hiouchi, Yreka and Redding in California, and Roseburg and Medford in Oregon.

Query 10.1.2 Mariana and Andean Style Subduction

This activity requires the identification of images and therefore the full activity may not be appropriate for all learners. To receive feedback for the text-based portion of the activity only, press the “check” button after sorting the first two items.

The Geological Tactile Image Repository contains a diagram illustrating Andean type subduction, called, “Continental Subduction Zone”. The “Oceanic Subduction Zone” graphic may also be helpful in explaining Mariana type subduction, although it does not include a back-arc spreading ridge.

Figure 10.1.3 Map illustrating the correlation between terranes of the Sierra Nevada, Klamath and Blue Mountains

This Figure is included for the purpose of showing the correlation between the geology of the Klamath Mountains with the Sierra Nevada and the Blue Mountains. Although the Klamath mountains are west of both the Sierra Nevada and the Blue Mountains (in northeast Oregon), the sequence of rocks occurs roughly in the same order, with a general westward younging trend. Due to the western location of the Klamath Range, a significant curve is required in the dotted lines that connect the geologic units.

Figure 10.2.1 Steps in the development of coral reefs

Development of a Coral Reef, in four stages:

1. A volcanic island with no coral reef.
2. A fringing reef develops.
3. After the volcano continues to subside, the fringing reef grows to a barrier reef and a lagoon separates the main island from the reef.
4. The original volcanic island has subsided below sea level, but the barrier reef continues to grow upwards, forming a ring that surrounds a lagoon.

Figure 10.2.3 Diorama of a Permian seafloor

The Permian community of organisms shown in the diorama occupy similar niches to modern seafloor organisms, but the individual species are quite different from those in the same niches today. Shown here are rugose corals, algae, gastropods, richthofenid brachiopods, and spiriferid brachiopods. The rugose corals featured in the center of the photograph are most relevant to this text. Large, radial, bright orange coral polyps emerge from a horn-shaped skeleton. Unlike modern corals, each individual skeleton is isolated and the polyps are much larger, like a small anemone.

Figure 10.3.2 Island arc with a back arc spreading center

The volcanic arc is divided into five zones. The subducting oceanic lithosphere dives beneath the continental lithosphere at the boundary between the ocean basin and the forearc. Inland from the forearc, the volcanic front sits above the zone where flux melting occurs as a result of subduction. Inland from here is the backarc, where a back arc convection cell in the asthenosphere produces a back arc spreading center in the lithosphere. This area is characterized by rising magma pushing upward through a parting in the crust. The oceanic lithosphere produced by this back arc spreading center is the ophiolite suite, which forms the back arc ocean basin.

Figure 10.3.4 Sea floor spreading at a mid-ocean ridge

This geologic cross-section illustrates how new oceanic crust forms at a mid-ocean ridge. It depicts the layers of crust and mantle beneath the seafloor, the upward movement of magma, and the structure of the oceanic lithosphere. The image shows depth from the seafloor down to about 9 kilometers and a horizontal distance of about 20 kilometers across the ridge axis. (A tactile graphic, Mid Ocean Ridge, is available from the Geological Tactile Image Repository.)

Key Features (Top to Bottom)

• Seafloor Topography
  • A rift valley sits at the center, labeled “Plate boundary,” where the seafloor is pulled apart.
  • On either side of the rift, layers of oceanic crust are shown, dipping away from the ridge axis.
• Oceanic Crust Layers
  • Pillow basalts: Green, lumpy structures near the seafloor, representing lava erupted underwater.
  • Sheeted dikes: Vertical dark bands that ferd lava to the surface.
  • Gabbro: Coarse-grained intrusive rock at deeper levels of the crust, crystallized from slowly cooling magma.
• Transition Zone between the crust and the underlying mantle
• Magma Chamber
  • Magma sits beneath the rift, feeding material into the crust above.
  • Wavy lines indicate the upward flow of magma toward the surface.
• Mantle
  • Beneath the crust and transition zone lies the mantle, which serves as the source of magma.

Scale and Orientation

• The vertical axis is labeled “Depth below sea surface (km)” and ranges from 0 to 9 km.
• The horizontal axis is labeled in kilometers, spanning from -10 to +10 km, centered on the ridge axis.
• The central black dashed line marks the plate boundary, with diverging arrows at the plate boundary indicating seafloor spreading.

Figure 10.3.12 Smith River X Post

A post by Redwood National and State Parks (@RedwoodNPS). The post reads, “‘I woke up like this #nofilter’ - the Smith River; The Smith River is the only undammed river in California, boasting an impressive title of one of the cleanest rivers in the lower 48. The spectacular turquoise color comes from the serpentinite rocks at the bottom.”

Figure 10.5.2 LaGrange Fault Map

The La Grange Fault is located along the south of the Klamath Mountains Province. The trace of the La Grange Fault is long and circuitous, running from southwest to northeast and closely following the topography on account of the low angle of dip. A large pocket of Weaverville Formation lies adjacent to the La Grange Fault along its southwest stretch. The Weaverville Formation is bounded on the south by the Democrat Gulch Fault, which extends southeast from the La Grange Fault. The Hayfork Basin lies to the southwest. Remnants of the Great Valley Sequence are scattered throughout this area. Due north of the La Grange Fault lies the Browns Meadow Fault, which extends in a line oriented northwest to southeast.