8.8: Detailed Figure Descriptions

Figure 8.1 Location of the Basin and Range Province of California

A physiographic map of California highlights the area that corresponds to the Basin and Range Geomorphic Province. The region is situated in eastern California, to the north of the Garlock Fault, and to the west of the Sierra Nevada Range. The regions' alternating basins and uplifted range extend into Nevada.

Figure 8.1.1 Basin and Range Province

The province is in the western U.S. and northern Mexico. It extends along the Gulf of Mexico on the Baja California to the Bajia de Los Angeles and also continuing along the east side of the gulf and along the Pacific coastline to the Trans-Mexican Volcanic Belt. In the U.S., the province is bounded by the Sierra Nevada Mountains on the west, and it extends to the east as far as the Wasatch Range of Utah and the Rio Grande Rift of New Mexico. Reno, Nevada, sits close to the northwest edge of the province; Salt Lake City, Utah, is at the northeast edge. The province curves west just south of Salt Lake City. Near Flagstaff, Arizona, the province curves west along the Colorado Plateau before moving north and then dipping back down south-west into Mexico. The northern boundary of this region is the Columbia Plateau of Oregon and it extends southward into Mexico, bordering the Sierra Madre Occidental.

Video 8.2.1 Tectonic Evolution of the Western United States

1. Present (last frame of movie). At Present the Pacific Plate fills most of the northeast Pacific Ocean basin with only the small Juan de Fuca and Cocos plates remaining from the previous configuration. The Pacific plate is moving northwest past North America. It has captured some slivers of the continental edge and is carrying them northwestward toward Alaska. Thus, the present Pacific-North America plate boundary lies within the continent, along the San Andreas fault system. It is connected to other plate boundaries at three-plate triple junctions, the so-called Mendocino and Rivera triple junctions; the locations and motions of these triple junctions help determine the on-shore geology in each time and place.
2. Early Cenozoic, 38 million years ago (first frame of movie). In the early Cenozoic, 40 Million Years Ago, other oceanic plates lay between the Pacific and North American plates. They were spreading away from the Pacific plate and subducting beneath the rim of the continent.
3. East Pacific Rise Migration. Coming forward in time, the eastern edge of the Pacific plate moved steadily northeastward as new seafloor was accreted by sea floor spreading. Eventually, the spreading center itself reached the subduction zone and the intervening plate was destroyed. The Pacific plate began to break off pieces of North America and carry them along the coast, creating the San Andreas-Gulf of California plate boundary inside the continent. Triple Junction Evolution. The Mendocino triple junction migrated steadily up the coast, attached to the Mendocino fracture zone on the Pacific plate. The Rivera triple junction hovered near the southern California borderland then, about 12 million years ago, when sea-floor spreading and subduction ceased off Baja California, it jumped to its modern position at the mouth of the Gulf of California.
4. San Andreas System Evolution. The past evolution of the San Andreas fault system occurred in two stage process. First the Salinian and borderland pieces of the continent were gradually transferred to the Pacific plate, later Baja California was transferred.
5. A third stage has begun: the Sierra Nevada/Great Valley block is in the early stages of being transferred and carried away. Pacific Plate Motion. It continually moved off to the northwest.
6. Basin and Range Expansion. In the early Cenozoic, western North America was much narrower. During the late Cenozoic it expanded, forming the Basin and Range province. This occurred during the time that the Pacific plate was pulling the rim of the continent away to the northwest, perhaps causing the expansion or, more likely, just making room for it. (It was already elevated, hot and weak, ready to fall apart given the chance.)

Video 8.4.1  Basin and Range: Extension, Erosion, Sedimentation

The silent animation is labeled "Extension, deformation, and sedimentation in a basin-and-range province"

The animation begins with a simplified diagram of a section of land with a few trees on it; bands of color indicate five layers of earth. On the left and right sides of the block, arrows are shown pointing away from each other. The arrows are labeled "Extension direction." The following text is also shown:

Typical rift features include:

1. Down-dropped faults and uplifted escarpments
2. Sediment-filled valleys from erosion and streams

The arrows move away from each other and the words "Extension direction" begin to stretch on a horizontal axis. Diagonal lines begin to appear on the surface of the land and extend underneath it; the text "Extension causes fractures and faults" appears.

The lines become more defined and each one is now labeled "fault." Now cut into sections, the land begins to move. Arrows point to areas on the surface where land has risen as the sections of earth tilt; these are labeled "deformation forms escarpments."

As expansion continues, the escarpments increase in height. Wavy lines to indicate streams appear under the shear walls; areas of sediment are indicated with yellow. The words "Mountains erode; streams carry sediment" appear on screen.

As the lateral movement continues, now the blocks of land are clearly set at an angle. Valleys have formed at the base of each escarpment and lakes begin to appear. The yellow sediment is clearly thickest in the deepest part of the valleys.

This text appears on screen: "Valleys fill with sediment. Lakes form on the valley floor."

The final scene shows a series of uplifted ranges (escarpments) and basins (valleys).

This text is repeated on screen:

1. Down-dropped faults and uplifted escarpments
2. Sediment-filled valleys from erosion and streams

Figure 8.5.3 Simplified map of the geology of the Long Valley caldera

This geologic map highlights the geological units, fault lines, and other significant geologic features in eastern California, primarily focused on Long Valley and Mono Basin. The region is bounded on the west by the Sierra Nevada Range, and on the east by the White Mountains. The Long Valley Caldera is an oval depression that is approximately 30 by 15 km in dimension. The town of Mammoth Lakes is on the western rim of the Caldera, and Mammoth Mountain is on the southwestern rim.

The Sierra Nevada Fault trends north-northwest, and cuts through the western portion of the caldera. The White Mountain fault is to the east of the complex.

Different geological formations on this map include:

1. Glass Mountain Rhyolite, which is found along the northeast border of the Long Valley caldera.
2. Bishop tuff occurs in large deposits to the north and southeast of the caldera.
3. Younger rhyolites and basalts occur in the western portion of the caldera and along it’s perimeter, toward Mammoth Lakes.
4. The Mono-Inyo Craters occur in a chain that extends between the Long Valley Caldera and Mono Lake approximately 25 km to the north.

An inset map indicates that deposits of Bishop ash (tuff) can be found in most of the western US, including Nevada, Utah, Arizona, New Mexico, Colorado, and Wyoming, as well as portions of Nebraska, Kansas, Texas and the panhandle of Oklahoma, as well as a portion of southeast Idaho.

Figure 8.5.5 The Inyo-Mono Craters chain

This satellite photograph shows a chain of volcanic domes and craters extending from Mono Lake south to Mammoth Mountain. Panum Crater lies in Pumice Valley, close to Mono Lake. Southeast we see Northwest Courlee and Upper Dome, with North Coulee southeast from that area. Due south from North Coulee is Crater Mountain, where we leave the valley floor. South from Crater Mountain we see an area that extends due east labeled South Coulee. The Devil's Punchbowl is south and slightly east from that area. Due south from the Devil's Punchbowl we see a string of features that are close together and arrayed in a rough line that moves south and slightly east along the mountain range: Wilson Butte, Obsidian Dome, Glass Creek Dome, North and South Deadman Creek Domes, Deer Mountain Crater, and Inyo Crater. Long Valley lies due east from this region. The southmost feature pictured is Mammoth Mountain, but the map indicates that Red Cones is southwest from Mammoth Mountain (off image).

Figure 8.6.1 A Diagrammatic Passive Margin

This simplified diagram shows a passive margin along the western North American continent. From left to right we see the ocean depicted above a thin layer of sediment, which rests on a thicker oceanic crust above a still thicker mantle region. The middle of the diagram shows the sediments quickly increasing in thickness as the ocean layer thins. The oceanic crust gives way as the continental crust thrust upward, pushing the mantle downward. When we reach the right side of the diagram the ocean has receded and the continental crust takes up most of the vertical space. The continental crust exhibits a series of listric normal faults, shown as diagonal lines slanted in the general direction of the thrust. A very thin layer of sediment overlays the continental crust, and the mantle has thickened below it.

Figure 8.6.2 Paleogeography of the Basin and Range of California

During the Paleozoic era what is now the Western US was in the deep ocean. The Colorado Plateau region was mostly covered by the shallower Inland Sea, with the easternmost regions of land exposed (this region is indicated with red dashed lines on the map). The zone that corresponds to the Basin and Range is due West of the Colarado Plateau Region, located offshore at the edge of the continental shelf transition to deeper water. It is indicated on the map by a black-dashed triangle oriented with its longest side along what is now the California-Nevada border and its shortest side beginning in what is now Death Valley and moving southwest to the approximate center of the current state. A black circle on the map in the southeast corner of the triangle indicates Death Valley, the approximate location of Paleozoic continental shelf rocks that are now exposed. The third side of the triangle runs almost due North-South. To the northwest of Death Valley and within the triangle, a white circle indicates the approximate location of the deep water sedimentary facies represented by Paleozoic rocks in the White-Inyo Range, east of the Sierra Nevada. The region that will become the future Colorado Plateau lies to the east of this area, straddling a shallow inland sea to the west and a coastal area to the east.

Inserted into the map is a cross-sectional diagram of a generic passive margin representing a geological cross-section that is shown by a solid black line. This line projects due east from what is now the San Francisco Bay Area and moves southeast to a mountainous area in the southern Colorado Plateau Region. In this diagram, from left to right, we see water above a thin layer of sediment, which rests on a thicker oceanic crust above the mantle. The middle of the diagram shows the sediments increasing in thickness as the oceanic crust subducts and thins. It gives way to the continental crust, at roughly the point where the water recedes. The sediment layer and mantle both thin as the continental crust expands. Rift valley strata are shown along the right third of the diagram. Arrows show the approximate location of the White-Inyo and Death Valley facies. This is discussed in the chapter text..