ContentsPutah and Cache: Entering

Previous piece More like this

Next piece

Next chapter

 

Geology of Putah-Cache: Introduction

Eldridge M. Moores and Judith E. Moores

Plate Boundaries

The Putah-Cache bioregion lies within the southeastern part of the northern California Coast Ranges. The California Coast Ranges are internationally famous for spectacular examples of rocks that were deposited and/or modified during ancient and continuing plate boundary activities along the West Coast of the U.S. The Putah-Cache rocks are part of this world-class plate tectonic exhibit.

The theory of Plate Tectonics holds that three kinds of boundaries (also called margins) exist between plates: Divergent Boundaries, where plates are moving apart and new ocean crust and mantle is formed by upwelling of new material from the interior; Convergent Boundaries, also called Subduction Zones, where two plates approach one another, and one descends beneath the other (as this happens, some material melts and rises to form granitic bodies, as in the Sierra Nevada, or volcanoes such as Mt. Lassen, Mt. Pinatubo, or Mt. Shasta); and Transform Fault or Conservative Boundaries, where two plates slide past each other without convergence or divergence. As one plate descends beneath another along a subduction zone, material is scraped off its upper part and attached to the over-riding plate. As the plate descends, however, it begins to melt, and molten rock rises to form granitic rocks in the crust and volcanoes at the surface of an island arc, such as Mt. Pinatubo in the Philippines, or a continental arc, such as Mts. Shasta, Lassen, and St. Helens.

At the present time, the plate boundary along western California south of Cape Mendocino is a Transform Fault boundary. This boundary is not a single line but a band of faults including the San Andreas and related faults which lie from west of the Golden Gate (San Gregorio-Hosgri Fault) to the Putah-Cache bioregion (e.g. the Wragg Canyon and Bartlett Springs faults) (see figure 1). Along these faults the Pacific plate (including Los Angeles and Santa Cruz) is moving northwest relative to the North American plate. In addition to these two major plates, there is also a small recently recognized microplate, the Great Valley-Sierra Nevada microplate on which we live (see figure 2). This plate is moving slowly NW with respect to North America, but less rapidly than the Pacific plate. Its separation from the North American plate seems to be a quite recent geologic phenomenon, 1-3 million years ago (Ma) or so.

The geologic record shows, however, that the Pacific/North American plate boundary has not always been the same as it is now. Before it became a transform fault boundary, the plate boundary along northern California was a convergent boundary. At that time, an oceanic plate, the Farallon plate, lay between the Pacific and North America plates and descended beneath North America. Material scraped off this plate became rocks found in the Coast Ranges. As the plate descended and began to melt under the North American plate, molten rock rose into the crust to form the granitic rocks of the Sierra Nevada, such as those in Yosemite or near Donner Summit, and the volcanic rocks of the Ritter Range. The rocks of the California Coast Ranges, the Great Valley, and the Sierra Nevada that formed from about 140 to 5 Ma together represent the classic example of an ancient convergent margin. The Sierra Nevada represents the continental arc (the curved region of volcanoes and granitic intrusions above the descending plate), the Franciscan represents the material formed in the subduction zone, and the Great Valley sequence was deposited in the gap between the arc and the trench or subduction zone. The rocks now exposed and their relationships have been used as a model for understanding the behavior of active convergent margins still buried beneath the sea, such as along the Pacific Northwest, southern Alaska, or southern Mexico and Guatemala.

Not long ago, geologically speaking, the convergent margin along North America began to change to a transform margin-this change is still taking place. Figure 2 shows (diagrammatically) how geologists infer that this change occurred. Before 30 million years ago (Ma), the Farallon plate descended beneath North America. and a subduction margin lay along the western edge of the continent. About 30 Ma, the spreading center beween the Pacific and Farallon plates intersected the subduction zone, dividing the Farallon plate into two pieces. The northern one became the Juan de Fuca plate, and the San Andreas fault system grew between the two surviving fragments of the subduction zone.

Structural Geology and Tectonics of the Putah-Cache Region

The Putah-Cache region displays a very complex geologic structure involving a number of separate features (particular rock types are discussed in subsequent chapters). The Great Valley rocks, originally horizontal, are bent upward so that they are inclined mostly towards the east. In addition, many folds and faults are shown on the map (Figure 3).

Folds

Folds are bends in rock layers or in major rock units. Anticlines are folds whereby the sides, or limbs, of the fold are inclined away from each other and the center of the fold is pushed up relative to its limbs. Synclines, conversely, are folds in which the limbs of the fold are inclined towards each other. Both types of fold generally indicate that the rocks have been compressed so that distance between two points on a rock layer on either side of a fold's limbs has decreased. In the Putah-Cache bioregion, folds are present along the west side of the Sacramento valley as well as in the Coast Ranges. The Sacramento Valley folds, west of Davis and northwest of Woodland, involve the youngest rocks in the area, the Tehama formation, and are thought to be presently active. The folds in the Coast Ranges may be presently growing as well, although it is also possible that they formed at least in part during Cretaceous time, about 90-100 Ma.

Faults

Faults are fractures in rocks along which one side has moved with respect to the other. Many faults are found in the area. The Bartlett Springs, Green Valley, and intervening faults are active parts of the San Andreas fault system. These faults (marked with half arrows on either side ) are ones where the major movement has been horizontal, with relative motion as indicated. Thrust faults, shown with triangular teeth on Figure 3, are faults where one side has moved up and over the other. As such, they represent also structures, like folds, that compress or shorten the Earth's crust.

Two originally subhorizontal, now highly folded, faults also are present in the Putah-Cache region. The Coast Range fault is a folded, originally approximately horizontal, fault that separates the Franciscan complex from the Coast Range ophiolite. It may represent a fault with a large amount of movement along which deeply buried rocks of the Franciscan complex have moved upward toward the surface. The Stony Creek fault zone is an originally subhorizontal fault, now highly folded, that separates the Great Valley sequence from the Coast Range ophiolite. It may have been a subhorizontal thrust fault active about 100 million years ago. Other faults are shown that may be subsidiary to these major features, or have some other origin.

Geologists try to "think in three dimensions". One means of achieving this is to construct interpretative vertical sections through the Earth's crust at depth, using information obtained on the surface or from the subsurface. Cross-section A-B on Figure 3 is an interpretation of the structure along a vertical section of the crust in the southern part of the Putah-Cache bioregion. The section portrays the inferred form at depth of the folds and faults shown on the map. From east to west, the section shows the "baby" active anticline of Plainfield Ridge, the tilting of the Great Valley sequence along the western side of the valley, the Wragg Canyon fault, and other folds and faults to the southwest. Such sections can be used to infer the nature of structures and their development.

The Coast Ranges are an active landscape. As a result of action along the North American-Pacific plate boundary, the active folds and faults within the Coast Ranges have produced vertical uplift of the rocks within the Putah-Cache region that still continues. In particular, the tilting of the Great Valley rocks along the eastern side of the Coast Ranges probably began within the past one to three million years. Much of the landscape is quite recent, geologically speaking. Both Putah and Cache Creeks probably existed prior to much of the uplift. They probably eroded-and are still eroding-their canyons even as the rocks come up around them. Such streams are called "antecedent", that is, they existed before the development of the topography around them. In the case of Cache Creek, this story is complicated by the history of the development of the Clear Lake basin, and the shifting of the outlet from the west to the east through time. Nevertheless, some stream must have existed to form Putah and Cache Creek canyons as the rocks rose around them. Movements still continue. We can see this process just beginning along Road 32 near West Plainfield Ridge. Most likely, if one were able to come back in a million years, there would be a new Coast Range along the west side of the Sacramento Valley between Davis and Winters and between Woodland and Capay Valley.

As the Coast Ranges move eastward to compress the western side of the Great Valley, the rocks themselves are also squeezed. As this happens, waters entrapped 10's to 100's of millions of years ago during deposition of sediments that became the rocks are squeezed out along fractures in the rocks and form perennial "salt" springs that dot the landscape. Underground waters heated near volcanic centers become hot springs. Some of these hot springs perhaps have played a role in the deposition of mercury and gold ores in mining regions, such as the McLaughlin mine.

Landscapes are there for a reason. The reason is their geology. The rocks and landforms are a historical archive that can reveal the history of a landscape's development if one can "read the language" of their geology.

Previous piece
Vegetation of Putah-Cache
More like this
Geology
Next piece
Human Population of Putah-Cache
Next chapter
Davis