Geologic History of California

Geologic History of California
A Brief Overview

Joel Michaelsen

I. Introduction

The diversity of California's natural landscapes is truly extraordinary. In no other state is the range of physical environments so wide. From the extreme heat of the deserts to the icy cold of the alpine areas, from some of the driest locales on the continent to some of the wettest, California has it all. Perhaps this is not surprising considering that the highest and lowest points in the lower 48 states occur in California less that 100 miles apart. In fact, the large topographic variation is one of the factors most responsible for the diversity, along with the state's position on the west coast of North America and its large latitudinal range. The last relates to the state's unusual human history, the first two relate to its unique geologic history.

California's geologic history is very complex, and there is much that is still poorly understood. The situation is complicated by the fact that most of the state did not exist as a coherent piece of the earth's crust until relatively recently in geologic terms. While the details are uncertain, the overall story appears reasonably clear. Most of California consists of a series of pieces of the earth's crust that have been pasted onto the western edge of North America over the last few hundred million years. These different fragments, or terranes to use the geologic term, traveled considerable distances from a variety of different directions before colliding with western North America. As a result, rock formations that are now adjacent often have very different histories. Some of the larger collisions were associated with major episodes of tectonic activity—intrusive and extrusive volcanic activity, folding and faulting, and mountain building. The most recent period of mountain building is still going on, and practically all of the current landforms and geographic features are very young in geologic terms—only a few million years old. Many of the rock formations exposed in the current mountain-building phase are much older, however.

II. Proterozoic and Paleozoic Era (2,500-245 million years ago)

Rocks older than 600 million years are rare in California. The oldest rocks, which are between 1,400 and 2,400 million years old, are located in the eastern deserts and the eastern Transverse Ranges (San Bernardino and San Gabriel Mountains). These rocks are considerably younger than the 4,500 million year age of the earth, but they are quite old compared to most of California. Tentative reconstructions of continental positions suggest that southeastern California and the rest of the southwestern U.S. were pasted onto the existing core of continental North America. The distribution of rocks of these ages suggests that the west coast of the North American Continent was well to the east of all but the southern end of what is now California (Figure 1). By around 1,200 million years ago the North American continent appears to have been joined with Gondwanaland (current Australia and Antarctica) to form a supercontinent named Rodinia (Figure 2). All of these very old formations have been extensively metamorphosed, so it is difficult to determine the conditions that existed when they were originally formed. Some of the oldest (around 1,800 million years old) are located in the mountains around Death Valley and are much like the rocks exposed in the inner gorge of the Grand Canyon. Another interesting correspondence occurs between matching exposures of old metamorphic rocks around in the San Gabriel Mountains and the Orocopia Mountains east of the Salton Sea. These two locations are about 130 miles apart, but they are on opposite sides of the San Andreas Fault—the San Gabriels on the west side, the Orocopias on the east side. (The Fault is the boundary between the North American and Pacific plates.)

Sometime around 700 to 800 million years ago, rifting split the Rondinia supercontinent, sending Gondwanaland off to the west and North America to the east. The long period after the split until about 400 million years ago appears to have been quiet in western North America since the western margin was on the trailing edge of the eastward moving continent. During this period eastern North America was the on the active leading edge of the continent, and the Appalacian mountain ranges were formed. The coastline remained east of California, probably in Utah and Idaho (Figure 3). Very thick sections of marine sedimentary rocks from this period are exposed in the mountains east of the Sierra Nevada. For the most part, these rocks are carbonates (limestone and dolomite), indicating shallow, warm ocean conditions with little incoming terrestrial sediment—possibly similar to the Gulf of Mexico today.

Around 400 million years ago the quiescent period in western North America came to an end with the first major mountain-building episode, known as the Antler orogeny (Figure 4). The evidence for this event is strongest farther east, but there are indications in the northern Sierra Nevada and Klamath regions of the development of an offshore island arc, possibly in an environment similar to Japan today. Another major period of mountain building in western North America, the Sonoma orogeny, occurred about 250 million years ago. This event is most evident in central Nevada, which may have been the location of the coast at the time (Figure 5). The orogeny seems to have marked the joining of the island arc containing rocks now found in the Sierra Nevada foothills and central Klamath Mountains with the rest of the continent. This event appears to mark the first time the coast moved west into most of California.

III. Mesozoic Era (245-65 million years ago)

Around 225 million years ago, the North American continent began to split away from Europe and Africa and started drifting westward, making the west coast the leading edge of the continent. A subduction zone was formed all along the coast from Baja California to British Columbia, as vast amounts of oceanic crust were shoved beneath the continent, melted, and rose to the surface in a chain of large volcanoes, probably similar to the modern Andes. The main body of intrusive granitic rocks that now make up most of the Sierra Nevada was created through this process and are probably the roots of the ancient volcanoes. The rocks that make up what is called the Sierra Nevada batholith are intrusive igneous rocks that formed as molten magma cooled and hardened slowly below the surface. They were only exposed after they hardened, in contrast to extrusive volcanic eruptions that bring still-molten magma to the surface where it cools relatively rapidly. The difference in cooling rate determines how large crystals will grow, with intrusive igneous rocks tending to have larger crystals.

The processes that produced the Sierran granites also initiated the mountain-building episode known as the Nevadan orogeny that uplifted the ancestral Nevadan Mountains in the site of the current Sierra Nevada. Structural features and rock types in the foothills west of the Sierra Nevada suggest that the Nevadan orogeny may have been caused by the arrival and docking of a complex island arc. By 150 million years ago the Nevadan Mountains composed the western coast of North America and the sea never again extended farther east than the eastern edge of what is now the Central Valley. At roughly the same time the granitic rocks were forming that are now found in the Peninsular Range of Southern California and in the Salinian block that runs along the western edge of the San Andreas Fault in the central and southern Coast Ranges (Figure 6). It has been suggested, however, that these rocks developed far to the south of their current locations. If this is true, the northern end of the Salinian block, now north of San Francisco, originally was attached to the southern end of the Sierra Nevada. The southern end of the Salinian block, now around Ventura County, was joined with the northern end of the Peninsular Ranges in Riverside County. Movement along the San Andreas and other faults would explain the current locations of these rocks.

Forces that created the intrusive granitic rocks of the Sierra Nevada and Southern California continued episodically until about 90-100 million years ago. During the same period sedimentary deposits were accumulating on the continental shelf, slope, and in an offshore subduction zone (Figure 7). By that time the subduction zone had shifted west to about the current location of the Coast Ranges (Figure 8). Deposits in the subduction zone were mixed with rocks from the upper mantle beneath the crust producing the Franciscan Formation with its serpentine rocks that extends from northern Santa Barbara to the northern California coast. The oceanic crustal material in the Franciscan formation was thrust up against the Sierran block by about 75 million years ago, extending the boundary of the North American plate further west.

IV. Cenozoic Era (65 million years ago to present)

By about 50 million years ago the ancestral Nevadan Mountains had eroded down to relatively low-lying hills. Large rivers flowed through the region and deposited gravels rich in gold. These dry gravels were extensively mined using hydraulic mining techniques during the gold rush. Climatic conditions were apparently quite warm and moist. The southern Sierra Nevada and Mojave region were elevated enough to allow for the erosion and deposition of thick layers of marine sedimentary rocks in what are now the southern Coast Ranges and western Transverse Ranges. This deposition continued without interruption until around 40 million years ago. Over the next 10 million years the coast shifted back and forth, producing a patchwork of marine and non-marine sedimentary rocks in the Coast Ranges and western Transverse Ranges.

Major changes began to occur about 25-29 million years ago. The oceanic plate that had been subducting beneath the western edge of North America became completely overridden, starting in the south, and the North American and Pacific plates came into direct contact for the first time. Tangential motion and expansion replaced convergent motion as the North American plate began interacting with the Pacific plate (Figure 9). The arrival of the spreading zone associated with the East Pacific Rise produced a substantial amount of stretching and thinning of the crust in eastern California and throughout the Great Basin. Along the coast, the San Andreas Fault system was formed. The region to the west of the fault zone on the Pacific plate began sliding northwestward relative to the area to the east on the North American plate. A block of the continent that includes Santa Barbara was apparently rotated clockwise more than 90^o to form the anomalous east-west trending Transverse Ranges. In far northern California and the Pacific Northwest, north of the Mendocino triple junction, convergent motion has continued right up to present times.

Volcanic activity, possibly related to extension and thinning of the crust, became widespread in the Sierra Nevada and Mojave regions around 20 million years ago. Around 10-15 million years ago a series of deep marine basins formed along the coast between Orange County and the San Francisco region. These basins apparently resembled current features in the Gulf of California which is being split open by the spreading zone associated with the East Pacific Rise. Possibly, some were also associated with stretching produced by the rotation of the Transverse Range block. The rocks formed in these basins (the shales of the Monterey Formation) are composed mainly of material derived from marine organisms, rather than terrestrial sediments.

About 5 million years ago mountain-building activity rapidly accelerated, and finally most of the modern mountain ranges were uplifted, including the Sierra Nevada and the large fault-block ranges to the east, the Coast Ranges, the Transverse Ranges, and the Peninsular Ranges. Subduction continued in the north forming the major volcanoes of the Cascades (Figure 10). Only in the last few million years (0.1% of all geologic history) did the California landscape began to resemble what we see today. In many areas the uplift has continued to the present. Other, more recent events which have further shaped the modern landforms have included: waves of Pleistocene glaciation in the Sierra Nevada and, to a minor extent, in the San Bernardino Mountains; recent volcanic eruptions in the Mojave and Great Basin regions; and the widespread volcanic activity that created the southern Cascade volcanoes (Mt. Shasta and Mt. Lassen) and the lava flows of the Modoc Plateau region.