Harden, Chapter 18
The Evolution of California Through Geologic Time
After looking at the highlights of California’s provinces, we can see a unifying theme in California’s geology: complexity caused by tectonics. The complexity has long been recognized by geologists attempting to decipher California’s geologic history. Beginning with the gold rush, the search for mineral and petroleum resources spurred the preparation of accurate and detailed geologic maps in most of California. By the 1960s, the nature and approximate age of many of the rock formations, and geometry of most of the faults and folds were fairly well established. Interpretations of California’s geologic history, however, still contained a great many uncertainties.
During the past 30 years, the concept of plate tectonics has given geologists a framework for interpreting the complicated assortment of California rocks and faults. Advances in technology have enabled us to better identify and date geologic materials at the surface and to collect information about the Earth’s interior. At the same time, societal concerns about seismic risks and other geologic hazards have led the public to provide the necessary resources to support geologic and geophysical investigations. Today, researchers are recognizing pieces of the tectonic puzzle with each new study. In turn, assembly of the pieces is shedding new light on California’s tectonic setting during different period in geologic time.
Because of the great complexity of California geology, our understanding of the evolution of California through geologic time is nowhere near complete. Nevertheless, it is useful to review California’s history and synthesize the tectonic setting of all of the provinces at different points in geologic time.
California’s Earliest History: Before 600 Million Years Ago
Rocks older than about 600 million years are relatively rare in California. They are found only in southeastern California – in the southern Panamint and Nopah Ranges near Death Valley, in some of the ranges of the Mojave Desert, and in the San Gabriel and San Bernardino Mountains. Because these rocks are found in only a few places, and because they have been subjected to many geologic events since their formation, little is known about the earliest geologic environment of southeastern California. Based on their composition, these rocks are thought to be remnants of very old terranes that were accreted early in the history of ancient continents that are preserved in today’s North American Plate.
The oldest rocks in southeastern California, termed the crystalline basement, consist of metasedimentary rocks, mainly gneiss, schist, and marble, and granitic plutonic rocks. The granitic rocks are about 1.4 to 1.7 billion years old, and, because they intruded into the metasedimentary rocks, those rocks are even older. At some localities, the oldest metamorphic rocks are as old as 2.4 billion years. Global reconstructions of the Earth at that time suggest that the ancient rocks of the southwestern United States were accreted to the most ancient core of North America.
About 1.2 billion years ago, an ancient supercontinent formed by the accretion of smaller continental fragments, including those represented by southeastern California’s most ancient rocks. Geologists have named this ancient continent Rodinia, a word meaning “homeland” in Russian. The ancient core of North America formed the core of Rodinia. Based on the similarities of rock sequences and their ages, geologist believe that the future west coast of North America was next to the future Australia and Antarctica – a totally different configuration than today’s.
About 900 million years ago, sediments were deposited on the crystalline basement rocks of the Rodinia supercontinent. Today these sediments are represented by the Kingston Peak Formation in the Death Valley region and by more widespread formation in northeastern Washington, southern Idaho, and northwestern Utah. The Kingston Peak Formation contains coarse-grained gravels and the size of the sediments indicates that they came from a landscape with considerable relief. Some of the cobbles in the formation appear to have been striated by glaciers, providing evidence that Rodinia was located in polar latitudes. In other regions, sedimentary rocks of this age are associated with volcanic rocks.
Late Proterozoic and Paleozoic California: The Passive Margin
By about 700 to 800 million years ago, the supercontinent Rodinia began to break up, and rifts developed within the continent. Sediments began to accumulate along the western margin of North America in response to the sinking of the continental edge.
Sedimentary rocks in several areas of southeastern California, including the San Bernardino Mountains and Death Valley regions, and in other areas in Washington, Oregon, Utah, and Nevada appear to have formed in shallow marine waters during the initial subsidence. Quartz sandstone and associated shale, later metamorphosed to quartzite and phyllite, are the most common sedimentary rocks deposited, most dating to about 600 million years ago in late Proterozoic time.
By early Paleozoic time, a stable continental shelf had developed along the continental margin of western North America. During the next 350 million years, thousands of meters of Paleozoic sedimentary rocks, mainly carbonates, accumulated on the carbonate platform. Geologists refer to this type of sedimentary environment, found along a tectonically quiet or passive margin, as a miogeocline. Today the rocks formed in the miogeocline can be seen in the ranges of the Basin and Range Province and the Mojave Desert, the San Bernardino Mountains, and the Peninsular Ranges. They are also preserved as metamorphic roof pendants in parts of the eastern High Sierra.
In general, the section of Paleozoic sedimentary rocks in southeastern California and adjacent parts of Nevada and Arizona increases in thickness from southeast to northwest. Assuming that the sediment accumulations decrease toward the ancient shoreline, geologist can reconstruct the continental margin. Based on the trends observed in Paleozoic sedimentary rocks, geologist believe that the continental margin faced toward the northwest in most areas.
Southeastern California’s relatively quiet early tectonic setting was interrupted during the Devonian and Mississippian periods, when an episode of plate collision and mountain building was recorded by Paleozoic rocks in western Nevada and adjacent parts of California. These disturbances resulted in the accumulation of sediments shed from newly uplifted mountains and caused preexisting sedimentary sequences to be folded and faulted. Clastic sedimentary rocks seen today in the northern Inyo Mountains and the Death Valley area record this episode of uplift, and the deformation is well preserved in Nevada, where geologist have named the ancient uplifted area the Antler Mountains.
Beyond the continental margin, other oceanic rocks were forming west of North America during Paleozoic time. Remnants of these rocks would be accreted to North America during Mesozoic time 100 to 200 million years later. Some of these rocks represent subduction-zone complexes, and others are remnants of oceanic volcanic terranes. During Mississippian and Permian time, a belt of island arcs lay west of the North American continent. Volcanoes from this chain erupted large volumes of andesite, and coral reefs ringed the volcanic island. Today the ancient island arc system is preserved in the Paleozoic rocks of the northern Sierra Nevada and the eastern Klamath Mountains of California.
Although California was tectonically quiet during much of Paleozoic time, it was a time of tectonic upheaval along what is today the eastern edge of North America. The tectonic collisions that produced the complex rocks seen in the Appalachian Mountains occurred during Paleozoic time. By the Permian, about 250 million years ago, another supercontinent had been assembled. This great mass was named Pangaea, meaning “one Earth” in Greek.
Mesozoic California
Early in Mesozoic time, about 225 million years ago, the ancient supercontinent know as Pangaea began to break up. Between about 100 and 50 million years ago, North America moved westward, and a rift developed along the eastern edge of the continent. This rift would eventually evolve into the northern Atlantic Ocean, splitting the North American Plate from the western edge of the European continent. Along western North America, a north-south subduction zone developed, roughly in the same area as today’s Sierra Nevada. A huge amount of oceanic lithosphere, equivalent to the entire Pacific basin, was subducted beneath western North America during Mesozoic time. Because of associated accretion, this was California’s most important geologic building period.
During Mesozoic time, belts of oceanic rock were added to California by a series of collisions and accretions The earliest rocks added to North America were Paleozoic sedimentary and volcanic rocks. These were accreted to North America during early Mesozoic time. Following each episode of accretion, the subduction zone shifted westward and accretion of a younger terrane began. The westernmost, youngest belt of accreted rocks in the Sierra Nevada is of middle to late Jurassic age, about 160 million years old.
Inland from the Mesozoic subduction zones, magma rose above the down-going plates, forming chains of andesitic volcanoes and granitic plutons beneath them. At its maximum development, the Mesozoic magmatic arc extended along western North America from Baja California, Mexico to British Columbia. In California, the arc was located approximately in the position of today’s Sierra Nevada. Plutonic rocks representing the roots of the Mesozoic volcanic chain are found today throughout the Klamath Mountains, Sierra Nevada, Basin and Range, Mojave Desert, and Penisular Ranges provinces. Mesozoic volcanic rocks, originally erupted along the arc, are also preserved in places in the same provinces. The greatest volume of magma was generated about 100 million years ago, during the subduction of the Farallon Plate.
By about 100 million years ago, the subduction zone had shifted westward to the approximate position of today’s Coast Ranges. Pieces of the Farallon Plate were being accreted along western California as it was being subducted beneath North America. Millions of years later, geologist would recognize these pieces as the Franciscan Assemblage of the Coast Ranges.
Between the trench and the Sierran volcanic arc, a marine forearc basin extended along most of California’s length. The Cretaceous Great Valley Sequence, including marine sedimentary rocks found today in the Transverse Ranges, accumulated in this linear basin. The sedimentary rocks of the Great Valley Sequence contain fragments of most of the immense volcanoes that once sat above the Sierra Nevada granitic rocks. The sedimentary rocks of the Great Valley Sequence accumulated on the Coast Range ophiolite, which geologist now recognize as representing the crust and upper mantle beneath the ocean floor.
The major tectonic events of the Mesozoic era in California had dramatic effects on the rocks that were already there. Folding and faulting accompanied the collision events of the Mesozoic, with the result that older rocks were folded and faulted during each episode. In many areas, Paleozoic and Mesozoic rocks were also metamorphosed during the intrusions of magma and episodic compressional events. Geologist attempting to understand California’s pre-Mesozoic history have had to “see through” these later changes to determine the original nature of Paleozoic and older rocks.
Cenozoic California
About 27 million years ago, the Pacific and North American Plates made direct contact for the first time when the closest part of Farallon Plate was completely consumed beneath North America. This event created the San Andreas transform boundary. North and south of the transform, subduction of the Farallon Plate continued. As increments of the Farallon Plate were consumed in the subduction zones, the transform margin lengthened. The development of the transform margin has been the dominant factor in California’s later Cenozoic history. Geologic events throughout California, even in provinces not directly at the plate boundary, have been strongly influenced by the San Andreas system.
The development of the San Andreas transform had major impacts on the western edge of California. Large blocks have shifted by as much as 315 kilometers along the right-lateral faults of the San Andreas system. For example, rocks of the Salinian Block, which probably formed near the southern end of the Coast Ranges, moved northwestward along the San Andreas fault to its present position along the central California coast. Right-lateral movement along the transform boundary also brought crystalline basement rocks from southeastern California to their present positions in the San Gabriel Mountains.
Cenozoic Basins
During the early history of the San Andreas system, large volumes of sediment accumulated in marine basins along the continental margin. Geologists believe that the sediments were deposited in pull-apart basins. The early boundary between the Pacific and North American Plates was oriented to create a transtensional tectonic regime in the area where the Transverse Ranges and Channel Islands are today. As a result, a number of fault-bounded basins formed and received sediments shed from nearby highlands. In the southern Great Valley, the San Joaquin basin was a very deep marine basin between 35 and 15 million years ago, and a shallow marine embayment persisted there until 2.5 to 3 million years ago.
Cenozoic Extension in the California Desert
In early Miocene time, about 22 to 17 million years ago, extension of the crust began along a roughly north-south zone that included parts of what is now the Mojave Desert and Basin and Range. Large crustal blocks were pulled apart along detachment faults. In some highly extended terranes, detachment faults completely removed the overlying rocks to expose the metamorphic rocks of the lower plate. Today the metamorphic core complexes can be seen in a belt that stretches from Canada to Mexico, including many that form the dome-shaped ranges of southeastern California.
Tectonic Rotation in the Transverse Ranges and Mojave Desert
Geologists have uncovered evidence that rocks in both the western Transverse Ranges and in the Mojave Desert Province have been rotated in a clock wise direction during Miocene time. Geologists believe that the western Transverse Ranges rotated as a result of right-lateral motion along the edge of the North American Plate. As the early San Andreas fault developed, blocks of the North American Plate were added to the north-west-moving Pacific Plate; these blocks lay west of today’s Transverse Ranges. The southern end of the western Tranverse Ranges broke free of North America, but the eastern end remained attached to the continent, with the result that block rotated clockwise.
Rotation of the western Transverse Ranges created a region of extension to the south, where today’s Los Angeles basin and offshore islands are. Geologist believe that Miocene extension caused the uplift and unroofing of metamorphic rocks from lower levels of the crust, such as the Catalina schist.
Magmatic Activity
One major consequence of the development of the transform margin was the shutting off of magmatic activity beneath North America. In areas east of the growing transform margin, no oceanic plate was being subducted. As a result, no magma was generated. After about 27 million years ago, magmatic activity began to cease inland of the transform margin. Where remnants of the Farallon Plate and the East Pacific Rise still exist today, north of the Mendocino Triple Junction and south of Baja California, Mexico, magmatic activity persists on the North American Plate. East of the plate boundary in the Basin and Range Province, crustal extension is responsible for recent volcanic eruptions in Death Valley, along the eastern side of the Sierra Nevada, and the Modoc Plateau.
The boundary between the Farallon and Pacific Plates, the East Pacific Rise, was an oceanic spreading ridge. As the Farallon Plate was being consumed by subduction, the East Pacific Rise approached closer to western North America. When the Farallon Plate was completely subducted, the East Pacific Rise itself encountered the subduction zone about 27 million years ago. Some geologist believe that this encounter might have triggered a period of volcanic activity close to the plate boundary. They hypothesize that the mid-Cenozoic volcanic rocks seen today in the Coast Ranges, near Point Conception, and in the Santa Monica Mountains, resulted from arrival of the East Pacific Rise at the western margin of the North American Plate.
California During the Past 5 Million Years
About 5 million years ago, the Gulf of California began to open, and the Baja California peninsula became part of the Pacific Plate. The transform boundary shifted inland in Southern California, and the transtensional tectonic regime was replaced by transpression. The change in motion created greater convergence between the Pacific and North American Plates, including formation of the Big Bend segment of the San Andreas fault. The new configuration of the plate boundary resulted in a series of left-lateral faults along the southern edge of the western Transverse Ranges blocks and along the Garlock fault. The rate of right-lateral motion along the San Andreas system also increased.
During the past few million years, the transform boundary between the Pacific and North American Plates has continued to lengthen. Today it extends along most of California in the form of the San Andreas fault system. Right-lateral motion along the active faults of the San Andreas system is carrying the Pacific Plate northwest and the North American Plate to the southeast. North of Cape Mendocino, remnants of the Farallon Plate, known as the Gorda and Juan de Fuca Plates, are sill being subducted beneath North America. As a result, a magmatic arc is still being generated north of Cape Mendocino, and Cascades volcanic chain marks the surface above it.
Most of California’s major mountain ranges were elevated about 4 to 5 million years ago in response to increased compression along the plate boundary. The present Sierra Nevada range rose in about the same position as the Mesozoic chain of volcanoes during the past 9 million years, but not to elevations as great as the Mmesozoic volcanic arc. In coastal southern California, reverse faults and folds developed, forming deep basins as well as newly uplifted mountains. Beginning about 4 million years ago, basin subsidence in southern California increased as a result of increased compression. Along the coast, flights of marine terraces add to the evidence of ongoing uplift, particularly where compressional forces are strengthened near the Big Bend and the Mendocino Triple Junction. Finally, earthquakes on both reverse and transform faults serve as constant reminders that both compression and lateral plate motion are active in California today.
Further east, in the Mojave Desert and Basin and Range Provinces, crustal extension continues, as evidenced by active normal faults. Recent earthquakes with right-lateral displacement indicate that the Eastern California Shear Zone is a developing part of the transform boundary.
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