Geography 390

Geography of California

A Brief Geologic History of California
(summarized after Debora Harden, Geology of California)
Before 600 my - earliest Precambrian crystalline basement rocks are found in western Utah cutting southwest across the southernmost part of Nevada and then across southern California (Mojave Desert area). The west coast of North America was along this line about 1 billion years ago. In other words, what is now the land of California was under the Pacific Ocean, except for southernmost California.

By 600 my, rifting of the North American Plate, with sinking of the western continental margin, resulted in the creation of a passive continental margin. Sediments accumulated over 350 million years, building up thousands of meters thickness of mostly carbonate sedimentary rocks (limestone). The continental margin went from southern California across southern Nevada then northward along eastern Nevada and western Utah, up through what is now eastern Oregon and Washington. Thus the California portion of the coast faced northwest. Later metamorphosed into quartzite and phyllite, today these rocks can be seen in the Basin & Range provinces (east of Modoc Plateau) and the Mojave Desert. They are also preserved in roof pendants in the eastern High Sierra.

Then came a period (in the Devonian and Mississippian periods, around 400-320 my) of plate collision and mountain building. Evidence of this uplift and rocks made up of sediment eroded from the resulting mountains are seen in western Nevada, but also in the Inyo Mtns. and Death Valley area. While this uplift was occurring, other oceanic rocks were forming off the continental margin, mostly from volcanic island arcs and subduction zone metamorphic rocks. These rocks were later accreted (plastered) against the western continental margin of the North American plate, forming parts of the northern Sierra Nevada and the eastern Klamath Mtns.

About 200 million years ago, in early Mesozoic time, Pangaea began to break up, creating the North American plate as we see it today. Between 100 and 50 million years ago the North American plate moved westward, breaking away from what is today the European continent. Approximately where today we see the Sierra Nevada, a subduction zone formed off the west coast of the North American plate. This subduction zone resulted in the collision and accretion of belts of oceanic rock that gradually built the continental margin westward. The youngest of these belts of accreted rock in the Sierra Nevada is about 160 million years old, middle to late Jurrasic in age.

During the Mesozoic subduction, magma rose up from beneath the descending plates, causing the formation of chains of andesitic volcanoes at the surface and plutons of granitic magma beneath them. Plutonic rocks from this period are found in the Klamath Mountains, Sierra Nevada, Basin and Range, Mojave Desert and Peninsular Ranges. During this time the Farallon plate was subducted and consumed beneath the North American Plate.

By 100 million years ago the subduction zone had shifted westward to the approximate position of today's Coast Ranges. Portions of the Farallon plate, along with oceanic sediments of volcanic and terrestrial detritus origin, were subducted briefly and then rose to be accreted against the North American plate, forming what we call today the Franciscan Formation.

The San Andreas transform fault system began about 28 million years ago with the collision of the Pacific plate and the North American plate. This collision caused the subduction zone along the coast to cease and the two plates began to slide past each other. The Pacific plate moves northwest relative to the American plate, a system referred to as a right lateral fault. The transform fault spread gradually northward from about the latitude of Los Angeles, carrying with it pieces of rock that crossed the plate boundaries. The Pinnacles in central California are an example of such transported materials. They are part of a volcanic complex about 23 million years old that has been carried 195 miles beyond where it was initially erupted. These rocks match a volcanic formation called the Neenach that is part of the Transverse Ranges 195 miles southeast of the Pinnacles. Other examples of features carried north along the fault include Point Reyes and Bodega Head, on the coast west of the SSU campus.

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Last updated July 23, 2003