2.6 Sierra Nevada and Coast Ranges, What is a Batholith?

During the Jurassic, when the North Atlantic began to open, a subduction zone formed along the western margin of the North American Plate. Evidence for this episode of subduction is found in a nearly continuous belt of igneous plutons that include Mexico’s Baja batholith, the Sierra Nevada and Idaho batholith found in the Western United States, and the Coast Range batholith in Canada.

Part of what formed this convergent plate boundary is now an excellent example of an inactive Andean type orogenic belt. It includes the Sierra Nevada and the Coast Ranges in California. These parallel mountain belts were produced by the subduction of a portion of the Pacific basin, Farallon plate, under the Western margin of California.

The Sierra Nevada batholith is a remnant of the continental volcanic arc that was produced by many surges of magma over tens of millions of years. The Coast Ranges represent an accretionary wedge that formed when sediments scraped from the subducting plate and provided by the eroding continental volcanic arc were intensely folded and faulted. Portions of the Coast Ranges are composed of a chaotic mixture of sedimentary and metamorphic rocks, plus fragments of oceanic crust called the Franciscan formation.

Beginning about 30 million years ago, subduction gradually ceased along much of the margin of North America as the spreading center that produced the Farallon plate entered the California Trench. The spreading center and subduction zone were subsequently destroyed. Uplifting and erosion that followed this event have removed most of the evidence of past volcanic activity and exposed a core of crystalline, igneous, and associated metamorphic rocks that make up the Sierra Nevada. The Coast Ranges were uplifted only recently, as evidenced by the young unconsolidated sediments that still mantle portions of these highlands.

California’s Great Valley is a remnant of the forearc basin that formed between the developing Sierra Nevada and the Coast Ranges. Throughout much of its history, portions of the Great Valley lay below sea level. This sediment-laden basin contains thick marine deposits and debris eroded from the continental volcanic arc.

From this example, we can see that Andean type mountain belts are composed of two roughly parallel zones of deformation. A continental volcanic arc, which forms along the continental margins, consists of volcanoes and large intrusive igneous bodies and associated metamorphic rocks. Seaward of the continental volcanic arc, where subducting plates descend beneath the continent, an accretionary wedge is generated. This feature consists mainly of sediments and volcanic debris that have been folded, faulted, and in some places metamorphosed. Between these regions of deformation lies a forearc basin, composed mostly of horizontal marine strata.

In summary, the growth of mountain belts at subduction zones is a response to crustal thickening caused by the addition of mantle derived igneous rocks. In addition, crustal shortening and thickening may occur along the continental margins as a result of convergence.
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Playlist of Geology and Earth Science Videos from Snow Mountain
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