ContentsPutah and Cache: McLaughlin Mine

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Geology of Putah-Cache: Coast Range Ophiolite

Eldridge M. Moores and Judith E. Moores

The McLaughlin mine sits along the contact between the Coast Range ophiolite and Great Valley rocks. The Coast Range ophiolite is also visible along the northwest edge of Lake Berryessa, along Highway 128 west of Berryessa near the Spanish Flat road turnoff, on Butts Canyon road, and along Highway 20, among other places in the region. The Coast Range ophiolite includes serpentine, associated intrusive rocks, lavas, and deep sea sediments that together formed the substratum on which the Great Valley sediments were deposited. Found world-wide, ophiolitic rocks generally form a uniform sequence from bottom to top of ultramafic rocks (peridotites,rocks low in silica and high in magnesium and iron), mafic intrusive rocks (igneous rocks, consisting of gabbro and/or diabase, that formed one or more km below the sea floor), and mafic extrusive rocks, often in the form of pillow lava, the remains of lavas extruded beneath water. Serpentine, the California state rock, is part of many ophiolites. The rock serpentine (or serpentinite) forms by addition of water to minerals in peridotite, changing them from olivine and/or pyroxene to the mineral serpentine.

Ophiolites are thought to form at oceanic spreading centers in the middle of the oceans, near or behind oceanic island chains (arcs) such as the Izu-Bonin or Marianas, or in narrow oceans such as the Gulf of California or the Red Sea. Most oceanic crust thus formed is returned to the Earth's interior by subduction. A small proportion of oceanic crust and mantle formed at a spreading center is preserved in the continents by collision of a continental margin with another plate in a subduction zone. Because continental crust is too buoyant to subduct, such a collision arrests the subduction there and results in a small amount of the over-riding plate being preserved on the continent's edge.

How did the Coast Range ophiolite come to be preserved between the Sierra Nevada and the Franciscan subduction zone? Recent work by Dr. Nikola Godfrey and her co-workers suggests that the entire Great Valley is underlain by a huge slab of oceanic crust and mantle that was thrust eastward over the edge of the North American Continent. This thrusting probably occurred when the North American continental margin collided with a west-dipping subduction zone, about 140-150 million years ago (Ma). This huge ophiolite basement beneath the Great Valley has probably preserved the Valley as a low-lying plain while the Sierra Nevada and Coast Ranges continue rising to the east and west of it.

The ophiolite exposed in the eastern Coast Ranges probably represents a tectonically-displaced remnant of this Great Valley ophiolite. Elsewhere in the Coast Ranges, ophiolite outcrops display a complete ophiolite sequence, but in the Putah-Cache watershed only serpentine, the lowermost part of the sequence, is preserved. Either erosion removed the upper parts of the sequence before the oldest Great Valley sediments were deposited, or the upper parts were never there at all. Similar incomplete ophiolite sequences are now known from many places in the world's oceans, especially along very slow-spreading ridges such as the Mid Atlantic and Southwest Indian Ridges, as well as in some fore-arc regions such as between the Marianas Islands and the Marianas trench to the east. Which, if any, of these environments formed the Coast Range ophiolite is a matter of vigorous debate. Most, if not all, serpentines in the Coast Ranges are part of the Coast Range ophiolite.

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