Central California: A Tectonic re-creation

Now that we better understand the mechanisms of plate tectonics, let us re-create our part of the world tectonically. 'Our part of the world' refers to the Santa Ynez, the Sierra Madre, and the Sierra Nevada Mountain Ranges in central California. One of our topo maps reconstructed a section of the Sierras while the other represented the Sierra Madres.

Let us recapitulate. About 50 million years ago the Pacific Plate was expanding in a northeasterly direction. It was leaning on the Farallon Plate, which was leaning on the North American Plate. The Farallon Plate had subducted under the North American Plate creating an underwater ridge as its top was scraped off as it entered the trench. This was the beginning of the Santa Ynez Mountain Range. For some reason at some point, the Pacific plate rotated around the Farallon Plate as an axis, now shifting to the northwest. The Farallon Trench was slowly consumed, between the Pacific and North American plates. The Pacific Plate continued to move ahead. Around 15 million years ago the Pacific Plate had reached the North American Plate, squeezing the Farallon Plate between, closing its trench creating the San Andreas Fault. At this time the Central Coast of California was either a sedimentary plane or under water.

As the Pacific plate, began leaning northwest, the remains of the Farallon Plate, in-between the two larger plates, and having nowhere to go, now that its trench was closed, began to rise above the ground. First the Santa Ynez Range rose above the ocean as islands. Then inevitably they crashed into the North American Plate. This pushed them further up and crumbled the sedimentary plain behind into the Sierra Madre Mountain Range.

“Three million years before the present—in the Piacenzian Age of late Pliocene time—neither the Coast Ranges nor the Sierra is above the horizon. From the hinge under the Great Central Valley, the Sierra fault block begins to rise. The tectonic behavior of the Coast Ranges is different. Sluggishly, they come up from the deep. They have no integral structure. They are a fragmentary mass, a marine clutter. They will be known in geology as the Franciscan melange. Appearing first as islands, the Franciscan pushes against the level sediments of the coastal plain and bends them upward until they are nearly vertical. Up through the melange come volcanoes that spew lava and tuffaceous ash in and around the Napa Valley. There are active volcanoes on the crest of the Sierra.” (McPhee, p181)

 

Remember that all this local mountain building activity only occurred within the last 3 million years. The Sierra granite batholith emerged above ground as a mountain due to a pulling action from the east. The Franciscan melange, remains of the Farallon Plate, emerged as islands then as the Santa Ynez Coastal mountains, which wrinkled the coastal plain creating the Sierra Madres. It is all there, in this one quote. Reiterating. “Appearing first as islands, the Franciscan pushes against the level sediments of the coastal plain and bends them upward until they are nearly vertical.” Three million years seems like a long time ago, but it is less than one thousandth the age of the earth. Hardly permanent. The age of the dinosaurs lasted hundreds of millions of years and they had been extinct for a long time already, when these local ranges became mountains. The Appalachian mountain range on the east coast of the North American Plate is hundreds of millions of years old. Our local mountains are just babies.

As the Pacific Plate collided with the North American Plate, North America held its ground creating a suture, a slip fault, called the San Andreas Fault. The connection between the Pacific and the North Atlantic began in the middle of Mexico. As the Pacific Plate slid northwest in a series of earthquakes over 50 million years along the San Andreas Fault, up the side of the North American Plate, it pulled this newly emerged land northward with it. This is Baja California.

Baja split off from the North American plate to join the Pacific Plate. However the split will continue up the San Andreas Fault, eventually splintering a large section off of the continent, turning it into a island off the coast of North America. As the Pacific Plate continues to move northward, the California Island will also be pulled northwards towards Alaska. After about 100 million years it will be opposite present day Alaska, then after another 50 million years it will reattach itself in the Arctic north.

After ripping Baja California off of North America, the Pacific Plate continued to push on North America. Finding nothing substantial in Southern California to block its course, it rippled up some mountains from the sedimentary plains. Suddenly, in tectonic plate terms, it ran into the 250 x 50 mile long granite block that was still under the surface of the earth at the time, the granite block, which is now called the Sierra Nevadas. The Pacific Plate was at that time and is still leaning upon the North American Plate. Finding this obstruction, the underground Sierras, in the way, it compressed the less dense south, bending the rippled mountains into the transverse mountain range. Simultaneously, its collision with the Sierras caused it to slide off to the west, creating the Garlock Fault.

“Central California appears to have moved westward along the Garlock Fault over the past ten million years. This movement is puzzling in a system of north-south faults but may be the result of being rammed from the south by Baja California.” (National Geographic February 1995)

 

Presently we are experiencing a collision of the North American Plate and the Pacific Plate along the San Andreas Fault. The Garlock Fault is a result of the northwest action of the Pacific Plate, sliding off the ‘immovable’ granite block to the east, pushing Central California west as would be expected as it slides around the obstruction. The Baja California peninsula connected to the Pacific Plate is ramming central California westward along the Garlock Fault, which runs roughly perpendicular to the San Andreas Fault. It seems that the push northward from Baja reached a resistance and hence faulted, cracked, where the granite block of the Sierra Nevada begin on their southern border. Supporting this idea is the fact that the Garlock Fault borders the Sierras on the south.

Barroom Speculations

Looking at the geological map of Santa Barbara, San Luis Obispo, Bakersfield area, (a United States Department of the Interior Geological Survey), it even looks like the Santa Ynez Mountains are an island arc colliding with the coast of California. This island arc called the Santa Ynez is preceded by the Santa Ynez Fault to the North, which runs right through the Santa Ynez Valley, right on the other side of the Santa Ynez Mountains. There are even stress fractures, i.e. unnamed faults, northward of the Santa Ynez Fault, indicating the waves of pressure exerted by this small island arc. This island arc is sliding up the coast, along the Santa Ynez fault. The Santa Ynez Fault runs from Point Arguello above Point Conception through the Santa Ynez Valley. Perhaps the Channel Islands and the Santa Catalina Islands are part of these string of island arcs sweeping towards the coast. The fault lines of the Channel Islands go the same way as the Santa Ynez Fault.

Furthermore at the intersection of the San Andreas Fault, the Garlock Fault, and the Santa Ynez Fault are all the highest mountains of this area: Pine Mountain at 7510, Mt. Pinos at 8831 and Frazier Mountain at 8013, While small by Sierra Nevada standards, they are a full 1000 feet higher than any of the nearby mountains.

One last supporting fact. There is evidence that whales have used the Santa Ynez Valley through to Lake Cachuma as one of their migratory patterns. Now if anyone has seen the Santa Ynez River, one cannot imagine a whale swimming up it. However if there were ocean between which was closed a million years ago when the island arc collided then it makes perfect sense.

Now we have our three diverse mountain ranges woven together in an integral way. The granite block called the Sierra Nevadas was first. The sediment from runoff settled on the plains. The Farallon Ridge was created by subduction. Then the pressure from the Pacific Plate pushed it up and into the coast simultaneously creating the Santa Ynez Mountain Range and the Sierra Madres. As the Pacific Plate collided with the immovable Sierra Nevada from the South it created the Garlock Fault, which moves the whole shebang westward. We are so interconnected. How can we escape to be truly by ourselves?

 

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