California is one of the few places in the world that lie on a transform fault. My question is why considering the fact that North American Plate is moving westward shouldn't California be a subduction zone like the Entirety of the west coast of the Americas say for Canada. Why is that the North American plate is simply rubbing up against Pacific plate in here as well as Canada.

  • $\begingroup$ short answer because there used to be an EAST pacific plate that much of North America is now sitting on, and California is sitting on a dead/dying spreading center. the west pacific plate is moving AWAY from north America. $\endgroup$ – John Oct 3 '20 at 2:48

Before the transform process of the San Andreas Fault zone we know today, there was indeed a subduction zone event taking place in this same region near now coastal California during the late Jurassic to late Cretaceous periods.

Today the city of San Francisco physically represents the aftermath of the ancient clash of mighty tectonic titans as the city and surrounding land and oceanic areas consist of and exists on several terranes and melanges which accreted, or formed, during this time, which make up the assemblage known as the Franciscan Complex. enter image description hereThese assemblages are an illustrative historical account of events that formed while the mighty Pacific Plate aggressively rammed its way in an easterly direction toward the equally colossal North American Plate. There was a key player who’s pretty much laid to rest now and that was the ancient tectonic Farallon Plate in this same region off of California before the - modern day, neverending, gorgeous, and ever-dynamic coastline. This ancient plate now rests somewhere underneath Utah or there about, theoretically. Although and obviously not as robust as either plate forcing it into its “Malachi crunch” (uh oh, now I’m dating my own “...Days”), it was sizable yet, so while it was forced to dive underneath the North American plate for as much as a mass apparently its size can be, until eventually becoming so plugged up and impeding its displacement from the ongoing movement of the Pacific Plate, forcing it to redirect its great energy force elsewhere. This systematic transition of backed up energy marks the birth of the famous, very active San Andreas transform fault! For more on SAF: https://geo.libretexts.org/Bookshelves/Geography_(Physical)/Supplemental_Materials%3A_Physical_Geography_(Lenkeit-Meezan)/08%3A_Shaping_the_Lithosphere/8.03%3A_Orogenesis%2C_Faults_and_Earthquakes

Equally fascinating is that the subduction of the Farallon Plate formed the Sierra Mountain Range orogenesis by way of folding and uplift, while tensional stress of the orogeny stretched things out on both Eastern and Western locations of the Sierra’s and “simultaneously” formed the Great Valley which California’s Central Valley of today was/is vastly part of, as well as the creation of the Great Basin on the far eastern side of the Sierra Nevada Mtn. Range.


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