[GOP_1900AD]

The plate being subducted beneath the North American Plate along the trench that runs from Humboldt County CA up to the Strait of Juan De Fuca is a rather small one. There is a minor spreading center a few hundred miles west of that trench. As the small plate subducts, volcanos in the Cascades happen. The Cascades run from Mt. Lassen, in far northern CA up to Mt. Baker, in N. WA (from about 41 N. lat up to about 49 N. lat.). The San Andreas and related faults run from the northern tip of the Gulf of CA up to the point where the trench and the spreading center join up in what is known as a triple junction, off the Humboldt Coast. It has a right lateral "strike slip" motion - e.g. the plates are grinding past each other in slight compression but there is little vertical or overiding motion. The SAF runs roughly NNW - SSW but takes a major jog between just north of Palm Springs and just north of Ventura. It also takes a smaller jog between Hollister and Woodside. As a result of the two jogs there are normal and thrust faults picking up some of the local strong compressive motion. Also, to accomodate the bends, there are Left lateral faults at ~ 30 / 60 degree angles to the SAF. The stuff going on in the Mojave Desert and east of the Sierra Nevada (also in Nevada, Idaho, Montana, Utah) in terms of geothermal output and minor volcanism have nothing directly to do with the SAF. Speaking of triple junctions there is a theory that there is a failed triple junction in the Gulf of CA the failed third arm of which accounts for this intra continental volcanism. Either that or the North American continent is slowly getting torn apart and a new ocean will form between the Gulf of CA and the Arctic at some point.

[capitan_refugio]

Back in the early to late 1980's I spent considerable time studying the structural geology of the Point Arena Basin, offshore Northern California. The study included coastal outcrop examaination, inspection of cores from oil wells drilled in the area (datiing from the early 1900's), inspection and correlation of oil well electrical logs from both onshore and offshore oil wells, and the mapping of the offshore geology from high resolution, CDP seismic data.

It is my professional opinion that the historic mapped "trace" of the San Andreas fault, based on the 1906 event, is incorrect. I believe that the modern trace of the San Andreas fault "horsetails" out into the central Point Arena basin, and that the plate boundary and junction with the Menodcino Fracture Zone is considerably more complicated than the previously thought.