Under the Volcano in my early days of blogging).
as shown in the last post. In others we find granitic and metamorphic rocks that look suspiciously like the rocks of the Sierra Nevada. Those parts lay on the west side of the San Andreas fault, which slices right through the Coast Ranges.
The biggest mess is the Franciscan Complex. It can be composed of greywacke sandstone (a sandstone with lots of clay and small rock fragments mixed in, generally, uh, gray in color), dark shale, red and green chert, some basalt, and even bits of limestone or marble. And the rock is a chaotic mix where the normal rules of stratigraphy have been tossed out the window. Geochemical studies show that some of these rocks have been many miles down in the crust, and have since been exhumed. Some of this chaotic mix, termed a melange, can be seen in the picture below, from along the coast north of Muir Beach.
On the ocean floor, sediments are scraped off the descending oceanic crust and mixed with sediments washing into the trench from the land. The resulting jumble is called an accretionary wedge. Between the accretionary wedge and the magmatic arc, a shallow ocean basin often forms, collecting thousands of feet of sediments like sand, silt and clay. This linear sea is called a forearc basin.
If you think about it, you might realize how difficult it is to directly sample the rocks in an active subduction zone complex. The volcanoes are there to see, of course, but it is difficult to drill into the active magma chambers. And the forearc basin and accretionary wedge are generally beneath the waves, meaning that research drilling would face even greater challenges. But there is no longer a subduction zone in Central California. It has been replaced by the lateral motion of the San Andreas fault (a transform boundary). The various parts of the subduction zone complex have subsequently been uplifted and eroded, revealing the deepest parts of a converging plate boundary.
The volcanoes of the Sierra Nevada magmatic arc were mostly eroded away millions of years ago (some eroded fragments cover parts of the northern Sierra Nevada). The sediments of the forearc basin are still largely in place, forming the basement of the Great Valley, but on the western edge, the sediments have been turned upwards, as seen in the hastily sketched diagram below (I forgot my chalkboard on the field trip last week, so I used this sketch in the field). We can literally drive into the earths crust for 20,000-30,000 feet all the way to the underlying ocean crust, which we call the Coast Range Ophiolite. We simply need to drive or walk up into the canyons of the Coast Ranges. In some places we find mantle rock, in the form of peridotite or serpentine. Likewise, we have numerous exposures of the accretionary wedge deposits that can be sampled with a hammer instead of a drill rig on a ship.