A long time ago (a few weeks; that's years in internet time) I began this series about driving through the most dangerous plate boundary in the world. Convergent boundaries with their subduction zones or gigantic thrust faults are the most dangerous boundaries that exist on the planet. We need no more reminder than the horrible tragedy that unfolded this week in Nepal.
California was once dominated by such a boundary, from about 200 million years to just a few tens of millions years ago. The subduction zone was replaced by a transform boundary known to most as the San Andreas fault. The San Andreas is dangerous in its own way, but the biggest quakes it causes pack only a thirtieth of the punch of a really big subduction zone quake (magnitude 8 compared to a magnitude 9+). The subduction zone still exists in the northern part of the state; it's called the Cascadia Subduction Zone.
The subduction complex includes three main features, an accretionary wedge, a forearc basin, and a magmatic arc. We have been traveling through parts of the accretionary wedge as we made our way through the Marin Headlands and Golden Gate. We've now entered another section of the wedge, the core of the Diablo Range. It's the domain of the Franciscan Complex, the "belly of the beast".
In the last post I likened the Diablo Range as being the structural equivalent of the alien baby popping out of John Hurt's chest in the movie Alien. Bear with me on this one. Material scraped from the ocean crust along with sediments washed in from the nearby continent accumulate in the trench, and get carried deep into the lithosphere where the rocks were subjected to intense pressure and elevated temperatures. The appearance and composition of the rocks was changed, and some of the minerals serve as proxies for estimating the depth the rocks reached: as much as 20 miles (32 kilometers)! The rocks churned upwards again, reaching close to the surface, but in the aftermath of the end of subduction the rocks were covered by the sediments of the Great Valley Group.
|After the narrow deserted road up the flank of Mt. Hamilton, the Lick Observatory complex felt positively urban.|
The rocks themselves are unremarkable to look at, being composed mostly of a clay-rich sandstone called graywacke (a German term for "gray barren rock"), and black mudstone and shale. A few interesting chunks of rock are exposed here and there, red or green chert sequences and pillow basalts like those of the Marin Headlands, and some other blocks called "blueschist knockers" (please don't ask). The blueschist is a form of metamorphic basalt or other volcanic rock that has been altered under high pressure into a bluish-gray rock containing the minerals glaucophane and lawsonite.
|The observatory complex is really a small village with dormitories and tourist parking.|
The earliest geologists to study the Franciscan Complex had a real challenge on their hands. The rocks didn't make much sense. They didn't "follow the rules" of normal stratigraphy. They were intensely faulted and folded, sometimes upside down, and thus were difficult to map. The geologists had no known process in their knowledge base that could explain such rocks. It wasn't until the formulation of the theory of plate tectonics in the late 1960s that these rocks could be explained as the product of deformation and metamorphism in the accretionary wedge of a subduction zone.
|The views from the summit of Mt. Hamilton are expansive, to say the least.|
|Some of the Manzanita bushes near the summit were more like small trees|
|San Antonio is a peaceful valley owned by some ranch outfit called "No Trespassing". His signs were everywhere.|
So, the thing is, I've been living here in the Great Valley for 25 years, and I've been up Del Puerto Canyon dozens of times, but until last month I had never traveled State Route 130 to Lick Observatory and San Antonio Valley. I finally did so, and I took the pictures gathered together in this post. There aren't all that many rocks in the pictures, because graywacke (the German "gray barren rock") is gray. And barren. The chaparral and oak woodlands were far more interesting, at least on a macro-scale. In a petrographic microscope, all kinds of interesting things have happened to these rocks due to their deep burial.
|An American Wigeon in a stock pond in San Antonio Valley|
|Mercury was mined from rocks of the Earth's mantle in the Red Mountain Mining District.|