Driving Through the Most Dangerous Plate Boundary in the World: Exploring the Belly of the Beast in the Diablo Range

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.

Then, as the pressures rose as a result of regional compression along bends in the San Andreas fault, the rocks of the Franciscan Complex were thrust upwards, piercing through the cover of Great Valley sediments, somewhat like the beast in Alien. They were exposed to erosion for the first time. And it is those rocks we traverse as we drive from Mt. Hamilton to the headwaters of Del Puerto Canyon.

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.

Although the rocks are similar in appearance, detailed mapping and chemical analysis allowed the researchers to distinguish several unique zones, or terranes, within the Franciscan, rocks that had formed together at a particular time. There are two of these sequences in the core the Diablo Range, the Eylar and Burnt Hills terranes. The Eylar is the older of two. It contains some fragments of the volcanoes that once existed at the summit of the ancestral Sierra Nevada. The Burnt Hills terrane includes bits of granite, indicating that the ancestral Sierra had undergone significant erosion by late Cretaceous time.

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

After all the tight curves and winding roads on the ridge of Mt. Hamilton, San Antonio Valley was a refreshing change with straight stretches of pavement and wide open oak woodlands and meadows. It was late March and the spring wildflowers were blooming, though maybe not for long in this drought year.

And then an abrupt transformation. The gentle landscape of San Antonio Valley is rimmed in on the east by the ridge of Red Mountain, and the bleached tailings piles from the old mercury mines indicated a major change in the rock structure. We are crossing the boundary between the rocks of the Franciscan Complex and the base of the oceanic crust and upper mantle. Del Puerto Canyon, lying a short distance east over a low pass, cuts a deep swath through the Coast Range Ophiolite and the overlying Great Valley Sequence. We'll be talking about that region in the next post.

Mercury was mined from rocks of the Earth's mantle in the Red Mountain Mining District.