We all admire the Bay when we visit high places, whether it’s here at Dunsmuir Ridge (click for a 2X version) or elsewhere in the Oakland Hills. But why is there such a big basin here?
One ready answer comes when you contemplate the major faults, the San Andreas and the Hayward, that bound the bay on the west and east sides respectively. They aren’t quite parallel, but fan slightly apart as you progress to the north. As the crust moves along these faults, the part between would sink, just like what happens at the smaller scale when a sag basin forms. Maybe it does. Why, then, are the Oakland Hills still rising from compression across the Hayward fault? Well, maybe things shift direction slightly from time to time and the sense of stress across the fault changes. So today we have compression while other times we have extension. But maybe the analogy of sag basins is the wrong one.
Geologists have the same question when we consider the Great Valley, that vast trough between the Sierra and the Coast Range. Most sedimentary basins are depressed by their load of sediments, but not to the point of lying at sea level like the Great Valley. The current working hypothesis about the Great Valley is that it’s a captured slab of oceanic lithosphere, a dense bottom layer, with thick sediments on top. It naturally, persistently rides lower than the continental rocks around it. Might the San Francisco Bay basin be a chunk of the same slab? That’s one hypothesis; we don’t really know.
The roots of the Bay are pretty deep for active-source seismic imaging, the kind of technique used at the Garrido property in Antioch to study a crime scene. The Bay is also pretty small for the passive-source seismic studies used to study the crust and mantle at the regional and continental scale. If you gave me a few million dollars, I could make a stab at an investigation, but no one can spare that for an idle question. So we’ll keep wondering as we stand on the high places.