26.1: Modern physiography suggests an ancient subduction zone

Figure \(\PageIndex{1}\): North-central California shows a three-part physiography that corresponds to the Mesozoic origins of this part of North America as a subduction zone at a convergent plate boundary.

At the latitude of San Francisco, California has three main physiographic provinces. From the Pacific coast on the west to more inland positions in the east, these are: the Coast Ranges, the Great Central Valley, and the Sierra Nevada. In spite of their subsequent geologic histories, these are all remnants of California’s former history as a convergent margin.

Today, the region is famous as a transform boundary, where the Pacific Plate and the North American Plate slide laterally past one another along the San Andreas Fault.

Figure \(\PageIndex{2}\): The modern plate boundary in the San Francisco region is a right-lateral transform plate boundary, seen here south of San Francisco, in an oblique view along the trace of the San Andreas Fault.

But the modern plate boundary in Califronia is not the same as the ancient plate boundary. During the Mesozoic, there was a third plate offshore: the Farallon Plate, which was in between the Pacific and the North American Plates. The relative motion between the Farallon and North America was quite different. Back then, coastal California was a convergent plate boundary, and the Farallon was subducting beneath western North America.

Figure \(\PageIndex{3}\): The tectonic setting of the west coast of North America in the Mesozoic was quite different than today, as shown in this interpretive map view. There was another plate present then, of oceanic lithosphere, between the Pacific Plate (also oceanic) and the North American Plate (continental lithosphere).

The boundary between the Pacific Plate and the Farallon Plate was an oceanic ridge, the site of seafloor spreading. In other words, it was a divergent plate boundary between two plates of oceanic lithosphere. The newly-minted Farallon traveled a short distance eastward, and then subducted beneath continental lithosphere of the North American Plate, producing key features of a convergent plate boundary: the accretionary wedge (focus of this case study) but also partial melting of the overlying mantle wedge. This melting was triggered by the release of water from the subducted plate. The resulting magma rose to produce a continental magmatic (and volcanic) arc. Weathering and erosion of the arc’s mountains produced sediment that was transported downhill, and accumulated in both forearc and back-arc basins.

Figure \(\PageIndex{4}\): A diagram showing key features in a Mesozoic-California-style subduction zone. (Modified from an original by KDS4444.)

The rocks of the Coast Ranges preserve a record of their past. Let’s take a look at both rock types and structures. To make the story somewhat simpler, geologists divide the rocks of the region up into packages of rocks that have seen relatively similar stories of formation, deformation, metamorphism, and exhumation. We call these packages terranes. On the peninsula of San Francisco itself, east of the modern plate boundary (i.e., the San Andreas Fault), there are three distinct terranes, separated by two map-scale bodies of sheared-out rocks called mélange:

Figure \(\PageIndex{5}\): A west-east cross-section of the rocks beneath San Francisco, California: east of the San Andreas Fault are a series of east-dipping terranes, each showing a distinct rock suites and metamorphic and deformational history. Zones of sheared-out rock (called mélange) separate the terranes.

Note the difference in orientation of the terrane contacts (each of them a manifestation of one instance of the plate boundary between North America and the Farallon Plate) and the modern plate boundary. In the Mesozoic, the plate boundary was a subduction zone dipping to the east under North America, and that shape is preserved as powerful evidence of the fundamental geometry of that boundary. The modern transform plate boundary, in contrast, is vertical.

Further to the north and east, there are more terranes still. Each terrane is a sliver-shaped package of rocks that was subducted (some rather shallowly, some quite deeply) and then transferred from the subducting Farallon Plate to the overriding North American plate.