26.5: Deformation of these rocks into geological structures

Convergent plate boundaries are stressful places: the rocks and sediments fed into the subduction zone are subject to compression and shearing, and the frequently deform in response to these stresses. Examples of deformation typical of subduction zone are well recorded in the rocks of California’s Coast Ranges: folding, faulting, and extensive zones of shearing that result in “mixed-up” rocks called mélange. Let’s examine each in turn.

Figure \(\PageIndex{1}\): Chevron-folded cherts of the Marin Headlands Terrane, exposed at Kirby Cove.

CRUMPLED LAYERS: Folding

Folding is particularly obvious in the regularly-layered cherts of the Marin Headlands Terrane, though it can also be spotted where bedding can be detected in the sandstone and shale layers in other adjacent terranes. Mechanically differentiated rock layers, such as chert/shale interbeds, are particularly susceptible to folding when compressed parallel to their layering: they buckle and crumple. Because the shale is weak and the chert is relatively stiff, layers of chert slide relative to one another on a “geological banana peel” of shale. This flexural slip allows much more folding than a homogeneous body of chert would be able to accommodate.

Folding occurs on many scales in California’s Coast Ranges. As the example below shows, millimeter-thin layers can be folded, but so too can entire terranes on the scale of kilometers. For instance, the Angel Island Terrane is near the axis of a regional synform, with more recently accreted terranes on either side of it as well as wrapping around below it.

Figure \(\PageIndex{2}\): Folds in deep sea vent deposits, Kayak Beach, Angel Island, California.

SLIP ALONG FRACTURES: Faulting

Another form of deformation that is common in the accretionary wedge is faulting. Through faulting, new slivers of the oceanic section are slathered onto the bottom of the accretionary wedge complex, building it up from below.

At Marin Headlands, north of the Golden Gate Bridge, there are no fewer than 17 thrust faults that have placed deeper rocks of the oceanic crust on top of near-shore and off-shore sediments, repeating the overall oceanic section again and again.

Figure \(\PageIndex{3}\): At Marin Headlands, basalt (rock of the oceanic crust) has been thrust atop trench sediments (sandstone and shale, sediments that must originally have been deposited on top of the crust). Sixteen such thrust faults have been mapped out at Marin Headlands, each repeating the oceanic “section” within the accretionary wedge.

Figure \(\PageIndex{4}\): At Marshall’s Beach in San Francisco, just south of the Golden Gate Bridge, visitors can observe the trace of the former plate boundary fault. This pencil-thin fault separated rocks already accreted to the North American Plate (in this case, former mantle that has been metamorphosed to serpentinite) from those that were being fed beneath, into the subduction zone (in this case, seafloor sediments of the Marin Headlands Terrane).

Sometimes these faults are discrete and crisp, where two vastly different rock units have been brought into contact along the slip surface. An example of this can be seen at right, where former mantle rock (serpentinite of the Hunter’s Point Shear Zone) has been emplaced atop seafloor sediments (shale and sandstone mainly) of the Marin Headlands Terrane. During the Mesozoic, this fault was for a while the plate boundary between North America and the Farallon Plate. As subduction proceeded, new faults formed within the subducting plate, and the relative motion between the two plates transferred from a shallower fault to a deeper one. Newly subducted sediments were shoved underneath the mantle sliver, as the Farallon moved downward relative to North America. Later, another new fault formed, becoming the new plate boundary, and this fault was abandoned, left inert in the geologic record, with the footwall rock below it now also accreted to the North American Plate.

Figure \(\PageIndex{5}\): Geologist John Wakabayashi of California State University, Fresno, examines the fault contact between metasedimentary rocks of the Marin Headlands Terrane (footwall, to the left/west) and serpentinite of the Hunter’s Point Shear Zone (hanging wall, to the right/east). This fault places mantle rocks on top of seafloor sediments. Sometime in the Mesozoic, this was the boundary between North America and the subducting Farallon Plate.

Figure \(\PageIndex{6}\): “Block in matrix” texture is characteristic of many Coast Ranges mélange outcrops. Marshall’s Beach, San Francisco, California.

THE MIXED-UP ROCK: tectonic shearing to make mélange

Other rocks may be sheared out, deforming in a mass. Serpentinites are particularly weak rocks under shear stress, and break into thousands of tiny faults, giving the overall rock a “scaly fabric,” which looks at first like a cleavage. However, many of the little flakes shows slickenlines on their surfaces, indicating they are little faults that have seen some slip. A large body of serpentinite can thus be transformed to a broad shear zone. If other rock types get mixed into the sheared-out serpentinite, a serpentinite mélange will result. Mélange is a term for tectonically-mixed-up rock, where varying blocks or “lozenges” of varying rock types get entrained in a sheared matrix. We call this texture “block in matrix.” Most commonly the blocks are of coherent blocks of the matrix rock, but in other cases, all sorts of exotic blocks can be introduced. At Ring Mountain in Tiburon, for instance, a serpentinite matrix hosts blocks of blueschist, eclogite, amphibolite, peridotite, and meta-chert.

Figure \(\PageIndex{7}\): Serpentinite mélange showing block-in-matrix texture. Perles Beach, Angel Island, California.

Figure \(\PageIndex{8}\): A mix of protoliths in mélange exposed at Pacifica State Beach, California. Mélange can have a metavolcanic or a metasedimentary matrix. Here, it’s both!

This pulpy mess is extraordinarily incompetent, and is the source of frequent landslides in coastal California. Shockingly, the south tower of the Golden Gate Bridge is anchored in serpentinite mélange of the Hunter’s Point Shear Zone.

Shale acts in the same slippery fashion as serpentinite, as the flakes of clay it contains slip relative to one another, creating a scaly fabric. Shale-based mélange is just as common as serpentinite-based mélange. In San Francisco itself, the Hunter’s Point Shear Zone is a wide swath of serpentinite mélange, which the City College Fault Zone has more of a shale basis to the scaly matrix. Near Pacifica, a town further south along the coast, the mélange has a distinctive mix of metavolcanic and metasedimentary source rocks, resulting striking mixtures of pale green and dark gray.