This is a fault in a quarry near Ensenada, Baja California, Mexico. My friend Annie Kammerer of the NRC sent me the image, and then I annotated it.

First off, notice that the rock is light-colored, with grey and pink tones. I suspect it's a granitoid of the Coast Ranges Batholith (one of many batholiths of the Mesozoic west coast, like the Sierra Nevada and much of Idaho.) The Coast Ranges Batholith extends from "mainland" Mexico to the Baja Peninsula, and up into southern California.

Second, a prominent cross-hatching pattern is seen in the rock. These fractures are two intersecting joint sets. Joints are fractures in rock along which there has been no movement. If the rock on opposites sides of the fracture does move, then it's not a joint; it's a fault. Joints are caused by stresses the rock experiences. Because tectonic stresses are often distributed over a large volume of rock, the rock often develops many joints in the same orientation. A bunch of parallel joints is called a joint set (here's an example from Utah). Joint sets are much more interesting than mere joints because (let's face it) joints are extremely common in rock: they are the most common geologic structure. Joint sets, on the other hand, speak about larger forces and bigger patterns.

The third and final reason for enjoying this photo is that it shows a fault well. Running right down the middle is a prominent fault. Note that the fault is wider than the joints: it's filled with some sort of pulverized goo. This is a material called fault gouge. Faults may or may not have fault gouge in them. It's essentially ground-up rock: any bits that stick out get crushed as the fault grinds over them: like a mortar and pestle smashes up spices. Sometimes when the fault is more planar, the rock rubs directly against its neighbor, producing slickensides. Sometimes, asperities (knobs & bumps on the fracture surface) get snapped off, but not ground into pulp: this produces a fault breccia (like this celebrated example in Death Valley).

I'm struck by how vertical this fault is. Faults come in all sorts of orientations, but there are four really common ones: (1) high-angle normal faults which dip at an average of 60º into the Earth, (2) low-angle reverse faults which dip at an average of 30º into the Earth, (3) extremely-low-angle thrust faults, which can be close to horizontal, and (4) vertical strike-slip faults, which dip at 90º, or close to it. This appears to be the latter. Annie, the photographer, tells me that there was a substantial (~100 m) offset along this fault. If you looked down on it from a bird's-eye view, you might be able to tell that, but it's impossible to gauge the offset from this cross-sectional view.

Final observations: the fault is oriented the same way as one of the joint sets. It's likely that the rock was jointed first, and then when tectonic stresses required strike-slip faulting, one of those joints (a pre-existing plane of weakness) was utilized as the site of movement. Note too up in the upper-left another "joint" seems filled with fault gouge, meaning it's really another (parallel) fault. That would be entirely expected if a jointed rock was being tectonically smeared out. The vertical "slices" of rock migrate past one another like a sheared loaf of (sliced) bread. Some of the slices adhere along their cut surface, whereas others move. Some of the joints become faults, but once they start moving, the stress is accommodated, and there's no reason for every joint to become a fault. The weakest link takes all the strain.

Labels: faults, joints, mexico