60.5: Folds and faults

Folds and faults are deformational structures, also called tectonic structures. They form as a consequence of tectonic deformation of pre-existing rocks. Where we have tectonic compression, we get folds and thrust faults. Where we have tectonic extension, we get normal faults.

Folds

Figure \(\PageIndex{1}\): Anticlines and synclines are different aspects of the same crumpling of a sedimentary layer.

Rock strata can be folded under compressional stresses applied over a long period of time. Generally, we recognize two major categories of folds, and one minor category: anticlines, synclines, and monoclines. Anticlines and synclines occur together, and share a “limb” in common. Where you find anticlines, you often find synclines, too.

Monoclines are features associated with the adjustment of sedimentary layers over top of basement uplifts. They are folds where the layers go from horizontal (on top of the uplift) to tilted, to horizontal again (next to the uplift). They are reasonably common in the Rocky Mountain west and Colorado Plateau region of North America, but not really anywhere else. Anticlines and synclines, in contrast, have a global distribution anywhere there is a mountain belt.

Figure \(\PageIndex{2}\): Monoclines, anticlines, and synclines are the three basic types of folds. Here they are shown in block diagrams, as well as in diagrams showing their essential geometry: the relationship of the strata to the axial surface.

It’s important to note that just because a fold goes down in the middle (i.e., a syncline), that doesn’t mean the landscape will also go down (i.e., a valley). The land surface’s shape depends on many variables, including the very important role played by the different rates at which different rock units weather.

For instance, Sideling Hill in Maryland is a mountain ridge (a “hill,” a topographic high) but a syncline (a structural low):

It’s only a mountain ridge because the strata at the center of the syncline are quartz-rich and therefore weather more slowly than the silty strata beneath them. The opposite situation is just as common: anticlines with easily-weathered material at the center are etched out so the structural high is weathered out to form a topographic low. Bottom line: There is no guaranteed relationship between the structure of a fold and the shape of the overlying landscape.

Faults

Faults are breaks in the crust along which movement has occurred. There are three main sorts of movement we observe on faults:

(a) the fault plane is dipping at some non-vertical angle, and the upper block of rock has slid down the fault surface under the influence of gravity. The movement is downward along the dip line of the fault, and is called normal faulting. It occurs in situations of tectonic extension, such as at divergent plate boundaries. Low-angle (close to horizontal) normal faulting is called “detachment faulting.”

(b) the fault plane is dipping at some non-vertical angle, and the upper block of rock has moved up the fault surface, getting shoved into position by tectonic compression. The movement is upward along the dip line of the fault, and is called reverse faulting. It occurs in situations of tectonic convergence, such as mountain belts and subduction zones. Low-angle (close to horizontal) reverse faulting is called “thrust faulting.”

(c) the fault plane is vertical, and the two blocks of rock move laterally past one another in opposite directions. These are “strike-slip” faults, and they occur at transform plate boundaries. All the slip is parallel to the strike of the fault. Strike-slip faults can be either right-lateral or left-lateral. If you imagine standing on one side, and looking across the trace of the fault at the opposite side, and the rock over there looks like it’s moving to the left, then the fault is left-lateral. To an observer on the opposite side, it will look like you are moving to their left!