Structural Geology?

Question

Folds and faults

Answer 1

Geologic faults, fault lines or simply faults are planar rock fractures, which show evidence of relative movement. Large faults within the Earth's crust are the result of shear motion and active fault zones are the causal locations of most earthquakes. The largest examples are at tectonic plate boundaries but many faults occur far from active plate boundaries. Faults can be categorized into three groups based on the sense of slip. A fault where the main sense of movement (or slip) on the fault plane is vertical is known as a dip-slip fault. Where the main sense of slip is horizontal the fault is known as a transform (or strike-slip) fault. Oblique-slip faults have significant components of both strike and dip slip.

For more information and images http://en.wikipedia.org/wiki/Faults

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (i.e. permanent) deformation. Synsedimentary folds are those due to slumping of material before deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones.

For more information and images http://en.wikipedia.org/wiki/Fold_%28geology%29

Answer 2

Folds are caused by compression of the crust of the earth. If the fold is in one direction (like a smooth stair step) it is called a monocline. Anticlines are upward folds (like a mound) and synclines are downward folds, like a bowl (usually all three are much longer, so are more like creases in a rug).

Faults are either Dip-Slip or Strike-Slip. Dip slip faults include those caused by compression (reverse and thrust faults) and those caused by extension (normal faults). In a reverse fault a part of the crust is pushed up and over it's neighbor and in a normal fault a part of the crust is lowered below the level of its neighbor. If the rocks were in a normal (older at the bottom) stratigraphic position prior to faulting then the reverse fault would emplace old rock on top of young rock, and a section of crust would also seem to be repeated, if you drilled a well through it. In the same situation a normal fault would seem to take away some of the rock layers, so you would seem to have a missing section of rock. A strike-slip fault (which may have it's plane at an angle and be called an oblique-slip fault) moves rocks sideways against one another, such as the San Andres fault in California, USA. Transform faults are faults that occur along and about perpendicular to many other faults and mid-ocean ridges, and change their fault type from place to place.