According to the American Geological Institute's Glossary of Geology, 4th edition, the definition of graben is the following:
"graben (n) An elongate trough or basin, bounded on both sides by high-angle normal faults that dip toward the interior of the trough."
Breaking it down a bit:
A fault is a crack in a rock body along which the rock has moved--i.e., the rock on each side of the fault has moved relative to the rock on the other side. Most faults are tilted relative to Earth's surface--they're not perfectly vertical. Therefore, there is a body of rock "above" the fault and a body of rock "below" the fault. The rock "above" the fault is called the hanging wall. The rock "below" the fault is called the footwall:
The hanging wall and the footwall can move in two different ways. In a normal fault, the hanging wall moves down relative to the footwall. In a thrust fault, the hanging wall moves up relative to the footwall:
A graben is a valley bounded by high-angle (i.e., steep) normal faults on both sides:
Sorry about the blurry images. Blogger's thumbnails aren't that great. But you should be able to see the images in all their glory by clicking on them...
7 years ago