Faults Folds and Joints Questions and Answers

Answer: C

Correct answer: displacement along the fracture

Both are fractures; the fault is the one along which the two sides have moved relative to each other. A joint may be open, mineral-filled or water-bearing and it is still a joint provided nothing has slipped along it. The distinction matters in practice: a fault carries crushed rock, a zone of weakness and often water, while a joint is a clean plane whose orientation and spacing govern block size.

Answer: D

Correct answer: vertical

Hade is measured from the vertical, and dip from the horizontal, so the two are complements:

A steep fault has a large dip and a small hade. The term survives from mining practice, where the departure of a fault or vein from the vertical was what mattered for driving a shaft, and it is a standard trap in examination questions.

Answer: A

Correct answer: above the fault plane

On an inclined fault plane, the block resting above it is the hanging wall and the one beneath is the foot wall - names taken from mining, where a miner working the plane would hang his lamp on the block above and stand on the one below. The distinction is what defines the fault types: the hanging wall moves down in a normal fault and up in a reverse fault.

Answer: B

Correct answer: down relative to the foot wall

A normal fault drops the hanging wall relative to the foot wall, which lengthens the section horizontally - so normal faulting records extension, or tension, in the crust. Rift valleys and the graben structures within them are built from sets of them. In a borehole crossing a normal fault, part of the sequence is missing, since the beds have been dropped past one another.

Answer: C

Correct answer: up relative to the foot wall

A reverse fault pushes the hanging wall up over the foot wall, shortening the section horizontally, so it records compression. In a vertical borehole the same bed is met twice, once in each block, because the sequence has been stacked on itself. A reverse fault of low dip is a thrust, which is the characteristic structure of a fold and thrust belt.

Answer: D

Correct answer: a low angle, commonly under 45 degrees

A thrust is defined by its shallow dip - conventionally below 45 degrees, and often only a few degrees over long distances. The low angle allows enormous horizontal displacement for a modest vertical one, so an entire thrust sheet may be carried tens of kilometres over younger rocks. This is how old rocks come to lie above young ones over large areas of a mountain belt.

Answer: A

Correct answer: horizontal, along the strike of the fault

In a strike-slip fault the blocks slide past each other horizontally, along the strike, with little or no vertical component, so the crust is neither shortened nor extended but sheared. Such faults are usually steep to vertical, and they are described as dextral or sinistral according to which way the far block appears to move to an observer facing it.

Answer: B

Correct answer: vertical component of the displacement

The total displacement along the plane is the net slip; its vertical component is the throw and its horizontal component the heave. For a slip \( s \) on a plane of dip \( \delta \),

Throw is the quantity usually quoted, because it is what determines how far the beds have been displaced up or down and therefore what a borehole or a map will show.

Answer: C

Correct answer: downthrown between two normal faults

A graben is a block dropped between two normal faults dipping towards each other, so it forms a trough. Grabens record extension and are the structural basis of rift valleys - the East African Rift and the Rhine Graben among them. The complementary structure, a block left standing high between two normal faults dipping away from each other, is a horst.

Answer: D

Correct answer: high between faults that dropped the adjacent ground

A horst stands above its surroundings because the blocks on either side have been dropped by normal faults, not because it has been pushed up. It is the counterpart of the graben, and the two alternate in an extensional province to give the basin and range topography of parallel ridges and troughs. Both are products of tension.