ASBOG Structural Geology and Tectonics 4

Question 1

What is a décollement in fold-and-thrust belt geology?

Accepted Answer

A subhorizontal detachment surface along which thrust sheets slide

Answer

A décollement is a subhorizontal detachment zone, often in weak rock like evaporites or shale, along which thrust sheets are transported.

Question 2

The critical taper model for accretionary wedges predicts that the wedge maintains a constant angle determined by what two factors?

Accepted Answer

Internal friction angle and basal friction angle

Answer

Critical taper theory (Coulomb wedge) shows that wedge geometry is controlled by the internal friction of the wedge and the friction along its base.

Question 3

Which stress state produces normal faults according to Anderson's fault theory?

Accepted Answer

σ1 vertical, σ2 and σ3 horizontal

Answer

Anderson's theory states that normal faults form when the maximum principal stress σ1 is vertical (gravitational load exceeds horizontal stresses).

Question 4

Passive continental margins are characterized by which structural feature?

Accepted Answer

Thick sedimentary wedges with listric normal faults

Answer

Passive margins develop on rifted continental edges with thick sedimentary prisms and listric normal faults formed during the rifting phase.

Question 5

What is the term for the angle between a fold hinge line and horizontal?

Accepted Answer

Plunge

Answer

Plunge is the angle measured in a vertical plane between the fold hinge (axis) and the horizontal.

ASBOG - Association of State Boards of Geology Practice Test