ASBOG Structural Geology and Tectonics 4 — Questions and Answers
Question 1: What is a décollement in fold-and-thrust belt geology?
- A steep reverse fault cutting basement rock
- A subhorizontal detachment surface along which thrust sheets slide (Correct answer)
- A type of fold with a horizontal axial plane
- An extensional fault bounding a graben
Correct answer: A subhorizontal detachment surface along which thrust sheets slide
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?
- Rock density and temperature
- Internal friction angle and basal friction angle (Correct answer)
- Sedimentation rate and subduction velocity
- Fluid pressure and overburden thickness
Correct answer: Internal friction angle and basal friction angle
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?
- σ1 vertical, σ2 and σ3 horizontal (Correct answer)
- σ1 horizontal, σ3 vertical
- All principal stresses equal
- σ1 and σ2 vertical, σ3 horizontal
Correct answer: σ1 vertical, σ2 and σ3 horizontal
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?
- Active volcanism and earthquakes
- Thick sedimentary wedges with listric normal faults (Correct answer)
- Accretionary prisms and subduction
- Fold-and-thrust belts and suture zones
Correct answer: Thick sedimentary wedges with listric normal faults
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?
- Dip
- Plunge (Correct answer)
- Rake
- Vergence
Correct answer: Plunge
Plunge is the angle measured in a vertical plane between the fold hinge (axis) and the horizontal.
Question 6: Mylonites are defined primarily by which characteristic?
- High porosity and permeability
- Strong foliation and lineation from ductile shear zone deformation (Correct answer)
- Random fabric from cataclastic grinding
- Coarse grain size from high-temperature recrystallization
Correct answer: Strong foliation and lineation from ductile shear zone deformation
Mylonites are ductilely deformed rocks with strong planar and linear fabrics formed by crystal-plastic flow in shear zones.
Question 7: The supercontinent Pangaea is estimated to have begun breaking apart approximately how many million years ago?
- 500 Ma
- 350 Ma
- 200 Ma (Correct answer)
- 100 Ma
Correct answer: 200 Ma
Pangaea began rifting approximately 200 million years ago during the Early Jurassic, eventually forming the modern continents.
ASBOG Structural Geology and Tectonics 4
What is a décollement in fold-and-thrust belt geology?