ASBOG Structural Geology and Tectonics 3 — Questions and Answers
Question 1: What does the term 'vergence' describe in fold analysis?
- The direction of plunge of fold hinge
- The direction toward which a fold or thrust is overturned or displaced (Correct answer)
- The strike direction of the axial plane
- The interlimb angle of the fold
Correct answer: The direction toward which a fold or thrust is overturned or displaced
Vergence indicates the direction of tectonic transport or overturning, typically toward the foreland in fold-and-thrust belts.
Question 2: Which seismic discontinuity separates the mantle from the outer core?
- Conrad discontinuity
- Mohorovičić discontinuity
- Gutenberg discontinuity (Correct answer)
- Lehmann discontinuity
Correct answer: Gutenberg discontinuity
The Gutenberg discontinuity at ~2,900 km depth marks the core-mantle boundary where P-wave velocity drops sharply.
Question 3: Slickensides on a fault surface provide information about what?
- Fault age
- Direction and sense of slip (Correct answer)
- Depth of faulting
- Rock type of fault zone
Correct answer: Direction and sense of slip
Slickensides are polished striations on fault surfaces that record the direction and often the sense of most recent slip.
Question 4: An ophiolite sequence, from bottom to top, begins with which unit?
- Pillow basalts
- Sheeted dike complex
- Ultramafic peridotite (Correct answer)
- Pelagic sediments
Correct answer: Ultramafic peridotite
A complete ophiolite sequence begins with ultramafic peridotite (mantle rock) at the base, overlain by gabbro, sheeted dikes, pillow basalts, and sediments.
Question 5: Isostasy explains which geological observation?
- The age of oceanic crust increases away from mid-ocean ridges
- Mountain roots extend deep into the mantle to achieve gravitational equilibrium (Correct answer)
- Continental crust is denser than oceanic crust
- Earthquakes occur at predictable depths along subduction zones
Correct answer: Mountain roots extend deep into the mantle to achieve gravitational equilibrium
Isostasy describes how crustal blocks 'float' on the mantle; mountain belts have deep crustal roots to compensate for their elevated topography.
Question 6: What type of metamorphic foliation forms by rotation and alignment of platy minerals perpendicular to the maximum compressive stress?
- Crenulation cleavage
- Fracture cleavage
- Slaty cleavage (Correct answer)
- Mylonitic foliation
Correct answer: Slaty cleavage
Slaty cleavage forms when phyllosilicate minerals rotate and recrystallize perpendicular to σ1, producing a smooth planar fabric.
Question 7: In a strike-slip fault, which term describes the block that moves toward the right when you face the fault?
- Normal fault
- Sinistral fault
- Dextral fault (Correct answer)
- Reverse fault
Correct answer: Dextral fault
A dextral (right-lateral) strike-slip fault causes the block on the opposite side to appear to move to the right when facing the fault.
ASBOG Structural Geology and Tectonics 3
What does the term 'vergence' describe in fold analysis?