ASBOG Structural Geology and Tectonics 5 — Questions and Answers
Question 1: Which structural feature is defined by a 180° change in dip direction over a short horizontal distance?
- Homocline
- Monocline (Correct answer)
- Anticline
- Kink band
Correct answer: Monocline
A monocline is a step-like fold where strata dip in one direction, creating a flexure with a change in dip, often draping over a basement fault.
Question 2: The Himalayan mountain belt formed from collision between which two plates?
- Pacific and Eurasian plates
- African and Eurasian plates
- Indian and Eurasian plates (Correct answer)
- Arabian and Eurasian plates
Correct answer: Indian and Eurasian plates
The Himalayas formed from the collision of the Indian subcontinent with the Eurasian Plate beginning ~50 Ma, creating the highest mountains on Earth.
Question 3: What does a stereonet (equal-area projection) primarily allow a structural geologist to do?
- Calculate age of rock formations
- Plot and analyze orientations of planes and lines in three dimensions (Correct answer)
- Measure fault displacement
- Determine rock composition from outcrop data
Correct answer: Plot and analyze orientations of planes and lines in three dimensions
Stereonets project 3D orientations of structural features (foliations, faults, fold axes) onto a 2D hemisphere for geometric analysis.
Question 4: In thin-skinned tectonics, deformation is confined to which part of the crust?
- The lower crust and upper mantle
- Only the basement crystalline rocks
- The sedimentary cover above a basal detachment (Correct answer)
- The entire lithosphere
Correct answer: The sedimentary cover above a basal detachment
Thin-skinned tectonics involves folding and thrusting only of the sedimentary cover, detached from basement along a décollement horizon.
Question 5: Which type of fold has a horizontal or near-horizontal axial plane with one limb overturned?
- Upright fold
- Inclined fold
- Recumbent fold (Correct answer)
- Reclined fold
Correct answer: Recumbent fold
Recumbent folds have essentially horizontal axial planes, causing one limb to be overturned so beds face downward.
Question 6: Positive flower structures in plan view are associated with which type of fault system?
- Normal fault systems
- Thrust fault systems
- Transpressional strike-slip systems (Correct answer)
- Transtensional strike-slip systems
Correct answer: Transpressional strike-slip systems
Positive flower structures form where transpression occurs along strike-slip faults, creating upward-diverging reverse faults and surface uplift.
Question 7: The concept of strain ellipsoid describes what aspect of deformed rocks?
- The original shape before deformation
- The three-dimensional state of finite strain showing principal stretch axes (Correct answer)
- The stress applied to the rock during deformation
- The orientation of cleavage relative to bedding
Correct answer: The three-dimensional state of finite strain showing principal stretch axes
The strain ellipsoid represents how a sphere deforms into an ellipsoid, defined by three principal axes of finite strain (X≥Y≥Z).
ASBOG Structural Geology and Tectonics 5
Which structural feature is defined by a 180° change in dip direction over a short horizontal distance?