ASBOG - Association of State Boards of Geology Structural Geology and Tectonics Questions and Answers — Questions and Answers
Question 1: On a geologic map, a series of parallel ridges and valleys are observed. Rock units on the map show a repeating sequence of older rocks at the core of the ridges and younger rocks in the valleys. The beds on both sides of the ridges dip away from the ridge crests. What structural feature does this pattern represent?
- A series of plunging synclines
- A horst and graben sequence
- A series of plunging anticlines (Correct answer)
- An imbricate thrust system
Correct answer: A series of plunging anticlines
This map pattern is characteristic of a series of plunging anticlines. Anticlines are up-folds where the oldest rocks are in the core, and the limbs dip away from the axial plane. When these folds plunge and are eroded, they create a V-shaped outcrop pattern on the map where the 'V' points in the direction of plunge. The ridges represent the more resistant rock layers within the anticlines.
Question 2: According to Anderson's theory of faulting, which stress regime is responsible for the formation of a normal fault?
- The maximum principal stress (σ1) is horizontal and the minimum principal stress (σ3) is vertical.
- The maximum principal stress (σ1) is vertical and the minimum principal stress (σ3) is horizontal. (Correct answer)
- The intermediate principal stress (σ2) is vertical, with σ1 and σ3 being horizontal.
- The maximum principal stress (σ1) and intermediate principal stress (σ2) are equal and horizontal.
Correct answer: The maximum principal stress (σ1) is vertical and the minimum principal stress (σ3) is horizontal.
Anderson's theory classifies fault types based on the orientation of the three principal stresses (σ1 > σ2 > σ3). For normal faults, which accommodate crustal extension, the maximum principal stress (σ1) is vertical (due to gravity), and the minimum principal stress (σ3) is horizontal, allowing the crust to stretch and pull apart.
Question 3: A structural geologist plots the poles to bedding from numerous outcrops across a folded region onto a stereonet. The poles form a great-circle girdle. What does the pole to this great-circle girdle represent?
- The strike of the axial plane
- The dip of the steepest limb
- The interlimb angle
- The trend and plunge of the fold axis (Correct answer)
Correct answer: The trend and plunge of the fold axis
For a cylindrical fold, the poles to the bedding planes measured along the fold lie on a great circle (a pi-circle or girdle). The pole to this great circle represents the orientation (trend and plunge) of the fold axis itself.
Question 4: A geologist is working in a continental rift zone, such as the East African Rift. Which of the following combinations of structural and volcanic features would they most likely encounter?
- Thrust faults, tight isoclinal folds, and andesitic stratovolcanoes.
- Strike-slip faults, pull-apart basins, and widespread silicic ash-flow tuffs.
- Normal faults, horsts and grabens, and bimodal (basalt-rhyolite) volcanism. (Correct answer)
- Reverse faults, nappes, and high-grade metamorphic rocks like eclogite.
Correct answer: Normal faults, horsts and grabens, and bimodal (basalt-rhyolite) volcanism.
Continental rift zones are characterized by extensional tectonics. This leads to the formation of normal faults, which create down-dropped blocks (grabens) and uplifted blocks (horsts). The thinning crust allows for magma to ascend, often resulting in bimodal volcanism, with both mafic (basalt) and felsic (rhyolite) lavas erupting.
Question 5: Which of the following conditions would most favor ductile deformation of a rock over brittle failure?
- Low temperature, low confining pressure, and rapid strain rate.
- High temperature, low confining pressure, and rapid strain rate.
- Low temperature, high confining pressure, and rapid strain rate.
- High temperature, high confining pressure, and slow strain rate. (Correct answer)
Correct answer: High temperature, high confining pressure, and slow strain rate.
Ductile deformation, or flowing/bending of rock, is favored by conditions found deeper in the crust. These include high temperatures, which allow minerals to recrystallize and flow; high confining pressure, which inhibits the formation of fractures; and slow strain rates, which provide time for the material to deform plastically rather than break.
Question 6: A geologic cross-section reveals older Paleozoic rocks have been thrust over younger Mesozoic rocks. What type of fault is responsible for this relationship?
- A growth fault
- A normal fault
- A reverse fault (Correct answer)
- A strike-slip fault
Correct answer: A reverse fault
A reverse fault is a type of dip-slip fault where the hanging wall moves up relative to the footwall. This compressional movement results in placing older rocks on top of younger rocks, causing a repetition of the stratigraphic section. Thrust faults are a specific type of low-angle reverse fault.
