Shortening Accommodation Mechanisms
Shortening within the Earth's crust is a response to compressional stress, which squeezes rock layers together.
Deformation Structures from Compression
Compressional stress results in specific types of geological structures:
- Folding: Rocks deform by bending without breaking, creating features like anticlines and synclines. This process shortens the horizontal extent of the rock layers.
- Reverse Faults: These are fractures where the crustal block above the fault plane (hanging wall) moves upward relative to the block below (footwall). This movement results in the shortening and thickening of the crust. Thrust faults are a low-angle type of reverse fault.
Evaluating Deformation Options
Analyzing the options based on stress types:
- Folds are caused by compression.
- Reverse faults are caused by compression.
- Normal faults accommodate extension (tensional stress), not shortening.
- Joints are fractures typically without significant displacement and don't primarily accommodate large-scale shortening.
Therefore, the combination of folds and reverse faults accurately describes the structures accommodating crustal shortening.
Important Questions from Folds
The following figure shows the lower hemisphere, equal area plots of the poles to bedding surfaces. Which one of the following is the correct interpretation?
- The intersection of bedding surface with upright slaty cleavage surface occurs as horizontal lineation. Which one of the following is the most likely interpretation? The bedding surfaces are folded into
Figures A and B show two cross-sections of a folded sandstone bed.
Which one of the following is the correct interpretation of the Figs. A and B?
Match the outcrop patterns of folds with corresponding types.
Outcrop Patterns Fold A. 
P. Plunging antiform B. 
Q. Plunging synform C. 
R. Reclined D. 
S. Recumbent - Which one of the following is the correct angle between the hinge line and the strike line of the axial plane in a reclined fold?
