Types of Faults

Why This Matters

Faults are where the action happens in geology—they're the fractures along which Earth's crust actually moves, releasing stress and reshaping landscapes. When you study faults, you're really studying how tectonic stress translates into motion: whether rocks are being pulled apart, squeezed together, or sliding past each other. This connects directly to plate tectonics, earthquake mechanics, mountain building, and basin formation—all core concepts you'll see tested repeatedly.

Don't just memorize fault names and definitions. For each fault type, know what stress regime creates it (tension, compression, or shear), how the rock blocks move relative to each other, and what landforms result. Exams love asking you to identify fault types from diagrams, explain why a particular fault forms in a given tectonic setting, or predict what surface features you'd expect to see. Master the underlying mechanics, and you've got this.

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Extensional Faults: When the Crust Pulls Apart

When tectonic forces stretch the crust, rocks fracture and slip along planes that accommodate that extension. The key mechanism is tensional stress causing the crust to thin and blocks to drop down along inclined fault surfaces.

Normal Fault

• Hanging wall drops down relative to the footwall—this is the defining characteristic and the most common exam question
• Forms in rift zones and divergent boundaries where tensional stress stretches the crust horizontally
• Creates valleys, basins, and grabens—think of the Basin and Range Province in the western U.S.

Listric Fault

• Curved fault plane that flattens with depth—the surface is steep near the top but becomes nearly horizontal at depth
• A special type of normal fault common in extensional settings like continental rifts and passive margins
• Produces half-grabens and rotated fault blocks—important for understanding sedimentary basin architecture and petroleum geology

Horst and Graben

• Horsts are uplifted blocks; grabens are down-dropped blocks—formed by parallel normal faults
• Created by extensional tectonics pulling the crust apart along multiple fault planes simultaneously
• Produces distinctive basin-and-range topography—the alternating valleys and ridges you see across Nevada and East Africa's Rift Valley

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Compressional Faults: When the Crust Shortens

When tectonic plates collide or converge, the crust shortens and thickens. Compressional stress forces rock blocks to override one another along fault planes, building mountains and stacking rock layers.

Reverse Fault

• Hanging wall moves up relative to the footwall—the opposite of normal faults, driven by compression rather than extension
• Forms at convergent plate boundaries where plates collide and crust is shortened horizontally
• Builds mountain ranges and elevated terrains—responsible for uplift in the Himalayas and Andes

Thrust Fault

• A low-angle reverse fault (typically less than 45°) where the hanging wall is pushed up and over the footwall
• Creates stacked rock sequences and nappes—older rocks can end up on top of younger rocks, violating normal superposition
• Common in fold-and-thrust belts of major mountain ranges—critical for understanding Appalachian and Rocky Mountain geology

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Shear Faults: When Blocks Slide Horizontally

When tectonic stress acts parallel to a fault plane, blocks slide past each other laterally rather than moving up or down. The dominant motion is horizontal, driven by shear stress at transform boundaries or within plates.

Strike-Slip Fault

• Horizontal movement with minimal vertical displacement—blocks slide past each other laterally along a vertical or near-vertical fault plane
• Classified as right-lateral or left-lateral depending on which direction the opposite block appears to move when you face the fault
• Creates offset features like displaced stream channels, roads, and fence lines—classic field evidence for identifying these faults

Transform Fault

• A strike-slip fault that forms a plate boundary—specifically connects offset segments of mid-ocean ridges or other plate boundaries
• Accommodates differential plate motion where plates slide horizontally past each other without creating or destroying crust
• Produces major earthquakes—the San Andreas Fault is the textbook example, marking the Pacific-North American plate boundary

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Complex Motion: When Faults Do Both

Not all faults fit neatly into one category. When multiple stress directions act simultaneously, faults can exhibit combined vertical and horizontal motion.

Oblique-Slip Fault

• Combines dip-slip and strike-slip motion—rocks move both vertically and horizontally along the fault plane
• Results from complex stress fields where tension, compression, and shear act together or change orientation over time
• Creates diverse geological features—harder to interpret in the field because offset patterns don't match simple fault models

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Quick Reference Table

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Self-Check Questions

1. If you observe the hanging wall has moved upward relative to the footwall, what type of stress regime created this fault, and what are the two possible fault types?

2. Compare and contrast normal faults and reverse faults in terms of stress type, relative block motion, and the landforms each produces.

3. A geologist finds older Paleozoic rocks sitting directly on top of younger Mesozoic rocks with a low-angle contact between them. What fault type best explains this, and why does it violate the principle of superposition?

4. Both strike-slip faults and transform faults involve horizontal motion—what distinguishes a transform fault from other strike-slip faults, and why is this distinction tectonically significant?

5. You're mapping a rift valley and observe alternating elevated ridges and down-dropped basins bounded by faults. What structural features are these, what fault type created them, and what does this tell you about the regional stress field?