Structural styles: extension, compression, strike-slip, salt, gravity

Anderson’s fault theory: the framework

In 1905, E. M. Anderson published a theoretical framework that still governs how interpreters think about fault systems. The key insight: at any point in the crust, there are three mutually perpendicular principal stress directions, conventionally called $\sigma_1$ (maximum compression), $\sigma_2$ (intermediate), and $\sigma_3$ (minimum compression, or maximum extension). Faults form along planes of maximum shear stress, which sit at roughly 30° to $\sigma_1$.

Anderson showed that in most crustal settings, one of the three principal stresses is approximately VERTICAL (gravity imposes this at Earth’s surface, where the free surface cannot support shear stress). Which of the three is vertical then determines the resulting fault geometry:

• $\sigma_1$ vertical — gravity is the maximum compression. The minimum stress $\sigma_3$ is horizontal. Faults form at 60° to horizontal (30° from $\sigma_1$) and are normal faults: the hangingwall moves DOWN relative to the footwall. This is the extensional regime.
• $\sigma_1$ horizontal, $\sigma_3$ vertical — horizontal compression exceeds gravity. Faults form at 30° to horizontal and are thrust faults (reverse faults at low angle): the hangingwall moves UP relative to the footwall. This is the compressional regime.
• $\sigma_1$ and $\sigma_3$ both horizontal, $\sigma_2$ vertical — horizontal shear stress dominates. Faults are nearly vertical and slip horizontally (transcurrent motion). This is the strike-slip regime.

Anderson’s three regimes cover most crustal faulting. Two additional styles (salt tectonics and gravity-driven listric faulting) complete the interpretive palette; they involve mechanisms that aren’t pure stress-driven Andersonian faulting but produce distinctive seismic signatures that every interpreter must recognize.

Exercise — learn each style's signature

• The widget starts in Extensional mode. Look at the cross-section: parallel beds cut by a normal fault, with the hangingwall (right side) dropped down and showing characteristic rollover curvature against the fault. The map view shows parallel N-S-trending normal faults with tick marks on the downthrown side. This is the North Sea Viking Graben in cartoon form — it is also the geometric setting of the F3 teaching dataset.
• Switch to Compressional. Now the beds are folded (anticline on the left, syncline on the right) and cut by a thrust fault dipping ~30°. The teeth on the upper side of the fault indicate the hangingwall direction. On the map, arcuate thrust-front lineaments (with teeth pointing toward the overriding block) are the Rocky Mountain / Andean foothill signature.
• Switch to Strike-slip. The cross-section shows a positive flower structure: multiple faults diverging upward from a single deep root, with the central corridor upwarped slightly. The map shows en-echelon fault segments — the classic San Andreas-style fingerprint. Real strike-slip systems have more complex geometries than any cartoon captures (pull-apart basins, push-up ridges) but the positive flower is the core recognition pattern.
• Switch to Salt tectonics. A salt diapir rises from depth, with draping sediments arching over the crest and rim synclines on the flanks (where sediments thin over the salt and thicken into adjacent minibasins). Salt moves because it is far less dense than compacted sediment overburden — about 2.16 g/cc compared to 2.4–2.7 for typical clastics — so it rises buoyantly wherever overburden pressure differentials allow.
• Switch to Gravity-driven (listric). A single listric growth fault curves from steep at the top to horizontal at depth, where it detaches on a weak layer (shale, salt, or overpressured mud). The hangingwall shows rollover AND growth stratigraphy (beds thicker on the downthrown side, because they were deposited DURING faulting; the fault was creating new accommodation space as sediments came in). This is the Niger Delta / Gulf of Mexico shelf signature.
• Take a moment to compare the cross-sections side by side (mentally or by flipping back). The differences are systematic: extensional normal faults dip steeply and throw hangingwalls DOWN; compressional thrusts dip shallowly and throw hangingwalls UP; strike-slip faults are near-vertical; salt bodies disrupt the layered pattern entirely; listric growth faults produce curved fault planes with growth strata. These are the fundamental patterns interpreters use to classify a basin at first look.

The global distribution of structural styles

Each style is geographically associated with specific tectonic settings:

• Extensional: passive continental margins (e.g., Gulf of Mexico, West Africa, North Sea), intracratonic rifts (East African Rift, Rhine Graben), and back-arc basins.
• Compressional: active orogenic belts (Andes, Himalayas, Rocky Mountains, Alps) and their foreland basins. The Zagros Mountains of Iran are one of the world’s largest hydrocarbon provinces, entirely compressional in origin.
• Strike-slip: transform plate boundaries (San Andreas, North Anatolian Fault, Dead Sea Transform) and their localized pull-apart basins. Strike-slip alone rarely produces large hydrocarbon provinces; combined with extension (transtension) or compression (transpression) it becomes more productive.
• Salt tectonics: basins with Paleozoic or Jurassic salt deposits that later got buried (Gulf of Mexico Louann Salt, North Sea Zechstein Salt, Santos Basin Aptian Salt, Persian Gulf Hormuz Salt). Many of the world’s largest hydrocarbon accumulations are salt-related.
• Gravity-driven listric: major prograding deltas and slope systems (Niger Delta, Nile Delta, Mississippi Delta). The combination of large sediment supply + weak detachment layer + gravitational gradient is what drives listric faulting here.

A single basin often combines multiple styles in sequence. The North Sea: Permian extension formed graben structures, Triassic-Jurassic extension created the main reservoir-bearing basins, Cretaceous-Paleogene thermal subsidence deposited the chalk, and Mio-Pliocene salt movement of the underlying Zechstein continues to deform the section. A careful structural interpretation identifies which style is responsible for which feature at each interval.

Polycyclic basins: multiple structural events overprinted

Real basins rarely preserve only one structural style. Successive tectonic events overprint earlier structures, producing composite signatures:

• Inverted basins: extensional basins that are later compressed. The original normal faults reactivate as reverse faults; rollover anticlines become compressional anticlines. Classic inverted basins: Sunda Shelf (Indonesia), Northwest European basins.
• Salt with later tectonics: a salt province that later experiences extension or compression. Salt diapirs form welds (pinched-off near-vertical salt stringers); rim synclines get sheared; faults deform the salt flanks.
• Rifting then passive margin: a basin that rifts, opens, and then sits as a passive margin for hundreds of millions of years. Early rift-stage extensional structures get deeply buried; later gravity-driven listric faulting affects only the upper cover sequence.

Polycyclic interpretation requires you to mentally peel back structural events in reverse chronological order: start with the youngest deformation and work downward/backward through time. Structural cross-section balancing (which you'll encounter in §3.4) is the quantitative tool that enforces consistency in this reverse-ordering exercise.

Common style-recognition pitfalls

• 2D sections lie about strike-slip. A single 2D line cannot distinguish a strike-slip fault (which moves horizontally in the plane perpendicular to the section) from a steep normal or reverse fault that happens to intersect the section vertically. Confirm strike-slip by combining multiple lines in different orientations and checking the map-view en-echelon pattern.
• Growth stratigraphy vs post-faulting thickening. True growth sedimentation produces bed thickening that matches the fault throw at each stratigraphic level (upper beds record smaller throws if the fault is dying upward). Post-faulting thickening from compaction or salt withdrawal can mimic growth superficially but won't show the throw-vs-time correlation.
• Imaging failure looks like structural complexity. Sub-salt shadow zones, near-surface distortion, and velocity-model errors all produce seismic images that look "chaotic" or "faulted." Before committing to an interpretation of structural complexity, cross-check against the acquisition quality and processing history — the chaos may be an imaging artefact, not geology.
• Polycyclic basins are often misdiagnosed. An interpreter trained on a classic extensional basin may miss the subtle later-stage compression that inverted the rollover anticlines; an interpreter trained on compressional orogens may miss the earlier extension that provided the structural grain. Study the regional tectonic history before interpreting.