6.1 Structural geology: folds, faults, stress & strain

Stress and Strain

Stress is force per unit area applied to a body of rock, while strain is the resulting deformation — a change in shape, volume, or both. The relationship between them is the foundation of structural geology. Three fundamental stress regimes act on the crust:

• Compression squeezes rock, with stresses directed toward one another; it shortens and thickens the crust and dominates convergent plate boundaries.
• Tension pulls rock apart, with stresses directed away from one another; it stretches and thins the crust and dominates divergent boundaries.
• Shear involves stresses acting parallel but in opposite directions, so one block slides past another; it dominates transform boundaries.

Geologists distinguish confining (lithostatic) pressure, which acts equally in all directions and merely compacts rock, from differential stress, which is unequal in different directions and actually deforms it. The resulting strain can be measured as elongation or as a change in angles, and features such as deformed fossils and stretched pebbles serve as natural strain markers.

Rock deforms in predictable stages. Under low stress it behaves elastically, storing energy and springing back to its original shape when the stress is released — the very mechanism that later drives earthquakes. Once the elastic limit is exceeded, deformation becomes permanent. Plastic (ductile) deformation bends and flows rock without breaking it and is favored by high temperature, high confining pressure, slow strain rates, and greater depth. Brittle deformation fractures rock and is favored by low temperature, low confining pressure, rapid strain rates, and shallow depth. Because temperature and pressure both rise with depth, the same rock that shatters near the surface can flow like putty deep in an orogenic belt. On the ASBOG exams, correctly linking a structure to its stress regime — extension to normal faults, compression to folds and reverse faults — is a recurring theme.

Strike and Dip

Any planar feature — bedding, a fault, or foliation — is located in space using strike and dip. Strike is the compass bearing of the horizontal line formed where the plane intersects a horizontal surface. Dip is the angle of steepest inclination measured downward from horizontal, perpendicular to strike, together with the direction of that inclination. A bed described as N30 degrees E, 45 degrees SE strikes northeast and dips 45 degrees toward the southeast. On geologic maps this appears as a T-shaped symbol whose long line shows strike, short tick shows dip direction, and adjacent number gives the dip angle.

Folds

Folds are ductile, wave-like bends in layered rock produced mainly by compression. Anticlines are important petroleum traps because buoyant oil and gas migrate upward into their arched crests.

• Anticline — an up-arched fold whose limbs dip away from the axis; the oldest beds occupy the core.
• Syncline — a down-warped fold whose limbs dip toward the axis; the youngest beds occupy the core.
• Monocline — a simple one-limbed, step-like flexure in otherwise flat strata.
• Plunging fold — the fold axis (hinge line) is tilted rather than horizontal, so eroded beds form a nose-shaped outcrop that closes in the direction of plunge for an anticline.
• Overturned fold — one limb is rotated past vertical, so both limbs dip in the same direction, recording strong directed compression.
• Recumbent fold — the axial plane is nearly horizontal.

Fold Anatomy

Every fold has an axial plane that bisects it and a hinge where curvature is greatest, while the sloping sides are limbs. Folds are classed by symmetry: a symmetric fold has limbs of equal dip, an asymmetric fold has unequal limb dips, and an isoclinal fold has essentially parallel limbs. Circular structures follow the same age logic as linear folds: a dome has the oldest rocks in its center with beds dipping outward, whereas a basin has the youngest rocks in its center with beds dipping inward.

Faults

Faults are brittle fractures along which measurable displacement has occurred. On an inclined fault, the block resting above the fault plane is the hanging wall; the block beneath is the footwall.

Extension by many parallel normal faults creates alternating horsts (up-thrown blocks) and grabens (down-dropped blocks), the classic Basin and Range topography. Strike-slip faults are right-lateral (dextral) or left-lateral (sinistral) depending on the apparent motion of the opposite block; the San Andreas is right-lateral. Because thrust faults are low-angle reverse faults, they can stack older rock over younger rock and shorten the crust across entire mountain belts. Field evidence for faulting includes offset marker beds, polished and grooved slickensides, drag folds, and crushed fault breccia or gouge.

Joints

Joints are fractures along which no appreciable displacement has occurred, which distinguishes them from faults. They form by unloading and expansion as overlying rock erodes, by cooling contraction (producing the hexagonal columnar joints of basalt), and by regional tectonic stress. Because joints open pathways through otherwise tight rock, they strongly influence groundwater flow, weathering, and slope stability — the point where structural geology meets the applied questions on the ASBOG exams.