deformation

Sediments and rock structures are subject to deformation under the influence of imposed stresses.

Stress is defined as a force applied over an area, F/A.
Stress may be uniform and equal from all directions:

▪ confining pressure of overburden

▪ release from confining pressure due to exposure by erosion (diagenesis and retrograde metamorphism).

Alternatively, stress may be unequal from different directions (differential):

▪ compression

▪ extension (tensional)

▪ shear stress (applied obliquely)

Causes of stress include

▪ uniform confining, lithostatic stress due to overburden (burial)

▪ tectonic stress

▪ expansion of water that has frozen in rock cracks or soils (cryoseism)

When rocks deform in response to imposed stress they exhibit strain, which is the differential change in size, shape, or volume of a material. Materials differ in their responses to stress, depending upon composition, conditions of temperature and confining pressure, and strain rate. However, regardless of intrinsic degrees of brittle or ductile qualities, all strained materials pass through 3 successive stages of deformation: elastic, ductile, and fracture (failure, or brittle deformation). Provided that the strain rate is sufficiently slow to allow minerals to accommodate structurally, minerals can adjust to applied stresses by a variety of mechanisms.

Forms of deformation include:
unconsolidated sediments

▪ slumping

▪ folding (fold anatomy)

▪ mass wasting and landslides

▪ faulting (fault attributes)

consolidated rock

▪ brittle, ductile, or elastic deformation due to lithostatic or tectonic stresses

▪ --earthquakes

▪ --faulting

▪ --folding

▪ --cataclism, milling, and brecciation

▪ --orogenesis