What is "paleostress"

Palaeo means past, thus paleostress (or palaeostress) means stresses that acted in the geological past (i.e. thousands to millions of years ago). In Geology paleostress analysis is concerned with deriving the directions along which the stress acted and gave rise to present structural feature in the rocks on earth.

Paleostress is a subset of mechanical stress within geology. Variations in stress fields within the Earth's crust can result in a variety of mechanical responses:

• Microscopic:

:* Crystal deformation, including twinning,

:* pressure solution

:*Microfractures,

:*Aligned fluid inclusions.

• Macroscopic:

:* Folding

:* fracturing

:* faulting (fracturing accompanied by offset of rock bodies on either side of the fracture surface)

Traditionally, deformation—either folding or fracturing—without dissolution are collectively termed mechanical strain. Both macroscopic and microscopic strain may be elastic, and only exist as long as differential stress exists, or it may be inelastic -- that is the deformation due to a particular stress event remains even after the stress is removed. In the latter case, inelastic deformation, the stress field responsible for the deformation if it can be inferred, is, then, the paleostress. Anderson's classic analysis of faulting serves as a simple application of paleostress analysis in terms of principal components of stress.

Zoback and Zoback's (1986) synthesis of contemporary stress measurements in North America was subsequently expanded to a global study (Zoback et al., 1989) which continues as the World Stress Project. Only a small subset of measurements in the WSP database qualify as paleostresses.

A number of regional studies of paleostresses has been undertaken, including Europe (Bergerat, 1987); North America (synthesized by Bird, 2002; Pilger, 2003), Australia (Pilger, 1982).