northwestern South Island and central Otago, New Zealand, for example. Both the full-scale convergence zones, such as the Himalaya, and the zones subordinate to transform faulting, such as the Transverse Ranges, contain examples of active folds and low-angle thrust faults, and future research is likely to discover that active folds and thrust faults are as widespread in the active convergent zones as they are in extinct mountain belts. This chapter reviews the state of knowledge of structures related to folding in light of their impact on society.
The mechanical properties of fold-and-thrust belts were considered by Elliott (1976) and Chapple (1978), building on the earlier work of Hubbert and Rubey (1959). Chapple (1978) noted that fold-and-thrust belts, whether on land or offshore, should show (1) a basal surface of décollement below which there is little or no deformation; (2) an overall shape in cross section of a wedge tapering toward the edge of the mountain belt with its base, the basal décollement, sloping toward the interior of the mountain belt; and (3) extensive horizontal contraction in the tapered wedge above the basal décollement. Davis et al. (1983) considered the mechanics of a fold-and-thrust wedge to be analogous to that of the wedge of snow that forms ahead of the blade of a moving snow plow. The snow deforms until the wedge attains a critical taper, then slides stably, growing as new snow is accreted at the front of the wedge. Parameters essential to an understanding of the mechanics of a fold-and-thrust wedge include the angle of topographic slope of the wedge toward the frontal edge of the deformed belt, the angle of rearward slope of the basal décollement, the coefficient of internal friction within the wedge, the coefficient of sliding friction on the base (about 0.85 according to Byerlee, 1978), and the ratio of pore fluid pressure to the vertical stress imposed by overburden (λ). Davis et al. (1983) applied their mechanical model to the active fold-and-thrust belt of western Taiwan, where extensive subsurface information is available, and they determined the critical parameters to be angle of forward topographic slope 2.9±0.3°, rearward slope of the décollement 6°, and λ equal to 0.7. The fold-and-thrust wedge is above sea level, and the topography is at steady state: thickening of the wedge by contractile tectonics is balanced by erosion, which proceeds at a rate of 5 to 6 mm/yr (Li, 1976; Suppe, 1981). The model is sensitive to the nature of material comprising the décollement, where it is assumed that essentially pure frictional sliding occurs. However, evaporites characterize the fold-and-thrust wedges of the Zagros Mountains (Stöcklin, 1968) and the Salt Range of Pakistan (Seeber et al., 1981), and these may yield plastically rather than by pressure-dependent Coulomb friction. The snowplow model has two important implications for active tectonics: (1) the age of deformation should migrate outward toward the front of the wedge, and (2) rocks within the wedge are near the point of critical failure and are likely to exhibit pore pressures greater than hydrostatic.
Yeats et al. (1981) pointed out that faults that do not extend downward into rocks of high strength will not be expected to produce large-amplitude ground acceleration that is due to seismic shaking because such rocks under near-surface confining pressures are not capable of storing enough elastic strain energy to generate a large earthquake when that strain energy is released instantaneously. Because fold-and-thrust belts terminate downward at a basal décollement over an undeformed rigid basement, the question of their seismotectonic signature is an important one. Where the décollement contains rocks of such low strength that deformation may occur plastically under low confining pressure, as in the Zagros Mountains and the Pakistan Salt Range, internal deformation is probably not accompanied by large earthquakes (Berberian, 1981; Seeber et al., 1981; Seeber, 1983). In the thinner portions of fold-and-thrust wedges, the rocks are probably not under sufficient confining pressure to possess much shear strength, even though they behave according to Coulomb friction laws. However, in the thicker portions of wedges, the earthquake potential is not so clear. Seeber et al. (1981) suggested that the greatest earthquakes of the Indian Himalaya occur in front of the range in the Ganges foredeep, even though this area has been characterized by low instrumental seismicity in recent years. The large isoseismal areas of these earthquakes and lack of surface rupture in great historical earthquakes of the Ganges flood plain suggest that a large part of the décollement surface moves as a blind thrust, producing a detachment earthquake. The 1964 Prince William Sound, Alaska, earthquake also may be of detachment type (Seeber et al., 1981).
Most folds characteristic of foreland fold-and-thrust belts form by flexural slip. A stack of stiff beds alternating with thin, less stiff layers is end-loaded, and these beds buckle by slip on inherently weaker bedding surfaces separating stiffer beds (Ramsay, 1967, pp. 392–393). Slickensides on these weak surfaces are perpendic-