Several students of cryptoexplosion structures have emphasized the importance of very high fluid pressures. Data on the behavior of mud and sand under high pore-fluid pressures support this concept. Extending the analysis of Goguel and Bucher, it is proposed that such pressures in a specific rock formation can initiate a diapir and drive it to the cryptoexplosion stage. As this formation moves laterally and radially inward to supply the diapir core, viscous coupling with overlying strata will cause these strata to move inward too. Both in the focal zone where the diapir forms and in the collar region through which the central uplift must be forced, there are strong inward horizontal confining stresses. To understand the subsequent behavior of this highly compressed vertical column of rock, data on the behavior of compressed metals and rocks are used. When fluid under high pressure surrounds a metal billet in an extrusion container, increased hydrostatic stress induced in the billet makes the metal capable of an increasingly ductile behavior. Extrusion under these conditions is termed hydrostatic extrusion. The movement of the central uplift can be compared to a hydrostatic extrusion process. If a sufficient compressive strain energy is stored in the upward-moving rock column, Lüders band phenomena are expected where the compressed column approaches the earth’s surface and encounters a zone of sudden, marked relief from the horizontal confining stresses. The large deviatoric stress acts within a very narrow Lüders front, which moves downward relative to the rising extruded rock. Lüders fronts are a very special category of shock front, and this mechanism can account for the so-called shock deformation textures, characteristic of central uplifts of cryptoexplosion structures. Shatter cones are also explained through penetration of tensile joints into the plastic zone where Lüders bands form. Certain other workers have deduced that the shock front must be associated with meteorite impact, but this deduction is thought to be in error. Violently formed breccias and pseudotachylites are also accounted for in this category of diapir; it forms a consistent internal mechanism for generating cryptoexplosion structures.

According to the classical theory, earthquakes would be caused by fracture, or the release of static friction, followed by sliding between the fault walls and a consequent drop of stress around the fault. At a depth of 600–700 km, where the pressure is about 200,000 bars, the lowest possible estimate of the coefficient of friction (about 1) would demand principal stress differences of the order of 400,000 bars for overcoming the friction of the fault wall and producing a drop in seismic stress. Such high stresses cannot be assumed to exist, if only because the yield stress of the asthenospheric rocks can hardly have an order of magnitude exceeding a few tens or at most hundreds of bars. This estimate is obtained by extrapolation of the observed creep behavior of hard materials with the assumption that the effect of a hydrostatic pressure is similar to that of a correspondingly increased molecular cohesive pressure; its order of magnitude agrees with that of the stress drop estimated from the energies released in earthquakes. A detailed discussion of this difficulty shows that earthquakes, except at focal depths less than about 5–10 km, cannot arise in the manner implied in the classical theory. The only plausible alternative available at present is that they are due to an instability of plastic deformation (creep) such as gives rise to slip bands, Lüders bands, the Hanson-Wheeler creep-deformation bands, and many other similar phenomena. If creep produces structural changes that accelerate further creep, the deformation concentrates gradually into thin layers in which high flow rates can develop, and finally even shear melting may occur by the heat development due to plastic deformation. Such a mechanism would explain the sequence structure of earthquakes (fore- and aftershocks). If fracture and frictional sliding are impossible and faulting can occur only by the gradual concentration of creep deformation into thin zones, the stress concentration around the edges of a fault cannot propagate this fault immediately over the entire stressed volume, as would be the case with faulting due to fracture. The fault can extend only after the occurrence of a certain amount of creep leading to progressive strain concentration in the region put under increased stress by the preceding faulting. The first faults in a sequence are likely to be small; after several preparatory faults have thrown a high stress upon a larger volume, however, extensive faulting with larger shocks may develop. Toward the end of the sequence the intensity of the shocks is likely to decrease again by depletion of stress. Since creep instability seems at present the only feasible mechanism of deep and intermediate-depth earthquakes, the fact that it is a characteristic of crystalline materials indicates that the seismogenic parts of the Earth’s mantle are substantially crystalline.

Low-temperature deformation is characterized by heterogeneous strain in which the bulk of the material clearly retains its primary texture. Deformation is by grain-scale crystal plasticity, rotation, fracture, and pressure solution, and by transgranular mechanisms that crosscut numerous grains. The important low-temperature crystal-plastic features are twin lamellae, deformation bands, and undulatory extinction. Subgrain formation by recrystallization or crystal-plastic strain of more than 15% marks the upper limit of the low-temperature regime. Grain rotation may produce foliations in soft sediments or rocks. Microscopic to mesoscopic kinks and crenulations of bedding occur in soft clay and shale. Transgranular features include Lüders’ bands, cooling and desiccation cracks, joints, extension-fracture cleavage, clastic dikes, mineral-filled veins of several types, recrystallization/replacement veins, vein arrays, boudins, faults, stylolites, slickolites, solution cleavages that range from widely spaced to slaty and pencil cleavage. Pressure fringes form adjacent to relatively rigid grains and have fabrics analogous to those in veins. Faults include conjugate fault pairs (Andersonian faults) multiple simultaneous conjugates (Oertel faults), and Riedel shear-zone configurations. The sense of fault displacement is determined from bends, steps, trails, tails, and feather fractures. Superplasticity, especially if aided by diffusion in grain-boundary water, might be important at low temperatures. Fault textures are diagnostic of the environment of deformation but have yet to be uniquely correlated with the presence or absence of earthquakes. Riedel shears and pseudotachylite may form in earthquake source regions, although pseudotachylite is evidently rare in brittle fault zones. The best indicators of stress magnitudes are the critical resolved shear stress for deformation twinning and the presence of tensile fractures. Strain magnitudes and stress and strain tensor orientations can be determined with a variety of methods that are based on mechanical twins, platy grain orientation, grain center distribution, and fault geometry and slip directions. Different deformation mechanism associations, expressed by the partitioning of the total strain into different mechanisms, are related to the ductility and environment of deformation. Deformation fronts separating different mechanism associations are defined on the basis of changes in the crystal-plastic component of strain. Rocks do not suffer deformation; they enjoy it. Rob Knipe, 1982

Abstract Excellent quarry exposures have been studied to examine the controls on the growth of fault networks in Cretaceous high-porosity sands. An inverse correlation is found at any one locality between the frequency of faults of an earlier tectonic event and the frequency of later faults. The early faults are cataclastic deformation bands with displacements typically up to 300 mm, and have thicknesses approaching their displacements. Later faults are also deformation bands except where present within a high-frequency array of earlier faults, where they are typically clustered high-displacement ultracataclasite zones that are narrower (smaller width/displacement ratios) than for the deformation band faults. A mechanical model using critical state soil mechanics explains the observed distributions and fault zone characteristics in terms of strength changes in the deforming sand unit and the stress path by which the material is subjected to ‘clastic’–plastic yielding. Localized faulting by constant-volume cataclastic flow at the critical state line will result in deviatoric stress reduction as Coulomb plasticity softening occurs within the fault zone. Elastic unloading of the walls will suppress the continued formation of deformation bands. The point at which the stress state reaches the critical state line, governed by the stress state and position of the ‘clastic’–plastic yield envelope, is therefore crucial in controlling the final distribution of deformation bands and larger faults in the system. Within this framework, the field and microstructural data suggest that earlier deformation became distributed by hardening processes such as compaction and grain-size reduction, resulting in a higher bulk yield strength. In a later tectonic event, the unit behaves in a stronger manner and deformation quickly localizes by fault zone softening processes into fewer fault zones that individually grow larger.