North American Cryptoexplosion Structures: Interpreted as Diapirs which Obtain Release from Strong Lateral Confinement

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Abstract
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.

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