Sheet fractures: response to erosional offloading or to tectonic stress?

Abstract

It has long been accepted that sheet fractures are due to pressure release consequent on erosional offloading, and the fractures are widely known as offloading joints, pressure release joints, or some similar term. All fractures are influenced by offloading for they close and disappear in depth as a result of lithostatic pressure. They find expression only as erosion brings them closer to the surface. But whether sheet fractures are entirely due to pressure release is questionable. Field evidence is cited that is inconsistent with pressure release but compatible with an origin involving compressional stress. For example: expansive tendencies consequent on pressure release would have been accommodated along pre-existing partings. There are discrepancies between the age of the sheet fractures and that of the land surfaces parallel to which, allegedly, they were formed. Sheet fractures and structures are characteristic of bornhardts, many of which evidently survive because they are in compression, with few open fractures. Minor forms associated with the release of compressive stress coexist with sheet fractures. Displacement along sheeting planes is evidenced. Some bornhardts display synformal structures, which is impossible in terms of offloading. The cross structures developed in sets of sheet fractures within the same bornhardt are also difficult to explain in terms of pressure release. The development of sheet partings parallel to land surfaces can be explained in terms of adjustment to planes of least principal stress. The development of sheet structure in two-stage (or multistage) bornhardts can be simulated experimentally.