MECHANICAL STRATIGRAPHY: THE BRITTLE PERSPECTIVE

Mechanical stratigraphy refers to the subdivision of rock into discrete intervals (mechanical units) according to the structures found in those intervals. Insofar as the style and intensity of deformation depend upon internal rheological properties and externally applied forces, mechanical stratigraphy is directly related to lithology, P-T conditions, and strain history. Lithotectonic units such as regional decollements and imbricate thrust blocks represent mechanical stratigraphy at a grand scale. Over the past ~15 years, research has focused more intensively on relating mesoscale structures to intraformational variations in lithology. Emerging from this work is the widespread conclusion that stratigraphy impacts many aspects of brittle deformation, including fracture style, dimensions, distribution, mechanics and scaling relations. For example, joints in interbedded lithologies are often restricted to the more competent units, and are mostly absent from the incompetent intervals. Further, mechanical unit thickness often controls joint shape (highly elliptical) and joint spacing (often proportional to bed thickness). The result is a fracture population whose attributes and distribution differ markedly from populations found in more homogeneous rocks such as granites. A second example is the dependence of fracture style on lithology. Faults and joints can develop in adjacent lithologic units in response to the same applied boundary conditions, which may reflect differences in mechanical properties such as cohesive strength. As deformation progresses, large throughgoing faults and joints will develop across multiple units. This is accomplished through the linkage of the smaller, pre-existing bed-confined fractures. Thus, morphologic, mechanical, hydrodynamic and scaling properties of throughgoing brittle structures in layered rocks are influenced by their inherited, stratigraphically-controlled structures. Further research on the relationship between stratigraphy and brittle deformation will lead to a better understanding of fluid flow, fault mechanics and the development of three-dimensional structural models in layered rocks.