Structural Geology:
Reviews and Methodology

Professors Aydin and Pollard have contributed several articles and chapters in edited books reviewing historical and current topics in structural geology and tectonics.

Pollard and Segall (1987) review the quasi-static solutions for two dimensional cracks in a linear elastic whole-space subject to uniform stress (traction) boundary conditions. Complete displacement and stress field equations are given for mode I, II, and III. Applications are described for the 1906 San Francisco earthquake event; the deformation associated with veins and solution surfaces at the tip lines of small faults; spacing of joints in layerd rock; the dilational form of igneous dikes; and joints in the process zone of a dike.

Pollard and Aydin (1988) review progress in understanding jointing over the century (1888-1988) as part of the Geological Society of America's Centennial Year special issue of the Bulletin. This contains both an historical review of what was published (with critical commentary), and an outline of what can be synthesized from these published papers. The fundamental relationships from continuum and fracture mechanics that are relevant to the initiation, propagation, and termination of joints are reviewed. It is argued that these relationships combined with appropriate field observations provide the best prescription for future progress.

Aydin (1996) is the second chapter in the National Research Council Committee book on Rock Fractures and Fluid Flow: Contemporary Understanding and Application s published by the National Academy Press. In this chapter the physical characteristics of fractures and fracture patterns are reviewed. A geological and geomechanical understanding of fracture formation is described, and the characteristics of various fracture types, network patterns and the internal structure of fractures are documented and discussed.

Aydin (2000) reviews the practical aspects of fractures and faults in reservoirs, and discusses the nature of hydrocarbon entrapment, migration and flow as influenced by these structures. This is meant to address the large issue of the roles of structural heterogeneities in the migration and entrapment of hydrocarbons. The permeability of joints, deformation bands, and faults is described and it is argued that these structures should be included in large-scale basin models and reservoir-scale simulation models. This proposition is supported by case studies.

Recently, Professor Pollard has been an active proponent for the use of a complete mechanical analysis in the investigation of problems in structural geology and tectonics. A paper presented in the 20th Anniversary Issue of the Journal of Structural Geology (Fletcher and Pollard, 1999) addresses the question: "Can we understand structural and tectonic processes and their products with appeal to a complete mechanics?" In the abstract the authors begin: Our answer is 'no', and then they go on to defend this position using two examples, one addressing the development of chevron folds and the other the development of echelon veins. They conclude that further progress in understanding these (and other) products of structural and tectonic processes can be made through an integrative approach using a complete and self-consistent mechanics.

Pollard (2000) is a Discussion of one of the papers in the 20th Anniversary issue of the Journal of Structural Geology authored by Marrett and Peacock (1999). This discussion addresses the historical development of structural analysis and the conceptual role of stress and strain in the analysis of structures. Some of the opinions in the original paper are challenged and a position is taken that advocates a research methodology that integrates geometry, kinematics, and dynamics. It is argued that geometric observations do not constitute the foundation of all structural analysis, but rather that this foundation is constructed from the conservation laws of physics, so geometric observations are properly viewed as data, some of which may be useful for testing refutable hypotheses.

Partly in response to Fletcher and Pollard (1999) and to other articles in the 20th Anniversary volume that took a decidedly different stance Bill Dunne and Jim Evans organized a debate at the 2000 GSA Meeting in Reno with the topic: "Kinematics vs. Mechanics in Understanding Rock Deformation" sponsored by the GSA Structural Geology and Tectonics Division. As a part of that debate a play was presented called "A Complete Suite", a musical allegory of mechanics in one act.