A perspective on the emergence of modern structural geology: Celebrating the feedbacks between historical-based and process-based approaches

Structural geology has emerged as an integrative, synthetic science in the past 50 years, focused on deciphering the history preserved in the rock record and determining the processes of rock deformation. Owing to the nature of structural geology, studies focus on historical elements, such as structural inheritance and tectonic history, and increasingly involve theoretical, process-based approaches. The strength of the fi eld is that it uses these historicaland process-based approaches simultaneously in order to determine the three-dimensional architecture, kinematic evolution, and dynamic conditions of lithospheric deformation over a wide range of spatial and temporal scales. *[email protected]; [email protected]; [email protected]; [email protected]; [email protected]; steven.wojtal@ oberlin.edu. Tikoff, B., Blenkinsop, T., Kruckenberg, S.C., Morgan, S., Newman, J., and Wojtal, S., 2013, A perspective on the emergence of modern structural geology: Celebrating the feedbacks between historical-based and process-based approaches, in Bickford, M.E., ed., The Web of Geological Sciences: Advances, Impacts, and Interactions: Geological Society of America Special Paper 500, p. 65–119, doi:10.1130/2013.2500(03). For permission to copy, contact [email protected]. © 2013 The Geological Society of America. All rights reserved. CELEBRATING ADVANCES IN GEOSCIENCE 1888 2013 66 Tikoff et al. spe 500-03 1st pgs page 66 INTRODUCTION: HISTORY AND PROCESS Structural geology has progressed from principally documenting geometrical and historical relations via geologic maps to a diverse science that embraces and advances continuum mechanics approaches, regularly utilizes state-of-the-art analytical and imaging instrumentation, and increasingly relies on sophisticated numerical methods. Practitioners are intensely interested in the physical and chemical processes that result in geological structures and structural fabrics. Yet, structural geologists still make geologic maps, sometimes even with pencil and paper, and are interested in the tectonic evolution (i.e., history) of an area. Structural geology, as it is practiced currently, draws upon and interweaves both historical-based and process-based approaches to answer questions about how the Earth deforms. The historical approach is primarily concerned with the relative or absolute chronology of events; the process approach is mainly interested in mechanisms and models, including deformation mechanisms and the mechanics of deformation. The root of the difference between historicaland process-based approaches in structural geology lies in the broader empirical versus theoretical division apparent in many fi elds of science. To understand the state of the fi eld, and appreciate the unique perspective of this discipline, it is helpful to understand how these historical and process-based approaches productively coexist. Three principal goals motivate many structural geology studies: Understanding the three-dimensional architecture, kinematic evolution, and dynamic conditions of geological structures (Fig. 1). Either an empirical or theoretical approach is employed to understand geological structures; the choice depends on the viewpoint of the structural geologist and the tools that he or she uses. An empiricist typically chooses the inductive approach, proceeding from examples to general rules or principles. Consequently, for example, she or he evaluates three-dimensional geometry, then derives the kinematic evolution from a detailed In this contribution we focus on signifi cant progress made in understanding shear zones, fault zones, intrusions, and migmatites, both as individual features and as systems. Intrinsic to these advances are insights into the strain history, specifi cally through the temporal evolution of geologic structures. Increasingly sophisticated geochronological techniques have advanced the fi eld of modern structural geology by allowing age determinations to be linked to rock microstructure and deformational fabrics, from which displacement rates and strain rates can be estimated in some settings. Structural studies involving new approaches (e.g., trenching), and integrated with geomorphology and geodesy, have been applied to study active geologic structures in near surface settings. Finally, signifi cant progress has been made in constraining the rheology of naturally deformed rocks. These studies generally rely on results of experimental deformation, with microstructural analyses providing the connection between naturally deformed rocks and results of experiments. Integration of fi eldbased observations, laboratory-derived rheological information, and numerical models provide signifi cant opportunities for future work, and continues the tradition of simultaneously using historicaland process-based approaches.