Physical Experiments of Tectonic Deformation and Processes: Building a Strong Community
The recent revolution in the analysis of physical experiments of tectonic processes has provided new quantitative tools to analyze their outcomes. Physical experiments using scaled analog models are unique in providing information on complex three dimensional deformation where processes can be directly observed. These observations critically complement insights gained from field and analytical/numerical investigations. Recent innovations in rheologic testing, digital image processing, and data collection are revolutionizing how we use experiments to provide insight into crustal deformation. At the same time, we are seeing the benefits of physical experiments in classroom teaching by engaging students in hypothesis testing and hands on laboratory experience. Strengthening of the community of physical experimentalists and instructors using analog materials to simulate tectonic processes will enhance our understanding of these processes, lend more power both to interpretations of field observations and to validation of numerical models, and deepen student understanding of tectonic mechanisms. A step toward a stronger community has been made with a recent workshop on physical modeling of tectonic processes, and this report is one outcome of that workshop. THE REVOLUTION IN PHYSICAL EXPERIMENTS Two hundred years ago, Hall (1815) published the first research paper to use physical experiments using analog materials to investigate mountain belt formation. Since these very first experiments, physical models in earth science have not only been useful tools for visualizing deformation but also have great power to investigate physical processes that govern deformation. For example, the innovative experiments of Tapponnier et al. (1982) and Davis et al. (1983) each with over 2,000 citations, have transformed our thinking about tectonic processes. Carefully scaled analog models provide a means to directly observe deformational processes that within Earth's crust are too slow and too large to directly document (Hubbert, 1937) Furthermore, within such experiments we have control over boundary conditions and material properties so that we can directly assess their effect on deformation. While fieldwork and analytical and numerical models are essential tools for investigating crustal processes, they often do not inform all aspects of the deformational story. Using physical experiments in conjunction with field observations and analytical/numerical investigations provides a strong three legged stool upon which we can build a robust understanding of crustal deformation processes (Fig. 1) Advances in experimental procedures have been developed at physical modeling laboratories within both academia and the petroleum industry. The past 10 years have seen a revolution within physical modeling of crustal deformation spurred by the utilization of new innovative analog materials (e.g. Di Giuseppe et al. 2015) systematic rheologic testing (e.g. Klinkmuller et al. 2016) incorporation of laser and image processing techniques for data analysis (e.g. Haq, 2012) measuring in situ stress (e.g. Herbert et al. 2015) and reconstruction of the evolution of complex 3D structures (e.g. Colletta et al. 1991) These advances all strengthen the quantitative rigor of physical modeling of tectonic processes. The vanguard of this recent revolution has been in Europe, which has many active laboratories staffed with technicians implementing and advancing these new technologies. While presently a typical experimental laboratory in the United States is run by a single principal investigator with his or her students, European labs are run with a team of lead scientists with tens of students. Consequently, the core of the experimental community is in Europe, where experimentalists host regular workshops and conference sessions focused on physical modeling. Strengthening the U.S. GSA Today, v. 26, no. 12, doi: 10.1130/GSATG303GW.1. 0.5cm 1cm A) Field Observations B) Laboratory Experiments pr oc es s pr oo f pysics 0.5cm Tectonic Processes 2cm C) Numerical/Analytical Models Figure 1. Deep understanding of crustal deformation relies on three approaches: field measurements of deformation, physics based predictions of deformation, and direct documentation of deformational processes within laboratory experiments. To illustrate the power of the tectonics three legged stool, we show results from a fully integrated study on the development of sheath folds in simple shear. (A) Sheath fold from Cap de Creus, Spain. (B) Photo from a physical experiment investigating the impact of layer viscosity contrast on the fold formation (Reber et al. 2013) (C) Cross section from an analytical model investigating the effect of total shear strain (Reber et al. 2012)