Jacqueline E. Reber

Localization and delocalization of deformation in a bimineralic material

We investigate how localization and delocalization of deformation occurs in a bimineralic material composed of a strong plagioclase and a weaker quartz phase. We perform numerical, meter-scale shear experiments in which we vary the temperature and the ratio of the two mineral phases. Three micromechanical deformation fields are identified according to the mechanical behavior of the minerals at play (brittle or ductile when both phases are in the brittle or ductile regime, respectively, and semibrittle when one phase is in the brittle and the other in the ductile regime). Besides these micromechanical deformation fields, we identify three deformation types characterizing the degree of localization (type I: localized shear zone, type II: localized anastomosing shear zone, and type III: delocalized shear zone). Type I is expected in the brittle deformation field. In the semibrittle field, all deformation types can be observed depending on the amount of weak phase present. In the ductile field, deformation is dependent on the strength ratio between the two phases. For a low strength ratio, deformation of type III is always observed. For high-strength ratios, deformation of type II can be observed for a moderate amount of weak phase. A small amount of weak phase (<10%) reverses the mechanical behavior of the strong phase and leads to the formation of a narrow anastomosing shear zone (type II) where fully ductile (type III) behavior is expected. This highlights the importance of a bimineralic material for the deformation localization and overall large-scale deformation processes.

Stick-slip and creep behavior in lubricated granular material: Insights into the brittle-ductile transition

Crustal deformation can occur via stick-slip events, viscous creep, or strain transients at variable rates. Here we explore such strain transients with physical experiments comprising a quasi-two-dimensional shear zone with elastic, acrylic discs and interstitial viscous silicone. Experiments of solely elastic discs produce stick-slip events and an overall (constant volume) strengthening. The addition of the viscous silicone enhances localization but does not greatly change the overall pattern of strengthening. It does, however, damp the stick-slip events, leading to transient, creep-like behavior that approaches the behavior of a Maxwell body. There is no gradual transition from frictional to viscous deformation with increasing amounts of silicone, suggesting that the mixed rheology is in effect as soon as an interstitial fluid is present. Our experiments support the hypothesis that a possible cause for strain transients in nature is an interstitial viscous phase in shear zones.

Investigation of wing crack formation with a combined phase-field and experimental approach

Fractures that propagate off of weak slip planes are known as wing cracks, and often play important roles in both tectonic deformation and fluid flow across reservoir seals. Previous numerical models have produced the basic kinematics of wing-crack openings, but generally have not been able to capture fracture geometries seen in nature. Here, we present both a phase-field modeling approach and a physical experiment using gelatin for a wing crack formation. By treating the fracture surfaces as diffusive zones instead of as discontinuities, the phase-field model does not require consideration of unpredictable rock properties or stress inhomogeneities around crack tips. It is shown by benchmarking the models with physical experiments, that the numerical assumptions in the phase-field approach do not affect the final model predictions of wing-crack nucleation and growth. With this study, we demonstrate that it is feasible to implement the formation of wing cracks in large scale phase-field reservoir models.

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., Klinkmüller 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)

Introduction to special section: Analog modeling as an aid to structural interpretation

Analog modeling provides the exploration and production industry with one of the most powerful and visual tools to understand the 4D structural evolution of sedimentary basins and individual or families of structures within those basins. Knowledge of the model setup and timing of syn-kinematic