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.

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Tectonic motions give rise to destructive earthquakes and transient slip events. These movements are often described by friction laws for stick-slip motion on brittle fault surfaces and gouge-filled zones 1,2 . Yet, many transient slip events, such as slow earthquakes and aseismic creep, occur in rocks that exhibit mixed brittle-ductile rheology, where these friction laws are not clearly applicable 3,4 . Here we describe the flow and evolution of fractures as observed in a semi-brittle rock analogue exposed to shear stress in laboratory experiments. We find that, depending on the strength of the rock-analogue material, and thus the magnitude of yield stress, the material exhibits either creep-like or stick-slip behaviour. At low yield stress, deformation occurs as constant creep along a main fracture, whereas at high yield stress, the material exhibits stick-slip behaviour. However, the deformation does not involve frictional behaviour; it is instead accommodated by the initiation and growth of a system of tensional and shear fractures. The opening and interplay of such fracture systems could generate tectonic tremor and slow slip. Our laboratory experiments thus support a frictionless alternative mechanism for the development of tectonic strain transients.Semi-brittle deformation in geologic systems is generally defined micromechanically 5-7 , wherein fracturing and/or frictional sliding characterizes the deformation of one mineral phase while another flows through viscous deformation mechanisms. In the resulting meso-scale deformation, rather than accommodate strain via a single, brittle shear fracture or continuous, ductile creep, there is a combination of both brittle and ductile deformation. Field observations of semi-brittle rocks show that for a range of composition, temperature and pressure, the formation of fluid-filled brittle fractures and veins accompanies localized ductile flow 3,4,8 . At the scale of the lithosphere, the transition from brittle to ductile deformation is highly temperature-sensitive, and thus such semibrittle deformation is particularly important at a depth interval in the lithosphere near the base of the seismogenic zone referred to as the brittle-to-ductile transition (BDT; refs 6,7). A wide variety of slip behaviours have been geophysically resolved to originate in the BDT, and observations and physical experiments of semibrittle materials show that the coexistence of brittle and viscous behaviour gives emergence to some of the characteristics of strain transients and slow slip events 9-11 . However, most workers explain differences in slip transients with laws that describe how fluid pressure, temperature changes and friction interact 12 . Here, we explore an alternative mechanism where strain transients ranging from stick-slip to creep can be explained by the co-occurrence of brittle and ductile deformation.Semi-brittle deformation has been studied in a variety of rock mechanics experiments 5,13 . Although constraining flow laws under laboratory conditions, these experiments are...

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.

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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.

Faults can release energy via a variety of different slip mechanisms ranging from steady creep to fast and destructive earthquakes. Tying the rheology of the crust to various slip dynamics is important for our understanding of plate tectonics and earthquake generation. Here we propose that the interplay of fractures and viscous flow leads to a spectrum between stick‐slip and creep. We use an elasto‐visco‐plastic rock analog (Carbopol U‐21) where we vary the yield stress to investigate its impact on slip dynamics in shear experiments. The experiments are performed using a simple shear apparatus, which provides distributed shear across the entire width of the experiment and allows in situ observations of deformation. We record force and displacement during deformation and use time lapse photography to document fracture development. A low yield stress (25 Pa) leads to creep dynamics in the absence of fractures. An intermediate yield stress (144 Pa) leads to the development and interaction of opening (mode I) and shear (mode II) fractures. This interaction leads to a spectrum in slip dynamics ranging from creep to stick‐slip. A high yield stress (357 Pa) results in the development of many mode I fractures and a deformation signal dominated by stick‐slip. These results show that bulk yield stress, fracture formation, and slip dynamics are closely linked and can lead to a continuum between creep and stick‐slip. We suggest that rheology should be considered as an additional mechanism to explain the broad range of slip dynamics in natural faults.

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