Darien Florez1,2, Christian Huber1, Susana Hoyos2, Matej Pec2, E.M. Parmentier1, James A. D. Connolly3, Greg Hirth11Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA2Department of the Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA3Department of Earth Sciences, ETH Zurich, Zürich, SwitzerlandCorresponding author: Darien Florez ([email protected])Key Points:Continuum model fits repacking experiments data of Hoyos et al.(2022) despite their stochastic nature.At intermediate melt fractions, mechanical repacking of particles may contribute significantly to the resistance of mushes to compaction.Particle-particle friction, rather than hydrodynamic effects, dominates viscous resistance associated with mechanical repacking.AbstractBefore large volumes of crystal poor rhyolites are mobilized as melt, they are extracted through the reduction of pore space within their corresponding crystal matrix (compaction). Petrological and mechanical models suggest that a significant fraction of this process occurs at intermediate melt fractions (ca. 0.3 – 0.6). The timescales associated with such extraction processes have important ramifications for volcanic hazards. However, it remains unclear how melt is redistributed at the grain-scale and whether using continuum scale models for compaction is suitable to estimate extraction timescales at these melt fractions. To explore these issues, we develop and apply a two-phase continuum model of compaction to two suites of analog phase separation experiments – one conducted at low and the other at high temperatures, T, and pressures, P. We characterize the ability of the crystal matrix to resist porosity change using parameterizations of granular phenomena and find that repacking explains both datasets well. Furthermore, repacking may explain the difference in compaction rates inferred from high T + P experiments and measured in previous deformation experiments. When upscaling results to magmatic systems at intermediate melt fractions, repacking may provide an efficient mechanism to redistribute melt. Finally, outside nearly instantaneous force chain disruption events occasionally recorded in the low T + P experiments, melt loss is continuous, and two-phase dynamics can be solved at the continuum scale with an effective matrix viscosity. Further work, however, must be done to develop a framework to parameterize the effect of particle size and shape distributions on compaction.

Icy moons in the outer Solar System contain rocky, chondritic interiors, but this material is rarely studied under confining pressure. The contribution of rocky interiors to deformation and heat generation is therefore poorly constrained. We deformed LL6 chondrites at confining pressures ≤ 100 MPa and quasistatic strain rates, and recorded acoustic emissions (AEs) using ultrasound probes. We defined a failure envelope, measured ultrasonic velocities, and retrieved elastic moduli for the experimental conditions. Chondritic material stiffened with increasing confining pressure, and reached its peak strength at 50 MPa confining pressure. Microcracking events occurred at low stresses, during nominally “elastic” deformation, indicating that dissipative processes are possible in rocky interiors. These events were most energetic at lower differential stresses, and occurred more frequently at lower confining pressures. We suggest that chondritic interiors of icy moons are therefore stronger, less compliant, and less dissipative with increasing pressure and size.

As partially molten rocks deform, they develop melt preferred orientations, shape preferred orientations, and crystallographic preferred orientations (MPOs, SPOs and CPOs). We investigated the co-evolution of these preferred orientations in experimentally deformed partially molten rocks, then calculated the influence of MPO and CPO on seismic anisotropy. Olivine-basalt aggregates containing 2 to 4 wt% melt were deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa at shear stresses of τ = 0 to 175 MPa to shear strains of γ = 0 to 2.3. Grain-scale melt pockets developed a MPO parallel to the maximum principal stress, s1, at γ < 0.4. At higher strains, the grain-scale MPO remained parallel to s1, but incipient, sample-scale melt bands formed at ~20° to s1. An initial SPO and CPO were induced during sample preparation, with [100] and [001] axes girdled perpendicular to the long axis of the sample. At the highest explored strain, a strong SPO was established, and the [100] axes of the CPO clustered nearly parallel to the shear plane. Our results demonstrate that grain-scale and sample-scale alignments of melt pockets are distinct. Furthermore, the melt and the solid microstructures evolve on different timescales: in planetary bodies, changes in the stress field will first drive a relatively rapid reorientation of the melt network, followed by a relatively slow realignment of the crystallographic axes. Rapid changes to seismic anisotropy in a deforming partially molten aggregate are thus caused by MPO rather than CPO.

To investigate the mechanisms involved during semi-brittle flow, we deformed Carrara marble over a confining pressure (Pc) range of 10-300 MPa and room temperature to ≈10% strain. We apply triaxial loading to intact Carrara marble and collect mechanical, ultrasound pulsing, and acoustic emission (AE) data during pressurization and deformation stages. The pulsing and AE waveforms are recorded using a pair of piezoelectric sensors. At lower Pc, microcracking is the dominant deformation mechanism, whereas at higher Pc, crystal-plastic mechanisms such as twinning and dislocation glide are favored. These changes in the activity of defect populations are manifested in changes in mechanical properties, velocity variations, and AE characteristics. Samples at higher Pc exhibit higher strength and require more work for fault-development. Transition from localized faulting to distributed barreling is observed between 50 and 100 MPa Pc. We track precise velocity variations from the pulsing waveforms using correlation-based methods. During the pressurization stage, the velocity increases logarithmically with Pc between 0-100 MPa, followed by a linear increase at higher pressures. During the deformation stage, the compressional wave velocity initially increases before the yield point due to closing of crevices, and then decreases exponentially after the yield point. The rate of this velocity decay is smaller as Pc increases, owing to reduced microcracking with very little change at Pc ≥ 200 MPa. AE data show that individual defect types emit characteristic patterns. Twinning produces repetitive patterns of low amplitude, short signals localized in frequency space whereas microcracks are more energetic, emit over a much broader frequency range, and show more variation in signal shape and duration. The AE spectra shift from ≈ 500 kHz to ≈15 MHz mean frequency as Pc increases, which is associated with increasing twinning activity. This acoustic data agree with microstructural observations of microcracks and crystal-plastic deformation in the samples. By joint-analyzing the stress-strain and velocity evolutions with AE observations, we obtain detailed changes in the micro-mechanisms accommodating strain in the Carrara marble and constrain the deformation modes as it goes through the brittle-plastic transition.

Most of the mass loss from the Antarctic Ice Sheet occurs through glaciers and ice streams, where fast-flow is partially controlled by rapid ice deformation in the margins. Deformation drives thermomechanical and recrystallization processes that influence further deformation, a feedback which may destabilize glaciers. However, few models account for the feedback between deformation and recrystallization. We derive an idealized model for ice temperature and grain-size that partitions deformational work into dissipated heat and changes in strain and surface energy, all of which drive dynamic recrystallization. Under conditions common in glacier shear margins, we show that a large portion of deformational work is stored as elastic energy, with the remainder dissipated as heat. This result revises our current picture of the amount of heat generated in glacier shear margins and suggests that changes in internal strain through dynamic recrystallization of ice likely play an important role in facilitating fast-flowing glacial ice.

Slickenlines are lineations thought to record slip motion and mechanical wear within shear fractures. Their formation mechanisms and effect on friction and fault rheology are poorly understood. We investigate natural slickenlines from strike-slip, normal, and low-angle detachment faults formed in volcanic, quarzitic, and mylonitized sedimentary lithologies, respectively. Slickenline surfaces exhibit non-Gaussian height distributions and anisotropic self-affine roughnesses with corresponding mean Hurst exponents in directions parallel– 0.53±0.07– and perpendicular –0.6±0.1– to slip. However, there is a significant variability in the fractal roughness descriptors obtained from multiple hand samples per fault surface. Microstructural analyses reveal that the principal slip surface is formed by a thin (≤100 µm) nanoparticulate- and phyllosilicate-rich layer, followed by a ~10 μm thick layer of increased cohesion, wherein several smaller grains coalesce into bigger aggregates. These microstructures are present in most analyzed samples suggesting that they commonly form during fault slip regardless of lithology or tectonic setting. Our results 1) suggest that deformation immediately adjacent to fault surfaces is energetic enough to comminute the rocks into nanometric grains and 2) highlight the intricacies of fault systems not fully captured by current models, which are likely to impact stress distributions and frictional responses along faults.

We propose a reformulation of the wing crack model of brittle creep and brittle failure. Experimental studies suggest that the mechanical interactions of sliding and tensile wing cracks are complex, involving formation, growth and coalescence of multiple tensile, shear and mixed-mode cracks. Inspired by studies of failure in granular media, we propose that these complex mechanical interactions lead to the formation of micro shear-bands, which, in turn, develop longer wing cracks and interact with a wider volume of rock to produce larger shear bands. This process is assumed to indefinitely continue at greater scales. We assume the original wing crack formalism is applicable to micro shear-band formation, with the difference that the half-length, a, of the characteristic micro shear band is allowed to increase with deformation (i.e. wing crack growth). In this approach, the dimensionless shear band half-length A is related to the dimensionless wing crack length L by a function, A(L) = 1 + f(L), where f(L) embodies the entire process of shear band formation, growth and interaction with other shear bands and flaws and the problem is then to identify its proper form. We compare the model predictions for various classes of functions f(L) to experimental brittle creep data. Although a very large class of functions reproduce the classic sequence of tri-modal creep, we found that only the simple power law f(L) = (L/Λ)q generated creep curves consistent with published creep data of rocks. Similar accord was also obtained with experimental brittle failure data.

Accurate descriptions of natural fault surfaces and associated fault rocks are important for understanding fault zone processes and properties. Slickensides–grooved polished surfaces that record displacement and wear along faults– develop measurable roughness and characteristic microstructures during fault slip. We quantify the roughness of natural slickensides from three different fault surfaces by calculating the surfaces power spectra and height distributions and analyze the microstructures formed above and below the slickensides. Slickenside surfaces exhibit anisotropic self-affine roughness with corresponding mean Hurst exponents in directions parallel– 0.53±0.07– and perpendicular –0.6±0.1– to slip, consistent with reports from other fault surfaces. Additionally, surfaces exhibit non-Gaussian height distributions, with their skewness and kurtosis roughness parameters having noticeable dependence on the scale of observation. Below the surface, microstructural analyses reveal that S-C-C’ fabrics develop adjacent to a C-plane-parallel principal slip zone characterized by a sharp decrease in clast size and a thin (≤100 µm) nanoparticulate-rich principal slip surface (PSS). These microstructures are present in most analyzed samples suggesting they commonly form during slickenside development regardless of lithology or tectonic setting. Our results suggest that 1) PSS likely arise by progressive localization along weaker oriented fabrics 2) deformation along PSS’s is energetic enough to comminute the rocks into nanometric grains, and 3) fault geometry can be further characterized by studying the height distributions of fault surfaces, which are likely to impact stress distributions and frictional responses along faults.

Laboratory data are essential for testing and refining of theories and models in Earth sciences. Recent developments in data mining techniques and machine learning have made it feasible to utilize and digest large amounts of information; yet such data must be initially prepared and structured in a meaningful way. Recognizing the potential and challenges of data access, multiple efforts are underway in the development of digital data repositories (Strabospot, Epos). Currently most information in experimental Geophysics is not accessible in digital, searchable form. Such information may include: equipment capabilities and configurations, original and edited experimental data, laboratory calibrations as well as information regarding testing protocols and procedures. The LAPS project aims to design and develop resources to facilitate data workflow and access. It specifically focuses on the needs of laboratory researchers, students and managers to prepare data for use in digital data repositories. As laboratories use a wide variety of hardware and software solutions to acquire and process data, we focus our efforts on the development of web based tools that do not require specific local infrastructure and software. One of the main objectives of LAPS is to establish a coherent and effective way to describe equipment and experiments across a variety of testing rigs and devices. To simplify the workflow we are proposing a combination of selecting pre-configured equipment and experimental profiles and manual data entry via web form. A completed test protocol containing all experimental metadata may then be saved locally (e.g., as JSON file) or (optionally) to a database. Such test protocols can be re-loaded and modified as needed. To complete the workflow of a successful test, a link to original or processed data files may be inserted. The data file layout can be defined in the experimental profile. Upload to a digital data repository is optional but by adhering to the proposed Strabo/Epos data model it will not require additional input. While the web based workflow will be accessible to all users, we also recognize the need to integrate the system into specific work flow solutions in rock deformation laboratories. We therefore provide the framework that simplifies local data management systems and analytical applications.

The brittle – viscous transition in the lithosphere occurs in a region where many large earthquakes nucleate. To study this transition, we sheared bi-mineralic aggregates with varying ratio of quartz and potassium feldspar at temperature, T=750oC and pressure, Pc = 800 MPa under either constant displacement rate or constant load boundary conditions. Under constant displacement rate, samples reach high shear stress (τ = 0.4−1 GPa) depending on mineral ratio) and then weaken. Under constant load, the strain rate shows low sensitivity to stress below τ ≈ 400 MPa, followed by a high stress sensitivity (stress exponent, n = 9 − 13) at higher stresses irrespective of mineral ratio. Strain is localized along “slip zones” in a C and C’ orientation. The material in the slip zones shows extreme grain size reduction and flow features. At peak strength, 1-2 vol% of the sample is composed of slip zones that are straight and short. With increasing strain, the slip zones become anastomosing and branching and occupy up to 9 vol%; this development is concomitant with strain-weakening of the sample. Slip zones delimit larger cataclastic lenses, which develop a weak foliation. Our results suggest that strain localization leads to microstructural transformation of the rocks from a crystalline solid to an amorphous, fluid-like material in the slip zones. The measured rheological response is a combination of viscous flow in the slip zones and cataclastic flow in coarser-grained lenses and can be modeled as a frictional slider coupled in parallel with a viscous dashpot.