Magnetic measurements of the lunar crust and Apollo samples indicate that the Moon generated a dynamo magnetic field lasting from at least 4.2 until <2.5 billion years (Ga) ago. However, it has been unclear when the dynamo ceased. Here, we report paleomagnetic and 40Ar/39Ar studies showing that two lunar breccias cooled in a near-zero magnetic field (<0.1 μT) at 0.44 ± 0.01 and 0.91 ± 0.11 Ga ago, respectively. Combined with previous paleointensity estimates, this indicates that the lunar dynamo likely ceased sometime between ~1.92 and ~0.80 Ga ago. The protracted lifetime of the lunar magnetic field indicates that the late dynamo was likely powered by crystallization of the lunar core.

We tested samples cored from the Vaca Muerta shale reservoir using nanoindentation (2 min) and triaxial (12 hr) creep experiments in which confining pressure and differential stresses were <40 MPa. In all cases, we observed transient creep wherein strain increased as the logarithm of time. Creep was always compactional, led to increased moduli, was triggered by changes in either hydrostatic or deviatoric stress, and occurred under loads well below the failure stress. Our results are consistent with yield cap models proposed to describe shear‐enhanced compaction of sandstones and carbonates, assuming that the yield surface depends on strain rate. We compared our results to earlier studies that observed the transition from transient creep to approximately constant strain rate behavior. If short‐term creep can be quantified by a creep modulus, C, and long‐term creep, after a transition time, tc, by a linear viscosity, η, then η = Ctc. Owing to heterogeneity, the local values of Young's and creep moduli, E and C, measured during nanoindentation, varied by several orders of magnitude. Simple averaging of the indentation results overestimated E and C, as compared to their triaxial counterparts. The kinetics of log time creep, which is seen in various materials and loading circumstances, has been represented by a simple conceptual model incorporating the interplay of viscoelastic elements with widely distributed characteristic times. Thus, we argue that log time creep is an emerging phenomenon, independent of the identities of the underlying physical mechanisms.

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Summary Hydraulic fracturing is crucial to hydrocarbon recovery from resource plays and is essential to exploitation of geothermal energy. This process creates new tensile fractures and reactivates existing natural fractures, forming a highly conductive stimulated-reservoir volume (SRV) around the borehole. Although this process has been extensively studied and modeled for isotropic rock, only a limited number of studies have been performed for anisotropic rocks, such as shales, gneisses, and foliated granites. The fracturing process of anisotropic rocks such as shales is examined in this study. We divide the rock anisotropy into two anisotropic groups: conventional and veined. Two members of the conventional first group are Lyons sandstone, a brittle, quartz-dominated, low-porosity and -permeability, anisotropic (11%) material; and pyrophyllite, a monomineralic-clay-rich, strongly anisotropic (19%) metamorphic rock similar chemically and mechanically to shale with extremely low porosity and permeability. The second group consists of a suite of natural shale samples (18% anisotropy) from the Wolfcamp formation containing mineralized veins. Fracture initiation and propagation are studied during Brazilian tests. Strain gauges and acoustic-emission (AE) sensors record the deformation leading to and during failure. Scanning-electron-microscope (SEM) imaging and surface profilometry are used to study the post-failure fracture system and failed surface topology. Post-fracture permeability is measured as a function of effective stress. The influence of anisotropy on fracturing is investigated by rotating the sample-fabric direction relative to the loading axis through increments of 15°. The rock microstructure, lamination, and brittleness control the activation of the layers. Lyons sandstone shows a wide brittle fracture with larger process zone with twice as much layer activation at lower stress levels than pyrophyllite. The fracturing process in veined shale is, however, a coupled function of rock fabric and mineral veins. The veins easily activate at 15° orientation with respect to the loading axis at stress levels of 30% of the unveined-failure load. The resulting unpropped fracture has enhanced permeability by orders of magnitude. We suggest that fracturing from a deviated well reduces the breakdown pressure significantly (compared with a vertical well) and activates a large number of veins with enhanced conductivity without the need for excessive proppant.