Abstract Uncertainty in constitutive equations for brittle‐ductile deformation limits our understanding of earthquake nucleation and propagation at the base of the seismogenic lithosphere. To reduce this uncertainty, we investigate exhumed strike‐slip faults and related deformation features in the Lake Edison granodiorite (central Sierra Nevada, CA) that developed at 250–500°C and ~250 MPa. The Seven Gables outcrop contains a 10 cm wide contractional fault step separating 2 m‐scale left‐lateral faults. Within the step, an ~ 4 cm thick leucocratic dike is stretched and rotated, thus constraining the kinematics of deformation, and the dike and surrounding granodiorite are strongly mylonitized. Petrographic and electron backscatter diffraction analyses reveal evidence for brittle and plastic deformation mechanisms, including dislocation creep, diffusion creep, microfracturing, and cataclasis. We present a 2‐D finite element model of the Seven Gables outcrop that tests a series of candidate constitutive equations: Von Mises elastoplasticity, Drucker‐Prager elastoplasticity, power law creep viscoelasticity, two‐layer elastoviscoplasticity, and coupled elastoviscoplasticity. Models based on Von Mises yielding most accurately match the outcrop deformation. Frictional plastic yield criteria (i.e., Drucker‐Prager) are incapable of reproducing the outcrop deformation due to the elevated mean compressive stress and reduced plastic yielding within the model fault step. Furthermore, the power law creep viscoelastic model requires a high strain rate (~10 −4 s −1 ) to resolve slip on faults and fails to localize strain within the step region. Comparing model results and elastic stress fields with field observations suggests that deformation localizes in regions of elevated mean compressive stress and Mises equivalent stress.
Abstract The Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground-deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground-deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.
Abstract Kinematic theories of flat‐ramp‐flat folds relate fault angles to stratal dips in a way that allows prediction of structural geometries in areas of economic or scientific interest. However, these geometric descriptions imply constitutive properties of rocks that might be discordant with field and laboratory measurements. In this study, we compare deformation resulting from kinematic and mechanical models of flat‐ramp‐flat folds with identical geometries to determine the conditions over which kinematic models may be reasonably applied to folded rocks. Results show that most mechanical models do not conform to the geometries predicted by the kinematic models, and only low basal friction ( μ ≤ 0.1) and shallow ramps (ramp angle ≤10°) produce geometries consistent with kinematic predictions. This implies that the kinematic models might be appropriate for a narrow set of geometric and basal fault friction parameters.
Granitic plutons commonly preserve evidence for jointing, faulting, and ductile fabric development during cooling. Constraining the spatial variation and temporal evolution of temperature during this deformation could facilitate an integrated analysis of heterogeneous deformation over multiple length-scales through time. Here, we constrain the evolving temperature of the Lake Edison granodiorite within the Mount Abbot Quadrangle (central Sierra Nevada, CA) during late Cretaceous deformation by combining microstructural analysis, titanium-in-quartz thermobarometry (TitaniQ), and thermal modeling. Microstructural and TitaniQ analyses were applied to 12 samples collected throughout the pluton, representative of either the penetrative “regional” fabric or the locally strong “fault-related” fabric. Overprinting textures and mineral assemblages indicate the temperature decreased from 400–500°C to <350°C during faulting. TitaniQ reveals consistently lower Ti concentrations for partially reset fault-related fabrics (average: 12 ± 4 ppm) than for regional fabrics (average: 31 ± 12 ppm), suggesting fault-related fabrics developed later, following a period of pluton cooling. Uncertainties, particularly in TiO2 activity, significantly limit further quantitative thermal estimates using TitaniQ. In addition, we present a 1-D heat conduction model that suggests average pluton temperature decreased from 585°C at 85 Ma to 332°C at 79 Ma, consistent with radiometric age data for the field. Integrated with the model results, microstructural temperature constraints suggest faulting initiated by ∼83 Ma, when the temperature was nearly uniform across the pluton. Thus, spatially heterogeneous deformation cannot be attributed to a persistent temperature gradient, but may be related to regional structures that develop in cooling plutons.
