Johanna M. Nevitt

Impacts of off‐fault plasticity on fault slip and interaction at the base of the seismogenic zone

Direct observations of faults exhumed from mid-crustal depths indicate that distributed inelastic deformation enhances fault slip and interaction across steps. Constrained by field measurements, finite element models demonstrate that the slip distribution for a fault in a Mises elastoplastic continuum differs significantly from that of a linear elastic model fault. Lobes of plastic shear strain align with fault tips and effectively lengthen the fault, resulting in greater maximum slip and increased slip gradients near fault tips. Additionally, distributed plastic shear strain facilitates slip transfer between echelon fault segments. Fault arrays separated by contractional steps, which are subjected to greater mean normal stress and Mises equivalent stress, produce greater maximum slip than do those separated by extensional steps (with no fractures). These results provide insight into fault behavior at the base of the seismogenic zone, with implications for rupture dynamics of discontinuous faults.

Testing constitutive equations for brittle‐ductile deformation associated with faulting in granitic rock

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 two meter-scale left-lateral faults. Within the step, a ~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 2D 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.

What Do Kinematic Models Imply About the Constitutive Properties of Rocks Deformed in Flat‐Ramp‐Flat Folds?

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.