A. John Watkinson

Initiation of folding and boudinage in wrench shear and transpression

Abstract A linear stability analysis is performed on a deforming layered three-dimensional linear viscous system. The system consists of a single layer of viscosity μ embedded in a medium of viscosity μ′. The layer is oriented normal to the z-axis and both the layer and medium are subjected to uniform simple shearing parallel to the y-axis (wrench shear) with superposed uniform shortening parallel to x and extension parallel to z (transpression). The stability of cylindrical perturbations of the form ζ = A (t) cos (ax − βy) is examined. It is found that fold-type perturbations are unstable and pinch-and-swell disturbances are unstable in some cases. For the case of wrench shear alone the fastest growing buckling disturbances are oriented at 45° to the positive y-axis, while the fastest growing pinch-and-swell disturbances have positive growth rates and are oriented 90° to the fold axes. Additional shortening parallel to the x-axis (transpression) causes the fold axes to initiate at lower angles to the y-axis. Pinch-and-swell disturbances may or may not be unstable in transpression depending on the magnitude of stretching parallel to the z-axis.

Reactivation of pressure-solution seams by a strike-slip fault-sequential, dilational jog formation and fluid flow

This work examines how older pressure-solution seams become sheared or reactivated by slip because of movement on a younger fault segment. The reactivation then leads to the creation of secondary structures. These are followed by further changes in the local stress-strain field that result in slip reactivation on the secondary structures and in the creation of third-order structures. This sequence of deformation reflects and reveals transient changes in the stress-strain field along the margins of the fault during active slip on the fault. The reactivation may lead to enhanced rock permeability and/or porosity that allow for temporary periods of fluid movement. Thus, we believe that this serves as an important model to contribute to the understanding of movement of fluids such as oil and gas around active faults.

Deformational features and impact-generated breccia from the Sierra Madera impact structure, west Texas

Deformational features from the Sierra Madera impact structure, west Texas, were studied to estimate shock temperature and shock pressure values involved during its formation. Sandstone, limestone, and polymict impact-generated breccia samples occur within the central uplift. The sandstones host several shock-related deformational features including toasted quartz grains and planar microstructures (planar fractures and planar deformation features) in quartz. Zircon mineral grains separated from the sandstone samples provide evidence of shock deformation in the form of planar fractures and granular surface textures. Both sandstone and carbonate within the central uplift contain shatter cones, whereas polymict impact-generated breccia samples contain shatter cone fragments and clasts of mixed lithology. Most high-temperature features (e.g., high-temperature mineral phases and a melt sheet) typically associated with less eroded structures of similar size are lacking; however, some small (millimeter-size) possible devitrified glass clasts within polymict breccias occur at several locations. Deformational features within zircon mineral grains, shatter cones, and quartz grains with multiple planar microstructures record pressure and temperature conditions during the impact of ∼3–20 GPa and postshock temperatures between 350 and >1000 °C. Deformational conditions during the impact event were almost certainly higher, given that erosion has presumably removed most of the more severely shocked rocks.

Shear localization in solids: insights for mountain building processes from a frame-indifferent ideal material model

Abstract Tectonic and orogenic processes, reflecting the dynamic nature of the planet, provide myriad examples of the failure of Earth materials under load. Despite this wealth of data, the shear localization process remains a difficult physical modelling problem, lying at the frontiers of complex and non-linear systems research. We present a non-conventional continuum-physics approach to address this problem, based on the mathematical properties of differential grade-2 (DG-2) materials. We choose this material because it is both frame-indifferent, and general enough to include other, simpler materials as special cases. DG-2 materials in pure shear exhibit a dynamic rescaling mechanism, associated with localized shearing, which links the spatial and temporal scales of this process in a self-consistent manner, independent of the observer. On typical thermal timescales, the thermomechanical competence of DG-2 materials depends on the ratio of thermal to mechanical diffusivities, κ/χ. On this basis, we hypothesize the effective rigidity of Earth materials, pertaining when the thermomechanical competence is greater than unity. This theory, applied to the whole Earth, suggests the existence of isopycnal ‘detachment’ zones at systematic, globally correlated depths beneath orogens, consistent with a variety of geological data.