The influence of fault geometry on small strike-slip fault mechanics

Abstract

Meter-scale subvertical strike-slip fault traces in the central Californian Sierra Nevada exhibit geometric complexities that significantly contribute to their mechanical behavior. Sections of faults that opened at depth channelized fluid flow, as evidenced by hydrothermal mineral infillings and alteration haloes. Thin sections show a variation in the style of ductile deformation of infill along the fault, with greater intensities of deformation along restraining bends. Orthorectified photo-mosaics of outcrops provide model geometries and parameter constraints used in a two-dimensional displacement discontinuity model incorporating a complementarity algorithm. Model results show that fault shape influences the distribution of opening, and consequently the spatial distribution of fluid conduits. Geometric irregularities are present at many scales, and sections of opening occur along both releasing and restraining bends. Model sensitivity tests focus on boundary conditions along the fault: frictional properties on closed sections and fluid pressure within sections of opening. The influence of the remote stress state varies along a non-planar fault, complicating the relationships between remote stresses, frictional properties, slip, and opening. Discontinuous sections of opening along model faults are similar in spatial distribution and aperture to the epidote infill assemblages observed in the field. 1 Introduction Faults have irregularly shaped surfaces, and often are composed of discontinuous segments. Such irregularities cause the mechanical behavior of faults to deviate from that of the idealized planar structure. Understanding opening and slip on faults at depth is crucial to many economic and scientific applications. For example, economic geologists have noted ore deposits along geometrically complex faults for nearly a century (

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…The analytical model used above contains several important assumptions that affect interpretation of the results. The model does not account for numerous factors in the faulting process, including opening normal to the fault, nonuniform friction, displacement gradients, influence of fault tips, host rock anisotropy, selfaffine fault geometries, 3-D fault geometries, or pore fluid pressure variability [e.g., Dieterich and Smith, 2009;Griffith et al, 2010;Ritz and Pollard, 2012;Ritz et al, 2015]. As a result, it does not account for changes in elastic moduli in the damage zone resulting from accumulated inelastic deformation [e.g., Faulkner et al, 2006;Cappa et al, 2014;Xu et al, 2014].…”

Slip on wavy frictional faults: Is the 3rd dimension a sticking point?

“…• Slip on non-planar faults may promote local opening (or closing) of fault faces, and such movements can impact fault zone permeability, e.g. Figure 4 of Ritz et al (2015).…”

“…In this study, we use the term 'faults' for surfaces with shear displacement and the term 'fractures' or 'cracks' more generally for surfaces with low offset where both opening and/or shear displacement is observed, these terms are common in LEFM literature. We focus on metre-scale faults to avoid the additional complexities of gravitational stress gradients, inhomogeneous material interfaces, and damage (Ritz et al, 2015). Several mechanisms cause the faces of fractures in rock masses to deviate from planar, these can occur both during initial fracture growth, and later, as slip accumulates on the fault surface.…”

“…Visual examples of fault roughness show it is ubiquitous but varied e.g. Cann et al (1997); Griffith et al (2010); Jones et al (2009); Ritz et al (2015); Sagy et al (2007). Many previous studies of rough fault surfaces have focussed EarthArXiv postprint, accepted for publication in the Journal of Structural Geologyhttps://doi.org/10.1016/j.jsg.2018.11.009…”

“…3 is the area under the curve of static slip distributions typically shown for a 2D fracture, e.g. Ritz et al (2015). These terms can also be converted to seismic moment using:…”