Mesoscopic structure of the Punchbowl Fault, Southern California and the geologic and geophysical structure of active strike-slip faults

Abstract

We examine the distribution, density, and orientation of outcrop-scale structures related to the Punchbowl Fault, an exhumed ancient trace of the San Andreas Fault, southern California, in order to determine the structure of the fault zone. The Punchbowl Fault has 44 km of right-lateral slip, and cuts the Cretaceous Pelona Schist in the study area. The mesoscopic structures examined include fractures, small faults, and veins; they were inventoried using scan lines at closely spaced stations along three strike-perpendicular traverses 200–250 m long across the fault. The fault zone thickness is a function of the type of structure measured. Slip along narrow (<2 m wide) ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores: fracture and fault densities appear to increase 50–80 m from the fault cores, and vein densities are highly variable across the fault zone. Fractures and faults in the damaged zone have a variety of orientations, but most are at high angles to the main fault zone. When coupled with previous geochemical and microstructural data, these data show that large-displacement faults of the San Andreas system, are up to 200–250 m thick, and enclose zones of mineralogic and geochemical alteration that are 20–30 m thick. Extreme slip localization occurs over zones 1–5 m thick. When reconciled with geophysical imaging, our data suggest that trapped headwaves travel in the damaged zone, and that some aftershock events produce slip on faults and fractures, which often have orientations very different from the principal slip surfaces.

Introduction

Evaluating the mesoscopic structure of fault zones at scales of between one meter and hundreds of meters is important for a variety of applications, including hydrogeology (Forster and Evans, 1991; Haneberg, 1995; Lopez and Smith, 1995; Caine et al., 1996; Zhang and Sanderson, 1996; Matthai et al., 1998), hydrocarbon migration (Fisher and Knipe, 1998), waste isolation (Ferrill et al., 1999), ore deposits (Guilbert and Park, 1985; Sibson et al., 1988), earthquake nucleation and propagation (Sibson, 1989; Li et al., 1998), and the rheological/mechanical behavior of faults (Bruhn et al., 1994). Chester and Logan (1986) suggested that the three important mechanical, hydrologic, and structural entities in and around a fault zone are the protolith, the damaged zone, and fault core. The damaged zone consists of the region of deformation associated with the fault, and may consist of an increased concentration of fractures, faults, veins, and microstructural deformation (Chester et al., 1993; Goddard and Evans, 1995; Caine and Forster, 1999). The fault core is the portion of the fault which consists of highly deformed rock where much of the slip is accommodated (Chester et al., 1993). Where such tripartaite division of fault rock types exist, they are likely to have significant impacts on the mechanical and hydrologic behavior of faults (Bruhn et al., 1994; Evans et al., 1997), as well as produce important, recognizable geophysical and geochemical signatures. (e.g. Li et al., 1994a, Li et al., 1994b; Unsworth et al., 1999).

To date, most work on exhumed fault structure has focused on individual aspects of faulting, and little work has related data on the structure, mechanics, mechanisms and morphology of fault zones across a range of depths and scales, or considered how fault structure influences earthquake wave propagation and fault segmentation. The San Andreas Fault (SAF) is perhaps the most intensively studied fault zone in the world, with a variety of geological and geophysical investigations of the processes of seismic slip, rupture, and segmentation. Geologic studies of exhumed faults in the SAF system (Waters and Campbell, 1935; Oakshott, 1958; Anderson et al., 1980, Anderson et al., 1983; Chester et al., 1993; Evans and Chester, 1995) suggest that to at least 4 km depth, major faults of the SAF system are limited to thin, discrete slip surfaces commonly within a fractured zone up to 200 m wide. There may be multiple anastomosing slip surfaces that are confined to a relatively thin zone, outside of which there is little to no fault-related deformation (Chester et al., 1993). These slip zones are interpreted as being bounded by zones of enhanced deformation, as suggested by increased densities of fractures, veins, and small faults.

We examine the variation of mesoscopic structures across the Punchbowl Fault, an exhumed ancient trace of the SAF, developed in metamorphic rocks, and combine these data with previous investigations to evaluate how a fault zone may be imaged by direct and indirect methods. We examined the Punchbowl Fault at multiple scales from mesoscopic (outcrop) to microscopic (petrographic and scanning electron microscopes). The density and orientations of fractures, veins, and small faults and the reorientation of pre-fault schistosity were documented along traverses perpendicular to the fault zone and are presented in this paper. The geochemical, microstructural and mineralogical variations within the fault zone are summarized in Schulz and Evans (1998). In this paper we briefly describe our methods for detailed outcrop analyses of fault zones, then present our results, which consist of the qualitative description of the rocks, followed by the analysis of the quantitative data. Finally, we discuss the structural and geophysical implications of our results.