Along strike applicability of results from the Deep Fault Drilling Project, Alpine Fault, New Zealand

A large proportion of coseismic slip is thought to occur along narrow principal slip zones (PSZs) that are millimeters to centimeters- thick. Understanding the physical properties of PSZ gouges and cataclasites, and their along strike extent, is a primary goal of fieldwork being done in conjunction with the Deep Fault Drilling Program (DFDP), Alpine Fault. Core retrieved from two shallow boreholes at Gaunt Creek contains mylonites with a Pacific Plate Alpine Schist protolith, cataclasites derived from Pacific Plate mylonites and from Australian Plate felsic igneous rocks, gneisses, and possibly metasediments, and three PSZ gouges. These three PSZ gouges are: (1) DFDP-1a PSZ gouge (90.5 - 90.8 m depth), which occurs at the contact between Pacific Plate cataclasite and Australian Plate Late Quaternary gravel; (2) DFDP-1b PSZ1 gouge (128.0 - 128.3 m depth), which occurs below Pacific Plate cataclasite and above mixed Pacific and Australian Plate-derived cataclasite; (3) DFDP-1b PSZ2 gouge (143.83 - 143.94 m depth), which occurs below the mixed protolith cataclasite and above Australian Plate augen mylonite Microstructurally, PSZ gouges in the core are similar to those documented in a nearby outcrop at Gaunt Creek by Boulton et al. (2012). Brown PSZ gouge layers contain reworked fault gouge clasts together with subrounded to rounded clasts of mylonite, ultramylonite, quartz and carbonate-rich fragments. From quantitative XRD, the brown PSZ gouges in DFDP-1b comprise 18% dioctahedral smectite, along with quartz (25 - 29%), orthoclase/microcline (3 - 4%), albite (24 - 25%), calcite (8 - 9%), kaolinite (0 - 6%), muscovite/illite (12 - 17%), chlorite (0 - 1%), and trace amounts of pyrite. Boulton et al. (2012) reported similar mineralogy for Gaunt Creek outcrop and Waikukupa Thrust outcrop PSZ gouges and found that the PSZ gouges have lower friction coefficients and lower permeability than surrounding cataclasites. Mineralogically, microstructurally, and geochemically similar brown PSZ gouges also occur at localities north and south of Gaunt Creek, from Little Man River to Robinson Creek, an along strike distance of 120 km. Everywhere mapped, brown PSZ gouges form at the contact between Pacific Plate and Australian Plate-derived cataclasites, which, importantly, do not contain smectite. Smectite-bearing gouges are generally absent on shallow dipping dextral-reverse faults at the toes of large thrust sheets, where plate boundary cataclasites overlie Late Quaternary gravels in sharp contact. Our results suggest that PSZ gouges retrieved in the DFDP-1 cores are commonly present on moderately dipping (average orientation 043°/30°SE; Norris and Cooper, 2007) dextral-reverse faults along the central Alpine Fault, and we discuss modes of PSZ formation. References Boulton, C., B.M. Carpenter, V. Toy, and C. Marone (2012). Physical properties of surface outcrop cataclastic fault rocks, Alpine Fault, New Zealand. Geochem. Geophys. Geosyst., 13, doi:10.1029/2011GC003872. Norris, R.J., and A.F. Cooper (2007). The Alpine Fault, New Zealand: Surface Geology and Field Relationships, in A Continental Plate Boundary: Tectonics at South Island, New Zealand, edited by Okaya, D., Stern, T., and F. Davey, American Geophysical Union Monograph Vol 175, Washington, D.C., 159-178.