Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling

Abstract

The fi rst phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer-to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20-30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement. This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifi cally the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture.

Large‐displacement, hydrothermal frictional properties of DFDP‐1 fault rocks, Alpine Fault, New Zealand: Implications for deep rupture propagation

“…At the same time, our data suggests that (frictional‐viscous) creep under low stress occurs at a temperature of T = 450°C, at least for slip velocities of V = 0.003–0.03 µm/s or 94.7 to 947 mm/yr, at least half an order of magnitude faster than the time‐averaged slip rate of the Alpine Fault of 26 mm/yr [ Norris and Cooper , ]. In addition, it is unlikely that the active slip zone width of the Alpine Fault is only 1 mm, as the PSZs drilled in DFDP‐1A and DFDP‐1B are approximately 180–200 mm thick [ Sutherland et al ., ; Toy et al ., ]. Considering that the PSZ was most likely active in the brittle part of the crust, the actual width of the deforming zone is probably much larger.…”

“…Ultramylonites within the sequence have accommodated large ductile shear strains, whereas fault gouges within the principal slip zone reflect brittle deformation processes (see Toy et al . [] for details). During exhumation, the fault rocks have been altered by pervasive fluid flow and precipitation of authigenic calcite and, locally, smectite [ Boulton et al ., ; Menzies et al ., ].…”

The effect of gradational velocities and anisotropy on fault-zone trapped waves

“…unfoliated cataclasites) within a transversely isotropic country-rock (e.g. schist) which could be imagined from a structure such as New Zealand's Alpine Fault (Toy et al 2015). In this case the SV velocities in the country-rock are lower than the SH velocities, resulting in reduced trapping for the F R FZTW.…”

A comparison of the use of X-ray and neutron tomographic core scanning techniques for drilling projects: insights from scanning core recovered during the Alpine Fault Deep Fault Drilling Project

“…Broadly speaking, the DFDP-1 cores scanned comprise two types of lithology: ultramylonites (Units 1 and 2 of Toy et al, 2015) and cataclasites (Units 3, 4 and 6 of Toy et al, 2015). Ultramylonites contain a foliation defined by alternating millimetre-centimetre quartzofeldspathic and phyllosilicaterich (biotite, muscovite, and chlorite) layers.…”

“…Cataclasites are found to contain millimetre-centimetre quartzofeldspathic clasts surrounded by a phyllosilicate matrix. All lithologies are cross-cut by millimetre-centimetre thick clay-enriched fractures, which contain quartz, albite, muscovite, chlorite, calcite, and smectite, that constitute the damage zone of the Alpine Fault (Caine et al, 1996;Schleicher et al, 2015;Toy et al, 2015;Williams et al, 2016).…”