Abstract
The first 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, specifically 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.
Highlights
Characterizing the internal structures of major fault zones is an important step toward understanding the gross physical attributes of geological faults, including their mechanical, seismic, and hydraulic properties
We describe the fault rocks intersected in the Deep Fault Drilling Project (DFDP)-1 boreholes, within ~70 m of the fault’s principal slip zone, based on observations of ~130 m of core and 25 thin sections representative of a larger suite of sections from samples spaced at ~0.5 m intervals throughout the core
| RESEARCH Alpine fault lithologies from DFDP-1 modates oblique-reverse slip on a moderately southeast-dipping plane that penetrates into the lower crust (Davey et al, 2007; Norris et al, 1990; Okaya et al, 2007)
Summary
Characterizing the internal structures of major fault zones is an important step toward understanding the gross physical attributes of geological faults, including their mechanical, seismic, and hydraulic properties. We describe the fault rocks intersected in the DFDP-1 boreholes, within ~70 m of the fault’s principal slip zone, based on observations of ~130 m of core and 25 thin sections representative of a larger suite of sections from samples spaced at ~0.5 m intervals throughout the core. | RESEARCH Alpine fault lithologies from DFDP-1 modates oblique-reverse slip on a moderately southeast-dipping plane that penetrates into the lower crust (Davey et al, 2007; Norris et al, 1990; Okaya et al, 2007) It is the primary plate boundary structure, with secondary faulting distributed mostly to its southeast (Cox and Barrell, 2007; Cox and Sutherland, 2007).
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