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

Virginia G. Toy, Carolyn Boulton, Rupert Sutherland, John Townend, Richard Norris, Timothy Little, Dave Prior, Elisabetta Mariani, Dan Faulkner, Hannah Scott, Brett Carpenter, Catriona Dorothy Menzies

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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.
Original languageEnglish
Pages (from-to)155-173
Number of pages19
JournalLithosphere
Volume7
Issue number2
Early online date3 Feb 2015
DOIs
Publication statusPublished - 1 Apr 2015

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Toy, V. G., Boulton, C., Sutherland, R., Townend, J., Norris, R., Little, T., Prior, D., Mariani, E., Faulkner, D., Scott, H., Carpenter, B., & Menzies, C. D. (2015). Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling. Lithosphere, 7(2), 155-173. https://doi.org/10.1130/L395.1