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

Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling
Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling
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.
1941-8264
155-173
Toy, V.G.
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Boulton, C.J.
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Sutherland, R.
af9b1cda-b9cb-496f-b995-904967e9e28e
Townend, J.
b4d4a70d-a8ce-4125-8b20-4ba6714ab7c9
Norris, R.J.
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Little, T.A.
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Prior, D.J.
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Mariani, E.
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Faulkner, D.
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Menzies, C.D.
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Scott, H.
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Carpenter, B.M.
42a47ad4-f86e-4cff-ab9f-6fe3f4f6ce18
Toy, V.G.
6106cd1a-006b-4efc-b0f4-d8b415298acb
Boulton, C.J.
818f0095-12ea-476a-9d16-8b68e4659801
Sutherland, R.
af9b1cda-b9cb-496f-b995-904967e9e28e
Townend, J.
b4d4a70d-a8ce-4125-8b20-4ba6714ab7c9
Norris, R.J.
da11d4ba-dfed-429c-badf-b9825eb544c2
Little, T.A.
1d0f7359-025f-49aa-af0b-d6cbe9715f08
Prior, D.J.
a546abf5-3af3-4754-8271-d7298a9a10ec
Mariani, E.
676f83ea-5fed-4a65-a177-ad5fcd02f0ad
Faulkner, D.
3c67c1b9-cc95-49b7-8504-f3ed9a0c44c4
Menzies, C.D.
04bc2a62-064d-4911-9c45-17b47f3005de
Scott, H.
68f5d547-0d51-4747-bb8e-760e2cf3d545
Carpenter, B.M.
42a47ad4-f86e-4cff-ab9f-6fe3f4f6ce18

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

Record type: Article

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.

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Published date: 16 January 2015
Organisations: Geochemistry

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Local EPrints ID: 376927
URI: http://eprints.soton.ac.uk/id/eprint/376927
ISSN: 1941-8264
PURE UUID: 161940a4-29f2-429c-8e01-2150fc099d07

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Date deposited: 08 May 2015 11:04
Last modified: 01 May 2020 16:31

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