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Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand

Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand
Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand
Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a >50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.
0091-7613
1143-1146
Sutherland, R.
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Toy, V.G.
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Townend, J.
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Cox, S.C.
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Eccles, J.D.
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Faulkner, D.R.
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Prior, D.J.
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Norris, R.J.
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Mariani, E.
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Boulton, C.
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Carpenter, B.M.
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Menzies, C.D.
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Little, T.A.
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Hasting, M.
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De Pascale, G.P.
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Langridge, R.M.
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Scott, H.R.
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Lindroos, Z.R.
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Fleming, B.
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Kopf, A.J.
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Sutherland, R.
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Toy, V.G.
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Townend, J.
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Cox, S.C.
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Eccles, J.D.
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Faulkner, D.R.
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Prior, D.J.
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Norris, R.J.
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Mariani, E.
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Boulton, C.
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Carpenter, B.M.
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Menzies, C.D.
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Little, T.A.
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Hasting, M.
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De Pascale, G.P.
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Langridge, R.M.
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Scott, H.R.
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Lindroos, Z.R.
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Fleming, B.
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Kopf, A.J.
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Sutherland, R., Toy, V.G., Townend, J., Cox, S.C., Eccles, J.D., Faulkner, D.R., Prior, D.J., Norris, R.J., Mariani, E., Boulton, C., Carpenter, B.M., Menzies, C.D., Little, T.A., Hasting, M., De Pascale, G.P., Langridge, R.M., Scott, H.R., Lindroos, Z.R., Fleming, B. and Kopf, A.J. (2012) Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand. Geology, 40 (12), 1143-1146. (doi:10.1130/G33614.1).

Record type: Article

Abstract

Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a >50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.

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Published date: 2 August 2012
Organisations: Geochemistry

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Local EPrints ID: 346725
URI: http://eprints.soton.ac.uk/id/eprint/346725
ISSN: 0091-7613
PURE UUID: d908b284-8d1a-42e0-8866-240f9fbfab17

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Date deposited: 08 Jan 2013 10:43
Last modified: 16 Jul 2019 21:47

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Contributors

Author: R. Sutherland
Author: V.G. Toy
Author: J. Townend
Author: S.C. Cox
Author: J.D. Eccles
Author: D.R. Faulkner
Author: D.J. Prior
Author: R.J. Norris
Author: E. Mariani
Author: C. Boulton
Author: B.M. Carpenter
Author: C.D. Menzies
Author: T.A. Little
Author: M. Hasting
Author: G.P. De Pascale
Author: R.M. Langridge
Author: H.R. Scott
Author: Z.R. Lindroos
Author: B. Fleming
Author: A.J. Kopf

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