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

Rupert Sutherland, Virginia G. Toy, John Townend, Simon C. Cox, Jennifer Eccles, Dan Faulkner, Dave Prior, Richard Norris, Elisabetta Mariani, Carolyn Boulton, Brett Carpenter, Catriona Dorothy Menzies, Timothy Little, Mike Hasting, G. P. De Pascale, Rob Langridge, Hannah Scott, Zoe Reid Lindroos, Betina Fleming, A. Kopf

Research output: Contribution to journalArticle

100 Citations (Scopus)

Abstract

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 fl uid 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.
Original languageEnglish
Pages (from-to)1143-1146
Number of pages4
JournalGeology
Volume40
Issue number12
Early online date18 Sep 2012
DOIs
Publication statusPublished - 1 Dec 2012

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    Sutherland, R., Toy, V. G., Townend, J., Cox, S. C., Eccles, J., Faulkner, D., Prior, D., Norris, R., Mariani, E., Boulton, C., Carpenter, B., Menzies, C. D., Little, T., Hasting, M., De Pascale, G. P., Langridge, R., Scott, H., Reid Lindroos, Z., Fleming, B., & Kopf, A. (2012). Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand. Geology, 40(12), 1143-1146. https://doi.org/10.1130/G33614.1