HR: 1340h
AN: T53B-1428    [Abstracts]
TI: Geophysical Exploration of the Alpine Fault Zone (New Zealand) and Evidence for High Fluid Pressure.
AU: * Stern, T
EM: tim.stern@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6001 New Zealand
AU: Okaya, D
EM: okaya@usc.edu
AF: Dept. of Geological Science, University of Southern California, South Science Building, Los Angeles, CA 90089 United States
AU: Henrys, S
EM: s.henrys@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, 6009 New Zealand
AB: A fresh look at the problem of sustaining high fluid pressures in active fault zones can be obtained by studying a major continental transform other than the San Andreas Fault. The Alpine Fault (AF) of central South Island, New Zealand, is one such example. The AF is an unusual continental transform in that both strike-slip (~ 35 mm/y) and dip-slip (~10 mm/y) movement take place on the same fault plane. Moreover, geophysical evidence shows this fault plane dips between 40 and 60 degrees from the earth's surface down to a depth of at least 30 km, where it soles out into what we interpret to be a decollement surface. Detailed analysis of seismic travel time anomalies and seismic reflection amplitudes shows that above the dipping fault-plane lies a banana-shaped region where seismic P-wave velocities are reduced by 6-10%. This low-velocity region has dimensions of roughly 20 x 40 km with the shallowest and deepest points being at ~ 8 and 35 km, respectively. A magnetotelluric study shows a low electrical resistivity zone in the crust that correlates closely with the region of low seismic velocities. Interconnected water at or close to lithostatic pressure is one physical condition that would give rise to both the low resistivity and low P-wave velocities. The question then arises of how high fluid pressures are contained in the fault zone ? A simple answer may be that fluid is supplied to the root of the fault zone at a faster rate than it can be removed via porous flow through the crust. Greywacke-schist rocks that are transported into the fault zone are thickened as they travel down a decollement surface; this surface has been partially imaged with seismic reflection methods. As the crustal rocks are thickened, water will be released (zeolite -> amphibolite prograde metamorphism) at an estimated rate of 0.15 wt. % per km of burial. For a regular continental permeability of 10^-17 m², we estimate that water is supplied to the root of the Alpine fault zone at a rate that is about 40 times higher than it can bleed off through crust of regular permeability. For the bulk of the plate motion to be accommodated on a single inclined fault plane, the fault itself is likely to be intrinsically weak. Most continental transforms sustaining a component of convergence, partition motion onto a vertical strike-slip fault and distributed thrust faults: Southern California is one example of this sort of behaviour. We therefore argue that it is high fluid pressure, due largely to excess fluid-release accompanying prograde metamorphism, that reduces normal stresses within the Alpine Fault Zone, and it is this mechanical weakening that allows plate motion to take place on a single inclined plane.
DE: 8108 Continental tectonics: compressional
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005