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| Titel |
Frictional properties of DFDP-1 Alpine Fault rocks under hydrothermal conditions and high shear strain |
| VerfasserIn |
André R. Niemeijer, Carolyn Boulton, Virginia Toy, John Townend, Rupert Sutherland |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109772
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| Publikation (Nr.) |
 EGU/EGU2015-9709.pdf |
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| Zusammenfassung |
| The Alpine Fault, New Zealand, is a major plate-bounding fault that accommodates 65-75%
of the total relative motion between the Australian and Pacific plates. Paleoseismic evidence
of large-displacement surface-rupturing events, as well as an absence of measurable
contemporary surface deformation, indicates that the fault slips mostly in quasi-periodic
large-magnitude earthquakes (< Mw 8.0). To understand the mechanics of earthquakes, it is
important to study the evolution of frictional properties of the fault rocks under conditions
representative of the potential hypocentral depth. Here, we present data obtained on drill core
samples of rocks that surround the principal slip zone(s) (PSZ) of the Alpine Fault and the
PSZ itself. The drill core samples were obtained during phase 1 of the Deep Fault Drilling
Project (DFDP-1) in 2011 at relatively shallow depths (down to ~150 m). Simulated
fault gouges were sheared under elevated pressure and temperature conditions in a
hydrothermal ring shear apparatus. We performed experiments at temperatures of 25,
150, 300, 450 Ë C, and 600 oC. Using the shallow geothermal gradient of 63 Ë
C/km determined in DFDP-1, our highest temperature corresponds to a depth of ~7
km (Sutherland et al. 2012); it would correspond to 10 km depth using a more
moderate geotherm of 45 oC/km (Toy et al. 2010). All samples show a transition from
velocity-strengthening behavior, i.e. a positive value of (a-b), to velocity-weakening behavior,
i.e. a negative value of (a-b) at a temperature of 150 Ë C. The transition depends on the
absolute value of sliding velocity, with velocity-weakening dominating at lower sliding
velocities. At 600 oC, velocity-strengthening dominates at low sliding velocity,
whereas the high-velocity steps are all velocity-weakening. Moreover, shear stress
depends linearly on effective normal stress at 600 oC, indicating that shearing is
essentially frictional and that no transition to ductile (normal stress independent)
flow has occurred. Thus, depending on the background (nucleation) strain rate,
our data indicate that the Alpine Fault should be able to generate earthquakes at
all temperatures above room temperature. However, at the highest temperature
investigated (600 oC), the transition to velocity-weakening is postponed to slip
rates above 10 mm/s (strain rate ~10-2 s-1). This observation, combined with
the absence of strength recovery after long holds, suggests that seismic slip may
propagate into regions of the fault unlikely to nucleate earthquakes. We propose
that in our porous gouges, thermally activated processes operate simultaneously
with granular flow, postponing ductile flow to higher temperatures or lower strain
rates.
Sutherland, R., V.G. Toy, J. Townend, S.C. Cox, J.D. Eccles, D.R. Faulkner, D.J. Prior,
R.J.Norris, E. Mariani, C. Boulton, B.M. Carpenter, C.D. Menzies, T.A. Little, M. Hasting,
G.De Pascale, R.M. Langridge, H.R. Scott, Z. Reid-Lindroos, B. Fleming (2012), Drilling
reveals fluid control on architecture and rupture of the Alpine Fault, New Zealand,
Geology,40, 1143-1146, doi:10.1130/G33614.1.
Toy, V.G., Craw, D., Cooper, A.F., and R.J. Norris (2010), Thermal regime in the
central Alpine Fault zone, New Zealand: Constraints from microstructures, biotite
chemistry and fluid inclusion data, Tectonophysics, doi:10.1016/j.tecto.2009.12.013 |
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