Transient Deformation at the Seismic-Aseismic Transition in a Mature Plate Boundary Fault Zone - New Zealand's Alpine Fault
Abstract
During the seismic cycle, stresses and strain rates fluctuate in the viscously-deforming zones down-dip of large faults. These transient events produce geological records that can be preserved in exhumed fault zones that have experienced single ruptures (e.g. Sesia Zone, European Western Alps). On the other hand, in major faults that have not experienced a simple, single rupture history, coseismic structures are likely to be destroyed during subsequent cycles of postseismic creep. New Zealand's active Alpine Fault has likely experienced upwards of 20,000 Mw~8 earthquakes, on average one every 200-300 years, over the last ≥5 million years of dextral-reverse slip. Fault rocks generated during these events are exhumed in the hangingwall, exposing materials deformed throughout the seismogenic zone at the surface. We have recognised a structural record of transient events in these rocks that differs from that previously reported elsewhere. Mylonites were formed by viscous shearing of a metasedimentary protolith downdip of the seismogenic structure. Rheological models predict these mylonites should have passed through a crustal strength peak (τ ≥100 MPa) around the brittle-viscous transition. Immediately prior to passing through this transition, they should have developed a small recrystallised grainsize (~10-15μm) and a crystallographic preferred orientation (CPO) indicating slip on the basal system during quartz dislocation creep, as well as a retrograde greenschist-facies mineralogy. However, the high-strain mylonites preserve a large recrystallised grainsize (>~30μm), amphibolite-facies mineralogy and CPO characteristic of prism slip. This suggests they were not significantly deformed at temperatures below ~450°C, significantly above the lower temperature limit for quartz crystal-plasticity at steady-state strain rates in the fault zone Microstructural observations and textural data indicate variable deformation style through the seismic cycle. Large fault ruptures propagated down-dip of the interseismically locked portion of the fault, so that a significant amount of deformation within the shear zone was released by slip on a single surface or surfaces, sometimes generating pseudotachylyte. High-stress immediate post-seismic creep resulted in activation of the uncharacteristic (harder?) prism slip system during quartz dislocation creep. On the other hand, creep strain rates in the interseismic period were much lower, so flow stresses were an order of magnitude less than expected if the shear zone deformation was evenly distributed over time. This postseismic creep must have been sufficient to develop a steady-state microstructure, so that the large quartz recrystallised grainsize developed while the ratio of recovery to deformation was high, but may have been insufficient to modify the prism CPO.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T52A..04T
- Keywords:
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- 3902 Creep and deformation;
- 8012 High strain deformation zones;
- 8030 Microstructures;
- 8031 Rheology: crust and lithosphere (8159);
- 8150 Plate boundary: general (3040)