TY  - JOUR
A1  - Schuck, Bernhard
A1  - Janssen, C.
A1  - Schleicher, Anja Maria
A1  - Toy, Virginia G.
A1  - Dresen, Georg
T1  - Microstructures imply cataclasis and authigenic mineral formation
JF  - Journal of structural geology
N2  - The Alpine Fault is capable of generating large (MW > 8) earthquakes and is the main geohazard on South Island, NZ, and late in its 250–291-year seismic cycle. To minimize its hazard potential, it is indispensable to identify and understand the processes influencing the geomechanical behavior and strength-evolution of the fault. High-resolution microstructural, mineralogical and geochemical analyses of the Alpine Fault's core demonstrate wall rock fragmentation, assisted by mineral dissolution, and cementation resulting in the formation of a fine-grained principal slip zone (PSZ). A complex network of anastomosing and mutually cross-cutting calcite veins implies that faulting occurred during episodes of dilation, slip and sealing. Fluid-assisted dilatancy leads to a significant volume increase accommodated by vein formation in the fault core. Undeformed euhedral chlorite crystals and calcite veins that have cut footwall gravels demonstrate that these processes occurred very close to the Earth's surface. Microstructural evidence indicates that cataclastic processes dominate the deformation and we suggest that powder lubrication and grain rolling, particularly influenced by abundant nanoparticles, play a key role in the fault core's velocity-weakening behavior rather than frictional sliding. This is further supported by the absence of smectite, which is reasonable given recently measured geothermal gradients of more than 120 °C km−1 and the impermeable nature of the PSZ, which both limit the growth of this phase and restrict its stability to shallow depths. Our observations demonstrate that high-temperature fluids can influence authigenic mineral formation and thus control the fault's geomechanical behavior and the cyclic evolution of its strength.
KW  - Alpine Fault
KW  - Fluid-rock interaction
KW  - Fault-rock microstructures
KW  - Fault healing
KW  - Authigenic mineral formation
KW  - Brittle deformation
Y1  - 2018
U6  - https://doi.org/10.1016/j.jsg.2018.03.001
SN  - 0191-8141
VL  - 110
SP  - 172
EP  - 186
PB  - Elsevier
CY  - Oxford
ER  -