Boulder, Colo., USA - Rocks within plate boundary scale fault zones become fragmented and altered over the earthquake cycle. They both record and influence the earthquake process. In this new open-access study published in Lithosphere on 4 Feb., Virginia Toy and colleagues document fault rocks surrounding New Zealand's active Alpine Fault, which has very high probability of generating a magnitude 8 or greater earthquake in the near future.
Descriptions already suggest that the complex fault rock sequence results from slip at varying rates during multiple past earthquakes, and even sometimes during aseismic slip. They also characterize this fault before rupture; Toy and colleagues anticipate that repeat observations after the next event will provide a previously undescribed link between changes in fault rocks and the ground shaking response. They write that in the future this sort of data might allow realistic ground shaking predictions based on observations of other "dormant" faults.
The first phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer- to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20−30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement.
This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifically the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture.
Source:GSA
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