No abstract

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5-893.2 m Measured Depth (MD). Continuous sampling and meso-to microscale characterization of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites, and mylonites, terminating 200-400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz+feldspar, most markedly below ~ 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.

No abstract

Graphitisation in fault zones is associated both with fault weakening and orogenic gold mineralisation. We examine processes of graphite emplacement and deformation in the Alpine Textural changes of graphite 2 Fault Zone, New Zealand's active continental tectonic plate boundary. Optical and scanning electron microscopic observations reveal a microstructural record of mobilisation of graphite as a function of temperature and ductile then brittle shear strain. Raman spectrometry allowed interpretation of the degree of maturity of carbonaceous material (CM), which reflects thermal and mechanical processes. In the amphibolite-facies Alpine Schist highly crystalline graphite, indicating peak metamorphic temperatures up to 640 • C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in reworking of CM under lower temperature conditions (500 • C-600 • C) in a structurally controlled environment, resulting in clustered (in protomylonites) and foliation aligned CM (in true mylonites). In the brittlely-deformed rocks (cataclasites derived from the mylonitised schists) graphite is most abundant (<50%) and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphite. Tectonic-hydrothermal fluid flow was probably important in deposition of graphite throughout the examined rock sequences. The increasing abundance of graphite may be a significant source of fault weakening, allowing strain localisation, as the fault rocks are progressively exhumed.

The Alpine Fault, New Zealand, is a large plate boundary fault with history of major seismic events that frequently ruptured to the surface. To better understand earthquake slip at shallow depth, we analyzed the frictional properties of gouge samples collected at three field exposures distributed along 40 km of the fault trace. The samples are rich in phyllosilicates (30-55%) and quartz/feldspar (41-64%), with small amounts of calcite (4-12%). The gouge samples were sheared in a confined rotary cell under room conditions at a normal stress of 2-3 MPa, under alternating slip velocity steps from 0.002 to 1.47 m/s. We found initial friction coefficients of 0.6-0.8 and gentle velocity strengthening during early slip with short displacement and low velocity. A drastic weakening of more than 50% reduction of the frictional strength occurred after displacements of 2-5 m as slip velocity exceeds~0.2 m/s, and there was no strength recovery even when slip velocity reduced back to low levels. This weakening was associated with marked temperature rise and intense CO 2 and H 2 O emissions. Based on continuous monitoring of gas pressure, CO 2 , and H 2 O concentrations within the gouge chamber, our analysis revealed that the dynamic weakening was triggered by gouge-zone pressurization, which was driven by thermal decomposition of calcite and dehydration of absorbed water. The pressurization process was enhanced by slip localization and establishment of a low-permeability shear zone. We further envision that the gouge zone pressurization is likely to cause local overpressure and internal fluidization within of the fine-grain gouge layer that leads to dynamic weakening.

No abstract