Damaged beyond repair? Characterising the damage zone of a fault late in its interseismic cycle, the Alpine Fault, New Zealand

X-ray computed tomography (CT) scans of drill-core, recovered from the first phase of the Deep Fault Drilling Project (DFDP-1) through New Zealand\u27s Alpine Fault, provide an excellent opportunity to study the damage zone of a plate-bounding continental scale fault, late in its interseismic cycle. Documentation of the intermediate-macro scale damage zone structures observed in the CT images show that there is no increase in the density of these structures towards the fault\u27s principal slip zones (PSZs), at least within the interval sampled, which is 30 m above and below the PSZs. This is in agreement with independent analysis using borehole televiewer data. Instead, we conclude the density of damage zone structures to correspond to lithology. We find that 72% of fractures are fully healed, by a combination of clays, calcite and quartz, with an additional 24% partially healed. This fracture healing is consistent with the Alpine Fault\u27s late interseismic state, and the fact that the interval of damage zone sampled coincides with an alteration zone, an interval of extensive fluid-rock interaction. These fractures do not impose a reduction of P-wave velocity, as measured by wireline methods. Outside the alteration zone there is indirect evidence of less extensive fracture healing.DFDP-1 was funded by: GNS Science; Victoria University of Wellington; the University of Otago; the University of Auckland; the University of Canterbury; Deutsche Forschungsgemeinschaft and the University of Bremen; Natural Environment Research Council grants NE/J024449/1, NE/G524160/1 and NE/H012486/1 and the University Of Liverpool; and the Marsden Fund of the Royal Society of New Zealand.2018-07-2

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>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 characterisation 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 c. 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.</p

Fracture geometries and processes in andesites at Mt Ruapehu, New Zealand: implications for the fracture modelling of the Rotokawa Geothermal Field

Fluid flow in the high-temperature (300 C), andesite-hosted Rotokawa geothermal reservoir (Taupo Volcanic Zone (TVZ), New Zealand) is largely controlled by fractures and faults but their geometries are still poorly understood. The aim of this study is to measure and derive geometric parameters characterising fractures in andesitic formations in order to use these as input for dis-crete fracture network models (DFN) and predictive fluid flow models of the Rotokawa geothermal reservoir. We make use of two complementary fracture datasets. (1) The fracture geometry in-trinsic to andesitic formations are studied on outcrops at Mt Ruapehu (TVZ volcano), with the measurement of c. 200 fractures along a 100 m long scanline, and the acquisition of a Terrestrial Laser Scanner (TLS) scan acquired over the entire outcrop. (2) Fracture orientation, width and spacing are determined for three acoustic borehole televiewer (BHTV) logs and 33 m of cores from the Rotokawa Geothermal Field. Two types of fractures are observed at the Mt Ruapehu outcrop. The majority of fractures form sub-vertical cooling joints. The TLS scan samples six dip directions suggesting an hexagonal section typical of basaltic lava flows. The scanline survey did not fully sample the six directions. The preliminary analysis of fracture length on the scanline survey highlights the high degree of fracture connectivity and a weak spatial clustering. A subset of the fractures are sub-horizontal, highly clustered and are aligned with possible changes of crystallinity, viscosity and flow banding within the flow. Further analysis is required to make firm conclusions about the fracture length and spacing. Fractures are conchoidal which enhances the fracture linkages, which cannot be easily quantified from scanline surveys and will be evaluated on the TLS scans. The BHTV and core analysis reveals that fractures within the reservoir are predominantly steeply dipping and NE SW-striking, parallel to the trend of the maximum horizontal compressive stress (S Hmax) and the rift axis. Fractures in the reservoir are preferentially oriented with respect to the in-situ stress and the tectonic faults but may be locally inherited from cooling joints and fractures associated with the internal fabric of the lava flows. BHTV logs indicate that the 8 50 mm wide fractures follow an exponential distribution. The log-normal, power-exponential or power-law distributions have similar likelihoods for fracture spacing of 0.005 50 m. Low spacing are best fitted by either an exponential, gamma or power-exponential distribution. This change at c. 1 m spacing may correspond to the threshold at which fracture interaction occurs. The lithological controls on the fractures is observed at both the outcrop and core scale, with fracture being less numerous and more tortuous in breccias than in massive lava. The breccias are typically more permeable than the massive interior, and offer lateral and vertical connectivity in reservoirs. Breccias also affect the propagation of the fractures due to their heterogeneity. Integrating these observations into fracture models will be fundamental to the prediction capability of the fracture models of the Rotokawa andesitic reservoir. Observations made at Ruapehu and Rotokawa have wide implication for the successful development of geothermal resources in volcanic-hosted geothermal reservoirs

Controls on fault zone structure and brittle fracturing in the foliated hanging wall of the Alpine Fault

Three datasets are used to quantify fracture density, orientation, and fill in the foliated hanging wall of the Alpine Fault: (1) X-ray computed tomography (CT) images of drill core collected within 25 m of its principal slip zones (PSZs) during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole televiewer images, (2) field measurements from creek sections up to 500 m from the PSZs, and (3) CT images of oriented drill core collected during the Amethyst Hydro Project at distances of  ∼  0.7–2 km from the PSZs. Results show that within 160 m of the PSZs in foliated cataclasites and ultramylonites, gouge-filled fractures exhibit a wide range of orientations. At these distances, fractures are interpreted to have formed at relatively high confining pressures and/or in rocks that had a weak mechanical anisotropy. Conversely, at distances greater than 160 m from the PSZs, fractures are typically open and subparallel to the mylonitic or schistose foliation, implying that fracturing occurred at low confining pressures and/or in rocks that were mechanically anisotropic. Fracture density is similar across the  ∼  500 m width of the field transects. By combining our datasets with measurements of permeability and seismic velocity around the Alpine Fault, we further develop the hierarchical model for hanging-wall damage structure that was proposed by Townend et al. (2017). The wider zone of foliation-parallel fractures represents an “outer damage zone” that forms at shallow depths. The distinct < 160 m wide interval of widely oriented gouge-filled fractures constitutes an “inner damage zone.” This zone is interpreted to extend towards the base of the seismogenic crust given that its width is comparable to (1) the Alpine Fault low-velocity zone detected by fault zone guided waves and (2) damage zones reported from other exhumed large-displacement faults. In summary, a narrow zone of fracturing at the base of the Alpine Fault's hanging-wall seismogenic crust is anticipated to widen at shallow depths, which is consistent with fault zone flower structure models