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Title: Physicochemical evolution of an active plate boundary fault, the Alpine Fault, New Zealand : insight from the Deep Fault Drilling Project
Author: Allen, M.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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The Alpine Fault, a transpressional plate boundary between the Australian and Pacific plates, bounding the western edge of the Southern Alps on New Zealand's South Island, accommodates over 70 % of the relative motion between the colliding Australian and Pacific Plates with dextral-reverse movement offsetting basement rocks laterally by ~470 km. This motion manifests itself as quasi-periodical ruptures of large magnitude earthquakes (Mw ~8), with the fault currently thought to be nearing the end of its most recent interseismic period. Seismicity in the upper crust occurs due to frictional instabilities on faults; with earthquake nucleation, propagation, and inhibition governed by the properties of the fault zone and the tectonic forces acting upon it. The Deep Fault Drilling Project (DFDP) is an ongoing international effort, undertaken in order to provide insight into the current geological and seismological state of a plate boundary fault at the end of its latest seismic cycle. The first phase of the DFDP comprised two vertical pilot holes (DFDP-1A and DFDP-1B) drilled into an active thrust segment of the Alpine Fault Zone at Gaunt Creek, Westland in January – February 2011. The boreholes reached depths of 100.6 m and 151.4 m, respectively, sampling a range of fault lithologies including ultramylonites, cataclasites and gouges from both the hanging wall and footwall of the fault; including multiple fault principal slip zones. This was followed by diverse downhole observations and the installation of sensors for long-term in situ monitoring. Drilling revealed the presence of a ~30 m thick alteration zone overprinting the fault core and transition into the damage zone. This alteration zone was defined based on a distinctive, light green colour in drill core and in field outcrop, produced by elevated concentrations of alteration minerals (e.g., phyllosillicates and carbonates) above regional levels. This thesis presents a multidisciplinary approach of experimental physical property measurement, microstructural observations and fine-scale spatially resolved geochemical analyses performed with the aim to characterise and define the behaviour of a major plate boundary fault throughout its seismic cycle, with particular focus on fluids within the fault and the impact they have on fault processes. The study includes: the measurement of permeability and seismic velocity anisotropy of heavily cataclased and altered fault rock, in order to characterise the properties of the fault at the end of an interseismic period; the microstructural and geochemical observations of carbonate-mineralised fractures within the fault core/alteration zone, in order to better understand sealing processes and fluidrock interactions within heavily cataclased rock; and a study of the occurrence of frictional melt and ultracataclasis within the fault, with focus on the inter-relations of these processes as fault properties and fluid behaviour varies throughout the seismic cycle. These analyses were performed on Alpine Fault lithologies collected from the field and the DFDP. Our suite of experimental permeability and seismic velocity measurements, conducted with pore fluid (H2O) pressure held at 5 MPa over an effective pressure range of 5 – 105 MPa, show a zone of low permeability, from 10-17 to 10-21 m2 , and low seismic velocity, P-wave: 4400 to 5900 m/s; S-wave: 3900 to 4200 m/s, material bounding the PSZ of the central Alpine Fault, i.e. the fault core/alteration zone, under conditions approximating depths of 5 – 6 km. Measurements performed on protolith lithologies show lower permeabilities and higher seismic velocities than the fault-damaged rock owing to their intact, unfractured nature. Low permeabilities at low experimental pressures are explained by the presence of fine grained alteration products, carbonate precipitation and highly variable cataclastic structures within the fault zone. The PSZ, being composed of ultrafine-grained alteration products, acts as a barrier to cross-fault fluid flow postseismicity. This is in agreement with other petrophysical studies on other mature faults subjected to extensive comminution and fluid-rock alteration. Using SEM-based electron backscatter diffraction and optical cathodoluminescence together with secondary ion mass spectroscopy, we present fine-scale, quantitative microstructural and spatially resolved geochemical observations on the alteration zone carbonate-sealed fractures. We have shown that the hanging wall alteration zone is pervasively mineralised by carbonate precipitation, while the footwall is relatively devoid. This mineralisation is generated through a protracted cycle of fracturing and sealing, with ambient conditions fluctuating during exhumation and seismicity, as evidenced by variation in calcite trace element chemistry and deformation microstructures across vein generations. The vast majority of veins appear to be filled by blocky-equant calcite crystals with uniform chemistry, indicating that the veins were rapidly sealed by a single fluid pulse. Subsequent fluid-rock interaction and deformation has resulted in modification of vein microstructures and chemistry. These findings reflect the transient nature of permeability on the Alpine Fault, highly permeable co- and immediately post-seismically, and reduced to its current state in the late interseismic period. The evolution of fault rock microstructure with seismic shear and subsequent interseismic fluid-rock interaction has a great effect on the hydraulic and elastic properties of fault zones. This evolution is complex and cyclical, potentially promoting rupture via dynamic coseismic processes such as thermal pressurization. During interseismic periods fluid flow is likely concentrated upon pervasive, unconsolidated gouge-filled fracture networks at the contact of the fault core/alteration zone and the relatively unaltered damage zone in the hanging wall, allowing the flow of fluid along the fault and to the surface, as observed by abundant hydrothermal springs southeast of the fault. Using SEM-based microstructural observations and geochemical analyses, including: backscatter electron imaging; electron backscatter diffraction; and energy dispersive spectroscopy, presented is a study on the occurrence of pseudotachylytes and nanoparticulates within the fault zone. The focus is on the setting of these features, as well as the extent of mechanical reworking, chemical alteration and/or preservation they have undergone throughout the seismic cycle. It is shown that pseudotachylytes occur throughout the fault core/alteration zone cataclasites, predominantly as reworked, disassociated clasts, with intact pseudotachylyte veins constrained to the fault core/damage zone contact in the fractured ultramylonites. Frictional melt appears to occur in the presence of fluid, as evidenced by the cataclasis of calcite veins associated with, amygdules within and veins overprinting pseudotachylytes. With the lack of intact pseudotachylyte veins within and in the immediate vicinity of the principal slip zone it can be inferred that frictional melt did not occur during the most recent high magnitude Alpine Fault slip event. It is more likely that the pseudotachylytes observed are reworked relicts from older Alpine Fault ruptures below the Brittle – Viscous Transition, or rather, smaller magnitude earthquakes that have produced pseudotachylytes that have been subsequently incorporated into the widening alteration zone with exhumation. The presence of preserved nanoparticles within clasts in the principal slip zone could indicate that the Alpine Fault produced nanoparticulates during increments of slip at greater depths without the production of melt. This could indicate a transition in slip weakening mechanisms with exhumation; frictional melting may occur in the more competent rock close to the Brittle – Viscous Transition, nanoparticulate-aided slip may be important at intermediate depths, aided by thermal pressurisation, while cataclasis dominates at shallower crustal levels.
Supervisor: Mariani, Elisabetta ; Faulkner, Daniel Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral