Geochemical and microstructural evidence for interseismic changes in fault zone permeability and strength, Alpine Fault, New Zealand

Abstract

Oblique dextral motion on the central Alpine Fault in the last circa 5 Ma has exhumed garnetoligoclase facies mylonitic fault rocks from 35 km depth. During exhumation, deformation, accompanied by fluid infiltration, has generated complex lithological variations in fault-related rocks retrieved during Deep Fault Drilling Project (DFDP-1) drilling at Gaunt Creek, South Island, New Zealand. Lithological, geochemical, and mineralogical results reveal that the fault comprises a core of highly comminuted cataclasites and fault gouges bounded by a damage zone containing cataclasites, protocataclasites, and fractured mylonites. The fault core-alteration zone extends 20-30 m from the principal slip zone (PSZ) and is characterized by alteration of primary phases to phyllosilicate minerals. Alteration associated with distinct mineral phases occurred proximal the brittle-to-plastic transition (T 300-4008C, 6-10 km depth) and at shallow depths (T 5 20-1508C, 0-3 km depth). Within the fault core-alteration zone, fractures have been sealed by precipitation of calcite and phyllosilicates. This sealing has decreased fault normal permeability and increased rock mass competency, potentially promoting interseismic strain buildup.

Permeability and seismic velocity and their anisotropy across the Alpine Fault, New Zealand: An insight from laboratory measurements on core from the Deep Fault Drilling Project phase 1 (DFDP‐1)

“…Overall, the mineralogy across the fault, in both the hanging wall and footwall, remains largely constant. X‐ray diffraction results from Boulton et al [] show that calcite, precipitated as cements and veins, vary in proportion more than any other mineral phase, comprising ~5% of the modal mineralogy in the hanging wall, decreasing sharply across the PSZ to ~1.5% in the footwall. Phyllosilicates such as chlorite and biotite are present throughout the fault zone, while smectite, an authigenic swelling clay, appears only within the PSZs.…”

“…Other than a few discrete features such as the PSZ and pseudotachylyte veins, the bulk mineralogy remains largely consistent across the fault zone [ Toy et al , ; Boulton et al , ]. Hence, the bulk mineralogy does not appear to be a major factor controlling the observed variations in physical properties.…”

“…Warr and Cox [] determined two generations of phyllosillicates on the Alpine Fault, hydrous chloritization at ~320°C, followed by the generation of swelling clays at shallower depths and temperatures of ~120°C. The dominant clay phases within the Alpine fault hanging wall are white mica (illite/muscovite) and chlorite [ Boulton et al , , ; Schleicher et al , ]. Schleicher et al [] found that enhanced levels of authigenic illite and smectite are present in the PSZ in contrast to higher levels of metamorphic illite/muscovite in surrounding cataclasites.…”

“…Schleicher et al [2015] found that enhanced levels of authigenic illite and smectite are present in the PSZ in contrast to higher levels of metamorphic illite/muscovite in surrounding cataclasites. White mica and chlorite fill a large proportion of fractures in the hanging wall, while smectite is found solely in the brown PSZ fault gouges, generated from the low-temperature alteration of illite/muscovite and chlorite, which is further altered into kaolinite and goethite [Craw, 1984;Boulton et al, 2012Boulton et al, , 2017Schleicher et al, 2015], with the combination of ultracomminuted grains and swelling clays on the PSZ reducing permeability to~10 À20 to 10 À21 m 2 , becoming a hydraulic seal.…”

How phyllosilicate mineral structure affects fault strength in Mg‐rich fault systems

“…The materials selected for the study are palygorskite (PIF-1) (79% purity) [Chipera and Bish, 2001], sepiolite (SV) (91% purity) [Viseras et al, 1999], saponite (78% purity) [Boulton et al, 2017], and talc item 468003 from Ward's Scientific. All samples were sieved to obtain the <53 μm fraction, with the exception of the saponite sample in which the <2 μm separate was obtained.…”