Characterising the internal structure of major fault zones, using samples from outcrops and boreholes from the Alpine Fault Zone, New Zealand
It is important to characterize the internal structure of major fault zones to understand their gross physical attributes, including their mechanical, seismic, and hydraulic properties. Using the Deep Fault Drilling Project (DFDP)-1 core samples and outcrop samples from the Alpine fault zone, I characterized the structure of cataclasites by mineralogical and microstructural analyses. The constituent minerals of the protolith mylonites were comminuted during cataclasis generating particles with new size and shape distributions, but new minerals also grew authigenically during deformation. The developed method of SEM-EDS analyses improving spatial resolution using lower accelerating voltage enabled identification minerals and determination of major element compositions of particles as small as ~ 2 µm diameter. Compositions of phyllosilicates in fault rocks from the DFDP-1 measured using this method show pressures of ~100 - 200 MPa, and fairly high temperatures of ~350 ºC were estimated for the analysed ultramylonite sample, while variable temperatures in the range ~180- 350 ºC were estimated for cataclasites, and lower temperatures of ~160 ºC were estimated for ultracataclasites. These results demonstrate that cataclasis from a mylonitic protolith is a progressive process occurring throughout the brittle fault zone during exhumation. The particle size distribution in different types of fault rocks shows that higher fractal dimension (D) by foliated ultracataclasite/cataclasite represents a process of ‘selective fragmentation of particles’. Lower D by ultramylonite and unfoliated cataclasite represents a process of ‘constrained comminution’. Orientation distribution of particles shows that the textural transition from ultramylonite through unfoliated cataclasite to foliated (ultra-)cataclasite involves increasing the number of dominant orientations initially through fracturing and then by rotation of particles. Foliations in cataclasites most likely formed by progressive fragmentation of particles during multiple repeated increments of coseismic slip. Therefore, during exhumation, the fault zone develops a new mineralogy by hydrothermal alteration of pre-existing minerals, and precipitation of new minerals. The newly-formed minerals are mostly phyllosilicates, which are known to be frictionally weak, but velocity-strengthening. This means that during exhumation the fault gradually weakens and becomes less likely to accommodate earthquake ruptures.
Advisor: Toy, Virginia
Degree Name: Doctor of Philosophy
Degree Discipline: Geology
Publisher: University of Otago
Keywords: Cataclasite; Microstructure; SEM-EDS; Alpine Fault
Research Type: Thesis