Hydrothermally-treated, reactivated fault

One experiment (MIS038) has been undertaken to examine effects of hydrothermal conditions (6hrs at 900ºC, Pc = 250MPa, Pf = 150MPa) on the stability of frictional

melt, as well as fracture and fault zone healing. Following the reactivation of an unfavourably oriented fault (θr = 60º) at room temperature, under conditions that had

previously been demonstrated to reliably produce frictional melt, hydrothermal conditions were achieved by introducing pore fluids and increasing both confining pressure and temperature. Microstructural analysis of the slip interface following hydrothermal treatment indicates that the fault has undergone substantial healing (Figs. 18A-D). Gouge particles have become compacted with the destruction of porosity; all evidence of sub-micron-sized gouge particles is gone, and many of the extension fractures associated with deformation have healed. Other fractures, possibly those with wider apertures, are partly healed and can be identified by trails of fluid inclusions or by the presence of relict open fracture segments (see white arrow, Fig. 18B). The population of completely unhealed fractures (Fig. 18B) are thought to arise from either volume changes associated with the alpha-beta phase transition of quartz and/or depressurisation at the end of the experiment, or as a result damage caused during subsequent sample preparation.

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In places the fault is completely healed with the pre-ground slip interface being essentially indistinguishable from the undamaged rock when imaged in BSE-SEM mode. There is no evidence of the characteristic variation in back-scattered electron intensity associated with the presence of melt in untreated experiments. Fault geometry indicates that these entirely healed zones are within the fault core and are thought to exist where melt welding has previously occurred. In regions where the fault damage consisted mainly of gouge formation, porosity is significantly reduced with compaction of the high porosity gouge. Most gouge particles no longer have the angular and shard- like shapes associated with formation by cataclastic processes, but instead are rounded to polygonal and have faceted, euhedral grain surfaces adjacent to remnant pores.

Figure 18: Microstructures produced during hydrothermal treatment of a reactivated fault.

MIS038, θr = 60º: sample was loaded until the there was one reactivation on the existing fault. The

sample was then hydrothermally treated for 6 hours at 900ºC. After hydrothermal treatment, BSE-SEM images taken of the cross-section of the fault indicate extensive healing at the fault interface. (A) Low magnification image showing the extent of healing in the damage zone during hydrothermal treatment. In places the pre-ground fault is almost completely healed (i.e. no porosity). (B) Example of partially healed microstructure. White arrow indicates a microfracture that is segmented by the uneven precipitation of quartz along its length. Note the heterogeneous porosity distribution associated with partial cementation/compaction of gouge and the rounding of particles. (C) Image of an area of the original fault trace that is almost completely healed. (D) High magnification image of gouge zone that is partially healed. White arrows point to fractures along possible grain boundaries. The reduction in porosity and significant rounding of grains is evident.

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Where grain boundaries are evident within the partially compacted gouge (see white arrows, Fig. 14D) a number appear to show evidence of grain interpenetration, although it is unclear whether the grain interpenetration is the result of grain-to-grain dissolution, or caused by the overgrowth of hydrothermally quartz. However, BSE-SEM imaging is an unreliable technique for monitoring the evolution of grain shapes and grain boundaries under hydrothermal pressing as any precipitated overgrowths of quartz are indistinguishable from the original grains. Consequently, it is uncertain whether dissolution-precipitation creep is active over the timescales of the hydrothermal treatment. There is evidence of complete dissolution and removal of sub-micron gouge clasts within the fault core and silica precipitation in fractures and as overgrowths on clast surfaces, but, there is insufficient evidence to determine whether dissolution occurs preferentially at stressed grain contacts. Further, BSE-SEM imaging cannot resolve whether the melt dissolves or simply devitrifies and crystallizes in situ. High-resolution cathodoluminescence imaging and spectroscopy were explored as a possible technique to distinguish between the original quartz grains, melt and the hydrothermally precipitated quartz to provide insights into melt generation, dissolution, precipitation and overprinting. The results of this investigation are discussed in the following section.

3.3 2D Microstructural analysis using high resolution SEM-CL

Microstructural studies using high resolution SEM-CL have been undertaken on the fault cores of a number of samples to investigate how the high stress concentrations and transiently high temperatures associated with slip, have influenced the cathodoluminescence of material in the fault core. Representative samples of (1) melt- welded, unfavourably-oriented faults, (2) high-displacement favourably-oriented faults, (3) melt-welded faults produced at saturated conditions, and (4) the hydrothermally- treated fault, were investigated using both panchromatic imaging and monochromatic spectral analysis. The use of a low accelerating voltage (5kV) reduces the electron interaction volume and allows high resolution (sub-micron) CL imaging to be achieved. Corresponding BSE images were collected concurrently using a mirror detector (MD) located inside the objective aperture of the SEM. This detector is not located in a position to allow the collection of low angle backscattered electrons, so provides little information on crystallographic orientation and consequently the melt zones are more difficult to recognise than in the images described in Section 3.2. All samples were

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Figure 19: Cathodoluminescence analysis of a melt-welded interface created by the stress-driven reactivation of a θr=60º fault under nominally dry conditions.

Images and spectra acquired from MIS017, reactivated at Pc= 50MPa. (A) False colour CL image of a

melt-welded section of the fault. Quartz appears as either orange-brown or purple and the central melt layer appears as blue to blue-green. (B)-(C) BSE-SEM and panchromatic CL images (respectively) of the location shown in (A). Red arrows in (B) identify the location of the melt welded fault. Note the higher emissions along the edge of the fault in the CL image. (D) False colour image of another section of melt welded fault. Note the clasts within the melt layer that have the same CL emissions as the adjacent wall rock. (E)-(F) BSE-SEM and panchromatic CL images of the area shown in (D). Red arrows in (E) identify the location of the fault. The locations of spectral analysis are shown on the panchromatic image. Representative spectra for quartz and melt are shown in figures (G)-(I) and all spectra collected are shown

in Appendix 5. The formation of melt results in an increase in the broad peak centred at ~3.0eV and a

decrease in the 2.0eV peak. The intensity of the emissions produced by the melt varies considerably, although most spectra acquired suggests emissions of <10,000 counts.

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found to be sensitive to modification from the electron beam, with an observable decrease in the intensity over all wavelengths occurring during prolonged analysis. To minimise time-dependent change to CL responses, the analysis times were kept short (5 seconds) and the beam was blanked while analysis was not in progress.

The undeformed quartz in all samples has emissions in the red or red-blue wavelengths, forming broad spectral peaks between 1.9-2.0eV (λ = 620-640nm) with red-blue samples forming a secondary dominant peak at ~3eV (λ = 410nm) (Figs. 19-22). Narrow emission lines or peaks are rare. Using the panchromatic colour filters the original quartz is revealed in red-brown to purple hues. It has been suggested that the discrepancies in CL colours of the detrital grains arise from differences in provenance and metamorphic history of the sediments [Ramseyer et al., 1988; Gotze and Zimmerle, 2000]. Authigenic overgrowths are clearly visible in CL-imaging, being recognisable by their commonly-different intensity and wavelength luminescence compared with the detrital grains. These overgrowths form the euhedral grain surfaces that are visible around pores when using in other forms of SEM imaging. Within individual grains, a history of brittle deformation is evident from the presence of numerous, commonly cross-cutting, healed microfractures with different luminescent properties.