Abstract
The Alpine Fault mylonite zone (AFMZ) is the downward continuation of the present-day Australian-Pacific plate boundary through the South Island of New Zealand. It exhibits an exponential shear strain distribution across a 1-2 km thick belt that is interpreted as the result of strain localization in mid- to lower crustal levels.
Strain-assisted modifications of Alpine Schist (AS) Grt porphyroblasts encompass foliation-parallel dissolution features and retrograde Grt consumption but no Grt crystal plasticity. Textural and microchemical evidence strongly suggests the absence of syn-deformational Grt growth, but that high-strain deformation associated with bulk retrogression in the AFMZ was Grt-destructive. Grt is therefore considered the least useful proxy for P-T-t-D conditions of Neogene high-strain deformation in the AFMZ. The presence of high phase strength contrast (HPSC) phases like Pl, Grt and Cc in the Qtz-Fsp AS protolith at T<600°C/P<1.0 GPa plays a central role for strain weakening in the AFMZ. Resulting stress and strain partitioning may not only amplify initial (hydrolytic) weakening of matrix quartz, but also accelerate pressure-sensitive deformation by fracturing during Arrhenius-hardening; in both cases promoting strain localization. Effective medium approximations for multiphase flow are not applicable to AF (ultra-)mylonites as the degree of interaction between phases with different PSCs is significant.
Gph-bearing AF ultramylonites show microstructural evidence for syn-deformational, grain boundary (GB)-focussed fluid flow by the presence of GB pores that decorate up to 30% of GBs. The extent of syn-deformational porosity is linked to the dislocation density of crystal-plastically deformed quartz. These pores developed along GBs as a consequence of dislocation-localized dissolution assisted by the primary metamorphic fluid. Decorated GBs may promote dynamic permeability assisted by grain boundary sliding and reduced bulk rock strength at mid-crustal conditions. The pre-kinematic primary metamorphic fluid is shown to be aqueous. Syn-kinematic fluids in GB fluid inclusions exhibit a predominant carbonic character with variable CO2/CO ratios. Aqueous fluids would very likely have hydrolytically weakened quartz at T>500°C before crossing the isothermal Chl-in isograd gradually consuming it. Carbonic fluids on the other hand would likely have helped strengthening matrix quartz through a decreasing water fugacity with decreasing temperatures.
Central section ultramylonites with no textural weakening indicative for a change of deformation mechanism provide clear evidence of rhomb <a> ± prism <a> slip and subgrain rotation crystallization. However, only low temperature quartz plasticity (i.e. basal <a> slip) is identified NE/SW along-strike in similar ultramylonites. Therefore, along-strike imprints in deformation fabrics and the across-strike CO2/CO proxy outline an extensive thermal perturbation in the central AFMZ.
The highest degree of modelled mid-crustal strain localization occurs with power-law flow combined with Von Mises yielding resulting in a constant deviatoric stress. In order to attain higher degrees of localization as seen in the AFMZ, the strain-softening component must result in a decrease in deviatoric stress. This is commonly referred to as a change from high stress, visco-plastic to low stress, linear viscous flow.
Strain weakening visco-plasticity is therefore the rheology of choice to numerically approach high-strain deformation of the Qtz-Fsp mid- to lower continental crust.