Dr Virginia G Toy
Rheology of the Alpine Fault Mylonite Zone: deformation processes at and below the base of the seismogenic zone in a major plate boundary structure
Brief summary of research topic
The Alpine Fault is the major structure of the Pacific-Australian plate boundary through New Zealand's South Island. During dextral reverse fault slip, a <5 million year old, ~1 km thick mylonite zone has been exhumed in the hanging-wall, providing unique exposure of material deformed at deep crustal levels under boundary conditions constrained by present-day plate motions. This study has focussed on the central section of the fault from Harihari to Fox Glacier. Here, mylonites derived from a quartzofeldspathic Alpine Schist protolith are most common, but slivers of Western Province-derived footwall material, which can be differentiated using mineralogy and bulk rock geochemistry, were also incorporated into the fault zone. These footwall-derived mylonites are increasingly common towards the north.
At amphibolite-facies conditions (T<650°C, P<8-10 kbar from Ti-in-biotite geo- thermometry and feldspar thermobarometry) mylonitic deformation was localised to the mylonite and ultramylonite subzones of the schist-derived mylonites. Most deformation was accommodated by dislocation creep of quartz, which developed strong Y-maximum crystallographic preferred orientation (CPO) patterns by prism<a> dominant slip. Formation of this highly-oriented fabric would have led to significant geometric softening and enhanced strain localisation. During this high strain deformation, pre-existing Alpine Schist fabrics in polyphase rocks were reconstituted to relatively well-mixed, finer-grained aggregates. As a result of this fabric homogenisation, strong syn-mylonitic object lineations were not formed. Strain models show that weak lineations trending towards ~090° and kinematic directions indicated by asymmetric fabrics and CPO-pattern symmetry could have formed during pure shear strains of ~3.5, coupled with simple shear strains > 30. The preferred estimate of simple:pure shear strain gives a kinematc vorticity number, Wk ~ 0.9997.
Rapid exhumation due to fault slip resulted in advection of crustal isotherms. New thermobarometric and fluid inclusion analyses from fault zone materials allow the thermal gradient along an uplift path in the fault rocks to be more precisely defined than previously. Fluid inclusion data indicate temperatures of 325±15°C were experienced at depths of ~4.5 km, so that a high thermal gradient of ~75°C / km is indicated in the near-surface. This gradient must fall off to <10°C / km below the brittle-viscous transition since feldspar thermobarometry, Ti-in-biotite thermometry and the absence of prism<c>-slip quartz CPO fabrics indicate deformation temperatures did not exceed ~650°C at ~7.0-8.5±1.5 kbar, ie. 26-33 km depth.
During exhumation, the strongly oriented quartzite fabrics were not favourably oriented for activation of the lower temperature basal<a> slip system, which should have dominated at depths <20 km. Quartz continued to deform by crystal-plastic mechanisms to shallow levels. However, pure dislocation creep of quartz was replaced by a frictional-viscous deformation mechanism of sliding on weak mica basal planes coupled with dislocation creep of quartz. Such frictional-viscous flow is particularly favoured during high-strain rate events as might be expected during rupture of the overlying brittle fault zone. Flow stresses supported by this mechanism are similar to those indicated by recrystallised grain size paleopiezometry of quartz (D>30mm, indicating maximum differential stress of ~44 MPa for most mylonites). It is likely that the preferentially oriented prism<a> slip system was activated during these events, so the Y-maximum CPO fabrics were preserved. Simple numerical models show that activation of this slip system is favoured over the basal<a> system, which has a lower critical resolved shear stress (CRSS) at low temperatures, for aggregates with strongY-maximum orientations. Absence of pervasive crystal-plastic deformation of micas and feldspars during activation of this mechanism also resulted in preservation of mineral chemistries from the highest grades of mylonitic deformation (ie. amphibolite-facies).
Retrograde, epidote-amphibolite to greenschist-facies mineral assemblages were pervasively developed in ultramylonites and cataclasites immediately adjacent to the fault core and in footwall-derived mylonites, perhaps during episodic transfer of this material into and subsequently out of the cooler footwall block. In the more distal protomylonites, retrograde assemblages were locally developed along shear bands that also accommodated most of the mylonitic deformation in these rocks. Ti-in-biotite thermometry suggests biotite in these shear bands equilibrated down to ~500±50°C, suggesting crystal-plastic deformation of this mineral continued to these temperatures. Crossed-girdle quartz CPO fabrics were formed in these protomylonites by basal<a> dominant slip, indicating a strongly oriented fabric had not previously formed at depth due to the relatively small strains, and that dislocation creep of quartz continued at depths ≤20 km. Lineation orientations, CPO fabric symmetry and shear-band fabrics in these protomylonites are consistent with a smaller simple:pure shear strain ratio than that observed closer to the fault core (Wk≥0.98), but require a similar total pure shear component. Furthermore, they indicate an increase in the simple shear component with time, consistent with incorporation of new hanging-wall material into the fault zone. Pre-existing lineations were only slowly rotated into coincidence with the mylonitic simple shear direction in the shear bands since they lay close to the simple shear plane, and inherited orientations were not destroyed until large finite strains (gamma < 100) were achieved.
As the fault rocks were exhumed through the brittle-viscous transition, they experienced localised brittle shear failures. These small-scale seismic events formed friction melts (ie. pseudotachylytes). The volume of pseudotachylyte produced is related to host rock mineralogy (more melt in host rocks containing hydrated minerals), and fabric (more melt in isotropic host rocks). Frictional melting also occurred within cataclastic hosts, indicating the cataclasites around the principal slip surface of the Alpine Fault were produced by multiple episodes of discrete shear rather than distributed cataclastic flow. Pseudotachylytes were also formed in the presence of fluids, suggesting relatively high fault gouge permeabilities were transiently attained, probably during large earthquakes. Frictional melting contributed to formation of phyllosilicate-rich fault gouges, weakening the brittle structure and promoting slip localisation. The location of faulting and pseudotachylyte formation, and the strength of the fault in the brittle regime were strongly influenced by cyclic hydrothermal cementation processes.
A thermomechanical model of the central Alpine Fault zone has been defined using the results of this study. The mylonites represent a localised zone of high simple shear strain, embedded in a crustal block that underwent bulk pure shear. The boundaries of the simple shear zone moved into the surrounding material with time. This means that the exhumed sequence does not represent a simple ‘time slice' illustrating progressive fault rock development during increasing simple shear strains. The deformation history of the mylonites at deep crustal P-T conditions had a profound influence on subsequent deformation mechanisms and fabric development during exhumation.
Publications related to this research
- Toy, V.G., Craw, D., Cooper, A.F., Norris, R.J. (2010) Thermal Structure of the Alpine Fault zone: constraints from microstructures, biotite chemistry, and fluid inclusion data. Tectonophysics, doi: 10.1016/j.tecto.2009.12.013.
- Toy, V.G., Prior, D.J. and Norris, R.J. (2008) Quartz fabrics in the Alpine Fault mylonites: Influence of pre-existing preferred orientations on fabric development during progressive uplift. Journal of Structural Geology 30(8), 602-621, doi: 10.1016/j.jsg2008.01. 001
- Sibson, R.H., Toy, V.G. (2007) The habitat of fault-generated pseudotachylyte: Presence vs. absence of friction melt. In: McGarr, A., Abercrombie, R., Di Toro, G. Earthquakes: Radiated Energy and the Physics of Faulting. AGU Geophysical Monograph 170, pp. 153-166.
Conference Oral Presentations
Toy, V. G., Norris, R. J., Prior, D. J. (2008) Transient deformation at the seismic-aseismic boundary in a mature plate boundary fault zone – New Zealand’s Alpine Fault. EOS Trans. AGU, 2008 Fall Meeting Supp. Abs. T52A-04.
- Toy, V.G., Norris, D.J., and Prior, D.J. (2007) Does strain become localized or delocalized during progressive deformation in New Zealand’s Alpine Fault zone? Oral presentation and abstract. 16th Conference on Deformation Mechanisms, Rheology and Tectonics. Rendiconti della Società Geologica Italiana v. 5 Nuovo Serie p. 208
Norris, R. J., Cooper, A. F., Toy, V. G., Prior, D. (2007) The deep structure of an active fault: the Alpine Fault mylonites, New Zealand. Proceedings, Continental Tectonics and Mountain Building conference, Ullapool, May 2007, Geological Society of London.
- Toy, V.G., Norris, R.J., Cooper, A., 2003: Interplay between brittle and ductile processes in the ‘Alpine fault’ mylonites. In: Manville, V., Tilyard, D. (eds.) Programme & Abstracts, Geological Society of New Zealand/New Zealand Geophysical Society/26th Annual Geothermal Workshop combined conference "GEO3", Taupo, New Zealand. Geol. Soc. of NZ Misc. Publ. 117A, 107.
Conference Poster Presentations
Toy, V.G., Craw, D., Cooper, A.F. and Norris, R.J. (2009) Thermal regime in the central Alpine Fault zone, New Zealand: Constraints from microstructures, biotite chemistry, and fluid inclusion data. Geodynamics of the Australian Plate Research Meeting, 8-9 June 2009, Melbourne, Australia.
- Toy, V.G., Norris, R.J., Cooper, A.F. and Prior, D.J. (2007) Rheological evolution of high strain fault rocks along a retrograde P-T path – The Alpine Fault mylonites. Abstract and poster. Geological Society of New Zealand Annual Conference. In: Mortimer, N. and Wallance, L. (eds.). Geological Society of New Zealand Miscellaneous Publication v. 123A p. 170
- Toy, V.G., Norris, R.J., Sibson, R.H. and Prior, D.J. (2006) Mechanical evolution of the Alpine fault zone. Abstract and poster, ISeS Summer School in Rheology of Earth Materials, Colorado College, Aug 5-12 2006. http://acad.coloradocollege.edu/dept/gy/ises/ISESabstracts.php#toy. (3.5 Mb pdf download)
- Toy, V.G., and Norris, R.J. (2006) Effects of pre-existing fabric, high strain and kinematic vorticity number on lineation development in the Alpine Fault zone. Abstract and poster, Geological Society of New Zealand and New Zealand Geophysical Society Joint Annual Conference. Stewart, B., Wallace, C., Lecointre, J. and Reyners. M. (eds.). Geological Society of New Zealand Miscellaneous Publication v. 112 A p. 89-90.
- Toy, V.G., Norris, R.J., 2005: Variation in deformation mechanisms during uplift of mylonites along the Alpine fault zone, New Zealand. Deformation mechanisms, Rheology and Tectonics. 2-4 May 2005, ETH Zurich. (3.5 Mb JPEG download)
- Toy, V.G., Sibson, R.H., Norris, R.J., 2004: Alpine fault pseudotachylytes. In: Manville, V., Tilyard, D. (eds.) Programme & Abstracts, Geological Society of New Zealand/New Zealand Geophysical Society/26th Annual Geothermal Workshop combined conference "GEO3", Taupo, New Zealand. Geol. Soc. of NZ Misc. Publ. 117A, 108. (3.5 Mb JPEG download)
- Toy, V.G. (2008) Rheology of the Alpine Fault mylonite zone: Deformation processes at and below the base of the seismogenic zone in a major plate boundary shear zone. 650pp. (212 Mb zipped pdf download)