Rheology of the Alpine Fault Mylonite Zone : deformation processes at and below the base of the seismogenic zone in a major plate boundary structure

Author:

Toy, Virginia Gail.

Year:

Project type:

Abstract:

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 to very high strains at deep crustal levels under boundary conditions constrained by present-day plate motions. The purpose of this study was to investigate the fault zone rheology and mechanisms of strain localisation, to obtain further information about how the structural development of this shear zone relates to the kinematic and thermal boundary constraints, and to investigate the mechanisms by which the viscously deforming mylonite zone is linked to the brittle structure, that fails episodically causing large earthquakes.

This study has focussed on the central section of the fault from Harihari to Fox Glacier. In this area, 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 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 degrees and kinematic directions indicated by asymmetric fabrics and CPO pattern symmetry could have formed during pure shear stretches up-dip of the fault of ~ 3.5, coupled with simple shear strains, gamma >=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 degrees C were experienced at depths of ~ 4.5 km, so that a high thermal gradient of ~75 degrees C/km is indicated in the near-surface. This gradient must fall off to <=10 degrees 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 degrees C at >=7.0-8.5±1.5 kbar, ie. 26-33 km depth.

Thesis description:

xxiii, 629 p. : ill., maps (chiefly col.) ; 30 cm + 2 sheets.

Department:

OU geology Identifier:

2007Toy

Author last name:

OURArchive access level:

Open Access

Location (WKT, WGS84):

MULTIPOLYGON (((170.097767729668561 -43.428227640892381,170.096750686110653 -43.444019934123311,170.05812467633109 -43.44226958941038,170.059142481419656 -43.426668266320256,170.097767729668561 -43.428227640892381)),((170.351277727865238 -43.318939157392663,170.342925564512996 -43.330895096800326,170.30686520757672 -43.31611458608716,170.315663788996005 -43.305243803766281,170.351277727865238 -43.318939157392663)))

Abstract PDF File

http://download.otagogeology.org.nz/temp/Abstracts/2007Toy.pdf

Collection

Citation

Toy, Virginia Gail., “Rheology of the Alpine Fault Mylonite Zone : deformation processes at and below the base of the seismogenic zone in a major plate boundary structure ,” Otago Geology Theses, accessed October 15, 2018, http://theses.otagogeology.org.nz/items/show/489.