Abstract
Abstract. Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. The Woodroffe Thrust in the Musgrave Block of central Australia is a regional-scale example that does not fit this model. It developed due to intracontinental shortening during the Petermann Orogeny (ca. 560–520 Ma) and is interpreted to be at least 600 km long in its E–W strike direction, with an approximate top-to-north minimum displacement of 60–100 km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in the mylonitization process, as can be demonstrated from the contrasting thorium signatures of mylonites derived from the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P–T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar. The distribution of pseudotachylytes, which acted as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these fault rocks are especially prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous fluids in the footwall. We also establish a qualitative increase in the abundance of fluids in the footwall along an approx. 60 km long section in the direction of thrusting, together with a slight decrease in the temperature of mylonitization (ca. 100 °C). These changes in ambient conditions are accompanied by a 6-fold decrease in thickness (from ca. 600 to 100 m) of the Woodroffe Thrust mylonitic zone.
Highlights
Continental fault and shear zone systems (e.g. Ramsay, 1980) with displacements on the order of several tens to hundreds of kilometres generally show an asymmetric mylonite distribution across the main fault horizon that is opposite for reverse faults or thrusts and normal faults or detachments
Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P –T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar
The juxtaposition of initially different crustal levels should result in a geometry that, for a thrust, preferentially preserves the broader ductile mylonite zone in the hanging wall, whereas, for a detachment, it should be in the footwall (e.g. Mancktelow, 1985, his Fig. 11; Passchier, 1984, his Fig. 2)
Summary
Continental fault and shear zone systems (e.g. Ramsay, 1980) with displacements on the order of several tens to hundreds of kilometres generally show an asymmetric mylonite distribution across the main fault horizon that is opposite for reverse faults or thrusts and normal faults or detachments. The juxtaposition of initially different crustal levels should result in a geometry that, for a thrust, preferentially preserves the broader ductile mylonite zone in the hanging wall, whereas, for a detachment, it should be in the footwall The mid- to lower-crustal Woodroffe Thrust of central Australia (Major, 1970) is an example that does not fit this model and predominantly developed a broader mylonite zone in the footwall (Bell and Etheridge, 1976; Camacho et al, 1995; Flottmann et al, 2004). Plagioclase dynamically neo-crystallized in the Woodroffe Thrust mylonites and associated shear zones (Bell and Johnson, 1989), forming typical core-and-mantle structures. Plagioclase studded with abundant inclusions of epidote and muscovite (Fig. 7d) This type is, with one exception, restricted to the northern locations (Fig. 8c)
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