Authors: Yassaghi, A.¹; James, P. R.²; Flöttmann, T.³; Winsor, C. N.²

Source: Australian Journal of Earth Sciences, Volume 51, Number 2, April 2004 , pp. 301-317(17)

Publisher: Taylor and Francis Ltd

Abstract:

Mechanisms are examined for the development of shear zones in the external portion of the southern Adelaide Fold-Thrust Belt, based on geometric and kinematic analyses, balanced cross-sections, microstructure, pressure, and temperature of deformation estimates. Kinematic indicators reveal the zones were overthrust towards the west-northwest. Shear-zone quartz-grain microstructures show mainly intracrystalline deformation and dynamic recrystallisation where there is greater grain dislocation. These features become less well developed in the shear zones closer to the hinterland, than in those on the foreland side. Primary fluid inclusions in deformed quartz veins reveal that average temperatures were in excess of 358°C and indicate an average pressure of about 300 MPa. Microprobe analyses of coexisting biotite-muscovite-quartz-feldspar reveal an average peak metamorphic pressure of 250 ± 100 MPa, compatible with the estimated pressure of 300 MPa from primary fluid inclusions. Modified fluid inclusions, however, suggest an average entrapment pressure of less than 150 MPa, which suggests that they suffered overpressuring during exhumation, implying that the amount of shear-zone exhumation decreases towards the foreland. Based on this information, an orogenic wedge model is the preferred mechanism for shear-zone development. The earliest generated and propagated shear zones in the hinterland were buried at greater depth and remained at higher temperatures and pressures before being exhumed. Later shear zones generated closer to the foreland propagated from shallower depths, implying that they experienced lower pressures and temperatures before uplift.

Keywords: Adelaide Fold-Thrust Belt; Delamerian; fluid inclusions; microstructure; shear zone; South Australia

Document Type: Research article

DOI: http://dx.doi.org/10.1111/j.1400-0952.2004.01059.x

Affiliations: 1: Department of Geology, Tarbiet Modarres University, PO Box 14115-111, Tehran, Iran 2: School of Earth & Environmental Sciences, University of Adelaide, SA 5005, Australia 3: SANTOS, GPO Box 1010, Brisbane, Qld 7001, Australia

Publication date: 2004-04-01

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