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STT-06 Marine and continental fold and thrust belts
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A kinematic, metamorphic and geochronological framework for intracratonic reworking in the western Musgrave Block, central Australia: Evidence for lower crustal channel flow?
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Tom Raimondo, University of Adelaide (Australia)
Alan Collins, University of Adelaide (Australia)
Martin Hand, University of Adelaide (Australia)
Althea Walker-Hallam, University of Adelaide (Australia)
Hugh Smithies, Geological Survey of Western Australia (Australia)
Paul Evins, Geological Survey of Western Australia (Australia)
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The crustal architecture of central Australia has been profoundly affected by periods of intracontinental deformation. Within the western Musgrave Block, Western Australia, the Neoproterozoic to Early Cambrian (600-520 Ma) Petermann Orogeny resulted in pervasive mylonitic reworking of Mesoproterozoic granites and granitic gneisses at deep crustal levels (P = 10-13 kbar and T = 700-780°C). SHRIMP and LA-ICPMS analysis of zircon and titanite indicate that peak metamorphic conditions were attained at c. 570 Ma, followed by progressive cooling to c. 600-660°C by c. 540 Ma driven by exhumation along the Woodroffe Thrust. A slight increase in average geothermal gradients moving south of this location suggests that deeper crustal sections experienced more rapid exhumation. This is supported by good correspondence between the record of thermal equilibration retained by equilibrium mineral assemblages and the crystallisation conditions of zircon and titanite identified using Ti and Zr thermometry.
Shearing conditions during deep crustal mylonitisation appear to be dominantly anhydrous, although evidence of fluid influx into discrete shear zones is indicated by solid-state zircon recrystallisation and relatively hydrous mineral assemblages. This suggests a complex pattern of fluid partitioning and limited structural connectivity between mylonitised domains.
At the outcrop scale, the correlation between distinctive structural expressions and strain intensity is interpreted to represent the simultaneous development of pure and simple shearing, resulting in the progressive partitioning of coaxial and non-coaxial strain components into discrete rock packages. This has implications for the genetic interpretation of lineations that plunge at oblique angles to the predominant regional orientation.
At the orogenic scale, the relationship between kinematic partitioning and an anomalous lobate geometry of the Woodroffe Thrust trace suggests that north-directed emplacement of a broad thrust sheet was accompanied by southwest-directed channel flow by gravitational collapse. This is indicated by the rotation of regional lineation patterns from orogen-parallel adjacent to the approximately linear fault trace to highly oblique at the point of greatest curvature further west, representing a change in the trajectory of material flow caused by lateral escape towards the orogen margin. Pervasive extensional deformation was thus produced in the hanging wall of the Woodroffe Thrust whose kinematic polarity is decoupled from the bulk tectonic transport of the Petermann Nappe Complex.
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