The anatomy of a deep intracontinental orogen
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[1] The crustal architecture of central Australia has been profoundly affected by protracted periods of intracontinental deformation. In the northwestern Musgrave Block, the Ediacaran–Cambrian (600–530 Ma) Petermann Orogeny resulted in pervasive mylonitic reworking of Mesoproterozoic granites and granitic gneisses at deep crustal levels (P = 10–14 kbar and T = 700–800°C). SHRIMP and LA-ICPMS dating of zircon indicate that peak metamorphic conditions were attained at circa 570 Ma, followed by slow cooling to ∼600–660°C at circa 540 Ma driven by exhumation along the Woodroffe Thrust. Strong links between regional kinematic partitioning, pervasive high shear strains and partial melting in the orogenic core, and an anomalous lobate thrust trace geometry suggest that north vergent shortening was accompanied by the gravitational collapse and lateral escape of a broad thrust sheet. Like the present-day Himalayan-Tibetan system, the macroscopic structural, metamorphic, and kinematic architecture of the Petermann Orogen appears to be dominantly shaped by large-scale ductile flow of lower crustal material. We thus argue that the anatomy of this deep intracontinental orogen is comparable to collisional orogens, suggesting that the deformational response of continental crust is remarkably similar in different tectonic settings.Keywords:
Orogeny
Mylonite
Thrust fault
Mylonite
Lineation
Flattening
Mesoscopic physics
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Mylonite
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Mylonite
Pegmatite
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Basal Gneiss Region, west Norway gives an Rb-Sr whole-rock isochron age of 1775 ±57 m.y. A foliated granodiorite in more homogeneous gneisses (Fetvatn gneiss) gives an age of 960 ±lO m.y. Similar ages from other areas indicate that most of the Basal Gneiss Region original ed during the Svecofennian and Sveconorwegian orogenies. The role of the Caledonian orogeny in the evolution of the Basal Gneiss Region remains unsettled. The 960 m.y. intrusion possesses a pronounced schistosity indicating recrys talliza tion in a stress field during the Caledonian orogeny or perhaps a late stage of the Sveconorwegian orogeny. The diminished role of the Caledonian orogeny in the formation of portions of the North Atlantic Caledonian System should modify theories for the evolution of this orogen.
Orogeny
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Abstract Mylonites are strongly deformed rocks which generally occur within thrust zones in many erogenic belts. In regard to their genesis, opinions are divided on the issues of the relative importance of mechanical and chemical processes and the type of deformation involved. A study of the mylonites derived from trondheimite in a part of the south-western Norwegian Caledonides and from basement granite in the eastern Himalayas reveals that shearing strain dominates in the initial stage of deformation, while ductile mylonitic microstructures are developed during a later flattening stage. Chemical mobility plays an important role in mylonite genesis.
Mylonite
Shearing (physics)
Flattening
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