Kinematics of bidirectional extension and coeval NW-directed contraction in orthogneisses of the biranup complex, Albany Fraser Orogen, Southwestern Australia
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Granulite-facies orthogneisses of the Mesoproterozoic Albany-Fraser Orogen from the locality of Bremer Bay, in southwestern Australia, record at least three phases of widespread, pervasive NW- and NE-trending bidirectional extension that alternate with shortening and/or shear related structures. Crustal extension occurred ca. 1180 Ma, based on SHRIMP U–Pb zircon geochronology of melts generated during deformation, which coincided with Stage II (1215-1140 Ma) of the Albany-Fraser Orogeny, a period of NW-directed contraction. Eight different deformation phases can be recognized in the Bremer Bay area: (1) formation of a pervasive migmatitic fabric, defined by alternating leucosomes and melanosomes, parallel to the main compositional layering, and axial planar to localized isoclinal folds of cm-wide melt bands; (2) first bidirectional extension phase, which formed cm-scale square boudins of mafic layers parallel to the main migmatitic fabric; (3) formation of open to isoclinal, upright to overturned, SW-plunging, NW-verging m-scale folds of early square and rectangular boudins and dominant migmatitic foliation; (4) renewed coeval NE- and NW-directed extension that produced intermediate (< 1 meter to a few meters) boudins of the migmatitic fabric and compositional layering; (5) formation of regional-scale, NW-verging, SW-plunging overturned folds of all previous structures; (6) third phase of bidirectional extension that formed large, decameter-scale boudins of the migmatitic fabric; (7) late folding phase that resulted in the formation of m-scale open to tight, SW-plunging, upright to moderately overturned, NW-verging folds; and (8) fracturing related to the intrusion of dominantly N-NW- and N-NE-trending intermediate and felsic few cm- to few dm-wide pegmatite veins. Melt generation was concurrent with all stages of deformation. The Albany-Fraser Orogen is reinterpreted as a diachronous orogen, resulting from the closure of the asymmetrically shaped ocean basin between the West Australian and Mawson cratons, which widens considerably from NE to SW along the length of the orogenic front. Subduction on the western side of the orogen was the driving force for NW-directed collision during Stage II of the orogeny. Slab breakoff and orogenic collapse following closure of an intracratonic ocean basin could account for the multiple phases of bidirectional extension, granulite facies metamorphism and pervasive partial melting throughout deformation.
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