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The Tabberabberan orogeny in Northwest Tasmania

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posted on 2023-05-27, 08:04 authored by Seymour, DB
In the Black Bluff region of Northwestern Tasmania, Cambrian sedimentary and volcanic rocks are distributed in an arcuate belt around the northwest margin of the Tyennan Geanticline, which is composed of Precambrian greenschistgrade metaquartzites and metapelites. The volcanics, which are dominated by heavily altered intermediate to acid pyroclastics, are concentrated in the southern part of the belt which borders the margin of the geanticline. The Cambrian rocks are unconformably overlain by Lower Ordovician quartzose clastics and Middle Ordovician limestones. Movements responsible for the angular discordance are inferred to have been dominantly vertical and responsible for steep tilting of the Cambrian sequences and considerable uplift of the Tyennan Geanticline, which became the source area for the sediments immediately overlying the unconformity. The quartzose clastics were mainly deposited in terrestrial alluvial fan environments, although marginal marine sedimentation became progressively more important upwards in the sequence. Intertidal sedimentary environments persisted during deposition of the limestone sequence, but relief of the Precambrian source areas had by early Hiddle Or.dovician time been reduced to such an extent that terrigenous influx was insignificant. Hiddle Devonian deformation of the Palaeozoic rocks, at high crustal levels, produced open, upright, horizontal to gently plunging folds of variable trend. The Tyennan Geanticline behaved as a relatively rigid block during this deformation, which has been correlated with the Tabberabberan Orogeny, of regional importance in southeastern Australia. The structural development now recognized involves a progressive sequence of regional folding on axes which trend east (Dl), northeast (D2), north (D3) and northwest to north-northwest (D4). The last three phases of folding were associated with development of axial plane cleavage in the more ductile lithologies. Localized folding on east-northeast axes and associated cleavage postdates 02, and may have occurred between D2 and D3. The relative timing of fold and cleavage development on west-northwest trends in the limestone at Loongana and Gunns Plains is uncertain. Previous conclusions regarding dating of the Tabberabberan deformation may need reconsideration in view of the newly proposed structural sequence. Development of tectonite fabrics in the altered volcanics \\

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