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Tectonic evolution of a forearc terrane, southern Scotia Ridge, Antarctica

Ian W. D. Dalziel
Ian W. D. Dalziel
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January 01, 1984

Metamorphic and sedimentary rocks of the South Orkney and South Shetland island groups and the northern tip of the Antarctic Peninsula appear to represent the products of subduction-related accretion and of sedimentation respectively in a forearc environment along the Pacific margin of Antarctica. Some of the rocks are imprecisely dated, but stratigraphic, paleontologic, and radiometric data indicate that the higher temperature part of the subduction complex was formed and all sedimentary strata deposited prior to the initiation of Gondwanaland break up. The remainder of the subduction complex, comprising comparatively high P/T assemblages, may be of late Mesozoic or even Cenozoic age.

Lithology indicates that the metamorphic rocks represent pelagic and volcanic material from the ocean floor tectonically interleaved with slices of oceanic lithosphere. The presence of penetrative polyphase tectonite fabrics and the metamorphic assemblages suggest that the deformation and metamorphism occurred under considerable cover. Thus the complex may be, in part, the product of underplating (“subcretion”) to a forearc accretionary wedge.

The sedimentary strata consist of graywacke and shale of turbidite facies associated with rare mafic pillow lava containing prehnite and pumpellyite. Bedding and primary structures are ubiquitously recognizable although the rocks are everywhere deformed by one major set of sub-isoclinal to tight asymmetric folds with associated axial planar slaty cleavage. The regional setting, structural style, and an observed transition structurally downward into the metamorphic rocks suggest that the strata were deposited partly in trench-slope basins, that is, within the zone of active deformation in the wedge.

The general parallelism between the dominant structural elements of all the sedimentary rocks and the continental margin along the length of the South Shetland Islands and on the adjacent Antarctic Peninsula indicates that the margin may have been established as a site of subduction-related deformation and metamorphism prior to the initiation of Gondwanaland break up, and that accretionary tectonism continued there after break up. The attitude of structures in the metamorphic rocks of the South Orkney Islands suggests that the microcontinental platform on which they are located may have rotated clockwise relative to the Antarctic Peninsula during opening of the intervening oceanic Powell Basin.

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