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.

Aseismic ridges on underthrusting oceanic plates often trend into cusps or irregular indentations in the trace of the subduction zone. For example, the Hawaii-Emperor Ridge trends into the Kuril-Aleutian cusp, and the Marianas arc is bounded by the Marcus-Necker Ridge on the north and the Caroline Ridge on the south. The association between ridges and cusps is too common to be due to chance; it is proposed that the extra buoyancy of the plate with its aseismic ridge gives the plate greater resistance to sinking. This would inhibit back-arc extension and thereby produce a notch in the subduction zone. Island arcs may, therefore, acquire their curvature by additional constraints than the Earth’s curvature. The geology of about 15 such cusp areas is examined for evidence to test the hypothesis that cusps were caused by subducted aseismic ridges. This hypothesis applies only to cases where extensional basins lie behind the arcs. There also appear to be cases where the trace of the subduction zone has been modified not by inhibited back-arc spreading but by splintering of the overthrusting and possibly the underthrusting plate as well. Extremely high, massive aseismic ridges might induce arc polarity reversals and thereby assume the role of protocontinental nuclei. Seismicity and volcanism are examined where aseismic ridges are being subducted; there are several examples of reduced seismicity that cannot be explained by insufficient sampling time. By modifying the geometry of the subduction zone, the downgoing ridges necessarily affect seismicity. In addition, the plate containing the ridge may be thinner and hotter and more likely to deform by creep. There is no systematic increase or decrease in the number of andesite volcanoes where the ridges are subducted. However, lines of volcanoes and sometimes other kinds of geologic and seismic provinces may stop or start at the arc-ridge intersections. This is attributed to segmenting of the lithosphere into distinct tongues, each tongue acting more or less independently. Aseismic ridges would act as lines of weakness along which the downthrust slab becomes detached.