Forearc tectonic evolution of the South Shetland Margin, Antarctic Peninsula
The main provinces of the South Shetland margin, Antarctic Peninsula characterized on the basis of multichannel seismic, long-range side scan sonar and swath bathymetry data, include from northwest to southeast (1) the oceanic crust of the former Phoenix Plate, flexed down toward the margin and affected by normal faulting in the upper crust; (2) a narrow trench, with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge; (3) an accretionary prism with a complex internal structure, the toe of which is overthrust above the youngest trench deposits; (4) a midslope forearc basin, with an asymmetric synformal structure; and (5) the continental shelf, which includes two distinct tectonic provinces. Calculations of the late Cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6.7 Ma from about 60 mm/yr, resulting from the slowing and eventual cessation of spreading on the Antarctic-Phoenix ridge. Once spreading had completely ceased (3.5–2.4 Ma), the convergence rate at the trench equalled the rate of extension in Bransfield Strait, which was probably less than 10 mm/yr at first but may have increased since 1.3 Ma, and this, in turn, would imply a corresponding increase in convergence rate. Above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges, laterally and vertically segmented by normal faults. Most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles. This suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope. The forearc basin records a long history of subsidence, during which the depocenter migrated landward, paralleling retrogradational erosion of the distal margin of the basin. Subduction of the Hero Fracture Zone at approximately the same location since 10 Ma may have enhanced tectonic erosion of the accretionary prism. In the forearc basin a synform is coincident with a bathymetric reentrant, but its axis is oblique to the trends of both the basin and the fracture zone. The origin of this synform and the final phase of uplift of the blueschist facies terrane on which Smith Island stands are probably related to their position above the subducting fracture zone ridge. The present elevation of Smith Island is thought to result from uplift which followed a ridge crest-trench collision directly southwest of the Hero Fracture Zone at 5.5 Ma. The northwestern edge of the Mesozoic to early Cenozoic arc batholith, trending northeast–southwest on the shelf, is proposed to have acted as a backstop to the accretionary prism during Cenozoic subduction.