Introduction
The Shetland (South Shetland) plate is a small tectonic microplate located off the tip of the Antarctic Peninsula that encompasses the South Shetland Islands and constitutes a distinct crustal block within the Southern Ocean. Bordered on three sides by the Antarctic plate and on the fourth by the Scotia plate, it occupies a relatively isolated position within a tight mosaic of larger lithospheric plates.
Its northwestern margin is defined by the South Shetland Trench, a remnant subduction zone that once accommodated consumption of the now-defunct Phoenix plate beneath the Antarctic Peninsula and the South Shetland Islands; material from the Phoenix plate has since been incorporated into the Antarctic plate. The plate’s southeastern edge is an extensional rift where continuing interaction with the Antarctic plate has produced the Bransfield Basin, a back-arc/rift basin formed by active rifting. The southwestern and northeastern boundaries are dominated by major fracture/transform features: the Hero fracture zone separates the Antarctic plate (to the southwest) from the Shetland plate (to the northeast), while the Shackleton fracture zone marks the transform boundary between the Shetland plate (southwest of the fracture) and the Scotia plate to the northeast. Together, these margins record a complex tectonic regime characterized by remnant subduction, active rifting, and significant transform faulting.
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Geologic history
The Shetland Plate is a late Pliocene–Pleistocene microplate that detached from the Antarctic Plate roughly 3–4 million years ago. Prior to its separation, the region northwest of the Antarctic Peninsula was characterized by convergence of the Phoenix Plate beneath both the Peninsula and the South Shetland Islands, producing the South Shetland Trench. Around 3 Ma seafloor spreading at the Antarctic–Phoenix spreading center in the Drake Passage ceased; with spreading halted, the Phoenix Plate lost measurable independent motion and is now effectively considered part of the Antarctic Plate. Subduction beneath the South Shetland Islands, however, continued and slab rollback of the former Phoenix slab generated extensional stresses and rifting along the Peninsula margin. Those extensional processes gave rise to the Bransfield Basin and the distinct Shetland Plate, and the active rifting centers within the Bransfield Basin continue to accommodate separation and maintain ongoing extensional deformation at the plate boundary.
South Shetland Trench
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The South Shetland Trench forms the northwest margin of the Shetland plate, manifesting as a linear oceanic trench where one lithospheric plate descends beneath another. At this convergent boundary the Antarctic plate to the north is being subducted beneath the Shetland plate to the south, and the trench represents the surface expression of that process. Subduction rates have declined substantially through the Neogene–Quaternary: over the past ~30 million years convergence averaged roughly 4–6 cm yr⁻¹ (1.6–2.4 in yr⁻¹), whereas during the last ~6 million years rates have fallen to about 1–2 cm yr⁻¹ (0.39–0.79 in yr⁻¹). Present-day plate-boundary kinematics are governed chiefly by northward motion of the Shetland plate together with ongoing slab rollback associated with the formerly active Phoenix plate, a dynamic that continues to influence trench behavior even though the Phoenix plate has been largely consumed.
Bransfield Basin
The Bransfield Basin forms the southeastern margin of the Shetland plate, occupying the marine trough between the Shetland plate to the north and the Antarctic plate to the south. As the intervening seafloor depression along this plate boundary, it functions both as a geographic divider and as an active tectonic element that records interaction between the two plates.
Tectonically, the basin is best described as a back‑arc rift basin developed in the wake of a volcanic arc, indicating that lithospheric stretching and rifting have occurred behind the arc in response to subduction‑related plate dynamics. This extensional regime has localized normal faulting and subsidence, producing a segmentation of deformation that reflects the interplay of arc‑trench processes and plate motions in the region.
Bathymetrically the Bransfield Basin is a pronounced depression with depths typically between about 1,300 m and in excess of 2,700 m, demonstrating significant vertical relief and lateral variability of the basin floor. Because it coincides with the plate boundary, the basin serves simultaneously as a morphological signature and a tectonic marker: it delineates the Shetland–Antarctic plate contact, focuses seafloor extension and subsidence, and thereby controls regional seafloor topography and the patterning of tectonic segmentation along the southeastern Shetland margin.
Fracture-zone structure on the Shetland Plate is dominated by two parallel transform systems that define its northeastern and southwestern margins. To the northeast, the Shackleton Fracture Zone comprises a suite of parallel transform faults forming the plate boundary with the Scotia Plate; unlike the other Shetland Plate margins, this boundary does not abut the Antarctic Plate. To the southwest, the Hero Fracture Zone likewise consists of aligned transform faults that separate the Shetland Plate from the Antarctic Plate and establish a coherent structural corridor linking the Bransfield Basin to the south with the South Shetland Trench to the north. Collectively, these two transform-fracture systems articulate the principal lateral boundaries of the Shetland Plate and mediate its tectonic interactions with both the Scotia and Antarctic plates.
Volcanism
Absolute K–Ar dating shows volcanic activity in the Shetland plate region has been continuous from the Cenozoic to the present, with contemporary eruptive and hydrothermal manifestations at Deception and Penguin Islands. Coeval with this volcanic record are large Cenozoic–Miocene plutons exhibiting calc-alkaline geochemical affinities characteristic of subduction-zone magmatism. The timing of these intrusive bodies corresponds to subduction of the Phoenix plate beneath Antarctica, supporting a genetic connection between plate-boundary subduction processes and long-lived intrusive magmatism in the area. Pliocene volcanism in the Bransfield Basin, however, records transitional geochemistry intermediate between calc-alkaline and mid-ocean-ridge tholeiitic signatures. This hybrid chemistry is best explained by back-arc rifting: extensional deformation promoted enhanced mantle upwelling and mixing between subduction-modified mantle domains and freshly upwelling asthenospheric mantle, producing magmas with intermediate calc-alkaline–tholeiitic characteristics. Together these observations document an evolution from dominantly subduction-driven magmatism toward rift-associated, hybrid mantle melting as tectonic conditions changed.
Earthquakes
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Since the early 1980s instrumentally recorded seismicity has established a multi‑decade catalogue for the Shetland plate, but that record is fragmentary and of limited resolution. The region’s isolation, frequent severe weather and the absence of a permanent seismic network reduce observation density and location precision, constraining continuity and the ability to resolve fine-scale seismotectonic features. Recorded events beneath the South Shetland Islands occur predominantly at intermediate focal depths (≈35–55 km), placing them beneath the shallow crust and within the subcrustal portion of the lithosphere beneath the island arc. The presence of such intermediate‑depth earthquakes is interpreted as strong evidence for ongoing subduction-related processes at the Shetland Trench and thus continued convergent plate interaction there; however, the sparse dataset limits detailed characterization of the slab geometry and temporal variability of these processes.