The South Orkney Islands are the exposed part of a continental
fragment on the southern limb of
the Scotia arc. The islands are to a large extent composed of metapelites
and
metagreywackes of probable
Triassic sedimentary age. Deformation related to an accretionary wedge
setting,
with associated metamorphism
from anchizone to the greenschist facies, are of Jurassic age (176–200
Ma).
On Powell Island, in
the centre of the archipelago, five phases of deformation are recognized.
The
first three, associated with the
main metamorphism, are tentatively correlated with early Jurassic subduction
along the Pacific margin of
Gondwana. D4 is a phase of middle to late Jurassic crustal extension
associated with uplift. This extension
phase may be related to opening of the Rocas Verdes basin in southern Chile,
associated with the breakup of
Gondwanaland. Upper Jurassic conglomerates cover the metamorphic rocks
unconformably. D5 is a phase
of brittle extensional faulting probably associated with Cenozoic opening
of
the Powell basin west of the
archipelago, and with development of the Scotia arc.