The southeastern border is a rift zone, with the Antarctic plate creating the Bransfield Basin.

[1] The Shetland plate started forming 3 to 4 million years ago.

Slab rollback of the former Phoenix plate underneath the South Shetland Islands caused rifting to develop in the Antarctic Peninsula creating the Shetland plate and the Bransfield Basin.

Rifting centers in the Bransfield Basin continue to separate the Shetland plate from the Antarctic Peninsula.

Plutons of calc-alkaline compositions, formed from a type of magma created above subduction zones, date from the Cenozoic to the Miocene and is coincident with the active subduction of the Phoenix plate under Antarctica.

More recent Pliocene dated volcanism in the Bransfield basin has transitional chemistry between subduction zone calc-alkaline and mid ocean ridge tholeiitic magmas.

The amount of data has been limited by remoteness, severe weather, and a lack of permanent seismic stations in the area.