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The separation of Zealandia from West Antarctica was the final stage in the Cretaceous breakup of the Gondwana Pacific margin. Continental extension resulting in formation of the Great South Basin and thinning of the Campbell Plateau leading to development of the Pacific‐Antarctic spreading ridge was partially accommodated along the Sisters shear zone. This east‐northeast striking brittle‐ductile structure exposed along the southeast coast of Stewart Island, New Zealand, is a greenschist facies extensional shear zone that separates a hanging wall of chloritic, brecciated granites, and undeformed conglomerate from a footwall of mylonitic Carboniferous and Early Cretaceous granites. This complex structure exhibits bivergent kinematics and can be subdivided into a northern and southern segment. The 40Ar/39Ar thermochronology indicates that cooling of the shear zone footwall began at ∼94 Ma with accelerated cooling over the interval ∼89–82 Ma. Structural and thermochronological data indicate a spatial and temporal link between the Sisters shear zone, initial sedimentation within the offshore Great South Basin, extension of the Campbell Plateau, and initiation of the Pacific‐Antarctic spreading ridge.

Geothermobarometric, radiogenic isotopic and thermochronologic data are used to track the influence of an ancient continental margin (Western Province) on development of an adjacent Carboniferous‐Cretaceous magmatic arc (Outboard Median Batholith) in Fiordland, New Zealand. The data show a record of complicated Mesozoic Gondwana margin growth. Paragneiss within the Outboard Median Batholith is of Carboniferous to Jurassic age and records burial to middle crustal depths in Late Jurassic–Early Cretaceous during subduction‐related plutonism and arc thickening. In contrast, Western Province metasedimentary rocks in the area of study immediately west of the Outboard Median Batholith are Late Cambrian–Early Ordovician in age, recrystallized at the amphibolite facies in the Late Devonian–Early Carboniferous and exhibit no evidence for Mesozoic textural or isotopic reequilibration. A phase of deformation, between 128 and 116 Ma deformed, exhumed, and cooled the Outboard Median Batholith to greenschist facies temperatures, while large parts of the Western Province underwent ≥9 kbar metamorphic conditions. Zircon grains from Mesozoic inboard plutons are isotopically more evolved (ɛHf(t) = +2.3 to +4.0) than those in the Outboard Median Batholith (ɛHf(t) = +9.4 to +11.1). The contrasting zircon Hf isotope ratios, absence of S‐type plutons or Proterozoic–Early Paleozoic inherited zircon, and the apparent absence of Early Paleozoic metasedimentary rocks indicates that the Outboard Median Batholith is unlikely to be underlain by the Western Province continental lithosphere. The new data are consistent with the Outboard Median Batholith representing an allochthonous (although not necessarily exotic) arc that was juxtaposed onto the Gondwana continental margin along the intervening Grebe Mylonite Zone.

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