Fiordland, southern New Zealand, contains two primary geologic components: an elongate core comprising high-pressure (>12 kbar) granulite facies orthogneisses (western Fiordland orthogneiss) and a structurally overlying mid-Paleozoic plutonic-metasedimentary complex with subordinate metavolcanics (Tuhua Sequence). The contact between these two components is a major structural discontinuity or decollement hitherto defined as the Doubtful Sound thrust, but reinterpreted here as a ductile shear zone separating lower and upper crustal plates generated during continental extension. Strongly lineated Lower Cretaceous mylonites occur throughout this zone; they formed prior to separation of New Zealand from Australia (at ∼80 Ma).

Continental rifting was accompanied by increased heat flow, silicic to mafic magmatism, and the widespread resetting of mineral isotopic systems. Thus, in terms of its overall tectonic setting and tectonothermal history, Fiordland bears a striking resemblance to the metamorphic core complexes of the North American Cordillera.

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