TIMING OF CONTINENTAL EXTENSION LEADING TO SEPARATION OF EASTERN NEW ZEALAND FROM WEST ANTARCTICA; 40AR/39AR THERMOCHRONOMETRY FROM STEWART ISLAND, NZ

Plate reconstructions of Mesozoic Gondwana place New Zealand at the Pacific margin, adjacent to southeast Australia and Marie Byrd Land, Antarctica. The Paparoa metamorphic core complex of South Island's Western Province records evidence for continental extension preceding separation of New Zealand and Australia. New field and thermochronometry data from SE Stewart Island is presented as evidence for a later stage of continental extension leading to separation of eastern New Zealand from Marie Byrd Land, West Antarctica.

A zone of ductile and brittle deformation in arc basement rocks is exposed along the SE coast of Stewart Island. Undeformed boulder conglomerates form the Sisters Islets 1 km offshore. The conglomerates dip NW towards SE dipping mylonitic rocks on the coast, and likely comprise part of the upper plate to an inferred intervening detachment fault. Brittle overprinting of ductiley deformed footwall rocks indicates exhumation through the brittle-ductile transition during extension.

Lineations in footwall rocks trend ~150-160^o, consistent with the spreading direction of the Pacific Ridge. This orientation is oblique to that observed in Paparoa footwall rocks and the spreading direction of the now-extinct Tasman Ridge. These orientations may be a result of Cenozoic rotation; however, the spatial/kinematic relationship between detachment faults and ocean basin spreading centers is maintained.

⁴⁰Ar/³⁹Ar thermochronometry of footwall minerals demonstrates a temporal relationship between extension in the Paparoa Range and Stewart Island. Paparoa footwall muscovite and biotite yield ages ranging from 102-92 Ma. K-feldspars record rapid cooling during the interval ~95-88 Ma followed by a change to slow cooling, which is likely coeval with inception of Tasman spreading. Muscovite and biotite from footwall gneiss on Stewart Island yield ages of 93 and 90 Ma, respectively, indicative of slow-to-moderate cooling at this time. K-feldspar models show a change to rapid cooling at 89 Ma followed by a return to very slow cooling at ~82 Ma. Rapid cooling from 89-82 Ma indicates extensional tectonism may have begun ~10 Ma later than in the Paparoa core complex. Return to slow cooling at 82 Ma is coeval with chron 33r, which marks the oldest seafloor formed during rifting of the Campbell Plateau from West Antarctica.