TECTONICS, VOL.8, NO.6, PAGES 1217-1234, DECEMBE• 1989 THE PAPAROA METAMORPHIC CORE COMPLEX, NEW ZEALAND: CRETACEOUS EXTENSION ASSOCIATED WITH FRAGMENTATION OF THE PACIFIC MARGIN OF GONDWANA A.J. Tulloch NewZealand Geological Survey, Lower Hutt D.L. Kimbrough • Department of Geological Sciences, University of California, Santa Barbara Abstract. In Westland-Nelson provinces of New Zealand, high-grade metamorphic and granitic basement rocks showing mylonitic ductile deformation are juxtaposed beneath low- grade metasedimentaxy rocks and undeformed granites by uplift onlow-angle detachment faults. Several metamorphic core complexes analogous to those described from western North America arerecognized. In thePaparoa Range, basement rocks include late Precambrian(?) paragneiss and granitic rocks of both Paleozoic and Cretaceous ages. Cover rocks includeOrdovician turbidites, Paleozoic and Cretaceous granites, andmid-Cretaceous breccia-conglomerates. Brittle deformation and hydrothermal alteration (silica, chlorite, hematite, carbonate +_ fluorite, uranium)characteristic of the detachment zone arealso superimposed onuppermost lower- plate mylonites. Kinematic indicators in themyloniticrocks including composite S-Cfabrics indicate thatthedetachment faults onthe northeast and southwest sides of the Paparoa Core Complex had opposite senses of shear, withcover rocks on both sides moving away from themetamorphic core. Ductile deformation postdates several 114+_ 18Ma granitic plutons but by108Ma hadceased to affect at least some of therocks currently exposed. Mylonitic rocks were uplifted to thesurface and eroded intoevolving half-grabens by105-100 Ma. Uplifted basement yields K-Ar dates as young as 88Ma, and tilting of thegraben sediments indicates detachment continued well into the Late Cretaceous, whenboth cover and •Now atDepartment ofGeological Sciences, San Diego State University, California. Copyright 1989 by theAmerican Geophysical Union. Paper number 89TC01258. 0278-7407/89/89TC-01258510.00 basement were intruded by alkali lamprophyre dykes. The Nelson-Westland core complexes occur within an Early Cretaceous granitic province characterized by relatively radiogenic strontium. Theboundary of this province, the NW trend to mid-Cretaceous half-grabens, theNNE trend of stretching lineations in mylonitic rocks, and theESE trend of late Cretaceous lamprophyre dykes indicate that regional extension was maintained in a NNE direction for much of the Cretaceous. This regional extension may be part of an "extension corridor"hich traversed the entire Gondwana continental margin fromNE Queensland, Australia, to Marie Byrd Land, Antarctica. Extension preceded opening ofthe Tasman Sea between New Zealand and Australia at approximately 84 Ma and closely followed long-lived compression on the Pacific convergent margin of Gondwana. Thepresence ofcore complexes in western NewZealand contrasts with theAustralian margin to theTasman Sea and lends support to simple shear models of continental rifting. INTRODUCTION In western New Zealand the structural relationship between high-grade rocks of the Charleston Metamorphic Group and low-grade metasedimentary rocks of the lower Paleozoic Greenland Group has been debated forsome 20 years. Laird [1967] and Hume[1977] discussed indetail the only known contact of these two units, in the southern Paparoa Range, where Greenland Group overlies the Charleston Metamorphic Group. Laird concluded that the Charleston Metamorphic Group represented metamorphosed Greenland Group, and this theme was extended by Shelley [1972], who proposed what amounted to a "mantied gneiss dome." Graptolite fauna which showed theGreenland Group tobelower Ordovician [Cooper, 1975], and a Rb-Sr radiometric age determination which suggested the Charleston Metamorphic Group tobelate