STRUCTURE AND KINEMATIC EVOLUTION OF THE LOWER CRUST AND UPPER MANTLE DURING LITHOSPHERIC EXTENSION FROM THE RESOLUTION ISLAND SHEAR ZONE, FIORDLAND, NEW ZEALAND

The recent discovery of deep (~18 kbar, ~60 km) granulite and eclogite exposures of Early Cretaceous age in Fiordland, New Zealand provides evidence of interactions between the lower crust and lithospheric mantle during a type continental extension that led to sea floor spreading and the opening of the Tasman Sea. These exposures have allowed us to determine how inherited rheologic heterogeneities in the lower crust and upper mantle controlled strain partitioning during deformation and the exhumation of mantle lithosphere. New structural data suggest that deformation in the deep lithosphere localized into an anastomosing array of high strain amphibolite facies shear fabrics (S2) that envelope lenticular pods of eclogite and granulite facies rocks. Some of the pods display long axes that are up to 2 km long. Inside the pods, eclogite and granulite facies foliations (S1) are cut by clinozoisite- and hornblende-bearing veins that record hydration of the deep lithopshere and brittle fracturing at high strain rates. The shear fabrics that envelop these pods display a regional-scale dome-and-basin pattern that reflects a constrictional strain field during subhorizontal stretching and vertical thinning of the lithosphere. This structural pattern resembles that which characterizes some Precambrian granite-greenstone terranes. Within the shear fabric (S2) that envelops the pods, aligned hornblende and plagioclase stretching lineations (L2) plunge toward ~25° and ~205°. Within high strain zones, the S1/L1 fabric is deflected into parallelism with S2/L2. Field relations also suggest that metasomatism of granulite and eclogite material representative of the upper mantle and lower continental crust localized strain into narrow (up to 500 m thick) zones during continental extension. This mechanism of strain localization reflects inherited mantle heterogeneity and resembles that which occurs in oceanic core complexes.