THE EVOLUTION AND EXHUMATION OF EARLY CRETACEOUS LOWER CRUSTAL GRANULITES DURING CHANGES IN PLATE BOUNDARY DYNAMICS, FIORDLAND, NEW ZEALAND

The Fiordland belt of New Zealand exposes over 5000 km2 of high-pressure (P=12-16 kbars) granulite facies orthogneisses that represent part of the lower crustal root of an Early Cretaceous orogen. Published geochronologic data show that these rocks were emplaced into the lower crust between 126 Ma and 116 Ma. We show how major changes in plate tectonic settings affected the evolution of these lower crustal rocks since their emplacement. Early Cretaceous convergence produced vertically stacked, gently dipping shear zones and a series of steep to moderately dipping granulite and upper amphibolite facies fabrics. In central Fiordland, these fabrics are crosscut by an interconnected network of subhorizontal, upper amphibolite facies shear zones that surround asymmetric lozenges of less deformed orthogneiss. Quantitative kinematic analyses of shear zone fabrics indicate that different time periods produced measurably different ductile flow regimes. Early Cretaceous (126-116 Ma) shear zones record vertical thickening and pure-shear dominated deformation. Shear zones that formed during Late Cretaceous continental extension record approximately equal components of pure shear and simple shear in within a vertically thinning crustal section. These latter shear zones tectonically denuded the granulites and exhumed them to upper crustal levels (~10 km paloedepths) by ~90 Ma. All Cretaceous structures are crosscut by curved strike-slip and oblique-slip faults that formed in the late Oligocene-early Miocene. Thermochronologic data combined with structural data and kinematic solutions show that Late Tertiary dextral transpression controlled the final uplift and exhumation of the Fiordland belt from ~10 km depth to surface levels. We show a strong correlation between the geometry and kinematics of deformation in the lower crust and changing tectonic regimes involving Early Cretaceous contraction and Late Cretaceous continental extension. We also show how Cretaceous and Tertiary regimes influenced the exhumation path of Fiordland’s high-pressure granulite belt.