FINITE STRAIN AND FLOW KINEMATICS IN THE MIDDLE AND LOWER CRUST DURING THE RIFTING OF GONDWANA

Exhumed sections of the middle and lower crust in western New Zealand reveal the type of deformation that occurred within the deep crust during the Cretaceous rifting of Gondwana. We present 3D finite strain and kinematic data from the upper and lower plates of a network of extensional detachment faults that formed during the rifting. The deepest shear zone juxtaposes lower crustal eclogite and garnet granulite facies orthogneiss (P ~17-19 kbar) against mid-crustal upper amphibolite facies schist and gneiss (P ~7-9 kbar). Kinematic analyses indicate that pervasive garnet-granulite fabrics truncate older eclogite facies gneissic foliations and mark the onset of extension and widespread lower crustal flow. Overprinting the eclogite and granulite facies fabrics are a series of upper amphibolite facies ductile shear zones. Individual branches of these latter shear zones reach 100 m thick. All shear zones record a symmetric style of lower and mid-crustal extension involving top-down-to-the northeast and -southwest displacements. Finite strain ellipsoids from the deepest part of the system are prolate, reflecting a regime of constriction. Preliminary geochemical data suggest that this deformation occurred with <10% volume loss. Bulk kinematic analyses at the scale of the lower crust confirm coaxial-dominated flow of the lower crust involving constriction and subhorizontal stretching. The constriction produced a regional dome-and-basin fold pattern during extensional denudation of the lower crust. Kinematic and finite strain ellipsoid data from shallower, mid-crustal levels of the crust (P ~7-9 kbar) show a 3D finite strain field that becomes progressively more oblate toward shallower depths, although the kinematic regime remaisn coaxial. We speculate that this vertical change in finite strain state reflects an increase in the importance of fluid transfer mechanisms and in volume loss in the middle-crust compared to the lower crust, which resulted in increased vertical flattening. The data also suggest a vertically coupled lower, middle and upper crust during the rifting of Gondwana.