Three-Dimensional Finite Strain and Kinematics of Flow from a Lower Crustal Extensional Shear Zone: Fiordland, New Zealand

Exhumed sections of the middle and lower crust in western New Zealand reveal how deformation was partitioned within the crust during the Cretaceous (~115-90 Ma) rifting of Gondwana. We present three-dimensional finite strain and kinematic data from the upper and lower plates of a newly discovered extensional detachment and metamorphic core complex that formed during this rifting event.  The Resolution Island Shear Zone juxtaposes lower crustal eclogite and garnet granulite facies orthogneiss (P ~17-19 kbar) in the core against mid-crustal upper amphibolite facies schist and gneiss (P ~7-9 kbar).  Our kinematic analyses indicate that pervasive garnet-granulite foliations (S₂) and lineations (L₂) truncate older eclogite facies gneissic foliations (S₁) and mark the onset of extension and widespread lower crustal flow.  Overprinting the S₂/L₂ fabric are 10 cm-100 m thick penetrative upper amphibolite facies ductile shear zones (S₃/L₃) that anastomose around dismembered lenses of rock. These shear zones record a symmetric style of deformation, involving both top-down-to-the northeast and -southwest displacements. Finite strain ellipsoids from the S₃/L₃ shear zones are prolate and geochemical data suggest that the deformation occurred with <10% volume loss. Bulk kinematic analyses at the scale of the lower crust confirm a type of coaxial-dominated flow of the lower crust involving constriction.  The deformation involved subhorizontal stretching toward the X-direction (NE-SW) of the finite strain ellipsoid and two directions of shortening that produced a regional dome-and-basin pattern. We also conclude that the deformation kinematics from the Resolution Island Shear Zone and metamorphic core complex resemble those described from shallower levels of the crust, which are also exposed in western New Zealand. These similarities suggest a kinematic coupling of deformation occurred vertically between the lower, middle and upper crust during the rifting of Gondwana.