OBLIQUE INTRA-ARC CONVERGENCE AND TRANSPRESSION ACCOMPANYING HIGH-FLUX MAGMATISM IN THE MID-LOWER CRUST OF A MESOZOIC CONTINENTAL ARC IN FIORDLAND, NEW ZEALAND

A deeply exhumed (40-65 km) section of continental arc crust exposed in Fiordland, New Zealand records a Devonian-Early Cretaceous history of subduction, arc magmatism and contraction on the Gondwana margin. During the Early Cretaceous, an outboard arc composed of Jurassic and older rock collided with the margin, leading to contraction, crustal thickening, and increased magmatism. We use structural analyses and preliminary SHRIMP-RG zircon ²⁰⁶Pb/²³⁸U dates to examine the interrelationships between magmatism and deformation accompanying and outlasting this event in eastern Fiordland.

Late Paleozoic magmatism in the study area is represented by 356.4 ± 4.4 Ma granite of the Cozette Pluton (CP). Mafic-intermediate magmatism that was coeval with Early Cretaceous contraction is represented by 119.9 ± 2.8 Ma hornblende diorite of the Misty Pluton (MP). The western CP is deformed by the Cozette Shear Zone, which operated through early emplacement of the MP. The shear zone is ≥1 km thick, dips gently to moderately to the NNW, and is composed of penetrative gneissic and locally mylonitic fabrics defined by aggregates of feldspar, biotite, and quartz. Gently plunging mineral lineations and kinematic indicators record sinistral oblique thrusting. The eastern side of the CP is deformed by thin (~200 m), gently dipping ductile thrust sheets. A subsequent phase of contraction (115-112 Ma) deforms these latter thrusts into upright, W- to NW-vergent folds and sets of SE-dipping thrusts after emplacement of the MP. These latter structures are cut by a 111.0 ± 1.1 Ma granite of the post-collisional Separation Point Suite. All ductile structures and the ~111 Ma granite are cut by brittle, cataclastic, pseudotachylite-bearing fault sets that deform ~100 Ma pegmatitic dikes.

Together the ductile structures define an Early Cretaceous mid-crustal transpressional thrust belt into which the MP was emplaced during oblique intra-arc convergence and the underthrusting of supracrustal material to lower crustal depths. Following pluton emplacement the thrust belt expanded to deform rocks along the eastern and western margins of the arc (including the Caswell Sound fold and thrust belt) and helped move magma through the crust. The brittle thrusts are interpreted to form a part of a late Tertiary, upper crustal transpressional fault system.