TIMESCALES OF INTRA-ARC CONTRACTION AND EXTENSIONAL COLLAPSE IN THE LOWER AND MIDDLE CRUST OF A CRETACEOUS CONVERGENT-MARGIN MAGMATIC ARC, CENTRAL FIORDLAND, NEW ZEALAND

The Fiordland orogen preserves an exhumed Mesozoic continental arc that records the transition from subduction-related magmatism and contraction along the Gondwana margin to extensional orogenic collapse that led to opening of the Tasman Sea. One of the least understood aspects of the orogenic development of Fiordland is the timescale of intra-arc contraction and its temporal relationship to magmatic arc construction and extensional collapse in the lower crust beginning at ca. 112 Ma. The contractional history of Fiordland is preserved in Caswell Sound, where contraction is manifested by a doubly verging fold-thrust belt characterized by garnet granulite and upper amphibolite-facies thrust faults that sole into a subhorizontal shear zone. In this region, a suite of biotite-hornblende diorite to granodiorite (western McKerr Intrusives: wMI) and their metasedimentary host rocks are thrust over the Misty Pluton (MP). Both MP and wMI are intruded by post-kinematic granitic dikes that cross-cut shallowly dipping mylonitc fabrics.

We sampled the wMI, MP, and a post-kinematic dike from the eastern end of the Caswell fold-thrust belt to determine the timing of magmatism and intra-arc contraction in the region. Two samples from the wMI yielded ²⁰⁶Pb/²³⁸U SHRIMP-RG zircon ages of 118.4 ± 1.3 Ma and 117.7 ± 1.7 Ma, respectively. A sample of the MP from the footwall of the Caswell fold-thrust belt yielded an age of 116.9 ± 1.8 Ma. Finally, a post kinematic granitic dike yielded an age of 112.2 ± 2.0 Ma. These data indicate that contractional deformation in the Caswell fold-thrust belt is restricted to ca. 117 to 112 Ma, and is partially coeval with the final stages of magmatic emplacement of the MP (ca. 122-115 Ma). The ages for the MP and wMI are statistically indistinguishable which suggests that the latter may be a shallower, more evolved phase of a single magmatic unit. These new data help better constrain the temporal transition from magmatic construction and intra-arc contraction from ca. 117-112 Ma to widespread orogenic collapse in the lower crust initiating at ca. 112 Ma, leading to the opening of the Tasman Sea at 83 Ma.