MANTLE DEFORMATION RECORDING SUBDUCTION INITIATION, RED HILLS, NEW ZEALAND

Fabrics, microstructures, and geochemistry of the Red Hills ultramafic massif, the South Island, New Zealand, provide evidence for subduction initiation along the Gondwanan margin. The oldest fabrics form on the eastern side of the massif. Here, the presence of consistently oriented L-tectonites indicates that the initial deformation was homogeneous and constrictional. Later kilometer-scale deformation zones overprint the early homogeneous fabric throughout the western portion of the massif. A new U-Pb zircon age of 274.55±0.43 Ma from a cross-cutting dike constrains high-temperature mantle deformation in the Red Hills to have occurred between 285 and 274 Ma. In addition, there are three identifiable phases of melting and/or melt migration in the massif. The second – Stage 2 – and third – Stage 3 events are correlated to L-tectonite development and later overprinting deformation zones, respectively. Melt interaction trends indicate that Stage 2 melts were relatively oxidized, at around log fO₂(ΔFMQ) +1, and boninitic in composition, suggesting a forearc subduction initiation setting for Stage 2. In contrast, Stage 1 and Stage 3 melts suggest a mid-ocean ridge setting and a forearc subduction-zone setting, respectively.

We present a kinematic model to explain the occurrence of the L-tectonites and subsequent deformation during subduction initiation. The three-dimensional boundary conditions for deformation in the incipient mantle wedge were transtensional, with a dominant trench-parallel component of motion. Trantensional deformation with low angles of oblique divergence is consistent with the constrictional fabrics preserved in the eastern domain of the Red Hills complex. Such a scenario indicates subduction likely initiated along an active oceanic transform fault. We test this model by kinematically restoring the Red Hills ultramafics to their Permian orientation and find the L-tectonites were oriented nearly parallel to the inferred trench. Using the restored position, the later overprinting deformation in the western massif also accommodated a component of normal motion (extensional deformation). The lack of evidence for contraction suggests that the initiation of subduction along the Permian margin of New Zealand occurred along a transform fault due to spontaneous, density driven processes.