DEFORMATION ANALYSIS OF ACCRETED SEDIMENTS AT A MESOZOIC SUBDUCTION WEDGE, SOUTH ISLAND, NEW ZEALAND

We present absolute deformation measurements from a suite of 54 samples from the Mesozoic subduction wedge of the South Island, New Zealand. The Otago Schist is exposed in a 150 km wide belt that strikes parallel to the ancient convergent margin and represents the most deeply exhumed parts of the Permian-Cretaceous accretionary wedge. Foliation is generally flat-lying near the core of the belt and steepens towards the flanks, forming a broad structural arch. We focus on samples from the low-grade flanks to investigate the pattern of deformation in both the pro- and retro- parts of the wedge. These data provide constraints on the three-dimensional pattern of within-wedge ductile flow on both the local and regional scales. Microstructural evidence indicates that deformation in the flanks of the Otago Schist was coaxial and dominated by solution mass-transfer processes. Local strains are both prolate and oblate with a mass-transfer volume loss typically about 20 to 25%.

Our work reveals a significant change in the pattern of the ductile deformation from the pro- to retro-side of the wedge. In the front of the wedge, deformation proceeded by horizontal contraction oriented perpendicular to the orogenic axis. On average, this minor shortening (Sz=0.84) was compensated by open-system mass-transfer volume loss (Sv=0.76), rather than by significant extension (Sx=1.0). In contrast, samples from the rear of the system show a well-developed sub-horizontal foliation and vertical shortening (Sz=0.86).

We propose that the difference in shortening directions across the Otago forearc high is a result of the distribution of accretionary fluxes into the Otago wedge. We infer that sediments in the north flank were mainly influenced by accretion at the front of the Otago wedge, thus accounting for the observed horizontal shortening. In contrast, we suggest that strong underplating beneath the Otago high drove vertical shortening, consistent with our deformation measurements from the retro-wedge as well as with the flat-lying foliation of the Otago Schist. The fore-arc high of the wedge was subaerially exposed, allowing both erosion and ductile thinning to contribute to exhumation.