Deformation of the Torlesse terrane in the Mt Hutt Range, Central Canterbury, New Zealand

Abstract

The highly deformed Permian and Mesozoic Torlesse Terrane forms much of the Southern Alps and the basement to the east in the central South Island of New Zealand. It is made up of a succession of quartzofeldspathic sandstones and siltstones deposited on oceanic crust and deformed into an accretionary prism at the active margin of the Gondwana supercontinent. At least four major periods of deformation may have affected the Torlesse Terrane, including deformation during subduction along the Gondwana margin, collision of the Torlesse and Caples Terranes, extensional deformation during break-up from Gondwana, and transpressive deformation related to the modem plate boundary through New Zealand. Structural work in the Mt Hutt Range at the Southern Alps range front in Central Canterbury, South Island, was directed at detailed outcrop-scale observations in two well exposed sections, to determine which episodes can be identified as leaving an imprint of pervasive deformation. Morphological evidence of rheological states and geometric analysis of overprint relationships were the primary data used in this analysis. Results suggest that virtually all deformation observed on outcrop scale can be most readily attributed to the accretionary phase associated with subduction along Gondwana. Pervasive extensional deformation occurred in the early stages of subduction, probably within underthrust sequences that were decoupled from the compressional stress field in the overlying accretionary wedge and experienced strong gravitational loading and fluid overpressures. Melanges developed locally at zones of high shear strain close to or within the decollements. After the underthrust sequences were underplated to the accretionary wedge, they experience the same compressional stress conditions as the frontally offscraped units, leading to pervasive outcrop-scale folding, thrusting and back-tilting of bedding. Gravitational forces occasionally influenced the morphology of structures. A second phase of folding and tilting of bedding acted oblique to the trench margin and is attributed either to collision of the accretionary prism with topographic irregularities on the subducting plate, or to a change in convergence vector. Subsequent tectonic events of collision and continental break up have not left any pervasive imprint of small scale structures. The deformation history postdating accretion of Torlesse rocks to the Gondwana margin has largely involved macroscale structures such as major faults and associated large folds, where strains on the outcrop scale are accommodated only by pervasive fracturing of varying intensity and external rotation and translation. Thus, attributing structures such as small scale folds, cleavage and evidence of high ductile strains to post-accretion events needs strong supporting evidence of timing.