Tectonic significance of ductile deformation in low-grade sandstones in the mesozoic Otago subduction wedge, New Zealand

Abstract

This study demonstrates how ductile strain measurements can determine the tectonic evolution of a large and long-lived subduction wedge. We provide a synthesis of the geology of the South Island of New Zealand, with an emphasis on a wedge tectonics perspective that contrasts with a traditional view that interprets New Zealand geology in the context of terrane collision and accretion. We argue that the Otago subduction wedge evolved in a steady fashion throughout its 290 to 105 Ma history in response to accretion of trench-fill and abyssal-plain sediments, and slow erosion of a subaerially-exposed forearc high. Maximum temperatures for rocks in the flanks of the forearc high were no greater than 150 to 300 °C, with solution mass-transfer active as the dominant ductile mechanism. The 54 studied samples provide information about the absolute ductile strains acquired all along their flow-path, from the site of accretion to exhumation in the forearc high. We use tensor-averages to estimate strain at a regional scale. These show plane-strain uniaxial flattening, given that the tensor-averages for <i>S_y</i> and <i>S_x</i> are close to one. On average, <i>S_z</i> is approximately 0.77, and this is balanced by a mass loss of about 23 percent. The average Z direction is sub-horizontal in the prowedge and moderately plunging in the retrowedge, a difference attributed to spatial variations in the mode of accretion. We infer that rocks presently in the pro-side of the Otago high were sourced by frontal accretion, and those in the retro-side were underplated. This result highlights the important role of accretion in determining the style of within-wedge deformation, and also demonstrates the benefit of using a tensor-averaging approach to examine regional strain.