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

Abstract

BSTRACT. This study demonstrates how ductile strain measurements can determine the tectonic evolution of a large and long-lived suhduction 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 ahyssal-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 masstransfer active as the dominant ductile mechanism. The 54 studied samples provide information ahout the ahsolute 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 S^ and S^ are close to one. On average, S. is approximately 0.77, and this is halanced hy 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 presentiy 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.

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

“…In addition, the spatially steep P – T gradient within the B unit suggests ductile thinning of this unit. Ductile thinning within an accretionary wedge may be accomplished by orogen‐parallel extension as proposed for the D1 stage of the Sanbagawa and Mikabu units in the study area (Wallis, ) or significant volume loss by pressure solution (Feehan & Brandon, ; Rahl, Brandon, Deckert, Ring, & Mortimer, ). Microstructural observation suggests that pressure solution mass transfer is the main deformation mechanism of the B unit (Figure d,e).…”

Ar‐Ar dating of K‐feldspar in low grade metamorphic rocks: Example of an exhumed Mesozoic accretionary wedge and forearc, South Island, New Zealand

“…Otago Schist tectonic models have changed from those emphasizing Jurassic terrane collision and crustal thickening [e.g., Mortimer , 1993] to those emphasizing Cretaceous extensional exhumation [e.g., Deckert et al , 2002; Forster and Lister , 2003; Gray and Foster , 2004] to those emphasizing an evolving accretionary wedge [e.g., Breeding and Ague , 2002; Rahl et al , 2011]. All are probably applicable, with convergent accretionary wedge structures preserved on the schist flanks and extensional fabrics in the core region of the Otago Antiform [ Stallard and Shelley , 2004].…”

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…Kamp (2001) suggested exhumation of Otago-Haast Schist began at ϳ135 Ma in response to the accretionary wedge achieving steady state conditions. The concept of different transport paths for different rocks was taken further by Rahl et al (2011), who suggested rocks on the eastern (prowedge) side of the Otago-Haast culmination had been frontally accreted, whereas those on the western (retrowedge) side had been underplated.…”