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Three-dimensional mantle lithosphere deformation at collisional plate boundaries: A subduction scissor across the South Island of New Zealand
Authors:RN Pysklywec  SM Ellis  AR Gorman
Institution:1. China Earthquake Networks Center, Beijing 100045, China;2. Institute of Geology, China Earthquake Administrator, Beijing 100029, China;1. Departamento de Geodinámica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Apartado 644, 48080 Bilbao, Spain;2. School of Geography, Environment and Earth Sciences, Victoria University, PO Box 600, Wellington, New Zealand;3. Centre of Isotopic Research, VSEGEI, 199106 St. Petersburg, Russia;4. Departamento de Geología, Facultad de Ciencias, Universidad de Salamanca, Plaza de la Merced s/n, 37007 Salamanca, Spain;1. Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, UMR6524 CNRS & IRD UR163, 5 rue Kessler, 63038 Clermont-Ferrand cedex, France;2. Earth and Environmental Sciences Section, University of Geneva, 13 Rue des Maraîchers, Geneva CH 1205, Switzerland;1. Instituto Geológico y Minero de España, Madrid, Spain;2. Instituto Andaluz de Ciencias de la Tierra, CSIC and Universidad de Granada, Armilla, Granada, Spain;3. Departamento de Geodinámica, Universidad de Granada, Granada, Spain
Abstract:The continental plate collision across the South Island of New Zealand is highly oblique (dextral) and bounded by oppositely verging ocean plate subduction zones. As such, the region can be considered as a type of ‘subduction scissor’. Within this tectonic context, we use three-dimensional computational geodynamic models to consider how convergent mantle lithosphere can be modified by scissor and strike–slip effects. Bounding subduction at both ends of the continental collision causes flow of the descending mantle lithosphere in the direction along strike of the model plate boundary, with thinning in the centre and thickening towards the subduction zones that bifurcates the continental mantle lithosphere root. With dipping bounding subduction, the mantle lithosphere root takes on a more complex morphology that folds over from one subduction polarity to the other, but remains as a continuous feature as it folds under the collision zone. In the absence of bounding subduction, the plate convergence causes a linear (along strike) mantle lithosphere root to develop. A rapid strike–slip motion between the converging plates transfers material in the plate boundary-parallel direction and tends to blur out features that develop in this direction—such as descending viscous instabilities. The along-strike variations in the morphology of the mantle lithosphere root that develop in the models—viz., thickening of the root towards the subduction edges, thinning in the centre—are consistent with recent, albeit poorly constrained, geophysical interpretations of the large-scale lithospheric structure of the South Island. We speculate that this reflects the nature of the evolution of the South Island collision as a limited continental segment of the plate boundary that it is dominated and guided by adjacent well-developed/developing ocean plate subduction.
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