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Deep Slab Collision during Miocene Subduction

                       Causes Uplift along Crustal-Scale Reverse Faults

                                        in Fiordland, New Zealand






          Keith Klepeis, Laura Webb, Hannah Blatchford*, Dept. of Geology, 180 Colchester Ave., University of Vermont, Burlington,
          Vermont 05405, USA; Joshua Schwartz, Dept. of Geological Sciences, California State University Northridge, 18111 Nordhoff
          Street, Northridge, California 91330, USA; Richard Jongens, Anatoki Geoscience Ltd., 64 Skibo Street, Dunedin 9012, New Zealand;
          Rose Turnbull, GNS Science, Dunedin Research Centre, Private Bag 1930, Dunedin 9054, New Zealand; Harold Stowell, Dept. of
          Geological Sciences, University of Alabama, 2003 Bevill Bldg., Tuscaloosa, Alabama 35487, USA


          ABSTRACT                           Puysegur Trench (Fig. 1A). Northeast of   ducted into the mantle millions of
            A new multidisciplinary project in   the trench, the subducted slab rotates and   years ago in unprecedented detail
          southwest New Zealand that combines   steepens to vertical below Fiordland,   (Wu et al., 2016; Reyners et al., 2017).
          geological and geophysical data shows   where it joins the Alpine fault (Reyners et   These imaged slabs can be integrated
          how and why deep lithospheric dis‐  al., 2017), an ~850 km transform that has   with surface geology and plate
          placements were transferred vertically   accumulated some 480 km of horizontal   kinematics to reveal previously hidden
          through the upper plate of an incipient   displacement since ca. 25 Ma (Sutherland   tectonic histories. Together, these and
          ocean-continent subduction zone. A key   and Norris, 1995). This region has   many other innovations are providing
          discovery includes two zones of steep,   generated great interest among geologists,   new opportunities to determine how
          downward-curving reverse faults that   in part because it is one of only a few   surface tectonic records connect to
          uplifted and imbricated large slices of   places where the surface tectonic record   processes occurring in the mantle as
          Cretaceous lower, middle, and upper    of an incipient ocean-continent sub‐  subduction zones form and develop over
          crust in the Late Miocene. Geochemical   duction zone can be observed directly   time (e.g., Liu, 2015; Liu et al., 2017;
          and structural analyses combined with   (Mao et al., 2017). It also represents   Kissling and Schlunegger, 2018).
              39
          40 Ar/ Ar geochronology and published   Earth’s deepest exposed example of an   In this article, we integrate structural,
          images from seismic tomography suggest   Andean-style continental arc (Ducea et   geochemical, and geochronologic data
          that the reverse faults formed at 8–7 Ma   al., 2015). Here, we use this unique setting   with images of the upper mantle derived
          as a consequence of a deep (~100 km)   to explore how Fiordland’s surface and   from seismic tomography to reconstruct
          collision between subducting oceanic   crust responded to events that occurred   the late Cenozoic tectonic history of
          lithosphere and previously subducted   deep within the lithospheric mantle since   Fiordland. The results provide new insights
          material. This collision localized   subduction began in the Early Miocene.  into the process of subduction initiation at
          shortening and reactivated two crustal-  Over the past few years, our under‐  continental margins, including the causes
          scale shear zones from the upper mantle   standing of the vertical links that   and consequences of vertical motions
          to Earth’s surface. The event, which    develop within the lithosphere has   within the overriding plate.
          is summarized in a new lithospheric-   benefitted from improvements in our
          scale profile, is helping us answer some   ability to extract information from the   PREVIOUS WORK
          long-standing questions about the origin   rock record. Innovative approaches to
          of Fiordland’s unique lower-crustal   studying fault zones that combine   Surface Geology
          exposures and what they tell us about    geochemistry and high-precision   The surficial geology of Fiordland is
          how inherited structures can transfer   geochronology with structural analyses,   dominated by exposures of the Median
          motion vertically through the lithosphere   for example, have enhanced our capacity   Batholith (Mortimer et al., 1999), which
          as subduction initiates.           to relate deformation histories to other   consist mostly of Carboniferous–Early
                                             processes across a wide range of scales   Cretaceous plutons. An eastern (outboard)
          INTRODUCTION                       (e.g., Haines et al., 2016; Schwartz et al.,   belt contains Jurassic and older rocks that
            In southwest New Zealand, oceanic   2016; Williams et al., 2017). At the same   accreted onto the Gondwana margin
          lithosphere of the Australian Plate   time, new methods in global teleseismic   during the Early Cretaceous (Tulloch and
          subducts obliquely beneath continental   tomography are revealing the geometry   Kimbrough, 2003; Marcotte et al., 2005).
          lithosphere of the Pacific Plate at the   and extent of material that was sub-   A western (inboard) belt exposes the Early




          GSA Today, v. 29, https://doi.org/10.1130/GSATG399A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.
          *Now at the Dept. of Earth Sciences, University of Minnesota,116 Church Street, Minneapolis, Minnesota 55455, USA.

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