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inboard batholith                            edge lies below central and northern
                        Alpine Fault  Misty Fault  Mt. Thunder Fault (projected)  Fiordland, where it appears to step to the
                A                                      Eocene-Oligocene basin A’  northeast. Interestingly, the pattern of
             0                                                                  Miocene reverse faults at the surface
                                       G                                        mimics this east-stepping geometry of
           -10                                                                  the plateau at depth (Fig. 1B), which

           -20                       G                                          provides an explanation for why the
                                  # #   # #              outboard batholith &   two ancient crustal boundaries were
           -30                      # #   # #  Gr, I                            reactivated in different places at 8–7 Ma.
                                        # #               accreted terranes       In their tomographic studies of the
           -40                                 # # # # # # # #                  subsurface, Reyners et al. (2011, 2017)
                                          # # # # # # #  # # # #    Moho
           -50   o c e a n i c   m a n t l e  # # # # # #  # # # # # # # #      concluded that a late Cenozoic collision
                                                                                between the subducting Australian Plate
           -60                                                                  and the western Hikurangi Plateau

           -70                                hydrated upper                    caused the underthrust plate to steepen
                                l             mantle wedge                      to vertical below 75 km (Fig. 3). A
                                 i
           -80                   t                                              reconstruction of the forward progress of
                                                                                this slab since ca. 25 Ma (Fig. 1A) shows
                                 h
           -90                    s                                             that the time when its leading edge first
                                  o
          -100                    p             subducted Hikurangi Plateau     encountered the plateau margin
                                  h
          -110                    e                                             coincides with both the surface location
                                  e
                                                                                and the timing of reverse faulting at 8–7
                                  r
          -120                                                                  Ma. This relationship suggests that the
           km                                                                   collision localized shortening and caused
                       50 km                                                    the reactivation of two Cretaceous shear
                                     V.E. = 1:1
                                                      faulted Cretaceous Moho of  zones as reverse faults. Their steep
                 Australian Plate lithosphere         the Median Batholith      orientations meant that the displace-
           a b   thickened a) lower and b) middle  limit of delaminated         ments were partitioned mostly into
                                                                                vertical motion, suggesting that this
                 crust of inboard batholith         lower crust                 event also should be visible in
            #    garnet pyroxenite (Cretaceous      V p /V s  < 1.70      subducted crust of  Fiordland’s exhumation history.
                 root of Median Batholith)                                  Hikurangi Plateau  In their compilation of Fiordland’s
                 mantle lithosphere (V p  > 8 km s )  V p /V s  < 1.72          record of surface uplift, Sutherland et al.
                                           -1
                                                   hydrated lower crust         (2009) inferred that topographic growth
                                                   and mantle (V p /V s  > 1.75)  and exhumation since ca. 25 Ma were
                                                                                associated with the inception of
          Figure 3. Vertical profile that combines new geological information with published geophysical   subduction and the downward deflection
          images (location in Fig. 1A). Colors match those in Figure 1A except where noted. The location and   of the Australian slab. However, one
          geometry of the subducted Australian Plate is from relocated hypocenters (Reyners et al., 2017).
          The location of the subducted Hikurangi Plateau is from Eberhart-Phillips et al. (2010) and Reyners   problem in trying to relate this history of
          et al. (2017). Gray lines are Cretaceous shear zones that penetrate the lower crust: G—George   uplift and exhumation to crustal
          Sound shear zone; I—Indecision Creek shear zone; Gr—Grebe shear zone.
                                                                                shortening is that neither topographic
                                                                                features nor zones of high exhumation
            These estimates closely match those we   crustal-scale shear zones, and estimates   rates could be linked to specific faults.
          obtained for Cretaceous crustal thick‐  of 12–15 km of late Cenozoic uplift   Our discovery of major reverse faults at
          nesses using metamorphic mineral   within their hanging walls.        the edge of Fiordland’s lower crustal
          assemblages combined with estimates of   Below the base of the crust, tomo‐  block helps to solve this problem. In
          the vertical offset across faults. This   graphic images show the 3D structure    particular, the close spatial and temporal
          similarity suggests that the Cretaceous   and subsurface extent of the subducted   agreement between vertical fault
          Moho approximately coincides with the   Hikurangi Plateau (Reyners et al., 2011,   motions at 8–7 Ma and the abrupt
          position of the current Moho, which has   2017; Davy, 2014). Images of Vp and    expansion of zones of rock uplift and
          been difficult to image using geophysical   Vp/Vs show that the plateau within the   high exhumation rates into eastern and
          techniques. It also suggests that the   Pacific Plate mantle is an ~35-km-thick   northern Fiordland suggests that these
          apparent shallowing of both the    zone of seismicity with a layer of high   uplift patterns were caused by reverse
          Cretaceous and the current Moho from   Vp (~8.5 km s ) eclogite crust at its base   faulting. We therefore conclude that
                                                        −1
          east to west beneath Fiordland is a   (Fig. 3). Above this layer, high Vp/Vs   fault-related uplift and topographic
          consequence of Late Miocene reverse   ratios (~1.75) probably reflect the   growth in the Late Miocene were direct
          faulting. This interpretation is compatible   presence of a hydrated mantle wedge,   consequences of the collision between
          with the steep orientation of the reverse   which may contribute to the poorly   the subducting Australian Plate and the
          faults, their reactivation of inherited   defined Moho. The plateau’s southwest   Hikurangi Plateau.

       8  GSA Today  |  September 2019




                      Depth sections down the dip (a and b) and along the strike (c) of the Hikurangi Plateau. The location of the sections is shown in Fig. 1b.
                      Vp is shown in the top panels and Vp/Vs in the bottom panels, with the white and yellow lines showing the limit of good resolution, as
                      represented by the SF =3.5 contour. Black crosses show inversion hypocentres near each depth section, and red circles similarly show
                      seismicity for the period 2001–2011 relocated with the new 3-D seismic velocity model. Letters indicate features discussed in the text,
                      namely: EOC eclogitized oceanic crust; HP Hikurangi Plateau; MBLC Median Batholith lower crust; OC oceanic crust; OSD Otago Schist
                      duplex.
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