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continent twice. The first underthrusting   A    Cross sec on of the Spey-Mica Burn fault zone
          occurred ca. 100 Ma when Fiordland    NW     lower crustal section         mid-crustal section     SE
          formed part of Gondwana; the second            72 (bt, hbl)                22 (psd)        0       1 km
          occurred in the late Cenozoic driven by   1.5 km          79 (hbl)               23 (psd)            1.5
          convergence between the Pacific and                            Misty Fault
          Australian plates (Davy, 2014; Reyners et   1.0                                                      1.0
          al., 2011, 2017). Currently, the western
          edge of the plateau lies below central and   0.5                                                     0.5
          northern Fiordland where it impacts the      Misty pluton (WFO)  Cozette pluton
          geometry of the subducting Australian   0    (118-115 Ma)      (~341 Ma)          Irene complex
                                                                                            (Cambrian)
          Plate (Reyners et al., 2017). South of the   B   George Sound shear zone spectra             Late Miocene
          line of section shown in Figure 1A, the   140                             C  Pseudotachylyte spectra  reac va on
          subducting plate parallels the Puysegur   130  72 (hbl)  79 (hbl)  GSSZ  10
          Trench and dips at ~68° below 50 km   120
          depth (Reyners et al., 2011). North of this   Apparent Age (Ma)  110    5       22 (psd)   23 (psd)
          line, the slab twists to the NE (040°) and is   100  72 (bt)
          vertical below 75 km (Reyners et al., 2017).  90                        0 0  20   40    60   80  100
                                                   0   20   40   60   80  100           Cumulative  Ar Percent
                                                                                                39
                                                       Cumulative  Ar Percent
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          INTEGRATED GEOLOGICAL
          STUDIES                            Figure 2. (A) Cross section of the Spey-Mica Burn fault zone (location in Fig. 1A). Profile shows steep
                                             reverse faults (dark black lines) that uplifted and imbricated the George Sound shear zone (orange-
                                             red-lined patterns), placing Cretaceous lower crust to the SE over Cretaceous middle crust. Yellow
          Reconstructing Fiordland           and blue represent undeformed portions of the Misty pluton and older Jurassic–Early Cretaceous
                                             igneous rock, respectively. Orange-lined pattern represents sheared Misty pluton; dark red-lined
            Many advances in our understanding of   pattern with plusses represents sheared Cozette pluton (samples 72 and 79). (B) Apparent  Ar/ Ar
                                                                                                           40
                                                                                                              39
          Fiordland’s deep-crustal exposures have   age spectra from hornblende (hbl) and biotite (bt) from sample 72 and hornblende from sample 79
          come from efforts to distinguish the age   indicate George Sound shear zone (GSSZ) deformation occurred at 117–110 Ma (dots are dated sam-
                                             ples). (C) Apparent  Ar/ Ar age spectra from 8 to 7 Ma pseudotachylyte (psd) within splays of the
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          and significance of various episodes of   Spey-Mica Burn fault zone (two runs each of samples 22 and 23). Similar ages were obtained from
          magmatism, metamorphism, and defor‐  pseudotachylyte in the Mt. Thunder fault (Figs. DR2 and DR3 in the GSA Data Repository [see text
                                             footnote 1] show detailed spectra and a detailed map of the Spey-Mica Burn fault zone, respectively).
          mation. In particular, the application of
          multiple geochronometers (e.g., Klepeis
          et al., 2016; Schwartz et al., 2016, 2017;   These crustal divisions are important   to an older period of Cretaceous
          Stowell et al., 2017; Tulloch et al., 2010,   because they provide an improved   extension. Consequently, it has sparked
          2019), combined with an improved   framework for determining how the   new investigations aimed at determining
          understanding of metastability in igneous   characteristics of magmatism, meta-  the age of faulting and its relationship to
          and metamorphic mineral assemblages   morphism, and deformation change   Miocene subduction and zones of high
          (Allibone et al., 2009a; Bhattacharya et   vertically within the lithosphere.  exhumation rates.
          al., 2018), have enhanced our ability to   One of the most significant outcomes
          correlate tectonic events across thousands   of our study is the discovery of a narrow   Reactivating Ancient Structures
          of square kilometers. These improve‐  zone of steep, downward-curving reverse   Determining the age and history of
          ments have allowed us to reconstruct   faults that placed a large, irregular slice of   faulting in Fiordland has been chal‐
          Fiordland’s crustal architecture with   lower crust up and to the east over the   lenging, mainly because the surface
          increased accuracy.                middle and upper crust (Figs. 1A and 2A).   expression of faults typically is narrow
            Figure 1A shows a new compilation of   The Spey-Mica Burn fault system, which   and eroded or concealed by sediment and
          Cretaceous paleodepths that provides a   is well-exposed in central Fiordland,   dense vegetation. To solve this problem,
          snapshot of Fiordland crust ca. 115 Ma,   extends for ~80 km and joins the Misty   we walked the surface traces of faults
          when it reached its maximum thickness of   fault (new name) along the eastern   and found high-quality exposures that
          ≥65 km. It also is the first to delineate the   boundary of the lower crustal block.    preserve kinematic information (Fig.
          boundaries of the various crustal blocks.   The fault zone then steps to the east in a   DR1, Table DR2 [see footnote 1]). Two
          The data derive from mineral assem‐  series of oblique-slip faults that connect   especially informative localities (stars in
          blages that represent the peak of Early   with another system of reverse faults,   Fig. 1B) expose pseudotachylyte-bearing
          Cretaceous metamorphism and estimates   including the Mt. Thunder fault (new   reverse faults at and near the eastern
          of the emplacement depths of plutons   name) and the Glade-Darran fault zone   boundary of the lower crustal block.
          whose age and history are known (see   (Fig. 1). This discovery is the first to   These sites show that the reverse faults
          Table DR1 in the GSA Data Repository ).   confirm that the last 12–15 km of the   reactivated two ancient crustal
                                        1
          Our reconstruction shows large blocks of   uplift and exhumation of Fiordland’s   boundaries that coincide with large,
          Cretaceous upper, middle, and lower   unique exposures is directly related to   ductile shear zones. The western
          crust, all of which are bounded by faults.   late Cenozoic reverse faulting rather than   boundary, which is centered on the


          1 GSA Data Repository item 2019195,  Ar/ Ar analytical methods and data tables, paleodepth data, and fault-slip data, is online at www.geosociety.org/
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          datarepository/2019.
       6  GSA Today  |  September 2019
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