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George Sound shear zone, is marked in   2B) yielded similar hornblende spectra   Over the past few years, several studies
          part by a linear belt of Late Carbon‐  and a 111.14 ± 0.76 Ma biotite plateau age.   (Decker et al., 2017; Milan et al., 2017)
          iferous granites (Ramezani and Tulloch,   These ages help establish that Cretaceous   have investigated the deep source regions
          2009) within the lower crustal block   magmatism and transpression over-   of the WFO batholith using isotopic
          (Fig. 1B). The eastern boundary    lapped in space and time, with pluton   systems and geochemical data. Decker et
          coincides with the old Carboniferous   emplacement occurring mainly at    al. (2017) showed that Early Cretaceous
          edge of Gondwana (Marcotte et al.,   118–115 Ma (Schwartz et al., 2016) and   plutons emplaced into the crustal boundary
          2005; Allibone et al., 2009b; Scott et al.,   deformation occurring at 117–110 Ma   marked by the George Sound shear zone
          2011; McCoy-West et al., 2014) and is   (Fig. 2B). They also help establish this   (Figs. 1B, 2, and 3) were sourced below
          deformed by both the Grebe and the   zone as a long-lived boundary that was in   the continental crust. Structural studies
          Indecision Creek shear zones (Fig. 1B).   place prior to subduction initiation at the   indicate that deformation aided magma
          All of these structures were infiltrated   Puysegur Trench during the Miocene.  ascent (Betka and Klepeis, 2013; Klepeis
          by magma and reactivated multiple times   To determine the age of the reverse   et al., 2016). Further work using oxygen
          since the late Carboniferous (e.g.,   faults, we collected two samples of   and hafnium isotopes (Andico et al., 2017)
          Marcotte et al., 2005; Scott et al., 2011)   pseudotachylyte from a well-exposed   indicates that strong isotopic differences in
          (Fig. 1B), indicating that they represent   segment of the Spey-Mica Burn fault zone   the lower crust existed across these shear
          long-lived zones of crustal weakness.  (samples 22, 23, Fig. 2A) and a third   zones during the Jurassic and Cretaceous,
            Figure 2A provides a detailed view of   pseudotachylyte sample (3A) from the    indicating they extended to lower crustal
          the superposed deformations caused by   Mt. Thunder fault (location in Fig. 1B;   depths during, and prior to, these times.
          the repeated reactivation of the western   results shown in the GSA Data   This work is important for understanding
          boundary. It shows that the Carboniferous   Repository [see footnote 1]). Multiple   Fiordland’s current crustal architecture
          Cozette pluton (pink) was intruded by the   runs of all three samples helped us cross   because it implies that the Spey-Mica Burn
          Early Cretaceous (mainly 118–115 Ma)   check the reproducibility of the apparent   fault system, which reactivated two ancient
          Misty pluton (yellow), both of which are   age spectra and interpreted ages. The   crustal-scale shear zones in the Late
          deformed by the George Sound shear   results indicate that the pseudotachylytes   Miocene, also transects the crust and
          zone (red-lined pattern). This same zone   all range in age from 8 to 7 Ma, indicating   penetrates into the upper mantle.
          also was the site of repeated magma   that faulting occurred approximately
          infiltration during the 170–128 Ma   simultaneously within both fault zones.  CONNECTING SURFACE
          interval (blue) (Allibone et al., 2009b).                             GEOLOGY TO DEEP
          Two phases of steep reverse faulting then   Probing the Deep Roots of Faults  LITHOSPHERIC STRUCTURES
          imbricated the shear zone, placing lower   One of the outcomes of the crustal   Our ability to investigate vertical
          crust to the east over middle crust.  configuration shown in Figures 1A and 1B   connections between Fiordland’s surface
            These findings have allowed us to   is an improved framework for determining   and the deep lithosphere requires a
          formulate many new questions, such as:   how structures and tectonic processes are   detailed knowledge of crustal archi‐
          How old is the crustal imbrication?    expressed at different depths within the   tecture, including when and how it was
          Why do faults deform only parts of the   lithosphere. For example, our work shows   assembled. Figure 3 shows a new profile
          Late Carboniferous boundaries? Our   that it is possible to walk continuously   that combines information from
          collaborative study aims to answer these   along the boundary between the Paleozoic   Fiordland’s rock record with recently
          questions and, in doing so, determine    Gondwana margin and the outboard   published tomographic models of the
          how the Paleozoic–Mesozoic history of   Jurassic arc from its location in upper   deep crust and upper mantle (Eberhart-
          Gondwana influenced Fiordland’s late   crustal exposures at the southern end of   Phillips et al., 2010; Reyners et al., 2017).
          Cenozoic tectonic history.         the Grebe shear zone to its lower crustal   The profile shows two narrow zones of
                                             expression in the Indecision Creek shear   reverse faulting directly above the region
          Unraveling the Timing of Fault     zone (Fig. 1B). This physical relationship   where the subducting Australian Plate
          Reactivations                      shows how narrow zones of Cretaceous   steepens to vertical against the
            An especially useful approach to   faulting in the upper crust gradually   Hikurangi Plateau. This discovery not
          distinguishing the age of superposed   change into thick zones of ductile shear in   only enhances our ability to reconstruct
          events at the boundaries of Fiordland’s   the lower crust (Fig. 1B). In addition, the   Fiordland’s subduction history, it also
          lower crustal slice has been through the   systematic mapping and dating of plutons   suggests a new mechanism by which
          use of  Ar/ Ar age spectra derived from   along the length of the shear zones shows   Fiordland’s crustal architecture and
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          step-heating experiments (Tables DR3   that magmatism and deformation were   surface record are linked to processes
          and DR4 [see footnote 1]). For example,   synchronous within them at all levels of   occurring at the base of the lithosphere.
          hornblende from the George Sound shear   the crust (Marcotte et al., 2005; Klepeis et   Estimates of crustal thickness beneath
          zone where it deforms the Carboniferous   al., 2016; Schwartz et al., 2017). This close   Fiordland, derived from isovelocity plots
          Cozette pluton (sample 79, Fig. 2B) yields   association is important because it allows   of Vp = 7.5 km s  (Eberhart-Phillips et
                                                                                             −1
          a 116.1 ± 1.1 Ma (1s) plateau age after an   us to use the geochemical signatures and   al., 2010; Reyners et al., 2017), suggest
          initial complex release pattern. A second   source regions of plutons to determine how   that Moho depths vary from ~30 km
          sample from where the shear zone   deep the George Sound and Indecision   below the WFO to more than 50 km
          deforms this same pluton (sample 72, Fig.   Creek shear zones once penetrated.  below the outboard batholith (Fig. 3).

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