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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
39
40
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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