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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
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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
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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