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Deep Slab Collision during Miocene Subduction

Causes Uplift along Crustal-Scale Reverse Faults

in Fiordland, New Zealand

Keith Klepeis, Laura Webb, Hannah Blatchford*, Dept. of Geology, 180 Colchester Ave., University of Vermont, Burlington,
Vermont 05405, USA; Joshua Schwartz, Dept. of Geological Sciences, California State University Northridge, 18111 Nordhoff
Street, Northridge, California 91330, USA; Richard Jongens, Anatoki Geoscience Ltd., 64 Skibo Street, Dunedin 9012, New Zealand;
Rose Turnbull, GNS Science, Dunedin Research Centre, Private Bag 1930, Dunedin 9054, New Zealand; Harold Stowell, Dept. of
Geological Sciences, University of Alabama, 2003 Bevill Bldg., Tuscaloosa, Alabama 35487, USA

ABSTRACT Puysegur Trench (Fig. 1A). Northeast of ducted into the mantle millions of
A new multidisciplinary project in the trench, the subducted slab rotates and years ago in unprecedented detail
southwest New Zealand that combines steepens to vertical below Fiordland, (Wu et al., 2016; Reyners et al., 2017).
geological and geophysical data shows where it joins the Alpine fault (Reyners et These imaged slabs can be integrated
how and why deep lithospheric dis‐ al., 2017), an ~850 km transform that has with surface geology and plate
placements were transferred vertically accumulated some 480 km of horizontal kinematics to reveal previously hidden
through the upper plate of an incipient displacement since ca. 25 Ma (Sutherland tectonic histories. Together, these and
ocean-continent subduction zone. A key and Norris, 1995). This region has many other innovations are providing
discovery includes two zones of steep, generated great interest among geologists, new opportunities to determine how
downward-curving reverse faults that in part because it is one of only a few surface tectonic records connect to
uplifted and imbricated large slices of places where the surface tectonic record processes occurring in the mantle as
Cretaceous lower, middle, and upper of an incipient ocean-continent sub‐ subduction zones form and develop over
crust in the Late Miocene. Geochemical duction zone can be observed directly time (e.g., Liu, 2015; Liu et al., 2017;
and structural analyses combined with (Mao et al., 2017). It also represents Kissling and Schlunegger, 2018).
39
40 Ar/ Ar geochronology and published Earth’s deepest exposed example of an In this article, we integrate structural,
images from seismic tomography suggest Andean-style continental arc (Ducea et geochemical, and geochronologic data
that the reverse faults formed at 8–7 Ma al., 2015). Here, we use this unique setting with images of the upper mantle derived
as a consequence of a deep (~100 km) to explore how Fiordland’s surface and from seismic tomography to reconstruct
collision between subducting oceanic crust responded to events that occurred the late Cenozoic tectonic history of
lithosphere and previously subducted deep within the lithospheric mantle since Fiordland. The results provide new insights
material. This collision localized subduction began in the Early Miocene. into the process of subduction initiation at
shortening and reactivated two crustal- Over the past few years, our under‐ continental margins, including the causes
scale shear zones from the upper mantle standing of the vertical links that and consequences of vertical motions
to Earth’s surface. The event, which develop within the lithosphere has within the overriding plate.
is summarized in a new lithospheric- benefitted from improvements in our
scale profile, is helping us answer some ability to extract information from the PREVIOUS WORK
long-standing questions about the origin rock record. Innovative approaches to
of Fiordland’s unique lower-crustal studying fault zones that combine Surface Geology
exposures and what they tell us about geochemistry and high-precision The surficial geology of Fiordland is
how inherited structures can transfer geochronology with structural analyses, dominated by exposures of the Median
motion vertically through the lithosphere for example, have enhanced our capacity Batholith (Mortimer et al., 1999), which
as subduction initiates. to relate deformation histories to other consist mostly of Carboniferous–Early
processes across a wide range of scales Cretaceous plutons. An eastern (outboard)
INTRODUCTION (e.g., Haines et al., 2016; Schwartz et al., belt contains Jurassic and older rocks that
In southwest New Zealand, oceanic 2016; Williams et al., 2017). At the same accreted onto the Gondwana margin
lithosphere of the Australian Plate time, new methods in global teleseismic during the Early Cretaceous (Tulloch and
subducts obliquely beneath continental tomography are revealing the geometry Kimbrough, 2003; Marcotte et al., 2005).
lithosphere of the Pacific Plate at the and extent of material that was sub- A western (inboard) belt exposes the Early

GSA Today, v. 29, https://doi.org/10.1130/GSATG399A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.
*Now at the Dept. of Earth Sciences, University of Minnesota,116 Church Street, Minneapolis, Minnesota 55455, USA.

4 GSA Today | September 2019