Deep Slab Collision during Miocene Subduction Causes Uplift along Crustal-Scale Reverse Faults in Fiordland, New Zealand

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