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

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 crustal-scale shear zones from the upper mantle to Earth’s surface. The event, which is summarized in a new lithospheric-scale 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.

Manuscript received 13 Dec. 2018. Revised manuscript received 15 Mar. 2019. Manuscript accepted 20 Mar. 2019. Posted 26 Apr. 2019.

© The Geological Society of America, 2019. CC-BY-NC.

10.1130/GSATG399A.1