Ductile fabrics in the zone of active oblique convergence near the Alpine Fault, New Zealand: identifying the neotectonic overprint

Abstract

The mid-crustal Alpine Schist in central Southern Alps, New Zealand has been exhumed during the past ∼3m.y. on the hanging wall of the oblique-slip Alpine Fault. These rocks underwent ductile deformation during their passage through the ∼150-km-wide Pacific–Australia plate boundary zone. Likely to be Cretaceous in age, peak metamorphism predates the largely Pliocene and younger oblique convergence that continues to uplift the Southern Alps today. Late Cenozoic ductile deformation constructively reinforced a pre-existing fabric that was well oriented to accommodate a dextral-transpressive overprint. Quartz microstructures below a recently exhumed brittle-ductile transition zone reflect a late Cenozoic increment of ductile strain that was distributed across deeper levels of the Pacific Plate. Deformation was transpressive, including a dextral-normal shear component that bends and rotates a delaminated panel of Pacific Plate crust onto the oblique footwall ramp of the Alpine Fault. Progressive ductile shear in mylonites at the base of the Pacific Plate overprints earlier fabrics in a dextral-reverse sense, a deformation that accompanies translation of the schists up the Alpine Fault. Ductile shear along that structure affects not only the 1–2-km-thick section of Alpine mylonites, but is distributed across several kilometres of overlying nonmylonitic rocks.