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

The mid-crustal Alpine Schist in central Southern Alps, New Zealand has bee n exhumed during the past similar to3 m.y. on the hanging wall of the obliq ue-slip Alpine Fault. These rocks underwent ductile deformation during thei r passage through the similar to 150-km-wide Pacific-Australia plate bounda ry zone. Likely to be Cretaceous in age, peak metamorphism predates the lar gely Pliocene and younger oblique convergence that continues to uplift the Southern Alps today. Late Cenozoic ductile deformation constructively reinf orced a pre-existing fabric that was well oriented to accommodate a dextral -transpressive overprint. Quartz microstructures below a recently exhumed b rittle-ductile transition zone reflect a late Cenozoic increment of ductile strain that was distributed across deeper levels of the Pacific Plate. Def ormation was transpressive, including a dextral-normal shear component that bends and rotates a delaminated panel of Pacific Plate crust onto the obli que footwall ramp of the Alpine Fault. Progressive ductile shear in mylonit es at the base of the Pacific Plate overprints earlier fabrics in a dextral -reverse sense, a deformation that accompanies translation of the schists u p the Alpine Fault. Ductile shear along that structure affects not only the 12-km-thick section of Alpine mylonites, but is distributed across several kilometres of overlying nonmylonitic rocks. (C) 2001 Elsevier Science Ltd. All rights reserved.