Seismic investigations of the lithosphere in an amagmatic back-arc region: North Island, New Zealand

New seismic constraints on crustal and upper mantle structures, kinematics, and lithospheric rheology are reported from an amagmatic back-arc region: the southwest North Island of New Zealand. Robust earthquake locations reveal a hypocentre 'downwarp' beneath the east-west trending Taranaki–Ruapehu Line. These earthquakes occur in the uppermost mantle, at depths of 30–50 km, and are distinct from shallower 8–25 km-deep earthquakes near Mt. Ruapehu in terms of focal mechanisms and principal stress directions.  A receiver function CCP stack shows that the mantle earthquakes occur beneath a large change in crustal thickness, where the Moho 'steps' from 28 to 35 km-deep and the steepest part of that step has a 20–50° dip. The mantle earthquakes are dominated by strike-slip fault movement and have a maximum compressive stress direction of NE–SW. The existence of mantle earthquakes beneath a steeply-dipping Moho step implies some sort of dynamic modication is occurring in the mantle lithosphere. One possibility to explain these features is the convective removal of the mantle lithosphere due to a Rayleigh–Taylor-type instability.  South of the Taranaki–Ruapehu Line, the Moho conversion weakens on both the receiver function CCP stack, and marine seismic reflection data under most of the Wanganui Basin (SAHKE02 and GD100 seismic lines). However, localised bright reflections at Moho depths can be seen in both near-vertical and wide-angle seismic data. Attribute analysis of near-vertical seismic reflections suggests that the rocks beneath the reflectivity are strongly-attenuating (Q ~20) with a negative velocity contrast relative to the lower crust. These observations are interpreted to be related to the presence of serpentinite (antigorite) and/or high pore fluid pressures in the mantle wedge.  The links between hydration of amagmatic back-arcs, serpentinisation and/or high pore fluid pressures, rock viscosity, and mantle instabilities are documented here for the southwest North Island of New Zealand. These associations may be applicable to other amagmatic back-arcs around the world.