Wide-angle seismic imaging beneath an andesitic arc: Central North Island, New Zealand

Structures within the deep crust and upper mantle of a continental back‐arc region are imaged with seismic wide‐angle reflection and refraction profiling. The data are from a new seismic experiment (MORC) in the Central Volcanic Region (CVR) of New Zealand, which is the active continental back‐arc system behind the Hikurangi Subduction Zone. Nine 0.4–1.3 ton dynamite shots were recorded on ∼700 seismographs spaced along a 120 km‐long array. There are two important discoveries from this study; a lower‐crust (15–25 km depth) “rift‐pillow” of rocks where P wave velocities (Vp) are ∼6.8–7.1 km/s, and strong reflections on reversed shots from a limited length (∼18 km‐long) reflector (R3) at a depth of ∼32 ± 1 km. Ray‐tracing shows the R3 reflector is in the upper mantle, beneath the eastern margin of the CVR, and is spatially linked to the active volcanic (andesite) arc and active geothermal fields. The relative amplitude of the R3 reflections indicates they are best explained by an interface across which there is a ∼80% drop in the S‐wave seismic velocity (Vs) and a modest (10%) drop in Vp. Such a change in wave speeds is consistent with R3 representing the upper surface of a body where the partial melt content may be as high as 12%. On the basis of these new data we propose a model of heat and strain transfer to explain the relative distribution of heat output, and andesitic and rhyolitic volcanism within a continental back‐arc basin. Key Points Discovery of a flat bright mantle reflector beneath an active andesite arc Discovery of a rift pillow of mafic rocks in lower crust of North Island, NZ Demonstration of wide‐angle reflection to explore lower crust‐upper mantle