Attenuation in the mantle wedge beneath super-volcanoes of the Taupo Volcanic Zone, New Zealand

Abstract

SUMMARYThe Taupo Volcanic Zone has a 120-km-long section of rhyolitic volcanism, within which is a 60-km-long area of supervolcanoes. The underlying subducted slab has along-strike heterogeneity due to the Hikurangi Plateau's prior subduction history. We studied 3-D Qs (1/attenuation) using t* spectral decay from local earthquakes to 370-km depth. Selection emphasized those events with data quality to sample the low Qs mantle wedge, and Qs inversion used varied linking of nodes to obtain resolution in regions of sparse stations, and 3-D initial model. The imaged mantle wedge has a 250-km-long 150-km-wide zone of low Qs (<300) at 65–85 km depth which includes two areas of very low Qs (<120). The most pronounced low Qs feature underlies the Mangakino and Whakamaru super-eruptive calderas, with inferred melt ascending under the central rift structure. The slab is characterized by high Qs (1200–2000), with a relatively small area of reduction in Qs (<800) underlying Taupo at 65-km depth, and adjacent to the mantle wedge low Qs. This suggests abundant dehydration fluids coming off the slab at specific locations and migrating near-vertically upward to the volcanic zone. The seismicity in the subducted slab has a patch of dense seismicity underlying the rhyolitic volcanic zone, consistent with locally abundant fractures and fluid flux. The relationship between the along-arc and downdip slab heterogeneity and dehydration implies that patterns of volcanism may be strongly influenced by large initial outer rise hydration which occurred while the edge of the Hikurangi Plateau hindered subduction. A second very low Qs feature is 50-km west above the 140-km-depth slab. The distinction suggests involvement of a second dehydration peak at that depth, consistent with some numerical models.