Abstract
Clustering of arc volcanoes in subduction zones indicates along-arc variation in the physical condition of the underlying mantle where majority of arc magmas are generated. The sub-arc mantle is brought in from the back-arc largely by slab-driven mantle wedge flow. Dynamic processes in the back-arc, such as small-scale mantle convection, are likely to cause lateral variations in the back-arc mantle temperature. Here we use a simple three-dimensional numerical model to quantify the effects of back-arc temperature perturbations on the mantle wedge flow pattern and sub-arc mantle temperature. Our model calculations show that relatively small temperature perturbations in the back-arc result in vigorous inflow of hotter mantle and subdued inflow of colder mantle beneath the arc due to the temperature dependence of the mantle viscosity. This causes a three-dimensional mantle flow pattern that amplifies the along-arc variations in the sub-arc mantle temperature, providing a simple mechanism for volcano clustering.
Highlights
Clustering of arc volcanoes in subduction zones indicates along-arc variation in the physical condition of the underlying mantle where majority of arc magmas are generated
This interpretation is based on the lack of strong contrast in seismic attenuation between low-velocity zones (LVZs) and the surrounding mantle because temperature is known to have a relatively large effect on seismic attenuation, and high attenuation is expected to occur in hot regions[6]
This type of mantle flow has been simulated within a thin low-viscosity layer of 4–8 km thickness imposed at 50–70 km depths immediately above the subducting slab in a numerical subduction model, whereby the along-arc variation in the extent of the return flow is initiated by numerical noise in the thermal field[17]
Summary
Clustering of arc volcanoes in subduction zones indicates along-arc variation in the physical condition of the underlying mantle where majority of arc magmas are generated. In NE Japan, the dense seismic networks and abundant earthquakes allow high-resolution imaging of seismic velocity structures, which indicate low-velocity zones (LVZs) in the mantle wedge that extend from the back-arc to the sub-arc region where volcanoes occur in clusters[1] Because of their strong spatial correlation with volcanic clusters, the LVZs have generally been interpreted as hot regions that feed melts to the overlying clustered volcanoes and have commonly been referred to as hot fingers[1]. Along-arc fluctuation in the slab-mantle decoupling depth is a possible mechanism to cause localized mantle inflow; the overlying mantle is stagnant and cold in regions with deepened decoupling depths whereas the overlying mantle is flowing and hot in regions with shallower decoupling depths[10] This contrasts with global thermal modelling studies that indicate a relatively uniform decoupling depth of 70–80 km (refs 11,12). Such condition combined with generally high temperature of the convecting mantle is likely to promote widespread melting
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