RELATIONSHIPS BETWEEN TEMPORAL-SPATIAL DISTRIBUTION OF MONOGENETIC VOLCANOES, CRUSTAL STRUCTURE, AND MANTLE VELOCITY ANOMALIES: AN EXAMPLE FROM THE ABU MONOGENETIC VOLCANO GROUP, SOUTHWEST JAPAN

Achieving an understanding of the nature of monogenetic volcanic fields depends on identification of the spatial and temporal patterns of volcanism in these fields, and their relationships to structures mapped in the shallow crust and inferred in the deep crust and mantle through interpretation of geophysical data. We investigated the spatial and temporal distributions of volcanism in the Abu Monogenetic Volcano Group (AMVG), Southwest Japan, and compare these distributions to fault and seismic data in the brittle crust, and mantle P-wave tomographic anomalies in the upper mantle and lower crust. Late Cenozoic monogenetic volcano groups in the southwest Japan Arc are thought to originate from the upwelling of mantle diapirs. The AMVG is one such monogenetic volcano group consisting of alkaline basalt and calc-alkaline andesite-dacite lavas and pyroclastics distributed over an area of 400km². Some of the 56 volcanoes comprised by the AMVG are located under the Sea of Japan, and are known primarily from bathymetry. Previous studies indicate that the AMVG is within a zone of predominantly E-W compressive stress. Essential characteristics of the volcano distribution are extracted by a nonparametric kernel method using an algorithm to estimate anisotropic bandwidth. Overall E-W elongate smooth modes in spatial density are identified that are consistent with upper mantle and lower crustal P-wave velocity anomalies, supporting the idea that the spatial density map of volcanic vents reflects the geometry of a mantle diaper. The maximum volcano density of this case is 4.0×10^-3 event/km². While estimated basalt productivity decreased after 0.2 Ma, andesite productivity increased and overall volume production is approximately steady-state. The estimated basalt productivity behaves as a volume-predictable model. Obvious increase in the area affected by volcanic activity did not occur at that time. This productivity also supports the idea that volcanism is related to a mantle diaper. These relationships hint that monogenetic volcanic fields in compressional tectonic regimes may be characterized as volume-predictable, rather than time-predictable as observed in many extensional settings.