Lithospheric Controls on Arc Volcano Distributions

Abstract

Distributions of arc volcanoes represent complex processes taking place at many depths. It has been suggested that the alignment of volcanoes in an arc can be described by a small circle geometry on a sphere. However, this assumption neglects the arc-specific tectonic framework that could influence arc volcanism. Here we investigated the distribution of arc volcanoes in three study areas (i.e. Mariana, Java and Lesser Sunda, and Sumatra) and further 16 arcs in global scale study, using quantitative tools to establish the preferred alignment model. We demonstrate that volcanoes at an arc are preferably described by segmented great circles – linear alignments on the Earth’s surface that we term ‘arc-segments’. This distribution model is best explained by control from the upper plate stress regime where the arc-segments are associated with arc-normal tension located at the base of flexed lithosphere. Lithosphere flexure is the result of down-pulling near the edge of the overriding plate by slab motion because plate-coupling maintains contact between the upper and lower plates. At the location of maximum downward flexure of the overriding plate, a lithospheric weakness zone partitions stress into compression in the near-surface and tension at greater depth. We suggest this is the site where magma pathways through the upper plate are created and lead to construction of a volcanic arc. Oblique convergence can influence the arrangement of arc segments into an en-echelon pattern, in addition to the lithosphere flexure. Where arc segments overlap and/or have been rotated an extended weakness zone with greater lithosphere thinning allowed the generation of more melt which can accumulate in the crust. In the latter case, the enhanced magma supply has contributed to the formation of a large-scale caldera (e.g. Toba). At global scales, a segmented great circle distribution model is statistically preferred at 16 arcs. Multivariate statistics display the importance of upper plate thickness in subduction dynamics where it correlates with the distance from trench to arc-segment, slab dip at shallow depth, and the rms-misfit of great circle fitting. Thus, control from the overriding plate on arc volcanism is important and should be considered in future arc studies.