Unraveling an enigmatic boundary along the Sunda-Banda volcanic arc

Abstract

This study utilizes a range of complementary geological, geophysical, and geochemical data to analyze the structural, deformational, and compositional variations along the Sunda-Banda volcanic arc transitional zone from oceanic to continental subduction. The core component is a synthesis of three station-based anisotropy inferences and two 3-D shear wave velocity models in the crust and upper mantle, all of which are derived based on ∼5 years of broadband seismic data collected across eastern Indonesia. These seismic results are based on receiver functions, shear wave splitting, ambient noise tomography, and a new teleseismic surface wave tomography model. We observe distinct along-arc spatial variations in the shear wave velocity and anisotropic structure within the crust and mantle wedge above the subducting slab. An intriguing arc-perpendicular orientation is observed in crustal fabrics and mantle anisotropy consistently in central Flores, contrasting with the arc-parallel orientations on the western and eastern sides. The orientations correlate with the change in strike of the highest topography as well as the presence of ∼N-S aligned cinder cones in central Flores. The 3-D shear wave velocity model in the crust and uppermost mantle also presents a sharp structural change in central Flores that indicates a relatively thinner crust. At deeper depths, the newly imaged upper mantle structure exhibits along-arc variations, but the change across central Flores appears to be distributed more broadly and outlines a relatively slow mantle wedge beneath eastern Flores. These new observations roughly correlate with previously identified changes in volcanic geochemistry characteristics across central Flores. Our study links the surface topography, geology, and volcanism with subsurface crust and mantle structures, unraveling an enigmatic structural and/or compositional boundary along the Banda volcanic arc.