Deformation and Mantle Flow beneath Indonesia from Seismic Anisotropy

Wedged between the Eurasian, Philippine Sea, Pacific, and Indo-Australian plates, Indonesia features several active and relict subduction zones in close proximity to one another. This makes for an ideal natural laboratory to study complex subduction processes. Seismic anisotropy is a direct result of deformational processes. Hence, it allows one to make inferences about the nature of mantle flow as well as large-scale lithospheric deformation, both past and present. Shear-wave splitting is one of the least ambiguous signatures of seismic anisotropy and can therefore provide important constraints on mantle and lithosphere deformation. Here, we present new shear-wave splitting results for the entire Indonesian region. We combine local-S, teleseismic SKS, as well as source-side splitting measurements for better depth constraint on the anisotropic fabrics. This is the first such comprehensive study of seismic anisotropy for the whole Indonesian region. A common feature for several subduction zones (for example, the Java-Sunda and the Sangihe) is that there appear to be two distinct regions of anisotropy: one within the lithosphere and one in or beneath the slab; the supra-slab mantle wedge is largely isotropic. We relate this to fossil anisotropy in the overriding plate and either fossil anisotropy in the slab or mantle flow beneath the slab. We compare the observed anisotropy to other tectonic and geodynamic information in the region to constrain subduction dynamics beneath Indonesia.