We investigate seismic anisotropy across southeastern Indonesia where the Indo‐Australian plate subducts beneath and collides with the Sunda‐Banda arc. Geochemical, geodetic, and tomographic studies reveal an along‐strike transition from oceanic subduction to continental subduction and collision near central Flores that is due to a change of lithospheric composition in the subducting plate. To investigate the anisotropic fabric and dynamics of the upper mantle surrounding this young (∼5 Ma) arc–continent collision, we perform shear wave splitting analysis on local and teleseismic S waves recorded by an array of broadband seismometers that crosses the subduction–collision boundary. Seismic anisotropy inferred from our local S dataset shows that anisotropic sources above the slab extend to depths exceeding 100 km. Analysis of teleseismic SKS and SKKS waves reveal a shift in subslab fast axes from trench‐parallel to trench‐perpendicular near the ocean–continent boundary in the lower plate, which we relate to regional subslab mantle flow being deflected around the subducted continental lithosphere. Along‐strike variations in anisotropic fast axes from teleseismic phases overlap with distinct structural and tectonic boundaries that divide distinct regions of the collision, implying the effects of the collision transcend any one dataset and highlighting the complexity of collisional boundaries. These results shed light on the interaction between tectonic structure and mantle dynamics in an emergent collision, and help constrain the nature of upper mantle deformation in the early stages of collision.

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