A 3D Analysis of the Indo-Australian Slab Subducting Beneath Sumatra
The Sunda Trench marks the oblique subduction of the oceanic Indo-Australian plate beneath the continental Eurasian plate along the western margin of Sundaland. This area of the Indo-Australian plate subducting at the Sunda Trench is part of the Wharton Basin, which preserves an fossil seafloor spreading ridge offset by north-south trending fracture zones. Currently, the subducted plate geometry is not well-constrained and is debated between authors. This study models the geometry and spatial extent of the subducted Indo-Australian plate utilizing tomography including recent local P-wave tomography (Liu et al. 2021), global P-wave tomography UUP07 (Amaru, 2007) and DETOX-P2 (Hosseini et al. 2020), and Benioff zone seismicity. This analysis highlights the relationship between the subducted slab geometry, known locations and ages of various fracture zones, and the Wharton Basin fossil ridge. Several 3-D mid-slab models of the subducting Indo-Australian slab were constructed using GOCAD software; tomographic velocities were also projected along the mid-slab surfaces. Slab depth and dip correlate with the age of the subducting lithosphere in all models; in regions where younger lithosphere is subducting, the slab is shallower, and has a shallower dip angle, whereas the slab is deeper with higher dip angles where older lithosphere is subducting. Both the UU-P07 and DETOX-P2 models identify a large "step" within the slab that correlates to a slower velocity zone indicating the presence of a slab tear. This slab tear aligns with the projected subducted fossil spreading ridge and supports previous interpretations by Hall & Spakman (2015) and Jacob et al. (2014). The pre-subduction length of the Sunda slab was estimated using the DETOX-P2 slab model (Hosseini et al., 2020) and restored to Earth’s surface. The restored DETOX-P2 model suggests a pre-subduction length of approximately 2,816 to 3,038 km. The restored slab was inputted into GPlates plate reconstruction software to estimate timing of initial subduction. The restored Sunda slab begins to subduct around 31 Ma in the southeast and progressively migrates northwest toward Sumatra by 24 Ma, which aligns well with the timing of renewed magmatism along the Sunda arc described by Lai et al. (2023).
Slab mapping, Seismic tomography, Indo-Australia, Sunda
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