Inherited Lithospheric Fabrics and Mantle Flow in Borneo from Splitting Analyses of Teleseismic and Local S Phases

The complex accretionary tectonics of Borneo record a history of multiterrane convergence and extension dating back to the Mesozoic. Seismic anisotropy can provide key insights into the regional tectonic evolution. Given the complicated tectonic history, applying a complex anisotropy model may help reconcile the different observations reported in previous studies. In this study, we use seismic data from five long-running stations with a recording period of about 20 yr to systematically investigate the layered anisotropic structure in the upper mantle, based on splitting analyses of teleseismic core–mantle-boundary refracted and local S waves. The joint analysis reveals two layers of anisotropy within the lithosphere and asthenosphere, showing different patterns. The lithospheric anisotropy is strongly correlated with major geological structures. In southern Borneo, the fast orientations are parallel to the strikes of the Kuching and Meratus Suture zones, which likely reflects the Late Cretaceous tectonic accretion along these boundaries. In northern Borneo, seismic anisotropy mainly records paleocollisional deformation related to the Sabah orogeny, which was induced by Proto-South China Sea subduction from the mid Eocene to early Miocene. The corresponding remnant slab is now located at depths that are too great to have a strong influence on the present-day mantle flow. This is supported by the asthenospheric anisotropy, which shows a dominant fast orientation of west-northwest–east-southeast that is consistent with the absolute plate motion of the Sunda plate. The anisotropy can be explained by the lattice-preferred orientation of olivine, which is related to a mantle flow field driven by the overriding plate motion and/or geodynamic processes within the asthenosphere associated with nearby subduction zones. Notably, the plastic flow is locally deflected by a lithospheric keel beneath eastern Borneo, which leads to the observed north–south-oriented anisotropy and a deflected flow pattern.