The Signature of Lithospheric Anisotropy at Post‐Subduction Continental Margins: New Insight From XKS Splitting Analysis in Northern Borneo

The relative paucity of recent post‐subduction environments globally has meant that, so far, little is known about tectonic processes that occur during and after subduction termination, as previously convergent tectonic plates adjust to the new stress regime. The region of Southeast Asia that now encompasses northern Borneo has been host to two sequential episodes of subduction—both now terminated—since the mid‐Paleogene. It is expected that these processes will have left signatures in the fabric of the upper mantle, which are manifest in the form of seismic anisotropy. We investigate the evidence for, and alignment of, anisotropic fabrics by measuring the splitting of a family of teleseismic shear phases. These observations provide a measure of the orientation of the effective anisotropic elastic tensor, in the form of the orientation of the fast shear‐wave polarization, ϕ, and add constraints on the strength of the anisotropic fabric, in the form of the delay time, δt. We observe two principal trends across northern Borneo that appear to be confined to the lithosphere. These patterns are likely related to tectonic processes associated with subduction, continental collision, and oceanic basin formation, events that can exert primary influence on the formation of post‐subduction settings. Plain Language Summary This study is concerned with understanding what happens to the upper 200 km of the Earth when subduction—the process by which one plate pushes beneath another and sinks into the Earth's interior—stops. We measure a property of the rock in the upper 200 km called seismic anisotropy, which tells us how fast earthquake waves move when traveling or polarized in one direction compared to another. Seismic anisotropy can inform us about both large‐scale plate tectonic events in recent geological history (10s of millions of years) and present‐day deformation. Northern Borneo has undergone two phases of active subduction followed by termination in the last 25 million years, making it one of the few places on Earth where we can explore this important stage of the subduction cycle. We find that tectonic compression and extension events related to termination and post‐subduction processes have left strong imprints in the upper 100 km of the Earth, leaving little‐to‐no remnants of signals we might have expected to observe from the active phase of subduction. Key Points New catalog of shear‐wave splitting measurements from a dense network in post‐subduction setti