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

Abstract

Northern Borneo-broadly coextensive with the Malaysian state of Sabah-lies near the north-eastern edge of the present-day Sundaland block, in Southeast Asia (Figure 1). This block, bounded by the seismically active Sunda and Philippines subduction zones, represents the southern extent of the slow-moving (∼20 mm year −1 ) Eurasian plate (Simons et al., 1999;Argus et al., 2011). Like much of eastern Borneo, northern Borneo was accreted onto the eastern margin of Mesozoic Sundaland between the Late Cretaceous and the Early Miocene (Hall, 1996). Though it now exhibits the characteristics of an intraplate setting, there is evidence in the geological record to suggest that it has been host to two opposing subduction systems since the start of the Neogene, both now terminated (see Figure 2). It is widely thought that the proto-South China Sea was subducted beneath the north-west continental margin of northern Borneo-continuing north-east along what is now Palawan-during the Paleogene, before terminating in the Early Miocene with continent-continent collision between the Dangerous

Seismic signature of subduction termination from teleseismic P- and S-wave arrival-time tomography: The case of northern Borneo

“…Post-subduction foundering in the form of a lithospheric drip is therefore our favoured explanation (Figure 10). While there could be a possibility that the sub-vertical high-velocity perturbation of the SD is due to, or significantly enhanced, by radial anisotropy, a high-velocity anomaly in correspondence of the SD drip is present in the S-wave model, which is much less influenced by radial anisotropy (see also Bacon et al, 2022). This indicates that the high-velocity perturbation associated to the SD in our tomographic models is likely due to a combination of thermal and compositional anomalies.…”

Ambient noise tomography of northern Borneo reveals evidence of subduction and post-subduction processes.

“…There have been a number of previous geophysical studies that have included northern Borneo, many of which involve the generation of regional velocity models of southeast Asia from seismic imaging. Although limited in resolution, they consistently reveal a high velocity feature in the upper mantle at around ∼100-300 km depth, which is generally attributed to remnant slabs Wehner et al, 2022;. More recently, targeted seismic studies have been performed using data from the northern Borneo Orogeny Seismic Survey (nBOSS) experiment .…”

“…found evidence for Sulu Sea extension continuing on-shore in northern Borneo by imaging crustal thickness variations using virtual deep seismic sounding. Bacon et al (2022) measured lithospheric anisotropy using XKS splitting and found that orientations of the fast axis of anisotropy were predominantly parallel to the Crocker Range in western northern Borneo, suggesting that anisotropy is dominated by recent continental collision. Fast axis orientations orthogonal to the opening of the Sulu Sea in central northern Borneo further support the idea of Sulu Sea extension propagating on-shore.…”