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 polarisation, 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.
The author contributions are as follows: C.A.B. – Conceptualization, Formal analysis, Methodology, Software, Investigation, Visualization, Writing – original draft, Data curation; N.R. – Funding acquisition, Supervision, Writing – review & editing; S.P. – Funding acquisition, Writing – review & editing; A.G. – Funding acquisition, Writing – review & editing; D.W. – Writing – review & editing, Formal analysis, Visualization; D.G.C. – Funding acquisition, Supervision, Writing – review & editing; F.T. - Funding acquisition, Supervision. The authors thank all those that contributed to the deployment, servicing, and recovery of the northern Borneo Orogeny Seismic Survey (nBOSS) network between March
2018 and January 2020. We also thank Miriam Reiss for assistance with SplitRacer and helpful discussions of the results presented herein. S. Pilia acknowledges support from the Natural Environmental Research Council (NERC) Grant NE/R013500/1 and from the European Union’s Horizon 2020 Research and Innovation Program under Marie Skłodowska584 Curie Grant Agreement 790203. Seismometers used in the nBOSS network were provided by the Universities of Cambridge and Aberdeen, and the Natural Environment Research
Council (NERC) Geophysical Equipment Facility (loan 1038). Waveform data recorded by the nBOSS network were extracted, quality checked, and archived by C. A. Bacon. We thank MetMalaysia for providing access to their restricted continuous waveform data recorded by their permanent MY network in Sabah. Finally, we would like to thank Eric Löberich and one anonymous reviewer for their insightful comments on this work. Department of Earth Sciences, Cambridge contribution ESC.XXXX.
Data Availability StatementSeismic data from the nBOSS network will be accessible through the IRIS Data Management Center (https://ds.iris.edu/ds/nodes/dmc/) from February 2023 under the network code YC (https://doi.org/10.7914/SN/YC_2018). Seismic data from the MetMalaysia station KKM are publicly available for download from the IRIS DMC under network code MY (no Digital Object Identifier). Supporting Information S1 datafiles, including cut and bandpass filtered waveforms, can be downloaded from https://doi.org/10.5281/zenodo.6461787. The shear-wave splitting analysis was performed with SplitRacer (Reiss & Rümpker, 2017), which can be downloaded from https://www.geophysik.uni-frankfurt.de/64002762/Software. We have made extensive use of Python (3.8) for our analysis, including the open-source packages: NumPy (1.21.5, Harris et al., 2020); SciPy (1.8.0, Virtanen et al., 2020); Pandas (1.4.1, pandas development team, T, 2021); Matplotlib (3.5.1, Hunter, 2007); and ObsPy (1.2.2, Beyreuther et al., 2010). A number of figures were produced using the Generic Mapping Tools (6.3.0, Wessel et al., 2019). Code for performing the multi-layer anisotropy modelling is available from https://doi.org/10.5281/zenodo.5931586 (Bacon, 2022). All code required to reproduce the visualisations presented in this study can be downloaded from https://doi.org/10.5281/zenodo.6480581.
Supporting Information can be found in the online version of this article.
- seismic anisotropy
- Southeast Asia
- shear-wave splitting
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