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Crustal velocity structure in Borneo Island using receiver function inversion

Syuhada Syuhada, Pranata Bayu, Anggono Titi, Ramdhan Mohamad, Zulfakriza Zulfakriza, Febriani Febty, Prasetio Aditya D., Dewi Cinantya N., Hasib Mohammad, Sulaiman Albertus

Acta Geophysica

2022

Vol. 70, no 6

2529--2553

Borneo is an island situated in a tectonically complex region and characterised by multiple arcs and continental blocks accreted during the Mesozoic and early Cenozoic. We analyse receiver functions of teleseismic events from 12 seismic stations around the island. In general, these stations sample a variety of geological environments, including Meratus Complex, Eastern Borneo, South–Western Borneo, North–Western Borneo, and Sabah Zones. We then derive the shear wave velocity models from the inversion of receiver functions using the stochastic non-linear approach. Inversion results indicate that the island is covered by sedimentary layers with thickness ranging from 1 to 3 km thick. The inversion solutions for most stations also show that the crustal thickness varies between 26 and 36 km around the region. The variation in the average crustal Vp/Vs values obtained for each seismic station addresses the geological diversity of the study area. Furthermore, the lowvelocity zone with high and low Vp/Vs in the lower crust observed beneath some seismic stations may be associated with the tectonic evolution and development of Borneo Island. The results inferred from our inversion are generally consistent with other previous geological and geophysical studies conducted in this region.

Lithospheric mantle anisotropy from local events beneath the Sunda–Banda arc transition and its geodynamic implications

Syuhada Syuhada, Hananto Nugroho D., Abdullah Chalid I., Puspito Nanang T., Anggono Titi, Febriani Febty, Soedjatmiko Bogie

Acta Geophysica

2020

Vol. 68, no. 6

1565--1593

Shear wave splitting analysis to characterise lithospheric mantle anisotropy has been performed to provide better knowledge about lithospheric deformation and mantle fow beneath the Sunda–Banda arc transition, Indonesia. The tectonic setting of the study area is very complex characterised by the transition from subduction along Sunda arc to collision in Banda arc. The splitting measurements show lateral and vertical variation in the fast directions of the S-waves in this region. When the splitting results are analysed through 2D delay-time tomography and spatial averaging, systematic patterns in delay times and fast polarisation become more visible. In the subduction domain, the spatial averages of fast directions are dominated by two distinct fast polarisations: perpendicular and parallel to the plate motion for shallow and deep events, respectively. The results suggest that anisotropy in this area is not only controlled by anisotropic source related to the simple mantle fow model, but also by anisotropic fabric in the mantle deformed under infuence of high stresses, high water contents and low temperatures. In addition, there might also be contribution from the anisotropic body in the upper layer. In the collision domain, spatially averaged fast directions show mostly perpendicular to the plate motion for all deep levels. For shallow level in this region, this trend is mainly governed by the lithospheric deformation process due to the continent-arc collision as also shown by delay time tomographic inversion. For deeper part of the region, the result of tomographic inversion and spatial averaging reveals a high anisotropy followed by rotational pattern of fast directions in the north of Timor. We suggest that this pattern might be related to the induced mantle fow due to lateral tearing of the slab.