P and S Wave Anisotropic Tomography of the Banda Subduction Zone

Abstract

In subduction zones with slab‐slab interactions, the pattern of mantle convection is very complex and still unclear. In this study, we jointly invert a large number of P and S wave arrival time data of local earthquakes for 3‐D isotropic and anisotropic velocity structures of the Banda subduction zone. Along the curved Banda arc, the subducting Indo‐Australian slab is detected clearly as a high‐velocity zone, and its azimuthal anisotropy changes along the arc strike, representing fossil anisotropy within the slab and modified anisotropy by the subduction processes. Around the northern edge of the Banda slab, a semi‐toroidal pattern of anisotropy appears in low‐velocity anomalies, representing mantle flow extruded from the Banda arc and escaped from a gap of the Banda‐Molucca slab toward the northeast. Our 3‐D anisotropic tomography uncovers the mantle convection pattern induced by the slab‐slab interactions, shedding new light on the complex dynamical processes in this curved subduction zone.

Big Mantle Wedge and Intraplate Volcanism in Alaska: Insight From Anisotropic Tomography

“…Recently, a novel tomographic technique has been developed to reveal tilting‐axis anisotropy, with the HSA freely orientated in 3‐D space (Z. Wang & Zhao, 2021). So far, this type of techniques has successfully revealed the presence of tilting HSA beneath the Japanese region (Z. Wang & Zhao, 2021) and its forearc area (Z. Wang et al., 2022), northern Fennoscandia (Munzarová et al., 2018), Central Mediterranean (Rappisi et al., 2022), and Cascadia subduction zone (Liang et al., 2023). These studies have demonstrated that the tilting‐axis anisotropy can reveal more complex 3‐D mantle flow by reconciling the contradictory assumptions of azimuthal and radial anisotropies.…”

Tilting‐Axis Anisotropic Tomography and Subduction Dynamics of the Java‐Banda Arc

“…One explanation of the intraslab anisotropy is preferred orientation of hydrated faults, which are developed in the upper part of slab due to plate‐bending prior to subduction (Faccenda et al., 2008, 2009). The trench‐parallel faults can cause trench‐parallel FVPs in the upper portion of the subducting slab, which has been observed in other subduction zones (e.g., Han et al., 2016; Liang et al., 2023; Wang, Zhao, & Chen, 2022). However, our Vp tomographic results indicate that the anisotropy in most parts (Regions E, G, and H) within the subducting Australian slab does not seem to match the characteristics of the FVPs caused by the hydrated faults, except for the local area between 120°E and 128°E beneath Timor Island (Region F).…”

“…Shear‐wave splitting measurements (Wang & He, 2020) and azimuthal anisotropic tomography (Hua et al., 2023) have observed that FVDs in the back‐arc region rotate from trench‐normal in the west (Java) to trench‐parallel in the east (Banda), reflecting a complex mantle flow pattern associated with the slab geometry. Under the assumption of a tilting hexagonal symmetry axis, the 3‐D Vp anisotropic model we obtained reasonably matches the features reported by the previous studies (Hua et al., 2022; Wang & He, 2020) and provides additional information that was not reported before, because we are able to better constrain 3‐D mantle flows and intraslab anisotropy.…”

“…It is crucial to investigate how its great curvature formed and how the related tectonic evolution in this complex subduction system actually proceeds, which can improve our understanding of mantle dynamics. To date, many studies have been made to investigate the structure and tectonics of SE Asia through various approaches such as geodetic measurements (Hinschberger et al., 2005; S. Zhao et al., 2023), geological surveys (Earle, 2023; Harris, 2006), and seismological investigations (Hua et al., 2023; Richards et al., 2007). However, there are two contrasting hypotheses about the cause of the curved arc: two different slabs subducting toward each other (Cardwell & Isacks, 1978; Das, 2004) or folding of one single slab (Pownall et al., 2016; Spakman & Hall, 2010).…”

“…For instance, fast velocity directions (FVDs) of azimuthal anisotropy in the Banda back‐arc mantle change gradually from nearly trench‐parallel to trench‐oblique (Wei et al., 2022), reflecting squeezed asthenospheric flow by the highly arcuate shape of the subducting slab. A semi‐toroidal anisotropic pattern was revealed around the northern edge of the Banda slab (Hua et al., 2023), probably reflecting extruded mantle flow.…”