The Geometry of the Subducted Slab Beneath Sumatra Revealed by Regional and Teleseismic Traveltime Tomography

Abstract

Sumatra is located on the western margin of Sundaland (Figure 1). According to plate reconstruction, Sumatra has been situated at its current location since the Late Jurassic (Hall, 2012). With the closure of the Neotethys Ocean, the intra-oceanic Woyla Arc accreted onto the western margin of Sundaland, forming present-day Sumatra (Advokaat et al., 2018; Hall, 2012, 2015) during the Late Cretaceous. Subduction beneath Sumatra was minimal from the Late Cretaceous to the Early Eocene, and Australia remained close to Antarctica during this period (Hall, 2012). Based on magnetic and deep-sea drilling data, the subduction of the Indo-Australian plate beneath Sumatra commenced at about 45 Ma when Australia separated from Antarctica (Hall, 2012). At that time, oblique subduction began in the Sumatra subduction zone (Beaudry & Moore, 1985; Sclater & Fisher, 1974). Studies on the deposition of sediment offshore of northern Sumatra and in the vicinity of the Sunda Strait indicate that rapid subduction may have begun in the early Miocene (∼20 Ma) (Beaudry & Moore, 1985; Malod et al., 1995). The convergence rate may have peaked in the Pliocene, as was inferred from the intense andesitic volcanism during that period; a high rate of convergence has characterized this subduction zone ever since (Hochstein & Sudarman, 1993). At present, the rate of oblique convergence in the Sumatra subduction zone is approximately 7.2 mm/yr in the south and decreases to 5.2 mm/yr in the north (Figure 1) (Sieh & Natawidjaja, 2000). Moreover, unlike the oblique subduction at the Sumatra trench, the subduction occurring in western Java is normal to the trench axis (Figure 1); consequently, there is an abrupt transition from normal subduction to oblique subduction at the Sunda Strait (Malod et al., 1995). As pointed out by Malod et al. (1995), this abrupt transition is governed by the convergence motion of the Indo-Australian and Southeast Asian plates.

A New Method to Estimate Slab Dip Direction Using Receiver Functions and Its Application in Revealing Slab Geometry and a Diffuse Plate Boundary Beneath Sumatra

“…These dip direction estimates agree well with the Slab2 model (Figures 6 and 7). The continuous variation of slab dip direction along the Sunda trench (Figure 7c) implies slab folding or tearing, also noted in travel time tomography studies (Liu et al, 2019(Liu et al, , 2021. The slab folding may be a result of the oblique subduction of the Australian plate (Liu et al, 2019;McCaffrey, 2009), or the lateral bending of the Australian plate, which is increasingly driven by gravity on the east side of the Wharton fossil ridge as the plate cools down.…”

A Slab Window Beneath North Sumatra Revealed by P‐Wave Mantle Tomography

“…An important clue is the low‐V zone in the subducted slab beneath Nias Island ( L4 in Figures 10a, 10b, and 11), which is confirmed by our extensive resolution tests (Figures 6c and 6d). Previous studies have suggested the presence of an intra‐slab low‐V zone caused by slab tearing under Toba (Hall & Spakman, 2015; Kong et al., 2020; Liu et al., 2021; Toyokuni et al., 2022), implying a somewhat extensive feature. In contrast, the low‐V anomaly L4 appears as an approximate cylinder with a diameter of ∼3° connecting the subslab mantle with the mantle wedge.…”

“…Previous tomographic studies in this region have unveiled the main features of the crust and upper mantle (e.g., Hall & Spakman, 2015;Liu et al, 2021;Toyokuni et al, 2022;Wang, Liu, et al, 2022). However, their low resolution prevented adequate imaging of the detailed structures.…”

“…Previous 3‐D tomographic models of North Sumatra have relatively low resolution, thus providing weak evidence for the slab tearing (Hall & Spakman, 2015; Widiyantoro & van der Hilst, 1997). A recent tomographic study used travel times of local and teleseismic events recorded at 57 stations to investigate the velocity structure beneath the North Sumatra region (Liu et al., 2021). However, their station coverage was still too sparse to capture the fine details of slab tearing beneath the Toba caldera.…”

P-wave velocity and radial anisotropy beneath Sichuan-Yunnan area revealed by teleseismic traveltime tomography

“…We use the teleseismic imaging method based on the eikonal equation to invert the crust-mantle velocity and the radial anisotropy model. The eikonal equation in any heterogeneous medium can be expressed as (Liu et al, 2021):…”