Mantle flow in regions of complex tectonics: Insights from Indonesia

Abstract

[1] Indonesia is arguably one of the tectonically most complex regions on Earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion. It is thus an ideal location to study the interaction between subducting plates and mantle convection. Seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes, such as mantle flow, for example. Here, we present novel shear wave splitting results across the Indonesian region. Using three different shear phases (local S, SKS, and downgoing S) to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features. For example, the block rotation history of Borneo is reflected in coast-parallel fast directions, which we attribute to fossil anisotropy. Furthermore, we are able to unravel the mantle flow pattern in the Sulawesi and Banda region: We detect toroidal flow around the Celebes Sea slab, oblique corner flow in the Banda wedge, and sub-slab mantle flow around the arcuate Banda slab. We present evidence for deep, sub-520 km anisotropy at the Java subduction zone. In the Sumatran backarc, we measure trench-perpendicular fast orientations, which we assume to be due to mantle flow beneath the overriding Eurasian plate. These observations will allow to test ideas of, for example, slab-mantle coupling in subduction regions.

Mid-mantle seismic anisotropy beneath southwestern Pacific subduction systems and implications for mid-mantle deformation

“…For Sumatra and the Philippines, our results are also generally consistent with previous work by Di Leo et al (2012) and Nowacki et al (2015). We document very similar delay times in these systems; furthermore, we observe similar trench oblique and trench-perpendicular fast directions in the Philippines and western Sumatra respectively as documented by Di Leo et al (2012) and Nowacki et al (2015).…”

“…10. Following our own previous work and that of others (e.g., Russo and Silver, 1994;Di Leo et al, 2012;Nowacki et al, 2015), we interpret our measurements as mainly reflecting anisotropy in the mid-mantle near the earthquake Foley and Long (2011) and Lynner and Long (2015). Beneath Tonga, the dominant observed fast directions are mainly trench-parallel for both up-dip and down-dip ray paths.…”

“…Well-documented patterns of mid-mantle anisotropy near subducting slabs would provide valuable constraints on patterns of deformation in the mid-mantle and could potentially constrain global models for mantle convection and material transport. Here we build on previous work in Tonga and elsewhere (e.g., Foley and Long, 2011;Di Leo et al, 2012;Kaneshima, 2014;Long, 2014, 2015;Nowacki et al, 2015) and present measurements of source-side shear wave splitting from deep earthquakes originating in the Tonga-Kermadec, Sumatra, New Britain, New Herbides, and Philippines subduction systems. We interpret these measurements as reflecting anisotropy in the transition zone and uppermost lower mantle.…”

“…Several studies have documented seismic anisotropy at mid-mantle depths using both body waves (Wookey et al, 2002;Chen and Brudzinski, 2003;Wookey and Kendall, 2004;Foley and Long, 2011;Di Leo et al, 2012;Lynner and Long, 2014;Nowacki et al, 2015) and surface waves (Trampert and van Heijst, 2002;Yuan and Beghein, 2013), although they are not plentiful compared to studies of upper mantle anisotropy. Global models by Trampert and van Heijst (2002) using dispersion measurements of Love wave overtones suggested the global presence of azimuthal anisotropy in the transition zone.…”

The Use of Direct Shear Waves in Quantifying Seismic Anisotropy: Exploiting Regional Arrays

“…Previous attempts to measure splitting parameters from direct shear waves depended on either the analysis of deep focus events, which are thought to be free from source-side effects (Savage et al, 1990;Fischer and Yang, 1994;Long and van der Hilst, 2005) or an appropriate correction for source-side anisotropy Anglin and Fouch, 2005). However, deep earthquakes are only found in a few selected subduction zones and are possibly subjected to both slab and subslab region anisotropy represented by delay times more than 1 s (Wookey et al, 2002;Di Leo et al, 2012). Thus, ignoring source-side effects for such events can be misleading.…”