Characteristics of the Mantle Flow System Beneath the Indochina Peninsula Revealed by Teleseismic Shear Wave Splitting Analysis

Abstract

Numerous geoscientific investigations have been conducted on the southeastern Tibetan Plateau and adjacent areas for understanding crustal and mantle deformation associated with the indentation of the Indian Plate into Eurasia. A number of key issues, such as the causes of a sudden change of fast polarization orientations from N‐S to almost E‐W at approximately 26°N revealed by shear wave splitting (SWS) studies, and the geodynamic implications of the transition still remain enigmatic, partially due to the lack of sufficient SWS measurements on the Indochina Peninsula. Here we employ the SWS technique to systematically illuminate upper mantle anisotropy beneath the Indochina Peninsula with an unprecedented data coverage. The resulting 409 SWS measurements from 29 stations show that upper mantle anisotropy beneath the vast majority of the study area is characterized by dominantly E‐W fast orientations which are nearly orthogonal to the strike of most of the major tectonic features in the study area, ruling out significant lithospheric contributions to the observed anisotropy. This observation, when combined with results from seismic tomography, numerical modeling, surface movement, and focal mechanism investigations, suggests that the observed azimuthal anisotropy is mostly the consequence of absolute plate motion or the westward rollback of the oceanic Indian slab. The flow system induced by the rollback or absolute plate motion may experience regional alteration from mantle upwelling along the eastern edge of the slab and through a previously detected slab window, leading to local variations in the observed splitting parameters.

Seismic Anisotropy in the Java‐Banda and Philippine Subduction Zones and its Implications for the Mantle Flow System Beneath the Sunda Plate

“…The summary of δt is presented in Figure S1. The resulting average δt is slightly smaller than the average value (1.25 s) from oceanic islands calculated by Király et al () from the Global Splitting Database (http://www.gm.univ-montp2.fr/splitting/) (Wustefeld et al, ) but which is in reasonable agreement with previous local studies, for example, 0.90 ± 0.14 s in the southernmost Indochina Peninsula (Yu et al, ), ∼1 s in the Sumatra wedge (specifically, measurements are made for the stations located above the 150‐km slab contour) (Hammond et al, ), and 0.34–1.08 s in the Banda wedge measured from local S splitting (Di Leo et al, ). The larger δt (of 1.6–2.4 s in Sumatra, Collings et al, ; ∼1.6 s in Java, Hammond et al, ; and 1.13–1.38 s in Banda, Di Leo et al, ) has been primarily attributed to subslab mantle flow, rather than the flow in the mantle wedge, and the larger δt all comes from SKS splitting measurements.…”

“…The lateral flow developed by highly arcuate subducting slab in Banda and trench‐oblique flow caused by the eastward subduction of the South China Sea Plate in Philippines will additionally contribute to the flow systems. Moreover, other flows with different geometry will certainly serve as important components in the flow systems, including nearly trench‐normal‐oriented flow (Figure ) (more likely a 2‐D corner flow) induced by the eastward subduction of the Indo‐Burman plate observed beneath the southern Indochina Peninsula (Yu et al, ); a 3‐D toroidal flow around lateral slab edges of the Celebes Sea slab suggested by Di Leo et al (); and a 3‐D toroidal flow in the Sumatra mantle wedge associated with slab tear observed by Huang et al (). The subslab mantle flow around the highly arcuate Banda slab, if it exists, may also contribute to the flow systems beneath the Sunda Plate.…”

“…Comparison of sS splitting observations in this study (blue bars) and previous SKS splitting results. Bars represent orientation of the fast direction, and its color represents different studies (lavender for SKS results of Di Leo et al, ; green for SKS results of Hammond et al, ; orange for SKS results of Yu et al, ; and cyan for fast SV directions of Debayle et al, , at a depth of 100 km). Their lengths are proportional to delay times, and the length of the cyan bars is proportional to the maximum amplitude of azimuthal anisotropy at this depth.…”

Mantle Flow and Deforming Continents: From India‐Asia Convergence to Pacific Subduction

“…The fit is however poor (red symbols) below Southeast Asia and the central and West Mediterranean realm. A more recent investigation over Indochina (Yu et al, 2018) confirms this poor fit. When using SL2013sv's seismic anisotropy inference instead (Figure 4), the quality of the match depends upon the depth considered.…”

“…This may imply that SKS-splitting anisotropy, as it corresponds to local measurements, shows here a smaller-scale flow that is not captured by the smooth SL2013sv model. The direction of this flow could be toward the west or southwest, corresponding to a tighter counter flow around the East Himalayan syntaxis toward the Burma slab as proposed by Wang et al (2013) and Yu et al (2018). At 100 km, the flow geometry (or of shearing below the lithosphere) indicated by SL2013sv anisotropy below Asia can otherwise be compared with the model of Sternai et al (2016;Figure 4a, inset) which was designed to study the 3-D interactions between slab retreat and collision for the India-Asia collision.…”