P wave tomography and anisotropy beneath Southeast Asia: Insight into mantle dynamics

Abstract

Southeast Asia is surrounded by subduction zones resulting from the interactions of several lithospheric plates. Its evolution has been also influenced by active tectonics due to the Indo‐Asian collision in the Cenozoic. In this study, we use a large number of arrival‐time data of local and regional earthquakes to determine 3‐D P wave tomography and azimuthal anisotropy in the mantle beneath SE Asia. High‐velocity (high‐V) anomalies representing the subducting slabs are clearly visible in the upper mantle and the mantle transition zone (MTZ). Low‐velocity (low‐V) zones with trench‐normal anisotropy are revealed in the uppermost mantle, which indicate back‐arc spreading or secondary mantle‐wedge flow induced by the slab subduction. In contrast, trench‐parallel anisotropy dominates in the deep upper mantle and reflects structures either in the subducting slab or in the upper mantle surrounding the slab. The trench‐parallel anisotropy is also significant in the lower MTZ, which may contribute to shear wave splitting observations. A low‐V body extending down to the lower mantle is visible under the Hainan volcano far away from the plate boundaries, suggesting that Hainan is a hot spot fed by a lower‐mantle plume. The low‐V body under Hainan is connected with low‐V zones in the upper mantle under SE Tibet and Vietnam. Our P wave anisotropy results reflect significant mantle flow existing in the asthenosphere from SE Tibet to Hainan and further southwestward to Vietnam. The present study, especially the 3‐D P wave anisotropy results, provides important new insight into mantle dynamics in SE Asia.

Evolution of the Southern Segment of the Philippine Trench: Constraints From Seismic Tomography

“…The Celebes Sea slab is visible down to ~150‐km depth along the Cotabato and Sulawesi trenches in cross sections AA′, BB′, and CC′ (Figures a–c), indicating a subduction duration of ~7 Myr assuming a constant convergence rate of ~35 mm/year and a low dip angle of ~35°, which is consistent with the Late Miocene to Pliocene age of the Cotabato Trench (Yumul et al, ). The Molucca Sea slab may have subducted down to ~650‐ and ~150‐km depths along the Sangihe and Halmahera trenches, respectively (Figures a, b, and d), in accordance with the previous tomographic results (Hall & Spakman, ; Huang et al, ). The high‐V anomalies associated with the subducted Philippine Sea slab are traceable to depths of 450, 550, and 600 km with an overturned dip angle in cross sections AA′, BB′, and CC′, respectively (Figures a–c), being consistent with the tomographic results of Fan et al ().…”

Teleseismic P‐wave tomography and mantle dynamics beneath Eastern Tibet

“…Extrusion of the low‐V and low‐density materials could uplift the topography around the Longmenshan fault zone. Such a flow model is clearly shown in Figures b and 13, which is supported by results of azimuthal anisotropy inferred from SKS splitting [e.g., Lev et al ., ; Wang et al ., ; Li et al ., ], surface wave tomography [e.g., Yao et al ., ], and P‐wave anisotropic tomography [e.g., Liang et al ., ; Wei et al ., ; Lei et al ., ; Huang et al ., ].…”

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

“…A similar slow shear wave velocity has been observed by many investigators in the same region [e.g., Huang et al ., ; Yao et al ., . Lei et al ., ; Huang et al ., ]. The strength and size of this slow velocity zone increase with depth and it terminates at 27°N and does not present at the Tibetan Plateau (Figures f–h).…”

“…This thermal erosion is probably confined in the area near the Tibetan border and in the shallow part (<70 km) below which the fast cratonic lithosphere is observed. The anomalous low‐velocity zone (LV3) in the EYC is also observed in P wave travel time tomography [ Lei et al ., ; Li et al ., ; Huang et al ., , ], which connects the low‐velocity zone beneath the Tengchong Volcano and the NIB in the south of our study region. The low to high low‐velocity pattern from NIB and WYC to EYC in our model could result from small‐scale convection in the mantle wedge above the subducted Burma slab, which was imaged as high‐velocity anomaly beneath the Burmese arc [ Li et al ., ].…”