ALBERT

Description: The Indochina block is important to our understanding of the extrusion model as a consequence of the Indo-Eurasia collision. The lithospheric structure of this block, however, remains obscured due to a lack of sufficient instrumentation for high resolution seismic imaging. We present a shear velocity model derived from Rayleigh wave phase velocity tomography using data from recently deployed seismic networks in this region. Our inversion results for lithospheric structure show strong correlations with tectonic history in this block. A prominent slow-velocity anomaly (5 per cent) is observed in northern Indochina along the Ailao Shan-Red River (ASRR) shear zone including Chuxiong basin, Lanping-Simao fold belt and Thailand rift basin, which has seen extensive deformation events since Eocene. The Khorat Plateau basin is characterized by thick continental keel type lithosphere, consistent with palaeomagnetic and geological observations indicating this basin has experienced much less deformation than the surrounding regions. Additionally, our inversion imaged a sharp, lithospheric-scale velocity contrast across the southeastern segment of ASRR, indicative of a thin and thus relatively weak lithosphere southwest of Red River Fault. The thin lithosphere, low asthenospheric seismic velocities we observe and the average crustal thicknesses in the region suggest that the topography high is dynamically supported by upwelling asthenosphere rather than thickening of the crust/lithosphere. Based on the occurrence of Palaeogene volcanism and its timing, we prefer an explanation of thinning of the lithosphere and allowing a throughgoing fault rather than emplacement of a thin terrane to explain the thin lithosphere. Therefore, the anomalously thin lithosphere between Khorat Plateau and the ASRR in conjunction with other geological observations is generally consistent with the extrusion model for Indochina, which requires localization of lithospheric deformation around tectonic blocks.