Distinct Variations in Seismic Velocity Structure of the Crust and Upper Mantle across the Ailao Shan-Red River Shear Zone in Northern Vietnam
Abstract
The tectonic evolution of the Aliao Shan-Red River shear zone (RRSZ) that runs from southeast Tibet through North Vietnam to the South China Sea and marks the boundary between the Indochina and South China blocks has been considered closely linked with the northward indention of the strong Indian plate into the Eurasian continent and the consequent uplift of the Tibetan Plateau. A variety of models have been proposed to explain the postcollisional deformation and magmatism of SE Asia and movement along the RRSZ. Since December 2005, Institute of Earth Science, Academia Sinica of Taiwan has deployed a regional broadband array with station spacing of ~50 km in Northern Vietnam for earthquake and seismic structure studies. We collect data from earthquakes with magnitude≥5.5 and epicentral distances of 30-90o between December, 2005 and June, 2008. Using this new dataset, we report 3-D variations of P- and S-wave speeds (δlnVP and δlnVS) and Poisson's ratios via δln(VP/VS) in the crust and upper mantle across the shear zone, obtained with tomographic inversion of P and S relative travel time residuals measured by inter-station cross-correlation of waveforms at both high- and low-frequencies. We employ physically realistic 3-D sensitivity kernels for frequency-dependent traveltime data and data-adaptive, multi-scale parameterization in the inversion. The resulting models reveal noticeable differences across the RRSZ, where the anomalies of distinctly low VS and VP/VS are widely-dispersed in the lower crust and uppermost mantle down to the depth of 100 km to the southwest of the RRSZ. This may indicate that ductile crustal mass has flowed out of Tibet into Indochina accompanying extrusion of relatively hot lithospheric mantle along the RRSZ related to Late Cenozoic volcanism in the region. Though less distinct in the S velocity model, an elongated fast anomaly about 60 km wide that strikes parallel to the RRSZ and subvertically extends to the depth of 60 km clearly emerges in VP. Such narrow high-wavespeed feature indicates the RRSZ may be strong and probably did not cut through the entire lithosphere. Besides, a prominent, localized low velocity region with positive free-air gravity anomalies is present in the crust between the RRSZ and subparallel Song-Ma suture to the south. The characteristics of low seismic velocity and high density suggest that it may originate from the Emeshian flood volcanism being offset along the RRSZ of several hundred kilometers. Moreover, two pronounced, larger-scale low-velocity anomalies are distributed at depths greater than 100 km. One is located to the south of the Tonkin Gulf, and the other situated in Guangdong and Guangxi Provinces, Southeast China. The latter is correlated with the deep, low-velocity anomaly found in the proximity of Hainan, which has been previously reported in global tomographic models.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T33B2414H
- Keywords:
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- 7203 SEISMOLOGY / Body waves;
- 7270 SEISMOLOGY / Tomography;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8178 TECTONOPHYSICS / Tectonics and magmatism