The Study of Crustal Heterogeneity in Ma River Fault Zone, Vietnam : An Application in Receiver Function

Abstract:

The Indochina block is located at the junction of four plates, which are India, Eurasia, Philippine Sea and Pacific plates, and the geological evolution at this area is also complex. Due to the well-known Red River fault zone play an important role in the evolution of the escape of the Indochina block, the southern part of this fault zone also exist another boundary fault zone, which is called Ma river fault. This area still exists many unknowns in the tectonic evolution. Therefore, this study analyzes teleseismic receiver functions to determine the crustal structure along the Ma river fault, northern Vietnam. We have selected teleseismic events with Mw≥5, and epicenter distance is between 30° and 90° and a Multiple-Taper Correlation (MTC) method is adapted to calculate receiver functions (RFs) for each station. The converted phase, such as P-to-S obtained from RFs, allows us to have insights on the characteristics of crustal structures including the dip of discontinuous interface and anisotropy as well. The above properties have significant effects on amplitudes and arrival time of RFs. Thus, we use Neighborhood Algorithm applied to receiver functions computed from 5 stations has yielded optimum crustal models that include anisotropy and has minimum misfit between the observed RFs and synthetic ones. Our preliminary results indicate that the depth of the Moho discontinuity in the Ma river fault zone is between 25 km and 35 km, the range of S-wave velocity is from 3.6 km/s to 4.5 km/s, and the orientations of crustal anisotropy are related with the local stress status as well. In addition, the low-velocity zone in the lower crust is observed beneath our study area. We hypothesize that the low-velocity zone in the lower crust might be caused by rising of upper mantle, or an extension of the low-velocity zone in the lower crust which is observed beneath the southeastern Tibet and the South China block.