[S-IT30] Tectonic collision systems in continents and oceans

[SIT30-P05] Seismic anisotropy in the deep continental upper mantle beneath Tibetan Plateau

*Cheng-Chien Peng^1,2, Ban-Yuan Kuo², Eric Debayle³, Ling-Yun Chiao¹ (1.IONTU Institute of Oceanography National Taiwan University, 2. Institute of Earth Sciences Academia Sinica, 3.Laboratoire de Géologie de Lyon, Université Claude Bernard Lyon 1)

Keywords:seismic anisotropy, upper mantle, Tibetan Plateau, Rayleigh wave tomography

Multimode Rayleigh wave tomography (Pandey et al., 2015) shows a depth-dependent azimuthal anisotropy in the Tibetan Plateau which may involve different stress field orientations in the lithosphere and the underlying mantle. The azimuthal anisotropy model could explain some characteristics of SKS splitting, but fails to predict the large delay time in central Tibet, probably owing to the lack of sensitivity of Rayleigh waves in the deep upper mantle. To reconcile the discrepancy in delay time between the model prediction and the SKS data, we search for new anisotropy parameters of the upper mantle layer (200~250 km) of the model to fit the SKS observations. In this process, we fix the upper layers of the Rayleigh model assuming they were well constrained. The best fit result shows that changing orientation slightly (<30°) and increasing anisotropy intensity to ~10% in the upper mantle layer can improve the SKS data fitting remarkably. Because 10% anisotropy intensity is unlikely within the 50 km thickness layer, we suggest that this anisotropy layer extends to deep upper mantle (>250 km) beneath the Tibetan plateau. This modified region is roughly beneath the north of the Bangong-Nujiang suture zone that coincides with the northern front of the underthrusted Indian lithosphere. The mechanisms that may cause this thick anisotropy layer in the mantle is under investigation.