Eastward Mantle Flow Field Underneath East Asia Quantified by Combining Shear-wave Splitting Orientations and Absolute Plate Motion Observations

Abstract:

Tractions due to mantle flow underneath plates are one of the main factors to be considered in force balance calculations, and a long-standing debate has been if those tractions “lead” (i.e., “drive”) or “trail” (i.e., “resist”) plate motion. Unfortunately, the mantle flow field, particularly its magnitude, cannot be observed directly. We quantified this sub-asthenospheric mantle flow field underneath East Asia by combining shear-wave splitting orientations (SKS/SKKS/PKS, referred to as SXS) and absolute plate motion (APM) observations. In our analysis, we firstly compared different APM velocities with SXS splitting orientations. We found a significant average misfit of larger than 24°, which suggests the present of sub-asthenospheric mantle flow which needs to be considered if one wants to understand the SXS splitting orientations beneath East Asia. Secondly, we quantified this flow field following Silver and Holt (2002) method, who assumed that asthenospheric anisotropy can mainly be attributed to the differential motion between lithosphere and sub-asthenospheric mantle, after considering 180° ambiguity in the SXS splitting orientations. We inferred an eastward mantle flow field of 1-2 cm/a, which is similar to flow driven by lateral density anomalies at the base of the asthenosphere. Our flow field is similar in direction but ~1 cm/a faster than lithosphere velocity, which suggests that sub-asthenospheric mantle flow drives Eurasia plate motions via exciting active tractions at the base of the lithosphere. However, below the Tibetan Plateau, the direction of the sub-asthenospheric mantle flow relative to the surface is orthogonal to the SXS splitting orientations; this remarkable pattern may need to be considered when explaining anisotropy in the Tibetan lithosphere.