Seismic evidence of the lithosphere-asthenosphere boundary beneath the Tonga area, southwestern Pacific

Abstract

We study the lithosphere-asthenosphere boundary (LAB) in the Tonga subduction zone using four deep focal events recorded by the Capital Seismic Network (CSN) of China and the Northeast China Extended Seismic Array (NECESSArray). The existence and depths of the LAB beneath the Lau Ridge are clearly revealed by the linear slant stacking of observed sP precursors (sdP). As illustrated by the reflected points of robust sdP phases from LAB, the depth of LAB in the northern Lau Ridge is about 63km with a range of 63–64km, in the northwest is about 77km with a range of 76–78km and in the south is about 72km. The uncertainty of determined depths is no more than 4km given the effects of the picking errors of arrival times and velocity heterogeneities. Our results suggest that the oceanic lithosphere of the Lau Ridge is thinner in the northern part than in the northwestern and the southern parts. According to the previous geodynamic and petrological studies, the oceanic lithospheric thinning in the northern Lau Ridge should be related to the strong erosion of the active mantle convection in the back-arc mantle wedge with the enrichment of the volatiles and melts.