UNDERSTANDING SEISMIC HETEROGENEITIES IN THE LOWER MANTLE BENEATH WESTERN PACIFIC FROM SEISMIC TOMOGRAPHY AND TECTONIC PLATE HISTORY

We combine results from seismic tomography and plate motion history to investigate the structure and evolution of slabs of subducted lithosphere in the lower mantle beneath western Pacific. We selected earthquakes and stations along a wide corridor from the Aleutians to Australia. We measured P wave differential travel times by waveform multi-channel phase cross-correlation (MCPCC). MCPCC enables to measure differential travel times without selecting a reference trace. We used phase cross correlation rather than standard cross-correlation as it is better suited to measure differential travel times on the first onset even if the first onset is not the most energetic peak. This dataset was complemented by ISC data and inverted to obtain a high resolution P wave velocity model.

Paleomagnetic data were used to reconstruct the plate motion of western Pacific from 130 My ago to present. 130 My ago, the Panthalassa Ocean was made of 4 plates: the Pacific plate surrounded by the Izanagi, Farallon and Pheonix plates. The Izanagi plate subducted beneath Eurasia until 60 My ago and then disappeared in the mantle. Australia and India plate moved toward the North since 70 My and the collision between India and Asia occurred about 30 My ago. This northward motion induced the East-West opening of the West Philippine Basin and then its rotation clock-wise between 50 and 40 My ago. This basin is now called the Philippine plate. The plate history is compared to the tomographic model in order to identify in the lower mantle slab fragments of the different plates and unravel subduction history over the last 130 My.