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2020-02-06 (Thu) Dr. Hisashi Utada: Pacific Array may reveal the cause of seafloor flattening

Speaker: Dr. Hisashi UTADA (Earthquake Research Institute (ERI), University of Tokyo, Japan. & Tongji University)

Date: 01/16 (Thu) 14:30

Title: Pacific Array may reveal the cause of seafloor flattening

Host: Ban-Yuan Kuo

Abstract: The Pacific Array is an international initiative to deploy an array of 10-20 seafloor geophysical arrays in the entire Pacific Basin, which provides us a new dataset for better understanding the oceanic mantle. Each array study is supposed to be designed to solve a regional geological problem by revealing the mantle structure. At the same time, this attempt of new observation concept, an array of arrays, will provide an opportunity of tackling problems of global scale by using data from entire Pacific Array. The present talk shows a possible research direction of the latter. Although we largely rely on the theory of plate tectonics to explain and understand various tectonic phenomena, there still remain a number of unresolved questions related to its framework. The basic theory of plate tectonics is well accepted and we can find its conceptual picture even in high school textbooks, in which the oceanic lithosphere is often drawn like a plate of constant thickness except near mid-oceanic ridges. This is neither an exaggeration nor a simplification but well reflecting the reality known as seafloor flattening. The feature of seafloor flattening is understood to be a consequence of thermal evolution of the lithosphere-asthenosphere system (LAS) as a general function of seafloor age. The typical feature of flattening can be simply explained by a model of cooling plate with a constant thickness artificially given, but other models are also possible. Moreover, even if we accept the plate cooling model, the physical mechanism of constant thickness is still controversial. This is because the problem has been argued mostly on the basis of near-surface data such as seafloor depth or heat flow. The recent progress in geophysical imaging may provide us an alternative approach to tackle this problem on the basis of deep mantle structure, which is supposed to be controlled by the temperature distribution in the LAS. Here I will present such a new approach to investigate the problem of seafloor flattening, and show ocean bottom geophysical data from the Pacific Array will enable us to answer this question. I also introduce the current status of the Pacific Array.