SEISMIC EVIDENCE FOR VERY DEEP ROOTS OF CONTINENTS

Abstract

A major problem in geodynamics with seismic data is discussed: How deeply do the continents penetrate into the mantle? Differential travel times of underside reflections from mantle discontinuities that appear as precursors to SS, in large parts of the globe, show a clear correlation with oceans and continents. They are significantly larger beneath the Asian and North American continents than underneath the neighbouring Pacific. From this observation we conclude that the Asian and North American continents affect the mantle well below 410 km. Changes in the thickness of the transition zone can explain our observations, which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities. Average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans. Moreover, our findings imply temperature variations about 100–200 K in the mantle transition zone. Weak reflections from 520 km depth corresponding to an impedance contrast of about 2% can be observed only in some areas of the Earth, while observations from other locations definitively show no signal from this depth. Therefore, we propose that the 520 km reflector is only a regional feature.