Evidence for a rheologically strong chemical mantle root beneath the Ontong-Java Plateau

Shear wave splitting measurements, in conjunction with studies of sheer wav e velocity structure, indicate that the Ontong-Java Plateau (OJP) large ign eous province (LIP) has a thick, compositionally distinct root that diverts asthenospheric mantle flow beneath the Pacific plate. The OJP, the largest of Earth's LIPs, stands 2 km above adjacent Pacific abyssal plains and is composed of mantle plume derived volcanics erupted at 122 and 90 Ma. Surfac e wave tomography of the Plateau reveals a seismically slow upper mantle ro ot that extends approximately to 300 km depth. The thickness and juxtaposit ion of the Plateau and the mantle root imply that the OJP is the preserved 'head' of a rising mantle plume formed in situ when the LIP erupted. Thus, it is a far-traveled body currently: moving northwestwards with the Pacific plate. Shear wave splitting at four seismic stations along the northern ma rgin of the OJP varies systematically: the fast asis of seismic anisotropy at three stations on the NE OJP margin trend NW, parallel to hotspot-define d Pacific absolute plate motion; at a fourth station? on the NW margin of t he Plateau, the fast shear wave trend is NE. Upper mantle flow directions d elineated by the sheer wave splitting could thus represent mantle flow dive rted around the leading, northwestern face of the theologically strong, che mically distinct OJP root. In sum, the Plateau and its deep root appear to be similar to continental tectosphere, except for contrasting seismic veloc ities. (C) 2001 Elsevier Science B.V. All rights reserved.