Crustal thickness of the Ontong Java Plateau and deep reflections near the base of its crust

Abstract:

The Ontong Java Plateau (OJP) is one of the largest igneous provinces (LIPs) on Earth (e.g. Coffin and Eldholm, 1994), covering an area five times that of the Japanese Islands. Moreover, studies describing the OJP as “the single largest oceanic plateau” (Taylor, 2006) and “Ontong Java Nui” (Chandler et al, 2012) suggest a much larger “original” OJP. The formation of such a huge feature does not fit plate tectonic theory, and no formation model explains all existing observations from the OJP. Formation of the OJP affected not only the solid Earth, but also the Earth’s environment. To understand its formation and environmental impacts, investigation of the crustal structure of the OJP is important. To date, crustal thickness has been ambiguous because values varied according to survey method. For example, seismic (e.g. Furumoto et al., 1976) and gravity (e.g. Sandwell and Renkin, 1988) methods yielded thicknesses of 35-42 km and 25 km, respectively. A seismic experiment using a large volume seismic source and ocean bottom seismometers (OBS) yielded a Moho depth of 35 km for the southernmost OJP via traveltime forward modeling (Miura et al., 2004), and an inversion analysis (Korenaga, 2011) showed similar results. To better understand the central OJP, we conducted an active source seismic experiment in 2010 using a large volume airgun array and 100 OBSs spaced every 5 km (Miura et al., 2011). Preliminary results with first arrival traveltime tomography and forward modeling have been reported previously (Miura et al., 2013). Recently, we applied traveltime inversion analysis (Fujie et al., 2013), incorporating the uncertainty approach of Korenaga (2011), for velocity structure modeling, especially for the depth of the Moho beneath OJP. Our analysis shows the greatest Moho depth at the center of the OJP. We will report on our latest results and propose a mechanism for formation of the OJP.