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P-wave velocity structure and deep crustal reﬂections in the central Ontong Java Plateau

The Ontong Java Plateau (OJP) is an elevated expanse of seaﬂoor in the western equatorial Paciﬁc outlined by 4000-m depth contour, and encompasses an area of 1.86 x 10 6 km 2 area (Mahoney et al., 2001), ﬁve times the size of Japan. Although thick crust has been inferred from OJP’s shallow water depths and size, the feature’s crustal thickness and structure have not been accurately determined. The OJP is the largest oceanic plateau on earth and is a typical large igneous province (LIPs) (Cofﬁn and Eldholm, 1994). The formation mechanism of LIPs does not ﬁt plate tectonic theory. No current alternative formation models can explain all of the observations. Understanding LIP formation is important not only for solid earth investigations, but also for environmental studies, because evidence suggests that LIPs may have had major environmental impacts (e.g. Tejada et al., 2009). The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) conducted an active source seismic survey using R/V K a i r e i to constrain the structure in the central OJP (Miura et al., 2011). The new multi-channel seismic (MCS) data show a clear reﬂection boundary about 1 s below seaﬂoor, which is thought to represent the contact between sediment and igneous basement (Mahoney et al., 2001). We observe several deep reﬂections below igneous basement. Two deep and strong reﬂections at 11-13 s and 14-15 s (two-way travel time) are thought to be signiﬁcant reﬂections with respect to OJP’s structure. Ocean bottom seismographic (OBS) data show clear ﬁrst arrival refraction phases at > 300 km offset distances and large amplitude later reﬂection phases. Refraction phases with an apparent velocity of 7 km/s are widely observed, indicating a thick layer. First arrival tomographic analysis reveals the velocity structure down to 40 km below sea level. We also employed a travel time mapping method that reveals deep interfaces using later reﬂection phases. From our analyses, we observe a continuous boundary at about 15 km depth where the P-wave velocity is 6.8-6.9 km/s, i.e., less than 7 km/s. Two continuous boundaries at about 33-35 km and 42-45 km depths are clearly imaged. Depth converted MCS data using OBS tomographic velocities show good agreement in distribution of the two deep reﬂections. Interpretation of the deep reﬂections is a subject for the future. According to Cofﬁn et al. (2006), the crustal structure of oceanic plateaus may be divided in two types according to tectonic setting at the time of formation: off-spreading axis and on-spreading axis. The former case incorporates original oceanic crustal structure. From our data, we cannot identify original oceanic crustal structure in the central OJP; therefore, it may have formed on-axis. In this presentation, we will show further data analyses and interpretations of OJP structure including formation mechanisms.