Expedition 502

The scientific objective of IODP³ Expedition 502 was to explore the nature of the acoustic basement in the outer rise area of the NW Pacific subduction system, where pelagic sediment of the subducting old (120–130 Ma) Pacific Plate is exceptionally thin. The hypothesis is that the acoustically thin sediment cover can be attributed to basalt sill intrusions or sheet lava extrusions in and on the pelagic sediment sequence by petit-spot basaltic magmatism. This hypothesis will be tested to see if petit-spot magmatism at the outer rise is more widely distributed than previously thought. The existence of widely-distributed petit-spots would change our understanding of subduction systems, including nucleation of megathrust earthquakes.

We successfully drilled down ~100 m beneath the seafloor, under a water column of about 5500 m! There were three drill locations, and the sediment-basalt boundary was captured at all three. Unfortunately, the extraordinary freshness of the basalts, which also had very low porosity, wore down the drillbits rapidly and we were unable to reach the original target depth of 160 m. Nevertheless, preliminary results by the paleomagnetism and micropaleontology teams have already given an estimate of the age of the overlying sedimentary sequence, and further analyses in the lab will shed light on the nature of the basalt samples.