PP-PRECURSOR STUDY OF LITHOSPHERIC AND UPPER-MANTLE DISCONTINUITIES BENEATH THE LINE ISLANDS OF THE SOUTH PACIFIC

PP and SS bounce point precursors have long been used to image the 410 and 660 km discontinuities beneath ocean regions but were not useful for shallow depths because noise in these data required application of low-pass filters. Array processing of the EarthScope transportable data improves the frequency content to 4Hz. We interpret the upper mantle beneath the Line Islands (LI), a chain of islands north of French Polynesia, as a function of distance to the LI using images produced by stacking PP data in 0.5 degree-wide lines parallel to the LI.

The 520 is observable beneath this region and is shallowest beneath the LI. To the SW of the island chain, the 410 is apparent at 430 km, then shallows by 6 km toward the NW. A weak 390 appears anti-correlated with the 410. A negative phase underlies the relatively flat 90-km-deep positive phase at the base of the lithosphere. A negative phase observed at ~80 km depth at the SW and NE ends of the profile shallows to ~60 km beneath the LI, suggesting the presence of either a lens of partial melt or depleted mantle due to past melt. The feature is thickest beneath the island chain. A positive phase (~50 km deep beneath the LI) lies above and parallel to this negative anomaly. A second lens of negative phases appears at a depth of 40 km to the SW but pinches out just north of the LI. We believe this to be the result of partial melt that accumulated beneath a semi-rigid layer (visible as a positive phase at a depth of 35 km beneath the LI). Vestiges of the melt appear to have collected SW of the LI, consistent with the presence of volcanic islands SW of the LI. The LI may have formed as a result of magma rising along a NW/SE-oriented line of weakness in the upper lithosphere. This may have prevented partial melt from migrating to the NE of the LI, as evidenced by a lack of volcanic islands to that side of the island chain. We observe the base of the crust at nominally 15 km but deepening to 20 km beneath the LI.