AN INVESTIGATION OF THE UPPER MANTLE AND LITHOSPHERE BENEATH THE HAWAIIAN ISLAND CHAIN USING PP BOUNCE POINT PRECURSORS

PP bounce point precursor data provide valuable information about oceanic mantle structure where few seismic stations are available. The plethora of data from EarthScope transportable array (TA) allows array processing, resulting in images with higher frequency content, up to 4 Hz. The improved frequency content means it is possible to image mantle structure to shorter wavelengths, enabling investigation of the lithosphere. under the Hawaiian island hotspot chain (HIC).

A profile perpendicular to the island chain is produced by stacking PP data along 0.5 degree wide stripes parallel to the island chain. A HIC average stack was produced with the chain in the center but the same data is restacked in 12 subdivisions of the original profile to produce 12 images perpendicular to different aged islands. A similar profile was constructed using data around an area further north that has extremely limited hotspot interference (referred to as control section).

Discontinuities are observed directly beneath the HIC at around 70 km and 110 km, which shallow by around 20 km. At around 40 km, a negative discontinuity (implying a velocity inversion), is present south of the hotspot but disappears near it. A slightly shallower, (~35 km) positive anomaly is similarly present to the north of the HIC but not to the south. A possible explanation for this is the melt which pushed up through the lithosphere when the islands were formed eventually pooled at this depth to the south but not to the north. Deeper layering can be seen in our control section (around 165, 215, 235, 260, 270, 290, 320, and 380, shallowing by 10 km away from the hotspot), but these layers progressively disappear beneath the islands. The cause of this may be the partial melting and mixing of hot or molten rock at hot spot causing the layering to be destroyed beneath the HIC by rising melt. A deep 410 km discontinuity was clearly present at around 420 km to 430 km.