UPPER CRUSTAL STRUCTURE ACROSS THE EASTERN LAU SPREADING CENTER USING P-TO-S CONVERTED SEISMIC WAVES

Abstract

Located alongside the active Tofua Arc within the Lau backarc basin, the Eastern Lau Spreading Center (ELSC) appears to generate two types of oceanic crust. Crust formed near the active arc, where the mantle is thought to contain relatively high amounts of slab-derived water, is abnormally thick (8-9 km) and compositionally stratified with relatively evolved lavas and an unusually mafic lower crust. Farther from the arc, where water contents are expected to be lower, the crustal structure is more typical of crust formed at intermediate-to-fast-spreading mid-ocean ridges, and lava samples are dominantly basaltic. In this study, P and S wave data from the L-SCAN active-source wide-angle reflection/refraction experiment are modelled to place additional constraints on upper crustal structure. A combination of ray tracing and finite difference numerical wavefield simulation is used to identify P and P-to-S converted seismic phases. The phases primarily arise from two shallow interfaces, one at only ~80 m depth or less, and the other at 500-650 m depth. The shallower interface is deeper than the sediment base, is observed all across the study area, and is interpreted as a ‘layer 2Aa’ boundary, proposed to result from a step change in crustal porosity. The deeper interface is interpreted as the layer 2A-2B boundary, corresponding to a transition from lavas to sheeted dykes. Layer 2A, on average, is 150 m thicker in crust that formed at the spreading center when spreading was located near the arc, as compared to when spreading was located farther away from the arc. Layer 2A thickness and Vp/Vs values indicate that a thicker and porous lava layer, dominated by basalts to basalt-andesites, cap near-arc crust, while a thinner and less-porous, mostly basaltic, volcanic layer caps the far-arc crust. These results are consistent with the waning influence of slab-derived volatiles on crustal formation as spreading moves away from the active arc.