Uppermost crustal structure across the eastern Lau spreading center from P-to-S converted waves
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ORIGINAL RESEARCH PAPER
Uppermost crustal structure across the eastern Lau spreading center from P‑to‑S converted waves Charu Lata1 · Robert A. Dunn1 Received: 21 March 2020 / Accepted: 12 October 2020 / Published online: 2 November 2020 © Springer Nature B.V. 2020
Abstract P and S wave data from the L-SCAN active-source wide-angle reflection/refraction experiment are modelled to investigate upper crustal structure in the Lau backarc basin. 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 ~ 80 m depth or less, and the other at 500–650 m depth. The shallower interface is deeper than the sediment base, is observed across the study area, and is interpreted as a ‘layer 2Aa’ boundary, proposed to result from a rapid change in crack density. 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 ( 70 km away). Layer 2A thickness and Vp/Vs values indicate that a thicker and more 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 seafloor spreading moves away from the active arc. Keywords Oceanic crust · S-waves · Backarc spreading center · Subduction zone
Introduction Along mid-ocean ridges, spreading rate is a dominant factor that controls crustal formation and structure (Reid and Jackson, 1981; Parmentier and Morgan, 1990). In contrast, along oceanic back-arc spreading centers, slab-derived volatiles, principally water, appear to influence melting processes and crustal formation, overprinting spreading-rate trends (Martinez and Taylor, 2002; Eason and Dunn, 2015). The presence of water during melting is expected to enhance melt production (e.g. Davies and Bickle, 1991; Stolper and Newman, 1994), with the water ending up in the melt (e.g. Hirth and Kohlstedt, 1996), and may lead to more silicic lavas (e.g. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11001-020-09419-5) contains supplementary material, which is available to authorized users. * Charu Lata [email protected] 1
Department of Earth Sciences, University of Hawai‘iMānoa, 1680 East‑West Road, Honolulu, HI 96822, USA
Sisson and Grove, 1992; Gaetani and Grove, 1994; Eason and Dunn, 2015). A principle observation is that when a back-arc spreading ridge is located close to an active arc system, the magma supply to the ridge appears to be relatively high, with a corresponding thicker upper-crustal extrusive layer and a greater total crustal thickness (Martinez and Taylor 2002; Dunn and Martinez 2011; Arai and Dunn 2014). In addition, lava
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