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 (< 50 km away), as compared to when spreading was located farther away from 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.

对来自L-SCAN有源源广角反射/折射实验的P和S波数据进行了建模，以研究Lau后弧盆地的上地壳结构。结合了射线追踪和有限差分数值波场模拟来识别P和P到S转换的地震相位。这些相主要来自两个浅界面，一个在约80 m或以下的深度，另一个在500-650 m的深度。较浅的界面比沉积物基底更深，在整个研究区域内都可以观察到，并且被解释为“ 2Aa层”边界，这是由于裂缝密度的快速变化所致。较深的界面被解释为2A–2B层边界，对应于从熔岩到片状堤坝的过渡。平均而言，第2A层 当扩展位于弧附近（<50 km）时，在扩展中心形成的地壳厚150 m，而扩展位于离弧更远（> 70 km）处的地壳厚。第2A层厚度和Vp / Vs值表明，一个较厚且多孔的熔岩层，以玄武岩为主的玄武岩-安山岩，盖住了近弧壳，而一个较薄且孔隙较少的，大部分为玄武质的火山层，盖住了弧形地壳。这些结果与平板状挥发物对海床形成的减弱影响相一致，因为海床扩展远离活动弧。