Three-dimensional crustal structure and magma chamber geometry at the intermediate-spreading, back-arc Valu Fa Ridge, Lau Basin - results of a wide-angle seismic tomographic inversion

In this paper the results of a tomographic analysis of a 3-D wide-angle seismic refraction data set acquired at the Valu Fa Ridge (VFR) in 1995 are presented. The VFR is an intermediate-spreading ridge located in the southern Lau back-arc basin in the southwest Pacific. The ridge comprises three morphological segments, the Southern, Central and Northern Valu Fa Ridges, separated by overlapping spreading centres (OSCs). Previous seismic experiments have identified a robust axial magmatic system beneath the central segment (CVFR) and the OSC with the northern segment (NVFR). The experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post-rift crust. A regularized inversion scheme that minimizes model roughness was applied to the first-arrival traveltime picks made from the wide-angle data. A quantitative approach for determining data uncertainties is described based on the signal-to-noise ratio of the arrivals. Several initial model assumptions were tested, including one with a thin melt lens, representing a seismic reflector identified in previous studies, explicitly included in the initial model. The inversion results suggest that crustal layer 2 exhibits northward thickening, which mirrors a similar northward thickening of the whole crust. In addition, local thinning of layer 2 is identified in the vicinity of the boundary between pre- and post-rift crust, which is thought to represent thinning of the crust prior to the onset of rifting. Axial low-velocity anomalies are identified in layer 2B/C and layer 3. The models are consistent with a continuous similar to6 km wide negative velocity anomaly in layer 3 with an amplitude of similar to0.7-0.9 km s(-1) relative to off-axis post-rift crust. This anomaly is consistent with the presence of an axial mush zone comprising a small percentage (< 1 per cent) of partial melt. The negative velocity anomaly in layer 2B/C is modelled with its largest amplitude (<similar to>0.5 km s(-1)) beneath the northern OSC. Possible origins for this anomaly include locally thicker crust or locally higher porosity near the OSC, or a high-temperature anomaly associated with the axial magmatic system.