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 sei smic refraction data set acquired at the Valu Fa Ridge (VFR) in 1995 are pr esented. The VFR is an intermediate-spreading ridge located in the southern Lau back-arc basin in the southwest Pacific. The ridge comprises three mor phological segments, the Southern, Central and Northern Valu Fa Ridges, sep arated by overlapping spreading centres (OSCs). Previous seismic experiment s have identified a robust axial magmatic system beneath the central segmen t (CVFR) and the OSC with the northern segment (NVFR). The experiment descr ibed 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 t o the first-arrival traveltime picks made from the wide-angle data. A quant itative approach for determining data uncertainties is described based on t he signal-to-noise ratio of the arrivals. Several initial model assumptions were tested, including one with a thin melt lens, representing a seismic r eflector identified in previous studies, explicitly included in the initial model. The inversion results suggest that crustal layer 2 exhibits northwa rd thickening, which mirrors a similar northward thickening of the whole cr ust. In addition, local thinning of layer 2 is identified in the vicinity o f the boundary between pre- and post-rift crust, which is thought to repres ent 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 consist ent with a continuous similar to6 km wide negative velocity anomaly in laye r 3 with an amplitude of similar to0.7-0.9 km s(-1) relative to off-axis po st-rift crust. This anomaly is consistent with the presence of an axial mus h zone comprising a small percentage (< 1 per cent) of partial melt. The ne gative velocity anomaly in layer 2B/C is modelled with its largest amplitud e (<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.