Seismic imaging of the axial region of the Valu Fa Ridge, Lau Basin - the accretionary processes of an intermediate back-arc spreading ridge

In 1995, a multidisciplinary geophysical experiment targetted the intermediate spreading Valu Fa Ridge (full rate 60 mm yr(-1)), which is centred on 22 degrees 20'S, 176 degrees 40'W in the Lau Basin. As part of this experiment, wide-angle and normal-incidence seismic profiles were collected both along- and across-axis to determine the crustal structure of the Central Valu Fa Ridge (CVFR) and its overlap with the Northern Valu Fa Ridge (NVFR). Controlled-source electromagnetic profiles and underway gravity, magnetic and bathymetry data were also collected. In this paper we describe the results of forward modelling of the along- and across-axis wide-angle and normal-incidence seismic data. An axial low-velocity block and its underlying slightly broader zone of depressed seismic velocities (low-velocity zone) have been identified, and these features are interpreted as corresponding to a melt lens and underlying magma chamber. The low-velocity block is 1-2 km wide and has a first-order upper boundary, from which large-amplitude reflections are observed; amplitude analysis of these indicates an interconnected melt fraction. The nature of the lower boundary is more poorly constrained, as no reflection event corresponding to the base of the low-velocity block is observed. Modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region (similar to 5.5 km s(-1)) are achieved by 250 m below the upper boundary, possibly suggesting a gradational lower boundary with high velocity gradient. The low-velocity zone (LVZ) is interpreted as an similar to 4 km wide magma chamber delineated by a seismic velocity anomaly of -0.2 km s(-1), extending down through layer 3 to within 1.5-2 km of the Moho. The velocity anomaly and dimensions of the LVZ are generally smaller than those observed at the East Pacific Rise (EPR) and Mid-Atlantic Ridge (MAR). The observed alone-axis continuity of the low-velocity block is remarkable, extending from the southern tip of the CVFR to the overlapping spreading centre (OSC) with the NVFR. A low-velocity block is modelled beneath the inside flanks (i.e. the slopes that dip into the overlap basin) of both ridges at the OSC, although the existence of a single low-velocity block beneath the overlap basin itself cannot be ruled out. The identification of a single LVZ centred on the overlap region, rather than two merged LVZs beneath each segment, implies that the material in each low-velocity block originates from the same crustal magma source. A reflection event from the Moho is observed from directly beneath the axis on both across-axis profiles, which indicates that a distinct crust-mantle boundary may be formed within the axial region. Many of the observations at the Valu Fa Ridge are consistent with those at the EPR and the Reykjanes Ridge (MAR), which implies that, regardless of spreading rate, crustal accretionary processes at mid-ocean ridges with similar magmatic budgets are also broadly similar.