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 intermedi ate spreading Valu Fa Ridge (full rate 60 mm yr(-1)), which is centred on 2 2 degrees 20'S, 176 degrees 40'W in the Lau Basin. As part of this experime nt, wide-angle and normal-incidence seismic profiles were collected both al ong- 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). Co ntrolled-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- an d across-axis wide-angle and normal-incidence seismic data. An axial low-ve locity 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 b oundary is more poorly constrained, as no reflection event corresponding to the base of the low-velocity block is observed. Modelling indicates that v elocities 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 bou ndary, 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 veloc ity anomaly and dimensions of the LVZ are generally smaller than those obse rved 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 c entre (OSC) with the NVFR. A low-velocity block is modelled beneath the ins ide 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 singl e LVZ centred on the overlap region, rather than two merged LVZs beneath ea ch segment, implies that the material in each low-velocity block originates from the same crustal magma source. A reflection event from the Moho is ob served from directly beneath the axis on both across-axis profiles, which i ndicates that a distinct crust-mantle boundary may be formed within the axi al region. Many of the observations at the Valu Fa Ridge are consistent wit h those at the EPR and the Reykjanes Ridge (MAR), which implies that, regar dless of spreading rate, crustal accretionary processes at mid-ocean ridges with similar magmatic budgets are also broadly similar.