Crustal structure, accretionary processes and rift propagation: a gravity study of the intermediate-spreading Valu Fa Ridge, Lau Basin

The Valu Fa Ridge is an intermediate-spreading (full rate of 60 mm yr(-1)) ridge located ill the Lau Basin. In 1995 this ridge was surveyed using a mu ltidisciplinary, geophysical approach to image crust and upper mantle struc ture, with the aim of investigating the processes of oceanic crustal accret ion in a back-arc tectonic environment. As part of this experiment a networ k of gravity profiles was acquired, together with seismic, magnetic, swath bathymetry and controlled-source electromagnetic data. Presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles, two oriented ridge-perpendicular and one ri dge-parallel, using the preferred seismic models of Turner et al. (1999) as a basis of initial model construction. In addition, the gravity data set i n its entirety has been used to calculate the mantle and residual mantle Bo uguer anomalies with the aim of investigating variability in crustal struct ure, both density and layer thickness, and the nature of the underlying upp er mantle. Of particular interest are the overlapping spreading centre betw een the Central and Northern Valu Fa Ridges, where seismic modelling implie s a generally thickened crust and a magma chamber located beneath the overl ap basin rather than separate chambers supplying each ridge, and the propag ating rift tip and associated basin-bounding pseudo-fault. Modelling results suggest that the pre- and post-rift crusts have different compositional origins, with lower densities required > 12 km off-axis to f it the observed free-air gravity anomaly. The locations of the transitions into regions of lower density correspond with those of Turner cr al. (1999) derived from seismic modelling, which in turn correspond in location to th e rift-related pseudo-fault identified by Wiedicke & Collier (1993). Calcul ation and interpretation of the mantle and residual mantle Bouguer anomalie s also confirms the lower off-axis densities and indicates a general increa se ill crustal thickness northwards towards the overlapping spreading centr e between the Central and Northern Valu Fa Ridge segments. The presence of thicker crust, corresponding to a region of negative mantle and residual ma ntle Bouguer anomaly, implies that the region surrounding the overlapping s preading centre has been in the recent geological past, or is presently, th e site of an increased magma supply. The residual mantle Bouguer anomaly al so reveals features related directly to off-axis lateral variation in densi ty and layer thickness, associated with the southward propagation of the Va lu Fa Ridge into the Havre Trough. Modelling of the north south variation in crustal thickness along-axis show s that the anomaly trend can be explained simply by thicker crust beneath e ach of the Valu Fa Ridge segments and the overlapping spreading centre betw een the Northern and Central Valu Fa Ridges. The observation of segment-cen tred crustal thickening is consistent with models of ridge segmentation tha t suggest that melt influx is located towards segment centres and that segm ent length is controlled by the maximum lateral extent to which this melt c an flow (Macdonald et al. 1988; Tolstoy et al. 1993). However, the overlapp ing spreading centre remains anomalous from these models in that it also ap pears to be associated with thicker crust and an increased melt supply.