Abstract
WE present normal-incidence seismic reflection profiles from a back-arc spreading centre in the southern Lau Basin, which show a bright, mid-crustal reflector beneath the ridge axis. Analysis of wide-angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low-seismic-velocity zone. We interpret the reflector as the top surface of a magma chamber. The Lau Basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre, the other being the East Pacific Rise between 9° and 13° N (ref. 1). The Lau Basin reflector is imaged beneath an entire 35-km-long ridge segment. It is also present beneath both ridges and the overlap basin of a small overlapping spreading centre. This is the first observation of a widespread magma body beneath an overlapping spreading centre, and indicates that the offset here is not related to a break in continuity of the magma chamber.
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