Depth extent of the Lau back-arc spreading centre and its relationship to subduction process

Dapeng Zhao, Yingbiao Xu, Douglas A Wiens, Department of Earth & Planetary Sciences,Washington University, St. Louis, MO 63130

LeRoy M. Dorman, John Hildebrand, Spahr Webb, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0215, USA.

Seismic tomography and waveform inversion reveal that large (up to 7%) slow velocity anomalies beneath the active Lau spreading centre extend to a depth of about 100 km, and are connected to lower-amplitude slow anomalies in the mantle wedge that extend to at least 400 km depth. This indicates that geodynamic systems associated with back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. The largest amplitude shallow slow anomalies are located just west of the Lau spreading centre, consistent with the observation that the current ridge propagation processes are moving the spreading centre westward, away from the Tonga Arc. The slow regions associated with the Tonga Arc and the Lau back-arc spreading centres are separated at the shallowest depths, but merge at depths greater than 100 km, suggesting that the slab components of back-arc magmas may originate at these depths.

Last Revised: May 7, 1997