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

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.

Knowledge of the seismic structure beneath back-arc spreading centres is of crucial importance for a variety of geodynamic problems. The width and depth extent of the slow velocity region below the spreading centres provide constraints on the origin of back-arc spreading, the geochemical source of arc and back-arc magmas, the interaction between subduction and back-arc spreading, whether the mantle upwelling beneath spreading centres is passive of active, and to what depth the upwelling persists. Here we present evidence from seismic tomography 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. The depth extent of the slow anomalies is also confirmed by waveform inversion of regional broadband seismograms. The subducting Tonga slab is imaged as a 100 km thick zone with a P-wave velocity 4-5% higher than the surrounding mantle. Our results indicate the geodynamic systems associated with back-arc spreading are not confined to the near-surface region, but are related to the deep processes in the mantle, 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: 18 Mar 1997