Abstract

Global positioning system (GPS) measurements in 1990 and 1992 from two sites on the southern New Hebrides island arc give convergence rates with the Australian plate of 103 ± 5 mm/yr and 118 ± 10 mm/yr. In contrast, GPS measurements in the central New Hebrides indicate anomalously low convergence rates of ≈42 mm/yr. On geologic time scales, the mean central New Hebrides convergence rate has been 85–132 mm/yr. Elastic fault models with a locked interplate thrust zone indicate that maximum possible rates of horizontal elastic strain are insufficient to account for the anomalously slow convergence. Therefore, we propose that the central New Hebrides segment is moving eastward relative to adjacent arc segments at a rate of ≈36–83 mm/yr. This displacement is accommodated by crustal shortening at the eastern margin of the arc and strike-slip faults crosscutting the arc. Resistance to subduction of the aseismic D'Entrecasteaux Ridge system is the likely cause for horizontal forces sufficient to shove a large segment eastward and fragment the arc. This process demonstrates that subducting bathymetric features can impose fundamental structural modifications on an arc that may represent the initial stages of arc polarity reversal.

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