Lateral variation of upper mantle structure beneath New Caledonia determined from P ‐wave receiver function: evidence for a fossil subduction zone

SUMMARY Short‐period teleseismic P ‐waves recorded at Noumea, New Caledonia, from intermediate and deep focus earthquakes in SE Asia and the SW Pacific are used to investigate crustal and upper mantle structure of the Norfolk ridge, which is currently described as a zone of convergence in its caledonian part. Radial components recorded from various source backazimuths show a succession of large P‐to‐S converted waves whose timing and polarity suggest the presence of a north‐dipping low‐velocity zone at approximately 60km depth below the station. The inferred strong lateral variations of structure in the upper mantle are also supported by observations of complex horizontal particle motions and large tangential components which exhibit clear P‐to‐S phases. The high velocity contrast observed across these dipping interfaces is interpreted as a structural remnant of the New Caledonia basin oceanic crust which would have been underthrust beneath the New Caledonia crust. Using geometrical ray theory in dipping structure, a model valid for the Noumea station has been derived. The crust is apparently quite homogeneous. It is 26 ±1 km thick beneath the station and seems to thicken eastward as deduced from the orientation of the Moho. The low average crustal velocities suggest a continental nature for New Caledonia. Toward the thrust zone, the Moho is a thick transition zone whereas it can be modelled by a sharp boundary elsewhere. The inferred structure can be interpreted in terms of either a dead subduction zone or an underthrusting zone. According to the history of compressive deformations observed in land, the underthrusting would likely have been syntectonic of the New Caledonia late Eocene obduction.