Shear wave velocity image of an ophiolite nappe in New Caledonia and its implications for Eocene subduction initiation beneath Zealandia

SUMMARY To better understand Eocene ophiolite emplacement and subduction initiation in northeastern Zealandia, we analysed ambient noise to image shallow (0–3 km) shear wave velocity structures of and beneath an ophiolite nappe in southern Grande Terre, New Caledonia. We assessed the uncertainties of each dispersion curve to obtain stable dispersion curves at short periods (<1 s) from a network of 17 seismic stations, whose average interstation distance is ∼15 km. We obtained 1-D velocity profiles and interpolated them to generate 2-D transects with a lateral resolution <2 km. Two velocity discontinuities were imaged at depths of 100 and 400–700 m, representing surface regolith and the base of the ophiolite nappe, respectively. The ophiolite nappe is underlain by continental basement rocks in the centre of the island and sedimentary rocks near the east and west coasts. The base of the nappe shallows at ∼2.5° westwards to the surface at its southwest flank. Based on the geometry of the ophiolite nappe, we suggest a down-going gravity-driven emplacement mechanism, and note similarities to allochthons in Reinga Basin and Raukumara Basin of northern New Zealand. The ophiolite nappe and underlying bedrock are more fractured on their east flank due to syn/post emplacement deformation, isostatic adjustment and present flexural bending.