EOCENE ARC-CONTINENT COLLISION IN NEW-CALEDONIA AND IMPLICATIONS FOR REGIONAL SOUTHWEST PACIFIC TECTONIC EVOLUTION

New Caledonia preserves evidence that constrains models for the tecton ic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subductio n-related terranes and their accretion to the Gondwana margin; (2) Cre taceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene ar rival of thinned Gondwana margin crust at a southwest-facing subductio n zone, resulting in collisional orogenesis and obduction of an ophiol itic nappe from the northeast; and (4) detachment faulting during exte nsional collapse, resulting in unroofing of metamorphic core complexes . The last phase explains supposedly anomalous metamorphic gradients i n the northeast of the island.