Eocene pre- and syn-obduction tectonics in New Caledonia (Southwest Pacific), a case for oblique subduction, transcurrent tectonics and oroclinal bending; structural and paleomagnetic evidence

Abstract

The structural analysis of the metamorphic belt and allochthonous terranes, which represent the lower and upper plates of the Eocene subduction/obduction complex of New Caledonia respectively, unravels a multi-step evolution. At subduction inception, the originally N-S trending northern Norfolk/New Caledonia Ridge was moving northward as recorded by sheared lower Eocene dykes of the Peridotite Nappe. Fast exhumation of the fore-arc mantle in transtensional conditions was recorded by the injection of fore-arc magmas through fast-cooling peridotites, followed by the occurrence of antigorite- and tremolite-bearing crack seals in small-scale transcurrent or oblique-slip faults. Meanwhile, slices of the oceanic crust of the lower plate were scrapped off and accumulated in the fore-arc region while some elements were dragged down into the subduction zone and contributed to the formation of eclogite facies mélange. When the tip of the New Caledonia Ridge reached the trench, it progressively jammed the subduction. Meanwhile, the oblique slip and coeval rotation of the arc due to the opening of the north Loyalty back-arc basin provoked the dextral transcurrent folding of already accreted ocean crust slices and metamorphic units on the course of exhumation. Bulk counterclockwise rotation of 56° ± 5° was recorded in the foreland basin by paleomagnetic data of sediments from the 45–35 Ma time interval. The final obduction of fore-arc peridotites occurred with top-to-the-SW kinematics when the island had already reached its present orientation.