Thermochronology of the New Caledonian high-pressure terrane: Implications for middle Tertiary plate boundary processes in the southwest Pacific

One of the youngest and best-preserved exposures of blueschist-and eclogite-facies rocks on Earth occurs in an elongate NW-SE-trending range in northeastern New Caledonia. This high-pressure (HP) terrane evolved within the Australia-Pacific plate boundary zone and records a middle Tertiary history of subduction burial, metamorphism, and exhumation. ⁴⁰ Ar and fission track thermochronology was undertaken in the New Caledonian HP terrane to further constrain the timing and rates of cooling and exhumation, as well as to evaluate tectonic models. Oceanic and sedimentary protoliths were subducted at rates of 6-16 mm/yr and metamorphosed under HP conditions at ca. 44 Ma. Subsequently, rapid cooling occurred from 40 to 34 Ma as the HP terrane was exhumed at rates of ∼5 mm/yr. The HP terrane was exhumed largely as a coherent block to relatively shallow crustal levels, primarily via ductile shearing associated with crustal extension. Since the early Oligocene (<34 Ma), exhumation rates decreased to <0.3 mm/yr as brittle normal faulting and erosional processes continued to exhume blueschists and eclogites from relatively shallow (i.e., cool) crustal depths. Exhumation of the HP terrane temporally coincides with obduction of ultra-mafic rocks in southern New Caledonia, and seafloor spreading in the North Loyalty Basin. We propose a model whereby HP metamorphism at ca. 44 Ma was followed by rapid exhumation from 40 to 34 Ma, translation, and Oligocene (<34 Ma) juxtaposition of the HP terrane against the other basement terranes of New Caledonia.,