CONSTRAINTS FROM LOW TEMPERATURE THERMOCHRONOLOGY ON EXHUMATION OF (U)HP TERRANES: THE EASTERN PAPUAN NEW GUINEA EXAMPLE

Exhumation of (U)HP rocks from mantle depths is influenced by factors that include the size of the (U)HP terrane, tectonic forces, and the relative buoyancy of subducted material. Low-temperature thermochronology does not usually constrain (U)HP exhumation as rocks may stall at crustal depths with final exhumation to the surface occurring much later, and via a different mechanism. In the D’Entrecasteaux Islands (DEI) of eastern Papua New Guinea (PNG) exhumation is active, and occurs within a rift-zone ahead of the westward-propagating active seafloor-spreading system of the Woodlark Basin. (U)HP protoliths were subducted at a north dipping subduction zone, and exhumed since ~8 Ma from depths of ~90 km, from beneath a former subduction thrust. That thrust is now represented by carapace shear zones that separate core zone rocks (amphibolite and eclogite-facies gneisses, Pleistocene granitoids) from upper-plate ophiolites.

Thermochronology data (AFT, ZFT, AHe, ZHe) constrain the late-stage thermal and exhumation histories of the DEI and rifted margins. With local variation, ages generally decrease westward from ~8 Ma at Misima Island, to ~1.5-0.5 Ma in the DEI. Track-length distributions, obtained via ²⁵²Cf implantation, allow inverse thermal modeling. Some samples from high elevations in the Goodenough core zone record residence within a PAZ, or reheating before final cooling/exhumation at ~0.75 Ma. Carapace zone and lower elevation samples rapidly cooled ≤0.75 Ma.

Density inversion, buoyancy enhanced by partial melt, reactivation of the former subduction thrust, normal faulting, and regional tectonics, including microplate rotation, all played a role in (U)HP exhumation in eastern PNG. However, the correlation of low temperature thermochronologic data with the timing of westward propagation of rifting indicate that exhumation of the (U)HP terrane is controlled by rifting prior to seafloor spreading in the Woodlark Basin. The regional tectonic control on rifting is counterclockwise rotation of the Woodlark microplate that removed the upper plate and facilitated exhumation from beneath the former subduction thrust. Geologic and tectonic similarities between the Eocene Western Alps and eastern PNG indicate motion of the upper plate away from the former trench facilitates (U)HP exhumation.