The thermo-tectonic evolution of the actively exhuming Mai'iu Fault footwall – Suckling-Dayman metamorphic core complex – in the Woodlark Rift of Papua New Guinea

Abstract

The Mai'iu Fault in the Woodlark Rift of southeastern Papua New Guinea is an active north-dipping low-angle normal fault (LANF) that has exhumed the spectacular Suckling-Dayman metamorphic core complex (SDMCC) in its footwall. While its youthful domal geomorphology suggests recent tectonic exhumation of the SDMCC, the cooling and exhumation history of this active metamorphic core complex have not been studied in detail before. Here we provide the first zircon and apatite fission-track (FT) and (U-Th)/He thermochronometric data on the SDMCC based on a suite of samples from slip-parallel footwall transects and syn-extensional alluvial deposits shed from the emerging dome. In addition, we double dated zircon using U–Pb LA-ICP-MS. These chronometric techniques are complemented by temperature and pressure estimates from Raman spectroscopy of carbonaceous material (RSCM) and Al-in-amphibole and Al-in-biotite barometry on footwall phyllites/schists and granitoids, respectively. The data indicate that slip on the Mai'iu Fault had initiated by ~4 Ma. We show that slip on the Mai'iu Fault has persisted at cm/year rates since the onset of extension, making it one of Earth's most rapidly slipping continental LANF. Integrating temperature estimates from FT and (U-Th)/He thermochronometry and RSCM, we calculated a down-dip, fault-parallel paleo-field temperature gradient along the now-exhumed part of the Mai'iu Fault of ~11 °C/km. For a likely range of pre-extensional geothermal gradients in the Woodlark Rift (10–20 °C/km), this paleo-field temperature gradient implies an average initial dip of the Mai'iu Fault of ~46°. Given the present dip of ~21° at the surface, the thermal data reinforce other evidence that the footwall of the Mai'iu Fault has been back-rotated to the south by ≥20° during progressive unroofing since fault inception, consistent with a rolling hinge style evolution of the SDMCC.