The project is a follow-on to the discovery that there are very young UHP rocks exposed on islands near the eastern tip of Papua New Guinea (PNG). A major objective is to use detailed study of this area to better understand how UHP rocks get to the surface from great depth. Because the rocks are in a tectonically active region, it can be argued that there is a good chance of improving models for UHP exhumation because this example is still in its tectonic context. There is evidence that the process is ongoing. Related to this objective is the study of the overall tectonic evolution of this area, which is needed in order to help understand how the UHP rocks got where they are and from whence they came.
Within this framework, this project involves a range of studies that can contribute to piecing together the UHP exhumation history and the tectonic evolution. One large study involves passive seismic data acquisition to image the crust and mantle beneath the area where partially exhumed HP/UHP rocks may occur at depth; a large fraction of the studies involve petrology, geology, mineral physics and geochemistry. In addition, GPS work and geodynamic modeling are included.
The distribution of earthquakes and volcanoes on Earth are primarily associated with convergent and divergent plate boundaries. Eastern Papua New Guinea is one of the best natural laboratories on Earth to study the evolution of a divergent plate boundary as it transitions from rifting to sea-floor spreading. With the discovery of the world’s youngest ultrahigh-pressure and high pressure ((U)HP) rocks at the western end of the westerly propagating Woodlark Basin sea flooring spreading system, eastern Papua New Guinea also provides a modern analogue for (U)HP terranes globally (i.e., formed during subduction/convergence). This project addressed two fundamental questions: How does lithosphere rupture? How are UHP terranes formed and exhumed? To investigate these questions requires understanding how mantle, lithospheric, and surface processes are linked in the rapidly evolving Australia (AUS)- Woodlark (WDK) plate boundary. We mapped geological units in the southern and northern-rifted margins of the Woodlark Rift, and the D’Entrecasteaux Islands where the (U)HP rocks, formed at close to 100 km depth, have been exhumed. Thermochronologic, geochemical, structural and petrologic data was used to determine the pressure-temperature-time-deformation histories (P-T-t-D) of rocks in the Woodlark Rift. We found that the (U)HP terrane formed within the larger obliquely convergent Australian (AUS)-Pacific (PAC) plate boundary zone, as the leading edge of the northern Australian plate was subducted to the north at the Pocklington Trough beneath oceanic lithosphere of the Solomon Sea (Woodlark microplate). Subduction of Cretaceous volcaniclastic sediments and basalts, derived from the Gondwana rifted margin (Australia), led to offscraping, subcretion/underplating, and formation of an accretionary wedge with rocks following many different P-T-t-D paths within the subduction channel. Remnants of the low-pressure part of the accretionary wedge occur on the southern-rifted margin of the Woodlark Basin (i.e., Louisiade Archipelago) where prehnite-pumpellyite to lower greenschist facies rocks (Calvados Schist) preserve a record of Early-Middle Miocene NNE-SSW convergence associated with AUS-PAC transpression. We determined the timing (~8 Ma to the present), rates (cm/yr), physical and chemical processes operative, as recorded in coesite eclogite metamorphosed at mantle depths (<750°C at >90 km) and subsequently exhumed to the surface. Integrated high and low-temperature thermochronologic data constrain T-t paths for exhumed eclogites in a continuous temperature window between 600 and 60°C. Thermokinematic models suggest rapid cooling (~8 Ma initiation), deceleration (~4-2 Ma), followed by rapid Pliocene cooling and exhumation. We determined the history of volcanism in the rift including 1) fragments of obducted oceanic lithosphere (Latest Cretaceous and Early Oligocene) and 2) those volcanic rocks that formed as the result of partial melting of subduction-modified mantle. The AUS-WDK plate boundary changed from convergence to divergence (rifting) as the Woodlark microplate rotated counterclockwise. Divergence between the upper plate (Woodlark Plate) and accretionary wedge led to removal of the upper plate and rapid (U)HP exhumation at cm/yr rates. Stream profile analysis, and low temperature thermochronologic data, suggest that final exhumation of the (U)HP terrane occurred during Plio-Pleistocene to Holocene time. Exhumation may still be ongoing as indicated by intermediate depth earthquakes (70-110 km) which occur ~100 km along strike to the west of the Late Miocene coesite eclogite locality. (U)HP rocks may occur at depth there, but have yet to be exhumed. The transition from subduction (to produce (U)HP rocks since 8 Ma) to rifting and seafloor spreading is diachronous, and proceeds from east to west. Many similarities exist between the (U)HP terrane of eastern PNG and the Eocene (U)HP rocks of the western Alps. These include the time scales of (U)HP metamorphism and subsequent exhumation (<8 Ma), rates (>cm/yr), and mechanisms (divergence within an obliquely convergent plate boundary zone). Removal of the upper plate -oceanic lithosphere in the case of PNG and a Cretaceous accretionary wedge in the case of the western Alps- led to buoyant rise of (U)HP rocks to form topographic highs (i.e., internal massifs of the western Alps and domes of the D’Entrecasteaux Islands). Funding provided significant research and training opportunities for undergraduate and graduate students, post-doctoral fellows and faculty. Results have been and continue to be published in peer reviewed journals and books. Research results were incorporated into mineralogy, petrology, structural geology and tectonics curricula at Syracuse University and the University of Vermont. Outreach activities included K-12 presentations on rocks, minerals, geochronology and plate tectonics. Numerous invited talks, based on results of this project, were presented at national and international conferences, and at universities in the U.S. and overseas, including at the University of Papua New Guinea. During field campaigns we taught local communities in eastern Papua New Guinea about volcanic and seismic hazards as well as the physical properties of minerals. This project contributed to understanding the complex tectonic evolution of a part of the larger Australian-Pacific obliquely convergent plate boundary zone that is rich in mineral resources.
