I am interested in the interplay between tectonics and metamorphism, specifically for young and active regions such as eastern Indonesia. I find myself working in a fairly diverse range of fields (mapping + ‘hard rock’ (mainly metamorphic) geology, petrology, tectonics, and geochronology), the idea being to have the geological context in which to interpret both large-scale tectonic processes and small-scale geochronological/geochemical/P–T systematics.
Prior to undertaking a postdoctoral position at the Australian National University (in May 2014), and later starting my ARC DECRA Fellowship (in January 2016), I graduated with a PhD in Earth Sciences from Royal Holloway University of London in June 2014.
(Ultra-)high temperature metamorphism and mantle exhumation on Seram, eastern Indonesia
Eastern Indonesia presents a rare snapshot in time of a complex tectonic system responding to the on-going collision of two continents. It therefore is an ideal region to investigate the mechanics of active subduction and the tectonic controls of crustal metamorphism. Episodes of ultrahigh-temperature (UHT, ≥900 °C) granulite metamorphism have been recorded in mountain belts since the Neoarchean. However, evidence for the tectonic mechanisms responsible for the generation of such extreme thermal conditions is rarely preserved. On Seram, 16 Ma UHT granulites—the youngest identified at the Earth’s surface—were recently discovered in the Kobipoto Mountains (Pownall et al., 2014). The generation of UHT conditions were attained through slab rollback–driven lithospheric extension caused core complex–style exhumation of hot subcontinental lithospheric mantle. Overlying continental crust, heated and metamorphosed by exhumed lherzolites, developed spinel + quartz (Fig. 1) and sapphirine-bearing residual assemblages, shown by phase equilibria modeling to have required temperatures of ~925 °C at ~9 kbar pressure. These findings would suggest that Seram may be a possible modern analogue for ancient orogens that incorporate UHT granulites.
Tectonic Evolution of Seram and the Banda Arc, Eastern Indonesia
The Banda Arc of Eastern Indonesia is one of the most striking topographic(/bathymetric) features on Earth, not only because of its extreme 180˚ curvature, but also because it encloses the 7 km Weber Deep – Earth’s deepest forearc basin. Subduction beneath the arc has been critical in accommodating the later stages of Australia–Southeast Asia collision, which initiated around 23 million years ago; however, disputes over whether one or two concave slabs comprise the subducted lithosphere, and the extent to which slab rollback operated during subduction has led to conflicting interpretations for how the arc evolved. On Seram, an episode of major lithospheric extension (Pownall et al., 2013; Pownall & Hall, 2014) that exhumed the upper mantle and drove UHT metamorphism (Pownall et al., 2014) attests to subduction rollback has having achieved the present-day slab geometry (Fig. 2). Rollback may also explain the formation of the enigmatic Weber Deep.
Subduction in Indonesia
Indonesia exhibits numerous active tectonic subduction zones that are for the most part seismically active (Fig. 3), allowing structural analysis of the slabs to be performed using information from earthquake hypocentres. The link between the subduction of hydrous oceanic crust and the generation of porphyry-forming magmas and fluids is well recognised although, at present, not fully understood. The aims of this project are firstly to investigate the tectonic controls of fluid release from slabs and the generation of porphyry-forming magmas in the slab and/or mantle wedge. Secondly, the project aims to decipher the tectonic evolution of the overlying plate across, for instance, Java and Nusa Tenggara in order to investigate the mechanisms of individual porphyry emplacement events. Both these aims require the investigation of the larger-scale tectonic evolution of the Indonesian region, which in addition to geological evidence may be inferred through interpretation of seismic tomographic models. Geochronology is the final element of the project, with 40Ar/39Ar dating of deposits and associated metamorphic and magmatic rocks providing important constraints on the timing of porphyry emplacement with respect to metamorphic and deformational events in the upper plate. The ultimate aim is to build a 4 dimensional (3 spatial dimensions + time) plate reconstruction for the Indonesia region as a tool to better understand how the ripping and tearing of slabs may lead to the generation and emplacement of Cu ± Au and other deposits, in addition to further understanding how the region has responded during the on-going collision between Australia and SE Asia.
- Pownall, J.M. & Hall, R., 2014. Neogene extension on Seram: A new tectonic model for the northern Banda Arc. Indonesian Petroleum Association, Proceedings, 38, IPA14–G–305.
- Pownall, J.M., Hall, R. & Watkinson, I.M., 2013. Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback. Solid Earth, 4, 277–314. doi:10.5194/se-4-277-2013
- Pownall, J.M., Hall, R., Armstrong, R.A. & Forster, M.A., 2014. Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia. Geology, 42, 279–282. doi:10.1130/g35230.1.
Pownall, J.M., Lister, G.S. & Spakman, W., 2017. Reconstructing subducted oceanic lithosphere by "reverse-engineering" slab geometries: The northern Philippine Sea Plate. Tectonics, 36, doi:10.1002/2017TC004686 http://onlinelibrary.wiley.com/doi/10.1002/2017TC004686/full
Pownall, J.M., Hall, R. & Armstrong, R.A., 2017. Hot lherzolite exhumation, UHT migmatite formation, and acid volcanism driven by Miocene rollback of the Banda Arc, eastern Indonesia. Gondwana Research, 51, 92–117. doi: 10.1016/j.gr.2017.07.003 http://www.sciencedirect.com/science/article/pii/S1342937X16304816
- Pownall, J.M., Forster, M.A., Hall, R. & Watkinson, I.M., 2017. Tectonometamorphic evolution of Seram and Ambon, eastern Indonesia: Insights from 40Ar/39Ar geochronology. Gondwana Research, 44, 35–53. doi: 10.1016/j.gr.2016.10.01 http://www.sciencedirect.com/science/article/pii/S1342937X16304178
Pownall, J.M., Hall, R. & Lister, G.S., 2016. Rolling open Earth's deepest forearc basin. Geology, 44, 947–950 http://geology.geoscienceworld.org/cgi/content/full/44/11/947?ijkey=r0IjGZXS8x9Ro&keytype=ref&siteid=gsgeology
Pownall, J.M., 2015. Miocene UHT metamorphism on Seram, eastern Indonesia: Reaction textures and P–T–t evolution of spinel + quartz granulites from the Kobipoto Complex. Journal of Metamorphic Geology, 33, 909–935 http://onlinelibrary.wiley.com/doi/10.1111/jmg.12153/abstract
Pownall, J.M. & Hall, R., 2014. Neogene extension on Seram: A new tectonic model for the northern Banda Arc. Indonesian Petroleum Association, Proceedings, 38, IPA14–G–305.
Pownall, J.M., Hall, R., Armstrong, R.A. & Forster, M.A., 2014. Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia. Geology, 42, 279–282. doi:10.1130/g35230.1 http://geology.gsapubs.org/content/42/4/279.full.pdf+html
- Pownall, J.M., Hall, R. & Watkinson, I.M., 2013. Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback. Solid Earth, 4, 277–314. doi:10.5194/se-4-277-2013 http://www.solid-earth.net/4/277/2013/se-4-277-2013.html
Watkinson, I.M., Hall, R., Cottam, M.A., Sevastjanova, I., Suggate, S., Gunawan, I., Pownall, J.M., et al., 2012. New Insights into the Geological Evolution of Eastern Indonesia from Recent Research Projects by the SE Asia Research Group. Berita Sedimentologi, 23, 21–27.
Pownall, J.M., Waters, D.J., Searle, M.P., Shail, R.K. & Robb, L.J., 2012. Shallow laccolithic emplacement of the Land’s End and Tregonning granites, Cornwall, UK: Evidence from aureole field relations and P-T modeling of cordierite-anthophyllite hornfels. Geosphere, 8, 1467–1504. doi:10.1130/GES00802.1