Abstract

Subduction initiation has long been believed to be an important setting for ophiolite formation. Stern and Bloomer (1992) emphasize the importance of sinking of the slab following subduction initiation. In their model, ophiolites form by sea-floor spreading during this sinking phase, which subsequently transitions into true subduction and arc volcanism. The problem is that there is no location where we can observe active sea-floor spreading associated with global scale subduction initiation and so test the Stern and Bloomer hypothesis. The best example is in the IBM (Izu-Bonin-Mariana) System where subduction began c. 52Ma ago. Because most of the key geological information is under water and beneath sediment, the International Ocean Drilling Program (DSDP, then ODP, then IODP) organized a number of drilling expeditions to the IBM region to help further understand subduction initiation. This talk will focus on the latest of these, IODP Expedition 352, which (in 2014) drilled through the entire volcanic sequence of the Bonin forearc in order to test the hypothesis that the fore-arc lithosphere created during subduction initiation is the birthplace of ophiolites. This Expedition successfully cored 1.22 km of igneous basement and 0.46 km of overlying sediment, providing diverse, stratigraphically controlled suites of fore-arc basalts (FAB) and boninite related to seafloor spreading and earliest arc development. Key geological observations were the fact that two holes (one FAB and one boninite) bottomed out in sheeted intrusions, likely dyke swarms, providing good evidence of formation by sea-floor spreading. The drilling confirms the time progression from tholeiites to boninites that is present in so many SSZ ophiolites, such as the Troodos, Hatay and Oman ophiolites in the 100-90Ma Tethyan ophiolite belt. In detail, however, the precise relationship of individual ophiolites to subduction initiation depends on a series of factors, including roll-back rate and the size of the ocean basin in which subduction initiates. The fact that compositions counter-intuitively become more MORB-like, rather than arc-like, towards the trench, has implications for the making of past plate reconstructions.

Biography

Julian Pearce is Emeritus Professor in the School of Earth and Ocean Sciences at Cardiff University. He is best known for pioneering and developing the technique of geochemical fingerprinting, still widely used by geologists to find out the original global tectonic settings of formation of the rocks in their study areas. Given that many rocks from the geologic record have experienced element mobility during weathering or metamorphism, a key prerequisite is the use of minerals or elements that are alterationresistant. His original work in 1973, while a research student at the University of East Anglia in the UK, was based on volcanic rocks, but his later research at the Open University and Universities of Newcastle, Durham and Cardiff in the UK, expanded this to a range of other rocks and minerals, such as granite whole rocks and chromites in mantle peridotites. Julian believes that, in addition to accurate, quality controlled analyses, the key to successful fingerprinting of any rock is a good understanding of its field geology, and so has spent a significant part of his career mapping, and collecting samples for analysis, on field expeditions and research cruises. His-land projects began with ophiolites and volcanic arcs (Cyprus and Oman ophiolites and the Central Andes), extending, in the mid-late 1980s, to participation in international geological traverses across collision zones (the Tibet Plateau, Himalayas and Eastern Anatolia). Much subsequent work involved the marine geology of marginal basins, as co-chief scientist on two IODP expeditions to the Western Pacific and co-PI of other cruises mainly to the Lau Basin (Tonga) and the Scotia Sea (off Antarctica). His leadership positions include the running of the JOIDES and ESSAC Offices for International Ocean Drilling and his awards include the Murchison Medal of the Geological Society of London in 2014.

Venue

Room 314/315, Steele Building (#03)