Identification and Implications of a Submarine Monogenetic Field in the NE Lau Basin

Short-lived, volcanism at discrete, closely spaced volcanic cones and low lying lava flows in the NE corner of the Lau backarc basin shares many characteristics with subaerial monogenetic fields. We use geological, morphological, petrological, and geochemical observations of this volcanic field made on five research expeditions since 2008, along with comparisons to well-known terrestrial monogenetic fields to assess whether the Mata volcanic group is best thought of as a submarine mongenetic volcanic field (a term rarely, if ever, applied to submarine settings). The volcanism has constructed a series of 9 small, very closely spaced, hydrothermally-active, elongate volcanic edifices near the east-west portion of the Tonga Trench, which are 1.5 to 7.5 km apart (summit to summit) and are 450 to 1400m tall. Only one of the volcanoes (West Mata) is currently active, erupting boninite pillow lavas along with explosively-generated volcaniclastic sediments. The ages of the youngest volcanics on the other Mata volcanoes are not yet determined but most are hydrothermally active and are surfaced with relatively young lava flows without significant sediment cover. The volcanoes are all formed predominantly of low effusion rate pillow lavas with variable amounts of pyroclastic deposits mantling the constructional topography, suggesting relatively long-lived volcanism (ca 100-200 yrs) at each center, similar to large lava shields in Iceland (e.g., skjaldbreidur). Detailed stratigraphic observations are as yet only available for one volcano (with more to come during an ROV field campaign in Sept. 2012). Bottom photographs provide no clear evidence for long-lived hiatuses at any of these cones and bathymetric data do not intricate overlapping constructional structures, resurgent construction, or large scale collapse or mass wasting structures, as might be expected for a protracted, many-eruption volcanic history at any single volcano. However, the oldest edifice does show evidence of post-volcanic tectonism and several of the smallest volcanoes appear to be built on a basement of either failed earlier volcanoes or rifted lithosphere. All but one of the cones are built of broadly boninitic volcanic products (the other is meimikite), yet major and trace element compositions are distinct enough (Glancy et al., this volume) to imply that each is fed by separate, poorly mixed, small magma batches, much like the MGVF in central western Mexico. The northern portion of the Lau Basin is the fastest opening backarc on Earth; this rapid extension combined with thin lithosphere, and episodic and dispersed magma supply from the nearby subduction system, appears to promote conditions favoring dispersed monogenetic volcanism over longer lived volcanic edifices or ridges.erspective view of the Matas from the west