Benham Rise unveiled: Morphology and structure of an Eocene large igneous province in the West Philippine Basin

The Benham Rise is an oceanic large igneous province at the western margin of the Philippine Sea. It has an ocean island basalt geochemistry, a surface area of similar to 0.11 Mkm(2) and a volume of similar to 0.13 Mkm(3). A crustal thickness of similar to 15 km is estimated from gravity anomalies. Volcanism was most active in the early stages of its formation, during the Lutetian from similar to 47.9 Ma to 41.3 Ma, although volcanic activity extended to similar to 26 Ma. Multibeam bathymetry with almost 100% coverage has revealed the morphology of Benham Rise for the first time. It consists of a main body (similar to 310 km by 330 km) with the Narra, Loro and Molave Spurs extending 100-200 km from its eastern side. The main body is built on a shield platform from similar to 5200 m to similar to 3800 m sub-sea. The platform flanks consist of similar to 3-15 km wide terraces with scarps as high as 100 m to 300 m. The platform is surmounted by a crest exhibiting caldera morphology at an average depth of similar to 2500 m. The crest is named Apolaki Caldera and may be the worlds largest known caldera with a diameter of similar to 150 km. Features like a breached rim, intra-caldera benches, and a resurgent dome indicate a multi-phase volcanic history consisting of both quiet and explosive eruptions. The bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of Benham Rise - shield-building, caldera formation, and post-caldera/late stage volcanism. The platform base is interpreted to have formed over a hotspot beneath the Central Basin Spreading Center during the shield-building phase between similar to 47.9 Ma to similar to 42.5 Ma. The arrowhead shaped Molave Spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot. The caldera formation followed and is interpreted to have occurred before similar to 41.3-41.5 Ma when Benham Rise was still joined with Urdaneta Plateau. The Narra Spur was formed by late-stage hotspot related volcanism, as Benham Rise and Urdaneta Plateau separated, and ended with the eruptions of the Vinogradov Seamount at similar to 26 Ma.