GEOLOGICAL SOCIETY OF THE PHILIPPINES
Benham Rise: A Large Igneous Province Accreted to Luzon
Benham Rise is a Large Igneous Province (LIP) that formed by voluminous outpouring of basalt above a hotspot in the vicinity of Central Basin Spreading Center in the West Philippine Basin. The elevation of 2000 m to 3500 m above the surrounding seafloor and the thickness of 15-18 km based on gravity data modeling point to its origin by eruption caused by larger than normal melting anomaly in the mantle. Based on its thickness and areal extent of 143,000 km2, the crustal volume of Benham Rise can be estimated to be about 2,145,000 to 2,574,000 km3, comparable to other smaller LIPs worldwide. Ridge morphology and overlapping spreading center fabric observed at the western end of the Central Basin Spreading Center also indicate fast-spreading rate from 55-46 (half-rate of 44 mm/y; [1, 2]). These observations are attributed to higher magma supply related to higher mantle temperatures that produced Benham Rise.
The geochemical composition of basement rocks recovered from Benham Rise and the shallower depth of the rise relative to the surrounding seafloor indicate an origin similar to other large plateaus and rises such as Shatsky Rise, Hess Rise, Manihiki, Hikurangi, and Ontong Java plateaus, and Mid- Pacific Mountains in the Pacific Ocean. Geochemical analysis of basalts from DSDP Leg 31, Site 292 on the Benham Rise showed a compositional signature different from those of rock samples taken from the deep ocean floor of the West Philippine Basin or the larger Atlantic and Pacific oceans. Tectonic plate motions from about 55 to 20 my ago indicate that Benham Rise formed southeast of its present position, moved, and collided with Luzon [3-5]. However, the greater than normal crustal thickness and buoyancy of Benham Rise led to its accretion with the Philippine landmass since 20 my ago.
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