Emplacement of Tamu Massif on the Margins of the Young Pacific Plate: The Importance of Regional Lithospheric Thermal Structure and Microplate Geodynamics

Abstract:

In the study of marine large igneous provinces (LIPs), a number of investigations have examined the importance and role of mantle plumes. Relatively fewer studies, however, have focused on the relationship between plate boundary geometry and LIP magmatism. Shatsky Rise, an oceanic plateau in the western Pacific, formed in a relatively distinctive tectonic setting. Shatsky Rise consists primarily of Tamu, Ori, and Shirshov massifs. The largest, Tamu Massif, was emplaced approximately 140-150 Ma at a triple junction consisting of the Pacific, Izanagi, and Farallon mid-ocean ridges. Moreover, the Pacific-Izanagi-Farallon triple junction was one of three triple junctions operating in close proximity at the time, as the nascent Pacific Plate (NPP) began to develop and expand in size. Triple junction kinematics studies as well as available magnetics data seem to indicate that many of the ridges bounding the NPP had spreading rates toward the high end of the spectrum, suggesting that the opening of the Pacific Ocean was characterized by a broad region of hot, thin lithosphere. This preliminary study uses available marine magnetics data to calculate lithospheric thickness during several stages of opening of the NPP, including the time at which Tamu Massif formed. To further understand the geodynamics of LIP magmatic emplacement at a triple junction network, we also employ bathymetry and gravity data to constrain the nature of isostatic compensation at Tamu Massif. Additionally, we compare the lithospheric structure of the Tamu Massif/NPP system to that of the currently-operating Easter plume/Easter microplate setting, in order to gain insight into different styles of plume-plate interactions. Overall, we conclude that the thermal conditions of the NPP, which likely resulted in a large area of thin or nearly-absent lithosphere, could have significantly facilitated widespread mantle melting as well as the magmatic expression of any nearby mantle plume.