Missing western half of the Pacific Plate: Geochemical nature of the Izanagi‐Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles

Abstract

The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb-Nd-Hf isotopes. The results showed that the subducted Izanagi-Pacific Ridge (IPR) formed from both Pacific (180-80 Ma) and Indian (80-70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the KurileJapan-Nankai-Ryukyu (KJNR) Trench at 60-55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian-Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165-130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian-Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous.

Origin of geochemical mantle components: Role of subduction filter

“…[] and Miyazaki et al . [], which is consistent with the classification made by Nd‐Hf isotope systematics in Figure 11. Small number of equatorial Atlantic MORBS may also be from Pacific DMM.…”

“…The proposed stationary boundary between Indian and Pacific upper mantles in Figure a is from Miyazaki et al . [].…”

Mantle Flow and Deforming Continents: From India‐Asia Convergence to Pacific Subduction

“…Liu et al () suggested that the distribution of low‐velocity anomalies below Asia is consistent with such an extrusion of mantle and they showed with numerical models that the continuous injection of mantle underneath the India‐Asia collision zone over the past 50 Ma could result in such an extrusion of mantle toward the east. Miyazaki et al () suggested instead that the boundary between the Indian and Pacific mantles has been stationary in the western Pacific since the Cretaceous. The long‐term subduction of the Tethys Ocean below the southern margin of Asia before collision may thus have allowed such an invasion on Indian mantle before collision started.…”

Origin of geochemical mantle components: Role of spreading ridges and thermal evolution of mantle

“…[] and Miyazaki et al . [], which is consistent with the classification made using the Nd‐Hf and Pb isotope systematics by Kimura et al . [] and the additional examination in Figure 6 of this paper.…”

“…In contrast, more than half of the Pacific mantle domain originated from PM younger than the average depletion age of 1.3 Ga old DMM (Figure a). The boundary between these two domains appears to have been stationary since the Mesozoic [ Mahoney et al ., ; Miyazaki et al ., ; Nebel et al ., ]. The more than half of the upper mantle in the NAIP appears to contain a large EDR component with depletion age <0.8 Ga, similarly young as in the Pacific mantle domain.…”