Abstract
Abstract. The architecture, formation, and modification of oceanic plates are fundamental to our understanding of key geologic processes of the Earth. Geophysical surveys were conducted around a site near the Hawaiian Islands (northeastern Hawaiian North Arch region; Hawaiian North Arch hereafter), which is one of three potential sites for an International Ocean Discovery Program mantle drilling proposal for the Pacific plate that was submitted in 2012. The Hawaiian North Arch site is located in 78–81 Ma Cretaceous crust, which had an estimated full spreading rate of 7–8 cm yr−1. This site fills a major gap in our understanding of oceanic crust. Previously drilling has been skewed to young or older crust (<15 or >110 Ma) and slow-spread crust. P-wave velocity structure in the uppermost mantle of the Hawaiian North Arch shows a strong azimuthal anisotropy, whereas Moho reflections below the basement are variable: strong and continuous, weak, diffuse, or unclear. We assume that the strength of the Moho reflection is related to the aging of the oceanic plate. The Hawaiian volcanic chain (200 km to the southwest of the proposed drill site) and the nearby North Arch magmatism on the proposed Hawaiian North Arch sites might also have affected recognition of the Moho via deformation and/or magma intrusion into the lower crust of the uppermost mantle. This workshop report describes scientific targets for 2 km deep-ocean drilling in the Hawaiian North Arch region in order to provide information about the lower crust from unrecovered age and spreading rate gaps from previous ocean drillings. Other scientific objectives to be achieved by drilling cores before reaching the target depth of the project are also described in this report.
Highlights
The architecture of oceanic plates is the fundamental question for understanding why plate tectonics has occurred on the present Earth
A workshop on developing a proposal for ocean drilling in the Hawaiian North Arch region was held in Kanazawa, Japan, on 6–7 November 2018. The goals of this workshop were (1) to share information about the Hawaiian North Arch region and other proposed hard-rock drilling sites that would use Chikyu, (2) to identify major scientific objectives for ocean drilling into the northeastern Hawaiian North Arch, and (3) to evaluate possible drilling sites; 37 researchers and students participated in this workshop
Seismic observations along the Galapagos Spreading Center (GSC) show thickening on-axis and thinning total extrusive rocks with axial magma chamber (AMC) depth, indicating that axial valleys, which are formed by dike intrusions and/or fault displacement, develop to trap thick on-axis flows with a deepening AMC and a decreasing magma supply rate (Fig. 7; Blacic et al, 2004)
Summary
A location in a young hot plate is, not suitable for mantle drilling Another major restriction is water depth, which should be shallower than circa 4000 m below sea level (m b.s.l. hereafter), because the anticipated maximum total length of the Chikyu riser system is 11 000 m (4000 m water depth + 7000 m penetration to the Moho in normal oceanic crust). Most of the oceanic plate in the Pacific Ocean is too deep for mantle drilling Considering these constraints, three candidate drill sites have been suggested: off the coast of Hawaii (off-Hawaii hereafter), off the coast of Mexico (off-Mexico hereafter), and the Cocos plate (Umino et al, 2013; Fig. 2). The goals of this workshop were (1) to share information about the Hawaiian North Arch region and other proposed hard-rock drilling sites that would use Chikyu, (2) to identify major scientific objectives for ocean drilling into the northeastern Hawaiian North Arch, and (3) to evaluate possible drilling sites; 37 researchers and students participated in this workshop
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