R6: Plutonic rocks, volcanic rocks and subduction factory

[R6-03] Muographic study of the oceanic crust-mantle density structure of the Oman Ophiolite

*Susumu UMINO¹, Hiroyuki Tanaka², László Oláh², Dezső Varga⁴, Tomoaki Morishita¹, Yoshihiro Hiramatsu¹, Yuki Kusano³ (1. Kanazawa University, 2. University of Tokyo, 3. AIST, 4. Wigner Research Center for Physics)

Keywords:Muography, Oman Ophiolite, Oceanic crust - Mantle, Moho transition zone, density structure

The oceanic lithosphere is divided into crust (layers 1-3) and mantle (layer 4) according to seismic wave reflection surfaces and velocity structures. The Layer 1-2 boundary corresponds to the boundary between sediment and basement rock. On the other hand, the Layer 2-3 boundary (upper-lower crustal boundary) and the Layer 3-4 boundary (Moho) are not well understood due to difficulties in observation and lack of basic understanding of geology and lithology. The Layer 2, 3, and 4 have been correlated with the extrusive rocks and sheeted dikes, gabbros, and peridotites, respectively, based on comparison with ophiolite geology. However, in Hole 504B, the only deep-sea drilling that penetrated the Layer 2-3 boundary, the boundary is located in the middle of the sheeted dike complex, which is attributed to alteration and reduced porosity^a. Hole 1256D penetrated into the gabbros, however, the Layer 2-3 boundary has not been reached yet^b. The drilling through the Moho and into the mantle is the flagship research project of the next IODP and the most important target of the Project Mohole, but it has not yet been accomplished. Therefore, comparative study of ophiolites is the only clue to understanding the relationships between seismic velocity structure and geology. In particular, the Oman ophiolite, the largest and best preserved in the world, has been intensely studied as an analogue of the fast spread crust and mantle. Kanazawa University, the University of Tokyo, Wigner Research Center for Physics in Hungary, and the Ministry of Energy and Minerals of the Sultanate of Oman are collaborating in an international joint project to image the lithologic boundaries of the Oman Ophiolite, corresponding to the crust-mantle boundary, upper-lower crust boundary, and layer 2A-2B boundary, on the scale of 100 m to km, which can be directly compared with the seismic structure. This presentation will give an overview of the project and the results of preliminary investigations.