[S-CG46] ハードロック掘削科学〜陸上掘削から深海底掘削そしてオマーン〜

[SCG46-02] Overview and Preliminary results from IODP Expedition 396: Mid-Norwegian Continental Margin Magmatism

*Sayantani Chatterjee¹、Sverre Planke²、Christian Berndt³、Carlos. A. Alvarez Zarikian⁴、Expedition 396 Scientists、Eiichi TAKAZAWA^1,5 (1.Niigata University, Department of Geology, Faculty of Science, Japan、2.Centre for Earth Evolution and Dynamics, University of Oslo, Norway、3.GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany、4.International Ocean Discovery Program Texas A&M University, USA、5.Japan Agency for Marine-Earth Science and Technology, Japan)

キーワード：Volcanic passive margins, Plume magmatism , Mantle source heterogeneity, PETM, Mid-oceanic ridges

Mobility of the fragmented lithosphere (or plates) over a weaker asthenosphere and subduction (destruction of plates) in our planet earth is somewhat unique in the solar system. Consumption of plates in the subduction zone is counteracted by the generation of the new lithosphere at mid-oceanic ridge settings, where divergent plate motion leads to mantle melting and subsequent formation of new oceanic crust by crystallization of magma. However, the magma outcome is not equally distributed throughout the Earth instead it is believed to be intrinsically linked with the rate of plate spreading. The making of a self-sustaining divergent plate boundary usually traces back from the continental rifting (due to lithospheric thinning and/or uprising mantle plume) and is considered a fundamental step in the Wilson cycle. Usually, continental rifting under influence of mantle plumes causes enormous magmatic outpour in spite of significant investigation spanned through the last few decades it is still unknown how such excess volcanism took place in a surprisingly short duration and the major controlling geodynamic factors behind it. According to the Wilson cycle disassembly and assembly of the supercontinents starts with initial pre-drift extension followed by rift-to-drift phase, the initial opening of an oceanic basin, seafloor spreading, widening of the basin, subduction of oceanic lithosphere, closure of the basin, and finally continent-continent collision. A classic example of continental extension, breakup, and formation of new mid-oceanic spreading centers can be found in the Atlantic ocean. The North Atlantic Igneous Province eruptions started around 61-62 Ma and formed the northern Atlantic passive margins to the present state. It is still poorly unresolved that, what is the key mechanism behind the excess amount of magmatism in a surprisingly short duration causes the breakup of the northeast Atlantic.
The main drive of the IODP Expedition 396 in August and September 2021, was to understand the relationship between rifting, enormous magmatism, and paleoclimate, and to resolve the relative contribution from plume upwelling, small-scale convection, and mantle heterogeneity and their relation to the formation of volcanic rifted margins during the northeast Atlantic continental breakup. Departing from Iceland, JOIDES Resolution sailed northeastward and drilled thick sequence of seaward dipping volcanics on the Mid-Norwegian continental margin on the western edge of the Vøring Basin. A total of 21 boreholes were drilled during Expedition 396 and successfully acquired cores of marine volcanic sediments and hard rock basement from nine boreholes along-strike and one cross-strike margin transect. The total drilling depth was 3950 with high recovery (overall about 57%), more than 500 m of basement rocks, and more than 2000 m of Paleogene sediment were drilled. A combination of RCB, XCB and APC drill bits were used during this expedition. The key paleoclimate interval the Paleocene-Eocene Thermal Maximum (PETM) in the Modgunn and Mimir transect; and the basalt stratigraphic intervals across the volcanic rifted margin, including both subaerial and deep marine sheet flows with inter-lava sediments were successfully recovered.