[SCG64-05] Oceanic lower crust and uppermost mantle of the Shikoku Basin: insights from expeditions to Mado Megamullion
Keywords:Mado Megamullion, Shikoku Basin, Backarc basin, Oceanic lower crust and uppermost mantle
The Shikoku Basin is considered as a typical backarc basin that ceased seafloor spreading at ~15 Ma. We noted the presence of OCCs there since early 2000’s. Following the first preliminary dredge survey in 2007, we conducted focused research programs as three cruises in 2018 and 2019 (KH-18-2, YK18-07 and YK19-04S) on the Shikoku Basin axial OCCs. These programs successfully confirmed the presence of two OCCs and a non-transform offset (NTO) massif in the southernmost segment of the Shikoku Basin extinct spreading axis. During these cruises, we performed geophysical mapping, dredging, as well as in situ observation and sampling with the DSV Shinkai 6500. One of the OCCs, named Mado Megamullion, is an ~20 km square domed high with axis-normal corrugations, located at an intersection of a short spreading segment (~30 km) and short transform fault (~45 km). Mado Megamullion and the NTO massif are both associated with high mantle Bouguer anomalies. This observation is consistent with the exposure of deep-seated gabbros and peridotites, commonly sampled with dredge as well as the Shinkai. Overall, the mineral compositions of Mado gabbros define chemical trends interpreted as derivative of melts modified by assimilation of mantle material at the crust-mantle boundary. The Mado peridotites include the plagioclase-bearing lherzolites that suggest melt stagnation and melt-rock reaction in the dying backarc spreading lithosphere. The scheduled YK20-05S cruise in 2020 April will collect more datasets on this OCC.
In this contribution, we will report the preliminary results of YK20-05S cruise, as well as the compilation of the available datasets, to understand the tectono-magmatic characteristics of Mado Megamullion.