Plumes in central Panthalassa? Deductions from accreted oceanic fragments in Japan

Abstract

Accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of Japan. Major accretion occurred twice; a late Early Carboniferous seamount chain acefeted in latest Permian to Middle Jurassic time, and a Late Jurassic oceanic plateau acefeted in the Early Cretaceous. The Late Jurassic plateau now in northern Japan and Sakhalin is here named the "Sorachi Plateau" and a Tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones. Petrochemical characteristics of basalts suggest two kinds of sources; depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling. All the accreted fragments in Japan can be traced back to the central part of the Panthalassa by using paleomagnetic data and plate "trajectory analysis." The timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change. Their place of origin, in the mid-Panthalassa, retains a positive residual geoid anomaly and slow Vp in the lower mantle, which suggest a past upwelling of hot material from the core-mantle boundary. Introduction Recently, large-scale igneous activities such as continental flood basalts and oceanic plateau volcanism have aroused interest because of their significance in understanding activity in Earth's interior and possible catastrophic effects on the global change of environment [e.g., Courtillot, 1990; Richards et al., 1989; Coffin and Eldholm, 1991; Duncan and Richard, 1991]. Periodicity of such igneous activity through geologic history has also been suggested [Courtillot and Besse, 1987; Rampino and Stothers, 1988; Larson, 1991a, b]. The central to southern Pacific Ocean is a place where several active hot spots are located. A positive residual geoid anomaly [Anderson, 1982; Chase and Sprowl, 1982; LePichon and Huchon, 1983] and slow Vp in the lower manfie [Morelli and Dziewonski, 1987; Tanirnoto, 1990; Fukao, 1992] suggest a broad upwelling of hot manfie beneath this region. The upwelling can be traced back to the Mesozoic by using hot spot tracks and plate kinematic reconstructions [Anderson, 1982; Chase and Sprowl, 1982; Tarduno et al., 1991; Anderson, 1990]. Cretaceous plume upwelling in this region is documented by studies of huge plateaus such as Ontong Java, Hess Rise, and others [Nakanishi et al., 1989, 1992a, b; Larson, 1991a; Tarduno et al., 1991]. Plume activity before the Cretaceous in the old Pacific Ocean (Panthalassa superocean) is obscured because few Jurassic parts of plates are preserved. The only way to clarify plume activity before the Cretaceous is through a geological analysis of accreted oceanic materials within circum-Pacific orogens. We have reviewed accretionary complexes in the western circum-Pacific orogens, especially in the Japanese islands, and have identified two periods of accretion of seamount chains or plateaus; accretion of a late Early Carboniferous seamount chain between the end of the Permian and the Middle to Late Jurassic and of a Lat...

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

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Formation and evolution of Shatsky Rise oceanic plateau: Insights from IODP Expedition 324 and recent geophysical cruises

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