Thermal evolution of the Tertiary Shimanto Belt, Muroto Peninsula, Shikoku, Japan

Abstract

Abstract The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto Sub‐belt (Eocene‐Oligocene) and the Nabae Sub‐belt (Oligocene‐Miocene). Both sub‐belts have been affected by thermal overprints following the peak of accretion‐related deformation. Palaeotemperatures for the entire Tertiary section range from ∼ 140 to 315°C, based upon mean vitrinite reflectance values of 0.9–5.0%Rm. Values of illite crystallinity index are consistent with conditions of advanced diagenesis and anchimetamorphism. Illite/mica b0 lattice dimensions indicate that burial pressures were probably no greater than 2.5kbar. In general, levels of thermal maturity are higher for the Murotohanto Sub‐belt than for the Nabae Sub‐belt. The Eocene‐Oligocene strata also display a spatial decrease in thermal maturity from south to north and this pattern probably was caused by regional‐scale differential uplift following peak heating. Conversely, the palaeothermal structure within the Nabae Sub‐belt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. There is a paleotemperature difference of ∼ 90°C across the boundary between the Murotohanto and Nabae Sub‐belts (Shiina‐Narashi fault), and this contrast is consistent with approximately 1200 m of post‐metamorphic vertical offset.Subduction prior to Middle Miocene probably involved the Kula or fused Kula‐Pacific plate and the background geothermal gradient during the Eocene‐Oligocene phase of accretion was ∼ 30–35°C/km. Rapid heating of the Shimanto Belt evidently occurred immediately after a Middle Miocene reorganization of the subduction boundary. Hot oceanic lithosphere from the Shikoku Basin first entered the subduction zone at ∼ 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. The background geothermal gradient at that time was ∼ 70°C/km. Whether or not all portions of the inherited (Eocene‐Oligocene) palaeothermal structure were overprinted during the Middle Miocene remains controversial.