Thermal evolution of the Tertiary Shimanto Belt, Muroto Peninsula, Shikoku, Japan
Thermal evolution of the Tertiary Shimanto Belt, Muroto Peninsula, Shikoku, Japan
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日本四国室户半岛第三纪四万十带的热演化
DOI：
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发表时间：
1992
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通讯作者：
Lee DiTullio
中科院分区：
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作者：
M. Underwood;M. Laughland;T. Byrne;J. Hibbard;Lee DiTullio
四国木本半岛上的岛曼托增生杂岩由两个主要的第三系地层单元组成：木本亚带(始新世-渐新世)和纳拜亚带(渐新世-中新世)。这两个亚带都受到了与吸积相关的变形峰值之后的热叠加的影响。根据镜质组反射率为0.9-5.0%Rm的平均值，整个第三系剖面的古温度范围为∼140-315℃。伊利石结晶度指数值与晚期成岩作用和安山变质作用的条件一致。伊利石/云母b0晶格尺寸表明，埋藏压力可能不超过2.5kbar。总体而言，Murotohanto亚带的热成熟度水平高于Nabae亚带。始新世-渐新世地层也表现出自南向北热成熟度的空间递减，这种模式可能是由于峰值加热后区域尺度的差异隆升造成的。相反，纳拜亚带内的古地热结构相当均匀，除了莫罗托角尖端的镁铁质侵入作用的局部影响。在Murotohanto和Nabae亚带(石池-奈拉希断裂)之间存在着∼90°C的古温差，这一对比与变质后约1200m的垂直错动相一致。
中中新世以前的俯冲可能涉及库拉或熔融的库拉-太平洋板块，始新世-渐新世增生阶段的背景地温梯度为∼30-35°C/km。在中中新世俯冲边界重组之后，岛曼托带的快速升温明显发生。来自四国盆地的热大洋岩石圈在∼15 Ma时首次进入俯冲带，这一事件也与日本海的开放和日本西南部的快速顺时针旋转相吻合。当时的背景地温梯度为∼70°C/Km。继承的(始新世-渐新世)古地热结构的所有部分是否在中中新世被叠置，目前仍存在争议。
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.