5:15 PM - 7:15 PM
[SMP28-P04] Peperitic textures along the Median Tectonic Line in Shikoku, southwestern Japan
Keywords:peperite, fault gouge, Median Tectonic Line
Peperite is a common rock type generated by the mingling of magma and wet unconsolidated sediment (Skilling et al., 2002). Numerous distributions of peperite have been reported in volcanic-related regions. There are a variety of peperitic textures depending on factors such as the mass interaction ratios of magma and sediments and the viscosity of magma. Since peperite could record processes that occur prior to explosions, the research has led to promoting understssanding volcanic explosion processes.
The Ishizuchi volcanic activity occurred in Miocene in the Shikoku region; dominantly, the central to western Shikoku. The Ishizuchisan mountain is the center of the volcanism. And then, the dikes, sills and lava flows disperse around the volcanic region. Chiba and Sakakibara (2006) reported the existence of peperite between the Middle Miocene Kuma Group and Ishizuchi Group. The peperite puts restriction on the deposition age for the Kuma Group.
On the other hand, we discovered two outcrops, Onda and Akeho outcrops, with peperitic textures along the Median Tectonic Line (MTL). Igneous rocks belonging to the Ishizuchi Group, andesite to rhyolite, with a variety scales distributes at some points along the MTL, and then mingled with fault gouge, which is widely accompanied along the MTL at the two sites.
We detected textures to characterize peperite under macroscopic or microscopic observation in these outcrops. Peperitic texture zones are accompanied by hydrothermal alteration and/or mineralization. The original textures of fault gouge are disturbed and obliterated in peperitic texture zones. Some quench bands distribute in contact zones of igneous intrusions and fault gouges. Peperitic breccia (Dsdd and Van Wagoner, 2002) of magma intruding wet gouge develop close to the margins of the igneous intrusions, but also more widely dispersed. There is a mixture of blocky and fluidal clasts with a variety of sizes and shapes.
The existence of peperitic textures provides beneficial information about the uplift rate around the north western Shikoku region. The volcanic intrusion into the fault gouge should occur in a shallow depth zone to form the peperitic texture, because the peperite formation needs wet unconsolidated gouge during Ishizuchi volcanism. The contact and mingling of the fault gouge and igneous rocks should occur less than 5 km depth (or maybe 1-2 km depth). Therefore, the peperitic texture can constrain the uplift rate of the western Shikoku region is estimated to be less than 0.3 mm/yr. Ohmori (1990) estimated that the western Shikoku region has risen by 0.3-0.5 mm/yr during the Quaternary. The uplift rate around the western Shikoku could be smaller than the previous thought values.
The Ishizuchi volcanic activity occurred in Miocene in the Shikoku region; dominantly, the central to western Shikoku. The Ishizuchisan mountain is the center of the volcanism. And then, the dikes, sills and lava flows disperse around the volcanic region. Chiba and Sakakibara (2006) reported the existence of peperite between the Middle Miocene Kuma Group and Ishizuchi Group. The peperite puts restriction on the deposition age for the Kuma Group.
On the other hand, we discovered two outcrops, Onda and Akeho outcrops, with peperitic textures along the Median Tectonic Line (MTL). Igneous rocks belonging to the Ishizuchi Group, andesite to rhyolite, with a variety scales distributes at some points along the MTL, and then mingled with fault gouge, which is widely accompanied along the MTL at the two sites.
We detected textures to characterize peperite under macroscopic or microscopic observation in these outcrops. Peperitic texture zones are accompanied by hydrothermal alteration and/or mineralization. The original textures of fault gouge are disturbed and obliterated in peperitic texture zones. Some quench bands distribute in contact zones of igneous intrusions and fault gouges. Peperitic breccia (Dsdd and Van Wagoner, 2002) of magma intruding wet gouge develop close to the margins of the igneous intrusions, but also more widely dispersed. There is a mixture of blocky and fluidal clasts with a variety of sizes and shapes.
The existence of peperitic textures provides beneficial information about the uplift rate around the north western Shikoku region. The volcanic intrusion into the fault gouge should occur in a shallow depth zone to form the peperitic texture, because the peperite formation needs wet unconsolidated gouge during Ishizuchi volcanism. The contact and mingling of the fault gouge and igneous rocks should occur less than 5 km depth (or maybe 1-2 km depth). Therefore, the peperitic texture can constrain the uplift rate of the western Shikoku region is estimated to be less than 0.3 mm/yr. Ohmori (1990) estimated that the western Shikoku region has risen by 0.3-0.5 mm/yr during the Quaternary. The uplift rate around the western Shikoku could be smaller than the previous thought values.