The Median Tectonic Line (MTL) is a strike slip fault system, and is the largest crustal break in southwest Japan. This region, which includes western Honshu (Chūgoku), as well as Shikoku and northern Kyushu, is divided into the Inner Zone, formed by complex faulting, and the Outer Zone, formed by warping. The Inner Zone is chiefly composed of ancient granites, rocks of Paleozoic age (250 to 540 million years old), and geologically more recent volcanic rocks, which are arranged in complicated juxtaposition.
The Outer Zone, consisting of the Akaishi, Kii, Shikoku, and Kyushu mountain groups, is characterized by a regular zonal arrangement from north to south of crystalline schists and Paleozoic, Mesozoic (65 to 250 million years old), and Cenozoic (formed within the past 65 million years) formations (4). The MTL bisects the island arc into the old terranes intruded by igneous rocks in the Inner Zone and partly metamorphosed accretionary complexes in the Outer Zone (1).
The Outer Zone, consisting of the Akaishi, Kii, Shikoku, and Kyushu mountain groups, is characterized by a regular zonal arrangement from north to south of crystalline schists and Paleozoic, Mesozoic (65 to 250 million years old), and Cenozoic (formed within the past 65 million years) formations (4). The MTL bisects the island arc into the old terranes intruded by igneous rocks in the Inner Zone and partly metamorphosed accretionary complexes in the Outer Zone (1).
The MTL separates anadalusite-sillimanite-type metamorphic rocks and granitic rocks within the Ryoke terrane from glaucophanic-type metamorphic rocks within the Sambagawa terrane (6). The magnitude of MTL displacement is difficult to estimate and poorly constrained in the order of around 200–1000 km (3) and its period of activity is as long as 100 m.y., and posses a highly complicated change in slip direction (1).
The MTL is found on the Eurasian Plate, with the Philippine Sea Plate at the South, the North-American Plate at the North-East and the Pacific Plate at the East. Mineral ages taken from Protomylonite samples in the MTL suggest that a significant ductile phase of movement occurred within it in the Middle Paleocene. This may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the Sambagawa and Ryoke belts (6).
The MTL is found on the Eurasian Plate, with the Philippine Sea Plate at the South, the North-American Plate at the North-East and the Pacific Plate at the East. Mineral ages taken from Protomylonite samples in the MTL suggest that a significant ductile phase of movement occurred within it in the Middle Paleocene. This may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the Sambagawa and Ryoke belts (6).
| The MTL constituted a larger fault zone together with the Central Sikhote Alin Fault, and had a left-lateral slip sense as a result of the quite rapid northerly motion of the Izanagi Plate (1). The MTL was probably formed in the Cretaceous with many subsequent reactivations, its western segment is still active with an almost pure right-lateral sense of motion and, although a great amount of geological information on the MTL has been accumulated, information about the subsurface, especially the deep-seated structure of the MTL, is still insufficient (5). |
References
- Neotectonic Intra-Arc Basins Within Southwest Japan http://www.intechopen.com/books/mechanism-of-sedimentary-basin-formation-multidisciplinary-approach-on-active-plate-margins/neotectonic-intra-arc-basins-within-southwest-japan-conspicuous-basin-forming-process-related-to-dif
- American Journal of Science Geological Map of the Median Tectonic Line http://www.ajsonline.org/content/310/10/1210/F1.large.jpg
- Internal structure and permeability of major strike-slip fault zones: the Median Tectonic Line in Mie Prefecture, Southwest Japan http://www.sciencedirect.com/science/article/pii/S0191814102000147
- Japan, The major physiographic regions http://www.britannica.com/EBchecked/topic/300531/Japan/23233/The-major-physiographic-regions#ref319150
- Gently North-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan http://www.sciencedirect.com/science/article/pii/S0040195196001175
- Middle Paleocene terrane juxtaposition along the Median Tectonic Line, southwest Japan: Evidence from 40Ar/39Ar mineral ages http://www.sciencedirect.com/science/article/pii/004019519190020S
- Application of the Bayesian Approach to Incorporate Helium Isotope Ratios in Long-Term Probabilistic Volcanic Hazard Assessments in Tohoku, Japan http://www.intechopen.com/books/updates-in-volcanology-new-advances-in-understanding-volcanic-systems/modeling-long-term-volcanic-hazards-through-bayesian-inference