By nature, Japan is a fault-riddled country, due to being squashed between the westward-moving oceanic plates and the eastward-moving continental plates on which the archipelago sits. The MTL, though, is different from pressure faults caused by this compression: it dates back to the Cretaceous and had a unique role to play in the composition of the Japanese Islands.
The MTL Museum for Japanese Geology
The Ohshika Geological Museum of Median Tectonic Line was established in 1993 in Oshika Village, Nagano Prefecture to showcase this massive fault line. The museum’s location is remote: not chosen for easy access but for proximity to the fault’s excellent exposure at two nearby outcrops. The museum building itself is of recent modern construction, built in 1992 and funded by Oshika Village. This is a one-man museum: Mr. Kazurou Kawamoto serves as Curator, researcher, and guide. To visit his museum is to avail oneself of his fathomless enthusiasm for Japanese geology.
The ground floor exhibits tell of the formation of the Japanese Islands and describe the several geological belts that make up the archipelago. Small rock samples of every kind are on display, while much larger rocks sit outside the museum. Upstairs in the centre of the room, there is a model of the region with buttons to push elevating the landforms along the fault. Though the rocks are labeled in both Japanese and English, most written explanations are in Japanese. However, Mr. Kawamoto speaks English from his five years in the US — be sure to visit on a day he is there and not out in the field doing research!
The rocks in the front courtyard are arranged in groups derived from the four geological belts that conveniently converge near the museum. A look at the map (below) reveals that these belts are:
- the Ryoke belt: a Jurassic sedimentary belt (accretionary complex) with Cretaceous volcanics lightly metamorphosed by high-temperature/low-pressure
- the Sanbagawa belt: a pelitic and mafic schist belt metamorphosed at depth by low-temperature/high-pressure
- the Chichibu belt: another Jurassic accretionary complex
- the Northern Shimanto belt: a Cretaceous accretionary complex
One might note from the map that these belts twist up and around the Kanto region, pushed out of line by the intrusion of the Izu Arc into central Honshu. The MTL generally forms the boundary between the Ryoke and Sanbagawa belts.
Outcrops of the Median Tectonic Line in Oshika Village, Nagano
A museum display photo (below) illustrates the fault line in red as it transverses the hills above the village and then dips under the alluvial plain as a blind fault. A cross-section diagram imposed on the photograph shows the Ryoke belt to the left in green, and the Sanbagawa belt to the right in purple-blue. The museum sits at the juncture of the cross-section and the fault line in the middle of the valley.
The fault exposures are somewhat removed from the museum, necessitating car travel. Going three kilometres downstream along the Koshibu River to the village hall and 14 kilometres upstream along Kashio River, one comes to the Kitagawa outcrop; here the fault zone is only 20m wide. Bands of fault gouge and the 6m wide Ryoke cataclastites can clearly be seen in the riverbank cutting. At the second Ankoh outcrop, the mylonite zone is about 1km wide, while Ryoke cataclastites are 150m wide.
The MTL: a Strike-Slip Fault through Southwestern Japan
Extending over 1200 km, the Median Tectonic Line runs east-west, parallel to the island arc: through the centre of Kyushu Island, across the top of Shikoku Island, and into Honshu Island from the Kii Peninsula up into the Kanto region. It has a complicated history and a regionally diverse nature today. Generally, the MTL fault zone can be characterized as consisting of high-angle right-lateral faults and low-angle reverse-slip faults, but in the Oshika area, the fault dips almost vertically.
The earliest mylonite produced from fault movement friction dates to the Late Cretaceous (ca. 90 million years ago); it is derived from granite that was buried 15km underground where the temperature was high enough for the quartz in the granite to be plastic. At this time, the fault had a left-lateral (sinestral) movement. Geologist Asahiko Taira hypothesizes that the landmass lying on the south side of the MTL was brought into its current position from over 2000 km to the south through strike-slip fault action.
In the Quaternary (from 2 million years ago), the MTL changed direction to move as a right-lateral (dextral) fault, turning the southern parts of Shikoku and the Kii Peninsula into a micro-plate that is travelling west. The Shikoku segment and western Kii Peninsula segment of the MTL are some of the most active faults among countless active faults in Japan. The MTL in the Oshika region is recognized as a class C active fault; in Shikoku and in western Kii, the fault is class A, moving 100 times faster than C. The Shikoku segment of the MTL is the only part active today, where it is the fastest moving land-based fault in Japan, moving at 5-10mm per year. Earthquakes along this fault recur between 1000 and 3000 years, and the MTL may have been involved in the great Keicho-Kinki earthquake of 1596.
Reference:
Taira, Asahiko (2001) “Tectonic evolution of the Japanese island arc system.” Annual Review of Earth and Planetary Sciences 28:109-34
Matsushima, Nobuyuki (1993) Cross-section of MTL trajectory, drawn for the MTL Museum (photo).
Acknowledgment:
Thanks to Kaz Kawamoto for certain data, corrections about the MTL fault, permission to use the annotated photograph with cross-section by Matsushima (1993), and his own added English overlay.
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