Introduction to the Landforms and Geology of Japan


Tei Mélange and Muroto
  -- Shimanto accretionary complex

Location of ShikokuFig. 1 Location of Shikoku

Southern Shikoku belongs to the Shimanto Belt (accretionary complex) in basement geology. Rocks of the Shimanto complex are exposed in many places along the coast of southern Shikoku. There are good spots to see the rocks including turbidites and mélange in the Muroto peninsula, southeastern Shikoku. The Tei mélange exhibits typical rocks of the Shimanto Belt. Cape Muroto is a very popular place for tourists. One of the features of the cape is a rugged coast landscape consisting of extremely tilted and deformed strata, gabbro, and landforms that resulted from uplift and wave erosion. Muroto was designated a Japan Geopark in 2009 and registered as a Global Geopark in September 2011. At Cape Muroto, there is a walking trail for looking round rocks along the beach and guide plates with English text. A visitor center provides information on the Muroto area.

In this topic, the Shimanto Belt is briefly explained first, and then geological spots are introduced, which include a beach in Nishibun, Geisei Village for mélange and rocks of oceanic plate stratigraphy and the Muroto area for turbidite, submarine-slide deposits, gabbro, and elevated coastal landforms. Access to each spot is shown on the last page.

Shimanto Belt

The Shimanto Belt is a accretionary complex zone formed 100 to 30 million years ago (Cretaceous to Paleogene), distributed in the outermost end (Pacific side) of southwest Japan from the Boso Peninsula in Kanto to the Nansei Islands (1800 km in length and 100 km in maximum width). The Shimanto Belt is characterized by a typical accretionary complex; countless thrust blocks tilted northward become younger toward south on the whole and formations consist mainly of sandy/muddy turbidites and mélange including basalt and chert that compose the oceanic crust of subducting plate (see “Cape Gyodo” for turbidite. Mélange is described below). The details of accretionary complex were first elucidated by land geological surveys in the Shimanto Belt. The characteristics and formation process are explained in the section “Accretionary prism”. Alternating beds of sand and mud (terrigenous sediments) are predominant in formations of this belt, which were formed by turbidity currents of mixture of sand and mud. Many of the beds were strongly deformed and tilted at a more than 60 degrees northward. Generally, about 1000 m thick alternating beds repeatedly appears by folding and faulting in the formations.

Mélange zones, the width of which ranges from several hundred meters to two kilometers, are inserted into the alternating beds of sand and mud formations. Faults are borders between the mélange and the alternating beds zones. Mélange is a chaotic mixture of rocks consisting of broken rock fragments of various sizes and muddy matrix. The word, “mélange” is a descriptive term, whose meaning has not how the mélange was produced. Mélange in the mélange zones contains blocks of basalt (pillow lavas), limestone, chert, and shale in terrigenous muddy matrix. The contained blocks constituted the oceanic crust formed far from land. They were scraped off and accreted to an accretionary prism on the landward plate when the oceanic plate was subducting (see the section “Accretionary prism”).

The basalt (pillow lavas) was produced at a mid-ocean ridge. The limestone includes nannoplankton fossils. The chert is dense siliceous rock mainly composed of shells of radiolarian and occurs as thin red or green beds. The shells deposited very slowly. The deposition rate is estimated to be about 5 m per one million years from the study of the chert, which is 1000-fold less than that of turbidite. The shale occurs as green, red, or white alternating beds (varicolored shale) on the whole. The shale is hemipelagic sediments, including radiolarian fossils, terrigenous silty clastic materials such as quartz and feldspar particles, and volcanic ash (white beds). In these mélanges in the Shimanto Belt, sheared structures with a constant sense are observed. Mélange that has the structure may be called “tectonic mélange”. Some mélanges in the Shimanto Belt were formed by debris flows.

Sheared structure
Fig. 3 Examples of sheared structure

Photo 1 Mélange (Tei Mélange) [Another window]
Deformed blocks in the mélange are basalt.

Since the Shimanto accretionary complex has few large fossils such as bivalves but an abundance of radiolarian fossils, the ages of formations were determined in detail using the radiolarian fossils. As a result, the alternating beds of sand and mud are classified into four age groups: 120 to 100 million years old (Early Cretaceous), 100 to 65 million years old (Late Cretaceous), 50 to 40 million years old (Eocene to Oligocene), and 30 to 20 million years old (Late Oligocene to Miocene) in this order from north. The Cretaceous part and the Tertiary part of the Shimanto Belt may be referred to as the northern Shimanto belt and the southern Shimanto belt, respectively. The oldest pillow lavas and chert in the mélange zones are aged 130 million years and the varicolored shale is aged 90 to 80 million years. The age of muddy matrix is similar to that of the alternating beds of sand and mud. Figure 4 shows the stratigraphy of the Shimanto accretionary complex (Late Cretaceous). In addition, paleomagnetic chronology revealed that the pillow lavas and limestone were formed near the equator and the alternating beds of sand and mud at about 30 degrees north, which is close to the present latitude of the Shimanto Belt. The pillow lavas and limestone travelled over 60 million years on a way of at least 3000 km by plate moving to meet proto-Japan.

Stratigraphy of the Late Cretaceous Shimanto belt
Fig. 4 Stratigraphy of the Late Cretaceous Shimanto belt
See also Fig. 7 in “Accretionary prism”. mya = million years ago (Based on Taira, 1990)

(Descriptions in this section mainly based on Taira, 1990 and 2004)

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