DEFINITION OF JAPAN’S PAIRED METAMORPHIC BELT

In a classic 1961 work, Akiho Miyashiro recognized that many circum-Pacific orogenic

belts can be divided into two component parts. On the

oceanward side of these orogens, Miyashiro recognized that

the rocks had experienced low-temperature high-pressure

metamorphism, whereas on the continentward side, he recognized

suites of high-temperature low-pressure metamorphism.

He named these orogens paired metamorphic belts and used

his native Japan as the prime example of such a paired metamorphic

belt.

Japan is an arc-trench system that rests as a sliver of the

North American plate above the Pacific plate outboard of the

Eurasian plate. This tectonic scenario has existed since the

mid-Tertiary. From the beginning of the Phanerozoic to the

Tertiary, Japan was part of the Eurasian continental margin

and was involved in interactions between Eurasia and the

Tethys and Panthalassian Oceans. A few occurrences of middle

Paleozoic rocks are known from Japan, but the vast

majority of strata are younger than middle Paleozoic. Most

of the rocks are aligned in strongly deformed structural belts

that parallel the coast for 1,850 miles (3,000 km) and include

fossiliferous marine strata, weakly to strongly metamorphosed

pelitic to psammitic rocks, and granitic intrusions.

Since most rocks are strongly deformed in fold-thrust belt

structures, it is inferred that the more strongly metamorphosed

units have been uplifted from deeper in the arc-accretionary

wedge system, or metamorphosed near the plutons.

The complexly deformed zones are overlain by littledeformed

Mesozoic-Cenozoic non-marine to shallow-marine

basin deposits. In addition, abundant Tertiary volcanic and

volcaniclastic deposits are present along the western side of

the islands, and in the Fossa Magna in central Honshu island.

In Japan the Sanbagawa belt represents a high-pressure,

low-temperature metamorphic belt, and the adjacent Ryoke-

Abukuma belt represents a high-temperature, low-pressure

metamorphic belt. Together, these two contrasted metamorphic

belts represent Japan’s paired metamorphic belt. In

Japan Miyashiro and others have deduced that these adjacent

belts with contrasted metamorphic histories formed

during subduction of the oceanic plates beneath Japan. The

low-temperature metamorphic series forms in the trench and

immediately above the subduction zone, where the cold subducting

slab insulates overlying sediments from high mantle

temperatures as they are brought down to locally deep highpressure

conditions. These rocks then get accreted to the

overriding plate and may become exhumed and exposed at

the surface in the high-pressure low-temperature belt. It is

not uncommon to find blueschist facies rocks containing the

diagnostic mineral galucophane, formed with pressures

greater than four kilobars and temperatures of 390–840°F

(200–450°C).

The adjacent Ryoke-Abukuma belt contains low-pressure

high-temperature, as well as medium-pressure, high-temperature

metamorphic rocks. These metamorphic rocks form

near the axis of the arc in close association with subduction-

derived magmas. Since the rocks in this belt form over a considerable

crustal thickness, and are associated with many

high-temperature magmas, they were metamorphosed at a

range of pressures and generally high temperatures.

A number of other paired metamorphic belts have been

recognized throughout the world. In the western United

States, the Franciscan complex contains rocks metamorphosed

at high pressures and low temperatures, whereas

rocks in the Sierra Nevada and Klamath Mountains contain

high-temperature low-pressure metamorphic facies. Other

paired metamorphic belts are recognized in Alaska, New

Zealand, Indonesia, Chile, Jamaica, and the Alps.

See also CONVERGENT PLATE MARGIN PROCESSES; METAMORPHISM.

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