paired metamorphic belts are mainly occurred in circum-pacific region,
particularly in japan. These metamorphic belts along which high-pressure
conditions. Late Mesozoic Ryoke-sanbagawa pairs in Honshu, late Paleozoic
hida-sangan and late Mesozoic kamuikotan-hidaka in Hokkaido are the three main
metamorphic belts. It also exposed along a northeastern-southwestern axis and
it parallel to the active subduction zone. It is mainly introduce the Sanbagawa
and Ryoke metamorphic belts
PAIRED OROGENIC METAMORPHIC BELTS OF JAPAN
The shikoku and Honshu are the islands of a
pair of metamorphic belts in japan is nearer to subduction zone. These belts
have different metamorphic signature but they are in same age.
METAMORPHIC BELT IN JAPAN
The ryoke belts lies to the NW direction, that
away from the subduction zone, these belts have low P/T conditions. They
containing rocks are formed in meta-pelitic sediments ranging up to sillimanite
zone. It represents low pressure, high- temperature condition
It composed of late
paleozoic volcanic or sedimentary fills, with grade increasing towards NW. The
belts which lies closer to the subduction zone.
zone is reached in the politic rocks. Basic rocks are formed with more common
than Ryoke belt. After it develop glaucophane with high-pressure or
low-temperature condition. Then the rocks formed in blue colour, that rocks
also called Blueschists.
This two belts are separated along a line by a
major fault zone is called median line. These belts are similar to sediments
that derived from relatively mature volcanic arc, during the sanbagawa belt
formed oceanward accretionary wedge with arc-derived sediments and volcanic mix
with oceanic crust and marine sediment.
ISOTHERMS AT SUBDUCTION ZONE
It shows a cross-section of subduction zone.
The 6000c isotherms are the thermal model of subduction zone, it is
deep as 100km in trench-subduction zone area. It can be shallow 20km beneath
the volcanic arc. The paired metamorphic belts have been widely accepted as
evidence for arc-trench system in the geologic past. There is a large
separating the two metamorphic belts in many pairs, examples are the median
tectonic line in western japan.
The highest-grade edge of the high P/T
metamorphic belts is directly cut by a faults of this kind in all paired belts,
whereas the lower P/T ratio metamorphic belts is at a distance from the fault
in some pairs, and is in direct contact with the fault in others.
The metamorphic belts on the ancient
continental side in paired metamorphic belts, which belong entirely to the low
P/T type in some cases, as in the Ryoke belt of japan. This belt formed in the
depth of ancient arc zones (Island arcs and active continental margins.)
In some pairs, the lower P/T ratio metamorphic belts is situated at a
distance from the above mentioned large faults separating the two belts. In
these cases the belts may show a broadly symmetrical thermal structure, with
metamorphic grade decreasing from a median axial zone towards both margins of
the belts. In other case the Ryoke belt pair is the ancient ocean-side half of
the belts has been lost by movement along the large faults, so that metamorphic
grade increases monotonically toward the fault. There should be common in any
ocean-continent or ocean-ocean subduction zone.
Figure.3- Metamorphic belts in the
(Figure.3) shows several examples in paired
metamorphic belts in circum-pacific region. These paired belts separated by 100
to 200km of less metamorphed materials, that area known as arc-trench gap, the
contact are typically shows major fault and it along dip-slip, but considerable
strike-slip are present.
The Japanese islands
are composed of many metamorphic belts. The Ryoke-sanbagawa paired metamorphic
belts are one of the best example. These Japanese belt sequence is indicating a
north-west subduction direction. The study of metamorphic belts of japan has
succeeded in allowing us to identify the geological and petrological
characteristics of these belts. It is helping to establish a more quantitative
understanding of regional metamorphism, which is the process going on in the
depth of the orogenic zone underneath the continental margin.