P
PAIRED METAMORPHIC BELTS
Wolfgang Frisch
Department of Geosciences, University of Tübingen,
Tübingen, Germany
Definition
As first identified in Japan, pairs of elongate, parallel belts
with significantly different metamorphic rocks are indicative of former subduction activity. One metamorphic belt
consists of rocks formed by high-pressure metamorphism
and the other consists of rocks of the Abukuma facies
high-temperature metamorphic rocks. These two belts of
metamorphic rocks, which were formed under extremely
different pressure/temperature relations, occur in direct
association and are called “paired metamorphic belts.”
Orogenic processes involve uplift that raises portions of
the metamorphic rocks toward the surface and into the
zone of erosion. From the spatial arrangement of the two
contrasting belts at the surface, it is possible to detect not
only former subduction activity but also the polarity of
the subduction zone (Figure 1). The high-pressure belt
(subduction metamorphism) marks the area of the subduction zone, and the high-temperature belt (Abukuma-type
metamorphism) marks the location of the magmatic
arc. In Japan, the paired metamorphic belts indicate subduction beneath the Asian continent since the Permian
epoch. Since that period, the subduction zone has shifted
twice toward the present Pacific Ocean. The presently
formed metamorphic rocks are still hidden at depth.
High-pressure or subduction metamorphism
Rocks in subduction zones are metamorphosed under conditions of high pressure at relatively low temperature,
hence the terms high-pressure (properly high-pressure/
low-temperature) metamorphism or subduction metamorphism (Figure 2). Such metamorphism results because
cool rock material is rapidly subducted to great depths.
Normally, at a depth of 50 km that corresponds to a pressure of 1.5 gigapascal, the temperature is approximately
800–900
C. In contrast, oceanic crust that reaches the
same depth in a subduction zone reaches temperatures
between 200 to more than 500
C. The lower value is valid
for old, cool, rapidly subducting lithosphere, and the
higher value is for young and warm, more slowly
subducting lithosphere (Stern, 2002). The isotherms in
the subduction zone are strongly declined downward
because rocks are poor heat conductors and they need millions of years to adjust to the higher temperatures normally
seen at depth (Figure 3).
Rocks that have been buried within subduction zones
(to even more than 100 km) and subsequently exhumed
during orogenic processes can be observed directly on
outcrop. Metamorphic conditions in a subduction zone
transform basalts, dolerites, and gabbros of the oceanic
crust into glaucophane schists (blueschists) and eclogites
(Figure 2). In addition, sedimentary rocks and splinters
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016
PAIRED METAMORPHIC BELTS
Wolfgang Frisch
Department of Geosciences, University of Tübingen,
Tübingen, Germany
Definition
As first identified in Japan, pairs of elongate, parallel belts
with significantly different metamorphic rocks are indicative of former subduction activity. One metamorphic belt
consists of rocks formed by high-pressure metamorphism
and the other consists of rocks of the Abukuma facies
high-temperature metamorphic rocks. These two belts of
metamorphic rocks, which were formed under extremely
different pressure/temperature relations, occur in direct
association and are called “paired metamorphic belts.”
Orogenic processes involve uplift that raises portions of
the metamorphic rocks toward the surface and into the
zone of erosion. From the spatial arrangement of the two
contrasting belts at the surface, it is possible to detect not
only former subduction activity but also the polarity of
the subduction zone (Figure 1). The high-pressure belt
(subduction metamorphism) marks the area of the subduction zone, and the high-temperature belt (Abukuma-type
metamorphism) marks the location of the magmatic
arc. In Japan, the paired metamorphic belts indicate subduction beneath the Asian continent since the Permian
epoch. Since that period, the subduction zone has shifted
twice toward the present Pacific Ocean. The presently
formed metamorphic rocks are still hidden at depth.
High-pressure or subduction metamorphism
Rocks in subduction zones are metamorphosed under conditions of high pressure at relatively low temperature,
hence the terms high-pressure (properly high-pressure/
low-temperature) metamorphism or subduction metamorphism (Figure 2). Such metamorphism results because
cool rock material is rapidly subducted to great depths.
Normally, at a depth of 50 km that corresponds to a pressure of 1.5 gigapascal, the temperature is approximately
800–900
C. In contrast, oceanic crust that reaches the
same depth in a subduction zone reaches temperatures
between 200 to more than 500
C. The lower value is valid
for old, cool, rapidly subducting lithosphere, and the
higher value is for young and warm, more slowly
subducting lithosphere (Stern, 2002). The isotherms in
the subduction zone are strongly declined downward
because rocks are poor heat conductors and they need millions of years to adjust to the higher temperatures normally
seen at depth (Figure 3).
Rocks that have been buried within subduction zones
(to even more than 100 km) and subsequently exhumed
during orogenic processes can be observed directly on
outcrop. Metamorphic conditions in a subduction zone
transform basalts, dolerites, and gabbros of the oceanic
crust into glaucophane schists (blueschists) and eclogites
(Figure 2). In addition, sedimentary rocks and splinters
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016