Question 9 of 13: Paired Metamorphic Belts

Question 9 (Part 2, Q3): Paired Metamorphic Belts (Part 2 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A paired metamorphic belt is a pair of parallel, broadly contemporaneous metamorphic belts of contrasting P/T type that develop together across a single subduction zone: an outer (oceanward) high-P/low-T belt (blueschist–eclogite facies) in the accretionary wedge/subduction channel, where cold subducted material is rapidly buried without time to heat up, paired with an inner (arcward) low-P/high-T belt (greenschist–amphibolite facies, locally Buchan-type) in the volcanic arc, where elevated heat flow from arc magmatism dominates.

Why they are important

Paired belts are direct petrologic evidence of an ancient subduction zone's polarity and asymmetry: the high-P/low-T belt marks the trench-side subduction channel and the low-P/high-T belt marks the arc, so mapping which belt lies on which side of a suture reconstructs the original subduction direction, even in old, deeply eroded orogens where the trench and volcanic arc themselves are long gone. This makes paired belts one of the primary tools for reconstructing pre-Mesozoic plate-tectonic history and identifying suture zones.

Examples

• Sanbagawa–Ryoke pair, Japan — the type example (Miyashiro, 1961): the high-P/low-T Sanbagawa belt (accretionary wedge) paired with the low-P/high-T Ryoke belt (magmatic arc), separated by the Median Tectonic Line.
• Franciscan–Sierra Nevada pair, California — the high-P/low-T Franciscan Complex (blueschist-facies accretionary wedge) paired with the low-P/high-T Sierra Nevada batholith/contact aureoles (the Cretaceous magmatic arc), separated by the Great Valley forearc basin.