Fig. 5.2.2: Temperature anomalies in a subduction zone (from Frisch & Meschede, 2025)

In the subduction zone, the pressure exerted on the rocks increases with depth. The deeper the rock is pulled down, the greater the pressure rises due to the ever-increasing overburden. However, temperatures also rise with increasing depth, but it takes time to heat the cool oceanic lithosphere. While the pressure increases immediately due to the overburden, the temperature rise lags behind the pressure increase.

This creates a thermal anomaly in the subduction zone because rocks are poor conductors of heat and only gradually adapt to a changing ambient temperature. The downward transport of the cool oceanic lithosphere occurs faster than the equalization of the rock temperature to the temperatures normally prevailing at depth. The temperature isogrades, which indicate areas of equal temperature, are significantly deflected downwards in the subduction zone, as can be seen in Figure 5.2.2 in the blue shaded areas. These areas are a zone where high pressures (HP) are accompanied by relatively low temperatures (LT). This is where the so-called high-pressure-low-temperature metamorphism (HP/LT metamorphism) occurs.

Metamorphism is essentially divided into three main types: high-temperature metamorphism, which is almost exclusively determined by temperature and occurs in the vicinity of magma intrusions, for example, in volcanic arcs above a subduction zone; regional metamorphism, which is particularly important in mountain-building processes where continental crustal material is forced to great depths; and pressure-driven high-pressure metamorphism, which is characteristic of the subducting plate in subduction zones.

The rocks formed during metamorphism change with increasing intensity. At low metamorphic stages, schists such as greenschists and glaucophane schists are initially formed, which are then replaced at higher stages by amphibolites, gneisses, or eclogites. Granulites are formed at the highest possible stages of metamorphism. At even higher temperatures and pressures, melting, or anatexis, finally occurs.

The different types of metamorphism can be assigned to different tectonic situations. High-temperature metamorphism is limited to contact metamorphism, which refers to metamorphism in the immediate boundary zone with magma bodies. Here, the focus is solely on an increase in temperature. Abukuma-type metamorphism, which characterizes temperature-driven metamorphism beneath volcanic belts, actually belongs to regional metamorphism, since pressure increases—albeit to a lesser extent—also play a role.

Metamorphic zones arise from different tectonic situations: In a subduction zone, “cold” oceanic lithosphere is transported relatively quickly to great depths. The pressure is equalized immediately, but the temperature lags behind. In a volcanic arc, the ascent of magma leads to a significantly increased heat flow, so that rocks at shallow depths are exposed to high temperatures. Temperature anomalies in the subduction zone reflect this: Typical for convergent plate margins is the coexistence of two specific metamorphic zones in paired metamorphic belts: high-pressure, low-temperature metamorphism (HP/LT) alongside high-temperature, low-pressure metamorphism (HT/LP).

In Japan, which has been situated on a subduction zone for hundreds of millions of years, paired metamorphic belts have been preserved, serving as indicators of past subduction activity. These include the Ryoke and Sanbagawa belts, now uplifted in mountains, which formed during the Jurassic and Cretaceous periods, and the Hida and Sangun belts, which were active from the Permian to the Jurassic. The Abukuma Plateau is part of the Ryoke Belt, which is characterized by temperature-dominated metamorphism. This plateau gives its name to the temperature-dominated Abukuma Metamorphism.