Thermal consequences of a subduction boundary jump: A numerical model for generating subduction‐related clockwise pressure‐temperature paths

Clockwise pressure‐temperature ( P‐T ) paths with a temperature increase after peak P have been reported from a number of subduction‐related metamorphic belts around the world. However, an adequate tectonic interpretation of these paths has been lacking. One of the principal difficulties is the lack of a sufficient heat source to account for the post peak P temperature rise. However, two‐dimensional thermal calculations incorporating accretion of a thick metamorphic unit in a subduction zone reveal that, in general, an accreted sheet with a width of several kilometers experiences a post accretion heating without having to appeal to any special internal heat source. The heating effect is due to the spatial shift of the subduction boundary, or subduction boundary jump. This heating can account for the generation of subduction‐related clockwise P‐T paths within thick metamorphic units. For such units the styles of clockwise P‐T paths vary considerably and can be used as a general indicator of the exhumation rate by considering the balance between conduction and advection during exhumation. In contrast, a thin unit with a width of ≅1 km tends to record a nonclockwise P‐T path regardless of its exhumation rate. This is because the thinness decreases the potential both for post accretion heating and for maintaining the higher achieved temperature during exhumation. In addition, a subduction boundary jump can cause a post accretion transition of metamorphic P‐T conditions from high to low P‐T ratios, and it may therefore be incorrect to use peak T metamorphic conditions as an indicator of the geotherm along the coeval subduction boundary.