How deep is the block-in-matrix formation in accretionary prisms?

The block-in-matrix fabric observed in subduction zones has been studied as an example of mixed brittle–ductile deformation potentially controlling the rheological behaviour of rocks. Many studies implicitly assume that most mixed deformation develops along the plate boundary interface or major fault at or below the brittle–ductile transition. We present four examples from the Northern Apennines and Western Alps, with maximum temperatures ranging from less than 100 to 350–475°C, which have been interpreted as having been deformed at the front of the upper plate of a subduction zone inside an accretionary prism and/or along the plate boundary interface. Our comparison of these examples suggests that block-in-matrix fabrics are not necessarily indicative of distinct pressure–temperature conditions or of deformation inside a major fault or shear zone. A pervasive mixed brittle–ductile block-in-matrix fabric can form at shallow structural levels in the upper-plate accretionary prism due to sediment accretion, isoclinal folding and boudinage of fold limbs before complete lithification. Out-of-sequence thrusting or the upward migration of the plate interface can cause the deep burial of frontally accreted and boudinaged sediments, and eventually the incorporation of already deformed block-in-matrix rocks along the plate boundary interface, leading to an increase in their pressure–temperature conditions.