Investigating Block-in-Matrix Formation in Accretionary Prisms

The block-in-matrix fabric observed in modern and ancient accretionary prisms throughout the world has been studied in recent years with respect to mixed brittle-ductile deformation processes, which in turn may be associated with slow earthquakes phenomena. Many studies focus on slow earthquakes near the downdip limit of the seismogenic zone (~350°C) and implicitly assume that most of the deformation observed in outcrops predominantly developed at metamorphic depths or during subsequent exhumation.  However, field observations indicate that the block-in-matrix fabric is not exclusively metamorphic in origin, but may also form through sedimentary or tectonic processes, already in the very frontal part of accretionary prisms and under diagenetic conditions.  We present three examples from the exhumed accretionary prisms of the Northern Apennines and Western Alps. From the shallowest to the deepest, the case examples are: (i) the Ligurian Units in the Northern Apennines, Italy (Tmax <100-150 °C), (ii) the Sestola-Vidiciatico and Subligurian tectonic units in the Northern Apennines, Italy (Tmax ~100–200 °C) , (iii) the Infrahelvetic Units in the Western Alps, Switzerland (Tmax ~170-320°C). Our comparison of these examples suggests that block-in-matrix fabrics can record a prolonged deformation history and are not necessarily indicative of distinct pressure-temperature conditions. A pervasive mixed brittle-ductile block-in-matrix fabric can form at shallow structural levels due to sediment accretion and isoclinal folding before complete lithification. The eventual lithification of accreted sediments promotes strain localization and results in a brittle-frictional overprint on the block-in-matrix fabric. Upward migration of the plate interface and out-of-sequence thrusting can cause the deep burial of frontally accreted sediments, eventually leading to metamorphism. Temperature-dependent ductile deformation processes can further overprint inherited structures and amplify the block-in-matrix fabric. Competence contrasts between blocks and matrix can evolve with changing pressure–temperature conditions, allowing either activation or “freezing” of mixed brittle–ductile deformation at different depths. The shallow formation of the block-in-matrix fabric may be related to shallow slow earthquake phenomena in the upper plate accretionary wedge but also contributing to mechanical heterogeneities that later influence deeper earthquake behaviour.