Internal architecture of the frontal part of subduction accretionary prism: the role of folding in brittle diffuse deformation

Studies of the shallow part (Tmax< 150°C) of ancient and active prisms documents that ductile (e.g., folds), brittle and localized (e.g., faults) deformation, and diffuse deformation (e.g. scaly fabric), likely contemporaneous at the scale of geological time, may occur in the same outcrop. Moreover, the source areas of precisely located shallow slow earthquakes (downdip locating the transition from brittle to ductile deformation) span a range of depths from <1 to ~15 km below the seafloor, overlapping the temperature range of megathrust earthquake rupture in some convergent margins (e.g. Nankai and Japan Trench), and propagating up to the seafloor in other ones (e.g. Costa Rica). This implies that the shallow part of the accretionary prisms cannot be uniquely defined as characterized by ductile/brittle behavior or as prone to localization or de-localization of deformation.

To better understand how deformation affects the frontal part of accretionary prisms, the general architecture and internal structure of the frontal part of accretionary prisms need to be considered.  Field evidence of the deformational structures occurring in the frontal part of active and ancient accretionary prisms from the mesoscale to the regional scale suggests that recumbent and isoclinal folding are frequent in the shallow part of accretionary prisms, developing in pre-lithification to poorly metamorphosed rocks. In this framework, diffuse scaly fabric and pervasive boudinage, traditionally considered as evidence of shear delocalization, could be alternatively seen as the result of the progressive deformation envisioned in literature for the formation of recumbent folds i.e.: buckling with the development of an overturned fold limb and subsequent kinematic amplification by coaxial strain components with vertical maximum shortening. However, the plethora of brittle structures accommodating the change in shape on the hinge and limbs of the folds, have a different distribution in space and accommodate a different strain than brittle fracture associated with faults and/or thick shear zone. Therefore, this interpretation has implications in terms of the distribution in space of brittle structures and in the distribution of shear strain in the frontal part of subduction zones.