Role of folding-related deformation in the seismicity of shallow accretionary prisms

Abstract

The sources of shallow slow earthquakes at subduction zone fronts remain unclear, but are commonly attributed to faults and shear zones. Structural studies of modern and ancient shallow accretionary prisms — wedge-shaped stacks of sediments and volcanic deposits scraped from subducting slabs and accreted onto the overriding plates at convergent plate boundaries — document a plethora of brittle structures associated with metres to plurikilometre-scale overturned and recumbent folds. These folds are the product of rock buckling and shearing at the front of subduction zones. At present, such structures are not commonly considered in models of the dynamics of accretionary wedges at the timescale of the seismic cycle, instead focusing on the role played by slip on major faults. Here we argue that fold-related brittle structures might also be associated with transient deformation events at elevated strain rates and in the presence of high fluid pressure. They have the potential to cause distributed microearthquake swarms occurring under low effective normal stress in accretionary prisms, and to affect the distribution of surficial displacement. Folding-related brittle deformation structures in accretionary wedges may contribute to shallow seismicity in subduction zones, according to a compilation of structural evidence.