Diffuse vs Localized Deformation in Smectite-Rich Scaly Clays: What can we Learn From the Study of an Ancient Shallow Accretionary Prism?

Abstract:

The mechanical properties of fault rock along the plate interface are an essential component influencing slip behavior of the subduction plate boundary. In fact, the shallow part of the intraplate fault zone which slipped during the Mw 9.0 Tohoku-Oki Earthquake in Japan had a very high concentration of smectite (60–80 wt%). We present here the analysis of different deformation structures recorded in smectite-rich pelagic clays (the Argille Varicolori, Northern Apennines, Italy) that deformed in the shallow (maximum temperature of 70°-80°C) frontal part of a Late Cretaceous - Eocene accretionary prism before continental collision. Throughout systematic chemical, mineralogical and micro- to meso-structural analyses we document that different styles of deformation, from diffuse to localized, accompanied the evolution of this exhumed accretionary prism, showing a close relation with the composition of pelagic clays. During early accretionary stages, a diffuse ‘ductile’ deformation occurred in association with the development of a pervasive scaly fabric, well expressed in the horizons that are richest in smectite, together with isoclinal folding and transposition of the foliation. Mesoscale chevron folding and pervasive boudinage, occurred where pelagic clays were alternating with siltstone or sandstone layers. Localized slip surfaces, less than 1 mm thick, juxtaposed decameter to hectometer-scale blocks of homogeneous lithology, sharply cutting the scaly fabric and the transposed foliation, and suggesting that the deformation concentrated at the boundary between different lithologies. Brittle faults with cm-thick cataclasis crosscut the early structures and are likely associated to the collisional stage. The results of our study document that: (i) the style and partitioning of deformation and the shear localization was strongly influenced by the composition of sediments even when small changes in composition occurred, (ii) partition of deformation and shear localization along lithological boundaries was relevant, and (iii) the inherited deformation acquired during earlier accretionary stages played a significant role in controlling subsequent deformation, even if smectite-rich pelagic clays (i.e., the Argille Varicolori) experienced deformation only at shallow depth.