Evolution of deformation bands, insights from structural diagenesis

Abstract

The vast majority of deformation bands (Dbs) studies focus on structural analysis only and neglect chemical/diagenetic effects. Here we report the structural diagenesis of Dbs occurring in the Tucano Basin (NE-Brazil). Although the occurrence of these structures in this basin has been well registered in literature, none of the researches related to them addressed the linking between structural and diagenetic processes. To fill this gap, we performed a multiscale (macro, meso and microscopic) analysis. Some key outcrops were selected and sampled for microscopic studies in order to investigate the deformational mechanisms active during Dbs formation and the coeval diagenetic processes. The diagenetic analysis reveals that the sandstones attained a minimum burial depth around 3 km and a temperature ≥100 °C, which means they reached the deep mesodiagenesis field. The nucleation of Dbs in these rocks started in the transition between the late eodiagenesis and initial (shallow) mesodiagenesis. In such conditions, the active deformational regime was the hydroductile flow, expressed by the dragging (bending) of rocks’ bedding (S0) by the Dbs. In the syn-lithification stage, transitional bands (hydroductile-cataclastic) developed exhibiting SC-type patterns and sigmoidal fractures. During deep mesodiagenesis, the cataclastic bands were formed by the cataclastic flow. These types of Dbs developed after mesodiagenetic quartz overgrowth and before (or synchronically) to chemical dissolution processes. The formation of tectonic matrix at this stage, along with the siliceous/ferruginous cementations, enhanced the strength of these structures.