In this study, we describe normal fault zones cutting foreland arkosic turbidites in deep diagenesis conditions (200°C). The microstructural analysis of the fault cores reveals a large amount of phyllosilicates (up to 34%) in the fault rock, derived from an intense feldspar muscovitisation. This process is favored by mechanical disaggregation of the feldspar grains following cleavage planes during deformation. Point-counting on thin sections sampled in the studied fault zones allows to quantify the proportion of feldspar transformed in white micas between 60-80%. We propose a modified version of the predictive Shale Gouge Ratio algorithm in order to integrate the proportion of phyllosilicates resulting from feldspar muscovitisation. We discuss the importance of feldspar alteration within faults of the deep brittle crust and its role on fault strength and high temperature reservoir partitioning.


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