The role of mechanical stratigraphy on the refraction of strike-slip faults
Authors: Carlini Mirko , Viola Giulio , Mattila Jussi , Castellucci Luca 
Published: August 2018

Abstract. Fault and fracture planes (FFP) that cut through multilayer sequences can be significantly refracted at layer-layer interfaces due to the different mechanical properties of the contiguous layers, such as shear strength, friction coefficient and grain size. Detailed studies of different but coexisting and broadly coeval failure modes (tensile, hybrid and shear) within multilayers deformed in extensional settings have led to infer relatively low confinement and differential stress as the boundary stress conditions at which FFP refraction occurs. Although indeed widely recognized and studied in extensional settings, the details of FFP nucleation, propagation and refraction through multilayers remain not completely understood, partly because of the common lack of geological structures documenting the incipient and intermediate stages of deformation. Here we present the results of a study on strongly refracted strike-slip FFP within the mechanically layered turbidites of the Marnoso Arenacea Formation (MAF) of the Italian Northern Apennines. The MAF is characterized by the alternation of sandstone (strong) and carbonate mudstone (weak) layers. The studied refracted FFP formed at the front of the regional-scale NE-verging Palazzuolo anticline and post-date almost any other observed structure except for a set of late extensional faults. The studied faults display coexistence of shear and hybrid (tensile-shear) failure modes and we suggest that they initially nucleated as shear fractures (mode III) within the weak layers and, only at a later stage, propagated as dilatant fractures (mode I-II) within the strong layers. The tensile fractures within the strong layers invariably contain blocky calcite infills, which are, on the other hand, almost completely absent along the shear fracture planes deforming the weak layers. Paleostress analysis was performed to constrain the NNE-SSW compressional stress field that produced the refracted FFP and to exclude the possibility that the present attitude of these structures may result from the rotation through time of faults with an initial orientation. Slip tendency analysis was also performed to infer the relative slip and dilation potentials of the observed structures. Mesoscopic analysis of preserved structures from the incipient and intermediate stages of development and evolution of the refracted FFP allowed us to build an evolutionary scheme wherein: a) Nucleation of refracted FFP occurs within weak layers; b) Refraction is primarily controlled by grain size and clay mineral content and variations thereof at layer-layer interfaces but also within individual layers; c) Propagation within strong layers occurs primarily by fluid-assisted development ahead of the FFP tip of a “process zone” defined by a network of hybrid and tensile fractures; d) The process zone causes the progressive weakening and fragmentation of the affected rock volume to eventually allow the FFP to propagate through the strong layers; e) Enhanced suitable conditions for the development of tensile and hybrid fractures can be also achieved thanks to the important role played by pressured fluids.