Progressive development of block-in-matrix fabric in a shale-dominated shear zone: Insights from the Bobbio Tectonic Window (Northern Apennines, Italy)

Block-in-matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary, tectonic and mud-diapiric processes, in many exposed orogenic belts. A true mélange originates when this fabric is associated with mixing of rocks of different ages and provenance, as that characterizing the main décollement shear zone developed between the highly allochthonous Ligurian nappe and its substratum of foredeep deposits at the margins of the Bobbio Tectonic Window (Trebbia Valley, Northern Apennines). Mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement. Evidence of polyphased deformations, spanning from liquefaction-related and hydroplastic structures, to pseudo-hydrofracturing features and mineralized veining, have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix. Crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure. A novel evolutionary scheme for block-in-matrix fabric development is here proposed, involving the progressive dismembering of already highly deformed units, originally developed in non- to poorly lithified conditions, through a generalized simple shearing achieved during the evolution of the shear zone, coupled with strain hardening due to tectonic compaction, synkinematic diagenesis and fluid expulsion.