Mélanges in flysch-type formations: Reviewing geological constraints for a better understanding of complex formations with block-in-matrix fabric

The geological term “mélange” is used to indicate a vast group of “chaotic rock” units with a block-in-matrix internal arrangement exhumed in orogenic belts. Geotechnically such units are often defined as “complex formations”, and are commonly associated with flysch-type formations. Such units range from coherent, bedding-concordant broken-formations to fully disrupted, discordant block-in-matrix rock assemblages. Mélanges derive from the progressive disruption and mixing of flysch-type successions from their earliest phases of development to their complete lithification and exhumation. They are the product of sedimentary (submarine mass transport), tectonic (thrust-related shearing) and mud-diapiric (fluid/gas driven remobilization of mud/shales) processes. Throughout a review of geological data, we show that the different intrinsic anatomical features and relationships with the host rocks shown by these various units allow differential internal anisotropy of the lithological properties, and, depending on their origin, size and distribution, they can variably influence the overall mechanical behavior of the host flysch-type formation. The review of geological data thus provides useful observations to distinguish different types of “complex formations” and block-in-matrix rocks with different geotechnical and mechanical characteristics, and then potentially suggesting the application of different rock or soil mechanics procedures.