Mass-transport processes and sedimentary mélanges: Deformation structures, stratal disruption and exotic blocks

Due to the recent advances in the investigating methods and the increasing work of comparison between mélanges in mountain chains worldwide, several mèlanges are now recognized as mass-transport complexes (MTCs), variously deformed by post-depositional, tectonic and/or mud-diapiric processes (sedimentary mélanges). We present here the results of structural and sedimentological studies of sedimentary mélanges and more classic MTCs in the Apennines of Italy and the Pyrenees of Spain. These bodies, especially the basinwide ones, are complex units involving the entire spectra of mass-transport processes. The down-slope motion of these bodies is enabled by the relative movement of discrete masses, with progressive stratal disruption of rocks/sediment involved and flow transformation. The results are twofold. First, three kinds of MTC are distinguished, in which the relative movements inside the bodies and respect the substratum are enabled by 1) shear-dominated viscous flows within a muddy matrix, 2) mud-silt-sandy matrix sustained by fluid overpressure, 3) concentrated shear zones/surfaces with advection of grains and fluids (overpressured basal carpets). The structural analysis of deformed horizons at the base and below the MTCs shows that the three types correspond to different kinematics of transport and emplacement and to different coupling with the substratum. Second, the potential of sedimentary processes in generating mélanges is still debated in the literature, with special emphasis on the mechanism of inclusion of exotic and HP-UHP blocks. The three types of MTCs display different potential of stratal disruption. The style of internal deformation shows that bed disruption progressively occurs from the early stages of slope failure to the stages of transport and emplacement. The final amount of stratal disruption depends on the sediment strength (composition and compaction), the activity of fluid (over) pressure, the length of en-mass transport and the rheology of the MTC-substratum interface. A review of the composition of MTCs of the Apennines and the Pyrenees shows that the capacity of rock mixing and the inclusion of exotics largely depend on the geometries and depth of slope failure, the geology of the basin margins and, again on the attitude of the MTCs to deform and rip-up the substratum. On the base of the studied field examples, the potential of mass-transport processes in generating rock mixingin some classic mélanges of other mountain chains and in present-day accretionary wedges is discussed.