MASS-TRANSPORT PROCESSES AND SEDIMENTARY MéLANGES: INTERNAL DEFORMATIONS, STRATAL DISRUPTION AND OCCURRENCE OF EXOTIC BLOCKS

Detailed studies in recent years of mélanges and chaotic deposits in various mountain ranges around the world have resulted in the recognition of a large number of sedimentary mélanges. These sedimentary mélanges mostly originated from sedimentary mass-transport processes and flows, although they were strongly deformed by post-depositional, tectonic and mud-diapiric events. Therefore, they commonly represent excellent fossil examples of mass-transport deposits (MTDs) to investigate the processes involved in their origin. Large scale, basin-wide fossil MTDs, including sedimentary mélanges, are complex units involving the entire spectra of mass-transport processes. Their down-slope movement is facilitated by: 1) shear-dominated viscous flows within a muddy matrix, 2) mud-silt-sandy matrix sustained by fluid overpressure, 3) mm-thick shear zones with advection of grains/fluids. These three types of MTDs display different potential of stratal disruption depending on the sediment strength (composition and compaction), the activity of fluid (over)pressure, the length of en-mass transport and the rheology of the MTD-substratum interface. The potential of mixing of rocks of different ages and metamorphic degrees, and of diverse structural units and paleogeographic domains is conditioned by: 1) the geometry and evolution of slope failure, 2) the depth of the headwall surface, 3) the geometry and tectonic setting of sedimentary basins and basin margins, 4) the effectiveness of tectonic structures and serpentinite diapirs in exposing/exhuming deep-seated HP rocks.

We discuss here the potential of mass-transport processes in mélange formation, based on both on-land and seafloor examples and on their distribution and volumetric relevance in the sedimentary record of accretionary wedges.