A C T Mélanges, intriguing rock units often found in accretionary complexes, consist of basalt lenses embedded in a highly sheared sedimentary matrix. The origin of mélanges remains a subject of vigorous debate, with consequences on our understanding of subduction processes. A first line of thought interprets mélanges as mixed lithologies intertwined by convergent tectonics. Supporters of this interpretation regard mélanges as fossilized witnesses of the lower-and upper-plate interface, with their rheological properties reflecting seismogenic subduction zones. However, a second line of thought is to consider that basalts and sediments were mixed prior to subduction by sedimentary and/or magmatic processes, this mix being only later incorporated into the accretionary wedge.

In this study, we present evidence supporting the pre-subduction mixing interpretation for mélanges from two paleo-accretionary complexes: the Kodiak complex in Alaska and the Shimanto Belt in Japan. In modern seafloor sediments in contact with basaltic submarine magmas, we show that the crystallinity of carbonaceous particles in sediments increases toward basalts, indicating a ~1 cm-thick aureole of contact metamorphism. Intriguingly, a comparable aureole of increased crystallinity is observed in four mélanges from the two paleo-accretionary complexes. Basalts were thus emplaced onto and into sediments by magmatism rather than by tectonics, challenging the notion of mélanges explored in this study as formed along the plate boundary interface. Moreover, the studied mélanges are made of mid-ocean ridge basalts, and deposition ages of mélange sediments coincide with proposed ridge subductions. This implies that the mid-ocean ridges at the trench were the source of the magmas that intruded into and extruded onto the clastic sediments and contributed to form the multilayered basalt-sediments architecture.