Melange‐forming processes in the development of an accretionary prism: Evidence from fission track thermochronology

Abstract

To give thermotectonic constraints on melange‐forming processes and to understand the significance of melange in the evolution of accretionary complexes, fission track analysis was carried out on samples collected from sandstone blocks in melange matrix and coherent sandstone sequences adjacent to melange units in the Cretaceous to Miocene Shimanto accretionary complex, southwest Japan, which is one of the typical accretionary complexes with an accumulation of geological information. Nineteen zircon ages and 15 track length distributions from melanges indicate with some local variation that materials with higher maximum paleotemperatures are exposed at the rear part of the prism, as is the case of the coherent units. Two apatite ages are contemporaneous with previously reported apatite ages of ∼10 Ma obtained from coherent units in the region, emphasizing the regional cooling through the apatite closure temperature of ∼100°C at that time. There is no evidence for substantial relative motion of the melanges with respect to the surrounding coherent units; melange formation is explained by processes that exclude large vertical transport (i.e., 5 km) of melange forming materials. These thermotectonic characteristics observed in the Shimanto accretionary complex are maintained during subsequent local internal deformation caused by the successive growth of the wedge.