“Block-in-matrix” fabrics that lack shearing but possess composite cleavage planes: A sedimentary mélange origin for the Yuwan accretionary complex in the Ryukyu island arc, Japan

Abstract

While melanges have been taken to be a significant component of accretionary complexes, the use of this term has been mostly descriptive, given the lack of consensus concerning their genesis. However, many workers have interpreted asymmetric shear fabrics in melanges as evidence of a tectonic origin (i.e., giving rise to tectonic melanges), in contrast to olistostromes, which are considered to be purely sedimentary in origin due (at least in part) to the lack of such shear fabrics. It is still uncertain whether accretionary processes include those that involve the intense shearing of lithologies along with, for example, the incorporation and entrapment of exotic blocks by muddy matrices during sediment deposition and deformation in axial deep-sea trench settings. Here, we argue that the so-called fabrics that characterize accretionary complexes are primarily sedimentary in origin and are distinct from shear fabrics represented by tectonic melanges. We also propose that, following their accretion, subsequent recycling of such oceanic materials is a common feature within evolving accretionary prisms. Lastly, we draw attention to the observation that D1 pressure-solution cleavage planes—notably widespread in the lower Cretaceous Yuwan complex, Amami Oshima Island, in the Ryukyu arc, SW Japan—are absent from the minor asymmetric shear structures that often characterize tectonic melanges. In contrast, we suggest that the Yuwan complex represents the effects of broadly symmetric flattening. This scenario is supported by our observation that the number of sedimentary and deformation events differs significantly compared to the single phase of shearing expected in tectonic melanges. D minus 1 (D −1 ) pressure-solution and axial-planar cleavages are only observed in exotic blocks and exhibit older accretionary deformation events within a preexisting accretionary prism. Of the existing structural associations, Sd1 is the oldest, predating the D −1 sedimentary episode, and it represents an sedimentary phase associated with the formation of a distinct type of block-in-matrix fabric produced by the accumulation of debris flows at seamount margins. Sd2 represents a succeeding sedimentary episode characterized by trench-axis debris flow, including inclusions derived from collapsed material from an older accretionary prism. The resulting Sd2 block-in-matrix fabric is one of the main lithologic components of the Amami Oshima Yuwan complex. Here, the Sd2 episode typically includes an igneous fabric, consisting of nonexotic intrusive and extrusive basalts within a terrigenous matrix, suggesting that the Yuwan prism may have also been associated with a trench-trench-ridge triple junction. D1 coaxial deformation is observed to overprint all of the episodes represented, although it has not destroyed or significantly sheared their respective fabrics, especially in regard to the main Sd2 block-in-matrix structures, which are therefore considered to represent original sedimentary structures.