“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 mélanges have been taken to be a signifi cant 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 mélanges as evidence of a tectonic origin (i.e., giving rise to tectonic mélanges), 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 oceanic 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 "block-in-matrix" fabrics that characterize accretionary complexes are primarily sedimentary in origin and are distinct from shear fabrics represented by tectonic mélanges. 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 planesnotably 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 mélanges. In contrast, we suggest that the Yuwan complex represents the effects of broadly symmetric fl attening. This scenario is supported by our observation that the number of sedimentary and deformation events differs signifi cantly compared to the single phase of shearing expected in tectonic mélanges. 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 oceanic sedimentary phase associated with the formation of a distinct type of block-in-matrix fabric produced by the accumulation of debris fl ows at seamount margins. Sd2 represents a succeeding sedimentary episode characterized by trenchaxis debris fl ow, including oceanic 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 signifi cantly sheared their respective fabrics, especially in regard to the main Sd2 block-in-matrix structures, which are therefore considered to represent original sedimentary structures.

Tectonic significance of different block-in-matrix structures in exhumed convergent plate margins: examples from oceanic and continental HP rocks in Inner Western Alps (northwest Italy)

“…Cowan and Page 1975;Cowan 1985;Dilek and Thy 2006;Burg et al 2008;Cowan and Brandon 2011;Wakabayashi 2011Prohoroff et al 2012;Festa et al 2013;Platt 2015) or after a complete cycle from subduction-collision, exhumation, and intra-continental deformation (e.g. Brandon 1989;Dilek 2006;Camerlenghi and Pini 2009;Osozawa et al 2009;Festa et al 2010bFesta et al , 2011Ogata et al 2012;Codegone et al 2012a). This is the case for different types of mélange and BIM occurrence worldwide (e.g.…”

Mechanisms and processes of stratal disruption and mixing in the development of mélanges and broken formations: Redefining and classifying mélanges

“…2; see also Figs. 4A and 4B), or by thrusting and folding in a collisional belt (e.g., Brandon, 1989;Pini, 1999;Dilek, 2006;Camerlenghi and Pini, 2009;Osozawa et al, 2009Osozawa et al, , 2011Festa et al, 2010aFesta et al, , 2010bCodegone et al, 2012b;Ogata et al, 2012b;Fig. 3A; see Types 6a2 and 6b2 in Tab.…”

“…2). Both of these mélange types may subsequently be overprinted and structurally reworked by tectonic processes such as shearing and tectonic mixing when placed in an accretionary wedge or in a subduction channel (e.g., Cowan and Page, 1975;Cloos, 1982;Cowan, 1985;Shreve, 1988a, 1988b;Medialtea et al, 2004;Dilek and Thy, 2006;Burg et al, 2008;Osozawa et al, 2009Osozawa et al, , 2011Cowan and Brandon, 2011;Wakabayashi, 2011;Fig. 2C; see Type 4b in Tab.…”

Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

“…The thickness of each horse of ocean floor stratigraphy has been reported to be up to a few hundred meters in the mélange zone focused on in this study Hashimoto and Kimura, 1999;Ikesawa et al, 2005;Hashimoto et al, 2012). Although mélanges have been suggested to be sedimentary, tectonic, and diapiric, most mélange zones in the Shimanto Belt are inferred to be tectonic mélanges (e.g., Kimura and Mukai, 1991;Onishi and Kimura, 1995;Hashimoto and Kimura, 1999), with some exceptions (Osozawa et al, 2009). The characteristics of tectonic mélanges are as follows: (1) the blocks have consistent asymmetric shapes and the surrounding shale matrices exhibit asymmetric flow textures around the blocks (asymmetric pressure shadow), without exclusively granular flow of sandstone, and (2) composite planar fabrics (generally called scaly fabrics) are well developed, especially in the shale matrices.…”