Deformation process and kinematics of mélange in the Early Cretaceous accretionary complex of the Mino‐Tamba Belt, eastern southwest Japan

Abstract

The Kanayama unit of the Mesozoic Mino‐Tamba accretionary complex in eastern southwest Japan is a thick mélange unit composed mainly of abundant muddy matrix and variously sized clasts of chert, hemipelagic siliceous mudstone, and sandstone. Two episodes of deformation formed the mélange fabrics: the first involved fragmentation of sandstone layers in response to mud injections; the second involved layer‐parallel, noncoaxial shear that resulted in the mixing of pelagic to terrigenous clasts and the formation of S‐C‐like asymmetric fabrics with scaly foliation. The S‐C fabrics exhibit a predominantly sinistral shear sense in plan view. After fold and tilt corrections, the restored slip motion inferred from the S‐C fabrics indicates a bulk top‐to‐the‐south shear. Combining the unit's terminal fossil age and radiometric age with a regional plate reconstruction model, these data suggest that the Kanayama mélange formed along a décollement during oblique subduction of the Izanagi plate in the Early Cretaceous.

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

“…Although extension of the mélange term to bodies that are non-mappable at 1:25,000 or smaller scales renders the term mélange useless (e.g., Raymond, 1984), it is not unusual to find this term used in describing small-scale or meso-scale mélanges (e.g., Bosworth and Vollmer, 1981;Bradley and Kusky, 1992;Wakita, 1988Wakita, , 2000Fukui and Kano, 2007). To avoid any confusion, we agree to the use of the terms "small-scale mélanges and broken formations" (see Codegone et al, 2012b) or "meso-scale mélange" (see Bradley and Kusky, 1992) in order to indicate not-mappable (at 1:25,000 scale) mélanges and broken formations, whereas "chaotic or disrupted (rock) units" (sensu, e.g., Yamamoto et al, 2009;Festa, 2011) must be considered a general term to indicate bodies apart from the nature of the embedded blocks.…”

Myths and recent progress regarding the Argille Scagliose, Northern Apennines, Italy

“…Scaly fabric, scaly foliation, or the more common scaly clay are terms that the current interest in convergent plate margins has led to wide usage in both modern and ancient subduction complexes. For example, on-land scaly clay was described as typical in mélanges from the Shimanto Belt in Japan (Kimura and Mukai 1991;Kiyokawa 1992;Kitamura et al 2005;Fukui and Kano 2007), from Nias Island in Indonesia (Pubellier et al 1992), from Taiwan (Chen 1997;Chang et al 2000Chang et al , 2001, and from Barbados (Enriquez-Reyes and Jones 1991). Scaly foliation has also been described in rock bodies of ancient orogenic belts such as the Northern Appalachians (Lash 1989;Waldron et al 1993;Whitehead et al 1995), the Antler Orogeny (Jansma and Speed 1993), and those related to the closure of the Palaeo-and NeoTethys (Babic et al 2002;Hara et al in press), to cite a few examples.…”

Mid-crustal horizontal shear zone in the forearc region of the mid-Cretaceous SW Japan arc, inferred from strain analysis of rocks within the Ryoke metamorphic belt

“…According to Fukui and Kano (2007), mélange in the Mino-Tamba terrane contains numerous isolated clasts entirely supported by mudstone matrix; sandstone/chert clasts are subangular to subrounded and ellipsoidal in shape, with an average aspect ratio of approximately 2, whereas clasts of other compositions (e.g., siliceous mudstone and felsic tuff) are lenticular in shape and have higher aspect ratios. The sandstone/chert clasts exhibit no preferred orientation, although other clasts are aligned parallel to the main foliation (Fukui and Kano, 2007).…”

“…Alternatively, it is possible that the plane strain to general flattening strain exhibited by the metachert and metapelite may have resulted in large part from the schistosity-forming deformation, because the radiolarian fossils are not deformed in the non-metamorphosed rocks in the Mino-Tamba accretionary complex but are deformed in the metamorphic rocks with a distinct schistosity (Yao et al, 1980;Okada, 1990;Fukui and Kano, 2007;Okudaira and Beppu, 2008). In such a case, the strain geometry inferred from radiolarian fossils in the metacherts (k = 0.6-1.2) would represent the strain geometry for D 1 in the Wazuka-Kasagi district.…”

“…In fact, bedding planes within rocks of the Mino-Tamba terrane developed with a shallow dip (e.g., Kimura and Hori, 1993;Isozaki, 1996;Niwa, 2006;Fukui and Kano, 2007); the boundaries of mineral zones and the schistosity are parallel/subparallel to the bedding planes (e.g., Okudaira et al, 1993;Ikeda, 1998Ikeda, , 2004Ozaki et al, 2000), suggesting that the schistosity developed horizontally at the time of the main metamorphism. Since a general combination of simple shear and shortening normal to the shear plane leads to a general flattening of the strain ellipsoid, the strain geometry of D 1 deformation indicates the likely formation of a horizontal shear zone with a shear direction parallel to the arc length; the shear zone would have developed at the depth at which the Ryoke metamorphic belt formed ($15 km).…”