Viscous flow and deformation of regional metamorphic belts at convergent plate boundaries

Abstract

[1] Viscous flow and deformation of a Newtonian fluid between rigid boundaries have been modeled for investigating deformation of a forearc wedge and a regional metamorphic belt as a weak deforming zone between the plates at convergent boundaries. Three types of analytic formulations together with numerical formulations for a Newtonian fluid with a constant viscosity are used for investigating various flows and deformation: (1) flows induced by squeezing between two parallel boundaries, (2) flows induced by squeezing and heterogeneous mass influx in the wedge between two oblique boundaries (e.g., accretionary wedge over the subducting plate with the continental plate as a backstop), and (3) flows induced by dragging along the boundaries and by homogeneous mass influx in the wedge as in type 2. All cases include strike-slip movements of the boundaries, which allows us to investigate three-dimensional (3-D) deformation (e.g., 3-D corner flow). The corresponding flow, deformation of infinitesimal and finite elements, and the timescale of flow are calculated for each configuration with various mechanical boundary conditions. The model results show that configuration and mechanism of the flow can be inferred from the spatial variation of finite deformation (e.g., deformed radiolarians as a strain marker and geometry of folding as finite deformation of a large block). In particular, prolate strain nearly parallel to the strike of subduction zone, together with a large-scale folding, can be a good indicator for deformation in the forearc wedge associated with 3-D corner flow induced by oblique subduction. Deformation observed in the Cretaceous regional metamorphic belts in southwest Japan can be explained by this mechanism.

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

“…Finally, if the Ryoke and Sambagawa metamorphic belts were associated with the three-dimensional corner flow in the forearc wedge, the low-P Ryoke metamorphic rocks that must be related to shallower flow and high-P Sambagawa metamorphic rocks related to deeper flow would locate at the ocean-side and continental-side in the forearc, respectively (Iwamori, 2003). However, the Ryoke metamorphic belt occurs at the continental-side (see Fig.…”

“…1); the three-dimensional corner flow model cannot produce this arrangement. Therefore, the models of the three-dimensional corner flow in the forearc wedge proposed by Toriumi (1985) and Iwamori (2003) are not suitable for the geodynamic process in the forearc region of the Cretaceous SW Japan.…”

“…The tectonic models proposed by Toriumi (1985) and Iwamori (2003) are of the three-dimensional corner flow in the forearc wedge, to produce the dominant prolate strain of rocks within the Ryoke metamorphic belt. We challenge the three-dimensional corner flow model based on the following three reasons.…”

“…As the elongated strain ellipsoid approaches the corner, the ellipsoid is rotated and stretched in the direction parallel to the rigid continental wall to produce prolate strain during its exhumation; this is the second stage deformation. Recently, by comparing the strain geometry data of Toriumi (1985) and Toriumi and Kuwahara (1988) with the results of the three-dimensional numerical simulations, Iwamori (2003) suggested that the most suitable geodynamic model for the forearc region of the Cretaceous SW Japan is a three-dimensional corner flow in the forearc wedge associated with subduction with moderate obliquity ($45°for the angle between the trench normal and the subduction direction). However, data of the strain analysis on the Ryoke metamorphic rocks (Toriumi, 1985;Toriumi and Kuwahara, 1988) is problematic, because the dominant strain is not prolate but plane strain to oblate in the western part of the metamorphic belt (Iwakuni-Yanai district; Okudaira and Beppu, 2008;Beppu and Okudaira, 2008).…”

“…Since the accretionary complex formed at a subduction zone and the low-P/high-T metamorphic belt formed near a volcanic arc, the deformation history of the metamorphosed accretionary complex, such as the Ryoke metamorphic belt, is a key to understanding the geodynamic processes in the forearc region at the eastern margin of Eurasia. Toriumi (1985) and Iwamori (2003) proposed the geodynamic models to explain the strain geometry/path of rocks within the low-P/high-T Ryoke and high-P/low-T Sambagawa metamorphic belts that have been considered to be formed simultaneously at the mid-Cretaceous arc-trench system in the eastern Eurasian continental margin. Based on the results of the strain analysis using the deformed radiolarian fossils as strain markers in metachert samples, Toriumi (1985) suggested that the geometry of plastic strains in the Ryoke and Sambagawa metamorphic belts dominates prolate strain.…”