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

作者：Okudaira Takamoto^*; Beppu Yuki; Yano Ryosuke; Tsuyama Mio; Ishii Kazuhiko

来源：Journal of Asian Earth Sciences, 2009, 35(1): 34-44.

DOI：10.1016/j.jseaes.2008.11.015

• Strain analysis
• Mid-crustal horizontal shear zone
• Detachment fault
• Forearc
• Median Tectonic Line
• Ryoke metamorphic belt
• SW Japan arc

The strain geometry of a Cretaceous low-P/high-T metamorphosed accretionary complex in SW Japan is one of the key geological constraints in understanding the Cretaceous tectonics of the forearc region at the eastern margin of Eurasia. We studied the strain geometry of upper greenschist to lower amphibolite facies metamorphic rocks from the mid-Cretaceous low-P/high-T Ryoke metamorphic belt, which during the mid-Cretaceous were located at mid-crustal depths close to the volcanic front. Strain analysis focused on deformed radiolarian fossils in metachert and sandstone/chert clasts within metapelite. The maximum stretching direction (X-direction) of the strain ellipsoid of the schistosity-forming deformation at the peak metamorphism is oriented E-W, and the XY-plane of the strain ellipsoid developed parallel to schistosity. The metachert and metapelite exhibit plane strain to general flattening strain that probably largely resulted from the schistosity-forming deformation. Based on the results of the previous strain analyses and this study, it reveals that the schistosity-forming deformation occurred under bedding-normal shortening, resulting in general flattening strain throughout the Ryoke metamorphic belt. The X-direction of the strain ellipsoid of the schistosity-forming deformation is oriented parallel to the length of the Ryoke metamorphic belt throughout the SW Japan arc. The schistosity formed parallel to bedding planes that developed horizontally in non-metamorphosed accretionary complexes of SW Japan, indicating horizontal XY-plane of the strain ellipsoid. These observations indicate the occurrence of horizontal shear deformation within the middle crust (similar to 15 km depth) at the time of the low-P/high-T metamorphism. The deep-seated horizontal shear zone and the shallower incipient Median Tectonic Line (MTL) fault system could result in a crustal-scale detachment fault in the forearc region of the mid-Cretaceous SW Japan arc. After the formation of the detachment, the left-lateral pull-apart forearc basins, Onogawalzumi Group distributed on the north side of the MTL and the southern marginal mylonite zone might be formed by the sinistral movement along the detachment fault.

• 出版日期2009-6-2