Interpretations of a Middle Miocene and late Quaternary steady dextral transpression in SW Japan and the opening tectonics for the Japan Sea

Abstract

An internal-rotation hypothesis for the deformation of SW Japan for Middle Miocene and Recent times is studied. Rotations are known to have occurred from paleomagnetic data for the earlier case; two geodetic flow field estimates, as well as another tectonic study, suggest that the rotations are also taking place at present. A further examination of the geodetic flow models suggests that the rotations contained in these flow models are intrinsic to the deformation of SW Japan, which occurs in an ∼E–W dextral-transpressive, simultaneous pure- and simple-shear, tectonic regime. Maps of rotation rates (one-half of vorticity) and shear gradients (the N–S variation of the E–W velocity field), constructed from the geodetic flow models, resemble one another in pattern. The estimates of the directions of the principal strain rates from the geodetic models are, on the whole, ∼N–S contractional and/or ∼E–W extensional, which match the geodetic reference coordinate system used in the adjustment of the geodetic data. These observations suggest that the vortical flow contained in the geodetic flow models is a frame-independent, shear-induced vorticity as the material is rotating with respect to the principal directions of the strain rates, and that the oblique motion of the Philippine Sea plate is partitioned between shear and arc-normal convergence components. The rotation rate is ∼10–12°/Ma. Furthermore, the kinematical vorticity number estimates from the geodetic data correlate well with the first-order tectonic fabric patterns in SW Japan. Super-simple-shear deformation for large parts of SW Japan, where the Median Tectonic Line and other large strike-slip faults are found, is also indicated in the (internal) kinematical vorticity number estimates. A new model for the formation of the Japan Sea is proposed: extrusion of Japan due to the Okhotsk–Eurasia collision. This model agrees with the fundamental observations of the Early–Middle Miocene deformation in the region. The application of an indentation–extrusion model to the Okhotsk–Eurasia collision results in a better agreement between the predicted and observed structures, and leads to simpler, more reasonable and coherent interpretations for the origin and development of the structures compared with the achievements of the previous models. The ∼NNE-trending Early–Middle Miocene rifts lying along the continental margin of western Hokkaido formed as pull-apart basins along a sinistral fault, related to the collision of the Okhotsk plate, which segmented, in a left-handed sense, because the collision was oblique. The ∼N–S-trending rifts and the normal faults found in the back-arc domain of the NE Honshu arc formed by eastward extrusion under the influence of the Mariana-type subduction in the Japan Trench. These rift basins were subsequently left-laterally displaced by movement along ∼NW-trending sinistral strike-slip faults. The Shikoku Basin opened from the east toward the west, and dextral shear was imparted to the SW margin from the Nankai Trough, leading to clockwise rotations in the Middle Miocene in SW Japan. The paleomagnetic rotations are accommodated as fully internal-structural rotations in the new model for the origin of the Japan Sea. The current episode of deformation in SW Japan began ∼1–5 million years ago, after a hiatus during a 15–5 Ma interval, and is likely a continuation of the Middle Miocene style of tectonics.