Paleomagnetism of the Ishikoshi Andesite: a Middle Miocene paleomagnetic pole for northeastern Japan and tectonic implications

Hiroyuki Hoshi,Yuki Teranishi

doi:10.1186/bf03352749

Abstract

Abstract To determine a Middle Miocene paleomagnetic pole for northeastern Japan and discuss its tectonic implications, we obtained new paleomagnetic results from Middle Miocene (about 14 Ma) dacite and andesite flows of the Ishikoshi Andesite. Characteristic remanent magnetizations were isolated from 12 lava sites by means of detailed alternating field and thermal demagnetizations. Analysis of demagnetization results and rock magnetic experiments indicated that magnetite or Ti-poor titanomagnetite is the main magnetic carrier. We obtained a formation mean direction (D = 9.2°, I = 65.9°, α95 = 7.3°, k = 40.5, N = 11) and compared it with published paleomagnetic results from other areas of northeastern Japan to discuss tectonic rotation. This comparison provided a mean paleomagnetic pole (85.9°N, 236.6°E, A 95 = 6.2°, K = 115.9, N = 6) that we consider represents the Middle Miocene pole for northeastern Japan. It is statistically indistinguishable from coeval poles for southwestern Japan, South Korea, and northern China, and we therefore conclude that northeastern Japan as a whole has not been subjected to tectonic rotation since the Middle Miocene. A reassessment of geologic and paleomagnetic data suggests that a previous model of the Late Miocene or later counterclockwise rotation of northeastern Japan is based on tilt-uncorrected paleomagnetic directions from tilted rock units.

Highlights

Paleomagnetic studies have contributed significantly to the establishment of a broad consensus that northeastern and southwestern Japan rotated counterclockwise and clockwise, respectively, in the Early to Middle Miocene to form the present arcuate shape of the Japan arc (Otofuji et al, 1985a) (Fig. 1(a))
We obtained a formation mean direction (D = 9.2◦, I = 65.9◦, α95 = 7.3◦, k = 40.5, N = 11) and compared it with published paleomagnetic results from other areas of northeastern Japan to discuss tectonic rotation. This comparison provided a mean paleomagnetic pole (85.9◦N, 236.6◦E, A95 = 6.2◦, K = 115.9, N = 6) that we consider represents the Middle Miocene pole for northeastern Japan. It is statistically indistinguishable from coeval poles for southwestern Japan, South Korea, and northern China, and we conclude that northeastern Japan as a whole has not been subjected to tectonic rotation since the Middle Miocene
A reassessment of geologic and paleomagnetic data suggests that a previous model of the Late Miocene or later counterclockwise rotation of northeastern Japan is based on tilt-uncorrected paleomagnetic directions from tilted rock units

Summary

Introduction

Paleomagnetic studies have contributed significantly to the establishment of a broad consensus that northeastern and southwestern Japan rotated counterclockwise and clockwise, respectively, in the Early to Middle Miocene to form the present arcuate shape of the Japan arc (Otofuji et al, 1985a) (Fig. 1(a)). This dramatic event is believed to be causally related to the opening of Japan Sea back-arc basins (Otofuji et al, 1985a; Tamaki et al, 1992; Jolivet et al, 1995). Detailed exploration of the Miocene rotation of northeastern and southwestern Japan is, crucial for illustrating the geotectonic history of circum-Japan Sea regions of eastern Asia and for studying active tectonic processes.

Results

Discussion

Conclusion