Early Miocene rotational process in the eastern part of south‐west Japan inferred from paleomagnetic studies

Abstract

This paper provides untilted paleomagnetic data obtained from the early Miocene strata around the Kanazawa‐Iozen area, in the eastern part of south‐west Japan. A thick pile of volcaniclastics and marine transgressive sediments underlie the area; they were deposited in the early stage of the Japan Sea opening event. Progressive thermal demagnetization tests isolated stable primary magnetic vectors from eight sites in the upper part of the Iozen Formation. Overall, the tilt‐corrected mean direction of this unit is D = 36.4°, I = 51.6° and α95 = 12.1. Together with a published paleomagnetic and chronological database, the present results suggest that clockwise rotation of south‐west Japan, linked to the back‐arc opening, commenced in the early Miocene and accelerated at the same time as rapid subsidence along the Japan Sea coast. Post‐opening, differential rotation within the eastern part of south‐west Japan is assumed, based on selected paleomagnetic data from the latest Early Miocene.

Paleomagnetism of the Lower to Middle Miocene Series in the Yatsuo area, eastern part of southwest Japan: clockwise rotation and marine transgression during a short period

“…This interpreta- Fig. 8 Tilt-corrected site-mean magnetic directions divided into each formation in the Yatsuo, Kanazawa-Iozen (Itoh and Kitada, 2003) and Mizunami (Hayashida, 1986) areas. Solid (open) symbols are on the lower (upper) hemisphere of the equal-area projections.…”

“…One reason is that more integrated stratigraphic correlation between both areas is still required as stated above. The other is that detailed correlation of formation-mean data in the Sunagozaka/Nanamagari Formation reported by Itoh and Kitada (2003) to the presented data in the Yatsuo area is difficult because their formation-mean data were obtained from limited horizons of only tuff layers. Itoh (1988) also proposed that the eastern part of south-west Japan had suffered differential rotation.…”

“…2), adjacent to the Yatsuo area. Although reversed polarity interval in the Sunagozaka/Nanamagari Formations was correlated to Chron C5Br (Itoh and Kitada, 2003), the Crucidenticula Kanayae Zone identified in the Sunagozaka/Nanamagari Formation (Yanagisawa, 1999a) is discordant to Chron C5Br on the basis of biochronology (Watanabe and Yanagisawa, 2005) (Fig. 7).…”

“…2) associated with ductile deformation in Japan arc. Later, Itoh and Kitada (2003) discussed more detailed rotational process in the Kanazawa-Iozen area (Fig. 2).…”

“…For the purpose of more detailed description of the rotational process related to Japan Sea opening, paleomagnetic data should be obtained from marine transgressive sediments along the back-arc margin. Itoh and Kitada (2003) executed paleomagnetic study intended for tilt-corrected samples in the Kanazawa-Iozen area. However their rotational process was not conclusive for the following two reasons.…”