Paleomagnetic rotations and the Japan Sea opening

Abstract

The apparent incompatibility of the classical "bar door" opening model of the Japan Sea based on paleomagnetic studies and the pull-apart geometry based on the observation of shear zones in and around Japan is symptomatic of a lack of understanding of the tectonic history of the Japan Sea. After a critical review of paleomagnetic data and a discussion on the possible error bars we present information concerning the internal strain of the Japan arc during the opening. We show that it is possible to integrate both sets of data in a single model, provided that some of the paleomagnetic rotations are due to distributed deformation of SW and NE Japan and not to a rigid body rotation of 600 km long blocks. Rotations occur at all scales and the 50 ø of clockwise rotation of southwest Japan is the sum of 30 ø of rotation of southwest Japan as a whole and of 20 ø due to internal deformation. The amount of paleomagnetic rotation is then compatible with the kinematics based on the internal structure of the Japan Sea and the geometry of major dextral shear zones. The rigid body rotation of southwest Japan is accommodated by extension and left-lateral motion along the Median Tectonic Line which is a second order fault between the two major dextral shear zones that bound the Japan Sea to the east and to the west. The problem of the apparent instantaneous rotation of southwest Japan as opposed to the progressive opening of the Japan Sea probably results from the way paleomagnetic data and radiometric dates are averaged. PROBLEM SETTINGThe past ten years have seen the development of a debate on the opening mechanism of the Japan Sea ( Figure 1). An important breakthrough was made by Otofuji et al.short period, less than 2 m.y. long, about 15 Ma. The model is that of a double rotation about two nearby poles and a double fan-shaped opening of the Japan Sea. The rigid behavior of the two continental blocks, each of them being more than 500 km long, implies that oceanic crust formation in the Japan Sea was contemporaneous with the rotation. A rapidly opening model was then postulated in concert with rapid rates of motion of the rotating blocks.Based on different data, Lallemand and Jolivet [ 1985] postulated that major dextral strike slip faults guided the opening of the Japan Sea in a pull-apart manner, in opposition with the fan-shaped kinematics of Otofuji et al.Later Jolivet et al. [1989, 1991] and Jolivet and Tamaki [1992] introduced block rotations in the pull-apart model, describing the whole region as a wide dextral shear zone (Figure 2). 356 PALEOMAGNETIC ROTATIONS AND THE JAPAN SEA OPENING JOLIYET ET AL. 357 JOLIVET ET AL. 359

Timing of clockwise rotation of Southwest Japan: constraints from new middle Miocene paleomagnetic results

“…These values are compatible with an approximate estimate for the amount of clockwise rotation of Southwest Japan (~45°; Otofuji and Matsuda 1987). Determination of the amount of rotation of an entire arc is not easy if, as claimed by Jolivet et al (1995), the arc has fragmented into several pieces during the rotation, and if each piece exhibits an indeterminate amount of intra-arc block rotation. However, results from Late Cretaceous rocks of eastern Southwest Japan suggest that intra-arc block rotation, if any, has been negligible (Fukuma et al 2003).…”

“…This model, hereafter called the 15-Ma rapid rotation model, is supported by several studies based on data from volcanic rocks (Otofuji et al 1991;Shimada et al 2001). However, this model has been questioned, given the results of a number of paleomagnetic and geochronological studies (e.g., Nakajima et al 1990;Jolivet et al 1995) that cite an absence or lack of data for the rotation of rocks dating from ca. 15 Ma in some areas of Southwest Japan (Hoshi and Sano 2013;Sawada et al 2013; and references therein), thus leading to an alternative view that clockwise rotation occurred before (not at) 15 Ma (e.g., Nakajima et al 1990;Hoshi et al 2000b).…”

“…Rotations are defined as positive if the observed declination is clockwise with respect to the expected declination calculated from the reference North China Block data (Otofuji and Matsuda 1983, 1984, 1987Otofuji et al 1991), and these results played an important role in the development of the 15-Ma rapid rotation model (these studies are reviewed by Otofuji 1996). However, Jolivet et al (1995) and Sawada et al (2013) have pointed out that the lithostratigraphy and chronostratigraphy of the volcanic succession investigated by Otofuji and colleagues remains unclear. Sawada et al (2013) revisited the succession and proposed a new stratigraphic architecture.…”

Anisotropy and Flow in Pacific Subduction Zone Back-arcs

“…d HASTON andFULLER (1991), TAYLOR et al (1991), KOBAYASHI et al (1995), CLIFT (1995). e FOURNIER et al (1994FOURNIER et al ( , 1995, JOLIVET et al (1994JOLIVET et al ( , 1995. f TAMAKI (1995).…”

Late Jurassic–Early Cretaceous intra-arc sedimentation and volcanism linked to plate motion change in northern Japan

“…The Sorachi-Yezo Belt and the Rebun-Kabato Zone were deformed by dextral slip and block-like rotation during the Oligo-Miocene due to the opening of Kuril Basin (Jolivet, Shibuya & Fournier, 1995;Takeuchi, Kodama & Ozawa, 1999). Furthermore, they were shortened by collision caused by the southwestward migration of the Kuril fore-arc sliver beginning in the Miocene (Kimura, 1985), resulting in the development of N-S-trending thrust and fold systems within the belts (Fig.…”