Because of very weak remanent magnetization less than 7.
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It is now well documented that K-Ar dating of plagioclase grains provides more reliable ages than the whole rock samples. Our results thus provide new insight into geochronological views of the Takadate and Ryozen Formations. The here presented data of K-Ar ages from the Takadate Formation show a unimodal distribution with mean value of Our present study only confirms the presence of younger ages as reported in the previous study. In this connection, ages of However, ages older than 20 Ma are not identified in the present study. Although we sampled at the same localities where the whole rock K-Ar ages of Since the studied samples contain low K-content, the whole rock dating method used by previous authors presumably gave erroneous dating results.
We thus conclude that the unimodel behaviour characterizes the age distribution of the Takadate Formation, which was formed at In comparison to the Takadate Formation, relatively older age of Previous dating results reported from the Ryozen Formation provide 10 different ages between The previously reported mean-age from the Ryozen Formation is 1.
Statistically speaking, the present study distinguishes two different age groups of Anomalously high value of Although the origin of older rocks is enigmatic, the Miocene volcanic activities have been presumably operative in the area since 34 Ma. The here presented palaeomagnetic results from the Cretaceous granites indicate that the Abukuma Terrane as a whole is characterized by westerly deflected direction with respect to north Fig. The southernmost site with northwesterly declination is located just to the north of the Tanakura Shear Zone.
Based on these directional similarities, it is suggested that the Cretaceous granite of the Abukuma Terrane was probably subjected to pervasive remagnetization. Nonetheless, consistent palaeomagnetic direction of the Cretaceous Abukuma granites indicates that the whole region was CCW rotated as a coherent block. Although some sort of sinistral movement has been reported from the Tanakura Shear Zone Otsuki , only little additional tectonic rotation is recorded. Cretaceous palaeomagnetic direction from the Abukuma Terrane is also consistent with those secondary magnetization reported from a variety of rocks in NE Japan Fig.
Similarities in directional behaviour from the Palaeozoic and Mesozoic rocks of the Abukuma and Kitakami Terranes with the Cenozoic rocks strongly suggest a CCW rotation of NE Japan after the occurrence of remagnetization. An excellent parallelism in palaeomagnetic directions from different regions of NE Japan strongly advocates a rigid rotational model, which extends from Yamagata to Oshima peninsula including the Abukuma and Kitakami Terranes , as a first order approximation.
Although the regional extent of NE Japan is still a debatable issue, the trend of rotational motion in the area pre-dominantly CCW strongly advocates the Tanakura Shear Zone as a southern margin of this province. Unfortunately, we could not get any meaningful palaeomagnetic results from the Inada and Tsukuba granites by the present study. This area is located to the north of the Nihonkoku-Miomote tectonic zone, which is considered as a northern extension of the Tanakura Shear Zone. Due to non-availability of reliable palaeomagnetic data, it is hard to speculate about the trend of rotational motion across the Tanakura Shear Zone in the south.
Taken in to consideration a difference in granitic petrology e. Recent geochemical study of the dioritic rocks from the Yamizo Mountains shows some sort of correlation with other results reported from the Early Cretaceous rocks of the Tamba belt Takahashi To enhance our understanding about the extent of NE Japan, rotational history of an area to southwest of the Abukuma Terrane should be revisited by any future palaeomagnetic endures. We compile all the available data including the present study in Table 3 , while the corresponding declinations are plotted as a function of radiometric ages in Fig.
Although the Miocene volcanic rocks of the Motegi area Shaded zones in the figure indicate mean palaeomagnetic declinations against the 10—14 and 25—27 Ma ages for NE Japan as well as the 20—30 Ma age for SW Japan. Based on six recent F-T dating results, Danhara et al.
Additionally, an age window of Mean ages based on multi age values are calculated by assigning unit weight to each age datum obtained through the K-Ar and F-T methods. Although, an F-T age of Otofuji has reported a mean declination of At These evidences thus indicate that at However, at about 16 Ma, which has been estimated through standard timescale of diatom biostratigraphy, a northerly declination is reported from the Matsushima Formation Yamazaki As noted by Yanagisawa and Akiba , up to 1.
An age of Even by taking in to account this difference, results from the Matsushima Formation do not contradict our prediction that the province of NE Japan was still undergoing CCW rotation at Based on these northerly declinations, we conclude that CCW rotational motion of NE Japan has completely ceased by Since the fault system along the Median Tectonic Line MTL is linear in the central part of SW Japan, a rigid block rotational model is an acceptable assumption for this province.
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After the summarization of rotational features in SW Japan by Otofuji , several geochronological results have been reported from this area. Using the multi age data set, mean ages are calculated from seven rock units by assigning a unit weight to each radiometric datum. These mean ages are then plotted against the declination values Fig.
Abrupt change in declination is indicated between Prior to formation of the Kumano volcanics at After the formation of Omori volcanics Comparison of these timings thus suggests that rotational activities in both these provinces SW and NE Japan had already come to an end by 14 Ma. This sort of difference between the SW and NE Japan implies that both these provinces underwent different mode of rotational motion prior to complete cessation. It seems a reasonable assumption that the province of NE Japan rotated slowly and gradually, while that of SW Japan experienced an extraordinary fast rotational motion.
Since the above mentioned ODP sites are located on the margins of Japan and Yamato basins, 40 Ar- 39 Ar age data give us some clues to an earlier stage in the Japan Sea formation. These data thus suggest that opening in the northern part of the Japan Sea has probably taken place earlier than its southern counterpart. In order to fully document tectonic history of the Japan Arc-Sea system, further ocean drilling data from the central part of these basins the Japan and Yamato basins are highly desirable.
Northwesterly palaeomagnetic declinations between Comparison of the available palaeomagnetic directions from both sides of the Tanakura Shear Zone implies as a first order approximation that an area to the north of this zone underwent a rigid body CCW rotation as a part of NE Japan. K-Ar dating of plagioclase grains revealed a Comparison of these directional behaviours with the corresponding age data indicates that the Abukuma Terrane was still in the process of CCW rotation at However, the 11—20 Ma palaeomagnetic data available from NE Japan indicate a complete cessation in CCW rotation of this province by Comparison of the available palaeomagnetic and geochronological data from NE and SW Japan suggest that rotational motion in both these provinces has already been ceased by 14 Ma.
We thank Dr L. Jolivet and an anonymous reviewer for their critical reviews that helped us to improve this manuscript. We are also thankful to three different organizations for their support in the completion of this work. Abe T. Aoki K. Petrology of Neogene Tertiary basic volcanic rocks of northeastern Japan 2. Takadate high-almina basalt series , J. Google Scholar. Besse J. Courtillot V. Apparent and true polar wander and the geometry of the geomagnetic field over the last Myrs , J.
Danhara T. Hoshi H. Iwano H. Yoshioka T. Orihashi Y. Japan , , — Enkin R. Fukudome T. Yoshida T. Nagao K. Itaya T. Tanoue S. Hasebe N. Tagami T. Nishimura S. The evidence of along-arc differential uplift of the Shimanto accretionary complex: Fission track thermochronology of the Kumano Acidic Rocks, Southwest Japan , Tectonophys , , — Hashimoto M. Geology of Japan , pp. Google Preview. Hayashi S. Ohguchi T. Hayashida A. Fukui T. Torii M. Hirooka K. Kawai N. Results of age determination of some Late Cenozoic rocks in southwest Japan , Ann.
In Japan , 69 — Takahashi M. Miocene counterclockwise rotation of Northeast Japan: a review and new model , Bull.
Paleomagnetic constraints on the extent of tectonic blocks and the location of their kinematic boundaries: implications for Miocene intra-arc deformation in Northeast Japan , J. Matsubara T. Early Miocene paleomagnetic results from the Ninohe area, NE Japan, implications for arc rotation and intra-arc differential rotations , Earth Planet Space , 50 , 23 — Ishii M. Japanese Assoc. Miwa K. Kawakami Y. Temporal relationship between the northern and the southern parts of the Kumano acidic rocks and the Muro pyroclastic flow deposit, Kii Peninsula, Japan, examined by comparison of paleomagnetic directions , J.
Japan , 53 , 43 — Tanaka D. Yoshikawa T. Paleomagnetism of the Nijo Group and its implication for the timing of clockwise rotation of southwest Japan , J. Ishii T. Yanagisawa Y. Yamaguchi S. Fission-track ages of the Miocene pumice tuff in the region around Matsushima Bay, Miyagi prefecture , Bull. Japan , 34 , — Doi M. Ohira T. Very low potassium analysis by flame photometry using ultra-low blank chemical lines: an application of K-Ar method to ophiolites , Geochem.
Inoue K. Honjou Y. Okada T. Ogata A. Argon isotope analysis by a newly developed mass spectrometric system for K-Ar dataing , Mineral. Ito H. Tokieda K. Tilting movement of the Japanese islands inferred from Cretaceous and early Tertiary paleomagnetic data , J. Jolivet L. Tamaki K. Neogene kinematics in the Japan sea region and volcanic activity of the northeast Japan arc , Proc. ODP, Sci. Fournier M. Japan Sea, opening history and mechanism: a synthesis , J.
Shibuya H. Natland J. Monograph 88, AGU , Washington. Kagami H. Transition of space and time of Cretaceous to tertiary igneous activity and lower crust of Honshu Arc—examination based on Rb-Sr whole rock isochron ages and Sr and Nd isotopes , Mem. Kaneoka I. Takigami Y. Takaoka N. Yamashita S. Kimura K. Kirschvink J. The least-squares line and plane and the analysis of palaeomagnetic data , Geophys.
Kitamura N. Sangawa A. Nakagawa H. Geology of the Sendai District, with geological map at , Konda T. Ueda Y. Yoji Kawamura, 65, looks at his page on www.
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