Hiroyuki Hoshi

Early Miocene paleomagnetic results from the Ninohe area, NE Japan: Implications for arc rotation and intra-arc differential rotations

We have carried out a paleomagnetic study on well-dated Early Miocene volcanic rocks from the Ninohe area in northern NE Japan. Dacitic welded tuffs ranging from 24 to 21 Ma possess westerly paleomagnetic directions with normal polarity (formation-mean: D/I = 294.5°/44.2° with α95 = 8.3°, 8 sites), while the andesite flows formed at 17 Ma exhibit southerly directions with reversed polarity (D/I = 186.6°/−61.9°, 2 sites). A positive conglomerate test assures the stability of high-temperature components of remanent magnetization. Our results demonstrate that northern NE Japan rotated counter-clockwise through more than 60° between 21 and 17 Ma, most likely in association with the opening of the Japan Sea. The rotation of northern NE Japan thus preceded the about 15 Ma rapid clockwise rotation of SW Japan, and was synchronous with the possible pre-16 Ma southward translation of it. In comparison with published data, we further suggest that intra-arc block rotations occurred in the back-arc region of NE Japan during and after the arc rotation.

Miocene counterclockwise rotation of the Abukuma Mountains, Northeast Japan

Abstract New paleomagnetic data have been obtained from Early to earliest Middle Miocene volcanic rocks in the Ryozen area, located in the northern part of the Abukuma Mountains in Northeast Japan. Our data show that the mountains have been rotated about 40° counterclockwise with respect to the Asian continent between 20 and 16 Ma. This rotation occurred synchronously with rifting of the lithosphere in Northeast Japan, which implies that the back-arc opening of the Japan Sea was the cause of this rotation. Combined with previously obtained paleomagnetic results from adjacent areas, a counterclockwise rotation of approximately 40° in the southern part of Northeast Japan is suggested to have occurred between 20 and 18 Ma. This result implies that a diachronism exists between the clockwise rotation of Southwest Japan (15 Ma) and the counterclockwise rotation of Northeast Japan (20–18 Ma).

Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction?

New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we find that island-arc tholeiite of the lower pillow lava sequence erupted first, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infill lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the development of a mature arc edifice greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction.

New age constraints on the Miocene tectonic evolution of southwestern Japan: fission track ages from the Shitara district, Aichi Prefecture

西南日本の時計回り回転や垂直運動などの事件に年代制約を与える目的で，愛知県設楽地域の北設亜層群と設楽火成複合岩体から32個の岩石試料を採取しジルコンのFT年代を測定した．北設亜層群上部の凝灰岩は17.5 Ma前後のFT年代を示し，これは生層序年代と調和する．他方，設楽火成複合岩体の珪長質噴出物のFT年代は15 Ma前後に集中し，これは膨大な噴出物が約15 Maの短期間に定置したことを示す．こうした結果と地質，古地磁気，古応力場に関するデータから，筆者らは次のようなモデルを提案する：（1）北設亜層群は西南日本の主要な時計回り回転の直前にあたる17.5 Ma頃に，西南日本マイクロプレートが沈降しながら南方へ移動していた状況下で堆積した；（2）西南日本の時計回り回転は15 Maには既に終了していた；（3）西南日本と四国海盆の接合，およびそれによって生じた広域的な地殻隆起は，15 Maより少し前に起きた．

Paleomagnetism of Miocene dikes in the Shitara basin and the tectonic evolution of central Honshu, Japan

Central Honshu, Japan, is conspicuous for a northward-convex form of zonal geologic structures. To clarify the formation of curvature of the zonal structures in the west of the convex form and further to discuss the tectonic evolution of central Honshu, we carried out a paleomagnetic study of dated (∼15 Ma) dike rocks of the Shitara basin. Samples were collected from 25 basalt dikes trending north-south with a nearly vertical intrusion surface. After stepwise alternating-field and thermal demagnetization, 24 site-mean directions were determined. Three dikes gave anomalous directions, probably resulting from instantaneous recording of a field transition or excursion. The other 21 site-means produced a mean direction, D = 9.7°, I = 54.5°, α95 = 5.2°, and a paleomagnetic pole at 82.3°N, 216.6°E, A95 = 6.2°. Although the reversal test is negative at the 5% significance level, the mean direction and pole are time-averaged ones in which the secular variation is averaged out. This was confirmed by studying the angular standard deviation of virtual geomagnetic poles. We conclude that the central Honshu curvature formed when the southwest Japan arc rotated clockwise between 17 and 15 Ma in relation to the opening of the Japan Sea, associated with differential rotation of the eastern part of the arc with respect to the central part. The differential rotation probably resulted from a sinistral shear on the eastern margin of the rotating arc. The formation of the curvature seems not to have borne on the collision of the Izu-Ogasawara arc with Honshu, although the collision probably caused large-scale clockwise rotation of the Kanto Mountains in the east of the northward-convex structure.

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

Southwest Japan rotated clockwise during the late stage of the opening of the Japan Sea, although the exact timing of the rotation is controversial. A recent biostratigraphic study has revealed that the Miocene Oidawara Formation in eastern Southwest Japan was deposited just before 15 Ma; consequently, its paleomagnetic direction may help constrain the timing of rotation. For this purpose, we collected fine felsic tuffs and siltstones at 71 stratigraphic sites (horizons) in the Oidawara Formation. An analysis of alternating field and thermal demagnetization results yielded characteristic remanent magnetization (ChRM) directions for 177 samples. Approximately 80 % (142) of the samples exhibit reverse polarity ChRM directions that are thought to be paleofield directions of reverse polarity Chron C5Br. Normal polarity ChRM directions in 35 samples include primary paleofield records as well as records of secondary magnetization. The data suggest that a short normal polarity interval (microchron or cryptochron) at ~15.8 Ma is present within the dominantly reverse polarity interval of Chron C5Br. Reliable site-mean directions for 19 sites yield a tilt-corrected formation-mean direction of D = 10.5°, I = 41.1°, α95 = 7.0°, and k = 23.9, indicating virtually no rotation with respect to a reference paleomagnetic direction for the Asian continent. A rotation versus age plot for Southwest Japan indicates that the clockwise rotation started after 17.5 Ma and ceased largely before 15.8 Ma, yielding a rotation rate of ~23°/Myr.

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

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, 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 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.

Erratum: Limited latitudinal mantle plume motion for the Louisville hotspot (Nature Geoscience (2012) 5 (911-917))

Nature Geoscience 5, 911–917 (2012); published online 25 November 2012. In the print version of this Article originally published, the present address for Toshitsugu Yamazaki was erroneously omitted. It is as follows: Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.