Yo-ichiro Otofuji

Paleomagnetic evidence for the Miocene counter-clockwise rotation of Northeast Japan—rifting process of the Japan arc

Paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of Cretaceous to Miocene age from Northeast Japan. The remanent magnetizations of welded tuffs with age between 32 Ma and 21 Ma old from 17 widely distributed sampling sites in Northeast Japan are fairly well grouped with a mean ofD = −41.2°, I = 56.5°, α95 = 7.2°. The remanent magnetization of welded tuffs of 14 Ma and 11 Ma from 4 sites shows a northerly direction (D = −11.7°, I = 60.5°, α95 = 15.3°). These data imply that Northeast Japan was apparently subjected to a counter-clockwise rotation through 47 ± 14° about a vertical axis on the representative point of Northeast Japan (141°E, 39°N) during the past 21 Ma. Comparing with the period of the clockwise rotation of Southwest Japan, the differential rotation between Northeast Japan and Southwest Japan is concluded to have occurred concurrently during the period between 21 Ma and 12 Ma. A rifting model of the Japan arc is proposed: Northeast Japan rotated counter-clockwise through 50° about a northern pivot at 146°E, 44°N, while Southwest Japan rotated clockwise through 54° about a southern pivot at 129°E, 34°N. The differential rotation about different rotation pivots gives rise to the opening of a diamond-shaped back arc basin, the Japan Sea.

Paleomagnetic evidence for the clockwise rotation of Southwest Japan

Over 500 oriented samples of felsic rocks of Cretaceous to Middle Miocene age were collected along the Go¯River in the central part of Southwest Japan, in an attempt to detect the process of tectonic rotation of Southwest Japan from the paleomagnetic view point. Thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples. Reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities. The paleomagnetic results establish that Southwest Japan began to rotate clockwise through58 ± 14° later than 28 Ma and ceased its motion by about 12 Ma. Southwest Japan has undergone no detectable north-south translation since 28 Ma. These results imply that southwest Japan was rotated about the pivot around 34°N, 129°E between 28 Ma and 12 Ma in association with the opening of the Japan Sea.

Asthenospheric injection and back-arc opening: Isotopic evidence from northeast Japan☆

Nd and Sr isotopic compositions were determined for the Tertiary volcanics from the back-arc side of the NE Japan arc. Sr isotopes show a linear trend through time from an enriched signature (87Sr86Sr = 0.705437) to a depleted signature (87Sr86Sr = 0.70270). In a complementary fashion, Nd isotopes start at low value (ϵNd = −0.80) and show a gradual increase (ϵNd=8.3) with decreasing age. Isotopic change of Nd and Sr from the enriched signature to the depleted one is synchronous with the opening of the Japan Sea at ∼ 15 Ma. This synchronism indicates that the opening of the Japan Sea was initiated by the injection of the asthenosphere. During the pre-opening stage, the mantle wedge was composed of a two-layered structure: the sub-continental lithosphere and the underlying asthenosphere. The volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub-continental lithosphere. The sub-continental lithosphere of the back-arc side was thinned by the injection of the depleted asthenosphere, which accelerated the growth of the MORB source within the mantle wedge of the back-arc side and resulted in magma generation with the depleted isotopic signature of Nd and Sr at the post-opening stage.

Amount of clockwise rotation of Southwest Japan — fan shape opening of the southwestern part of the Japan Sea

Abstract Paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 Ma and 112 Ma from Yamaguchi and Go¯river areas in the central part of Southwest Japan. The new data, together with those of younger igneous rocks published previously, define the change of paleomagnetic field direction during the late Mesozoic/ Cenozoic period for Southwest Japan. The paleomagnetic direction from this area has pointed 56 ± 3° clockwise from the expected field direction estimated from APWP (apparent polar wandering path) of the whole of Eurasia during the period between 100 and 20 Ma. In comparison with the expected one from the eastern margin of Eurasia (Korea, China, Siberia), the Cretaceous field direction of Southwest Japan shows the clockwise deflection by 44–49°. These results establish that while the eastern margin of Eurasia, including Southwest Japan, was rotated more or less with respect to the main part of Eurasia during last 100 Ma, Southwest Japan was rotated clockwise through more than 40° with respect to the eastern margin of Eurasia since 20 Ma. The large amount of rotation for Southwest Japan implies that it is rotated by an opening of the southwestern part of the Japan Sea, which widens northeastward (fan shape opening). The tectonic feature of Southwest Japan and the Japan Sea is analogous to that of Corso-Sardinia and the Ligurian Sea in the Mediterranean, indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim.

Opening of the Sea of Japan back-arc basin by asthenospheric injection

Abstract The position of a volcanic front on an arc is governed by dehydration reactions of amphibole + chlorite in down-dragged hydrated peridotite at the base of the mantle wedge. As these reactions are essentially pressure-dependent (ca. 3.5 GPa), the paleo-position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it. This paper evaluates volcanism in the Northeast Japan arc since the Oligocene and discusses the mass transportation in the mantle wedge during the episode of back-arc extension in the Sea of Japan with reference to the migration of the volcanic front. The volcanic front during the Oligocene (ca. 30 Ma) was located along the western coast of the present Northeast Japan are and migrated about 200 km trenchwards at 23 Ma before the opening of the Sea of Japan back-arc basin. It follows that the angle of subduction became steeper during the period of 30-23 Ma, triggered by the injection of the asthenosphere into the mantle wedge. The back-arc extension of the Sea of Japan between 21 and 14 Ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection. The temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere. This caused the characteristic volcanism at 23-22 Ma: (1) High-magnesian andesite volcanism took place in the near-trench region; (2) across-arc compositional variation was not observed; (3) the volume of volcanic materials increases towards the back-arc side of volcanic arc. The volcanic front has migrated over a short distance inwards (less than 100 km) since 15 Ma, possible as a result of cooling in the mantle wedge, because both amphibole and chlorite break down at slightly higher pressures at lower temperatures. The upper mantle materials which injected into the mantle wedge and caused back-arc spreading are identical to those of the mid-oceanic ridge basalt source in Sr-Nd isotopic compositions, because volcanic rocks in the back-arc region of the Northeast Japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 Ma.

Counterclockwise rotation of Northeast Japan: paleomagnetic evidence for regional extent and timing of rotation

Early to Late Miocene welded tuffs have been sampled from northeast Honshu Island and the Oshima Peninsula of Hokkaido Island, Japan for paleomagnetic study. Characteristic directions with a high unblocking temperature component above 560°C are isolated from 24 sites. Westerly declinations (D = −1° ∼ −91°) are identified in welded tuffs older than 17 Ma from these areas. When combined with previous paleomagnetic data from welded tuffs, the Early Miocene paleomagnetic poles derived from northeast Honshu Island and the Oshima Peninsula are statistically identical and indicate that northeast Honshu Island and the Oshima Peninsula were subjected to counterclockwise rotation as a single tectonic block. These results establish that the rotating block of Northeast Japan extends north to the Oshima Peninsula to the latitude 43°N. The dimension of this rotating block is of the order of 500 km. The paleomagnetic data provide a best curve for the rotation process, with the counterclockwise rotation reaching a climax at 15.0 Ma and a net amount of rotation of 46.4°. The 500 km long Northeast Japan block was rotated counterclockwise through more than 45° and accompanied a clockwise rotation of the 600 km long Southwest Japan block at about 15 Ma.

Clockwise rotation of the Red River fault inferred from paleomagnetic study of Cretaceous rocks in the Shan-Thai-Malay block of Western Yunnan, China

More than 150 samples were collected at 23 sites from the Lower Cretaceous Jingxing Formation near the city of Yongping (25.5°N, 99.5°E), which is located on the west side of the Red River fault. Sixteen sites have characteristic directions with a high-temperature component above 500°C. The high-temperature component of magnetization from twelve sites is of pre-fold origin, and reveals clockwise deflection in declination (D = 42.0°,I = 51.1° andα95 = 15.7°). The easterly declination of more than 40° at Yongping is consistent with the Cretaceous paleomagnetic direction (D = 45°) of Chuxiong (25°N, 101.5°E), which is located on the east side of the Red River fault. Both areas were subjected to about 25° clockwise rotation with respect to the eastern part of the Yangtze block since Cretaceous time. We conclude that the Red River fault was rotated through25° ± 16° with respect to the eastern part of the Yangtze block, associated with rotation of the Yongping and Chuxiong areas. Removal of the rotation from the Red River fault indicates that the latter and the Jinsha suture formed a straight line along N55°W–S55°E within the Asian continent from Vietnam to the Tibetan Plateau in the present coordinate system during the Cretaceous. The linear feature of the Red River fault and Jinsha suture was probably deformed to the present curved shape as a result of collision of the Indian continent with the Asian continent.

Paleomagnetic study of middle Cretaceous rocks from Yunlong, western Yunnan, China: evidence of southward displacement of Indochina

Middle Cretaceous red sandstones and siltstones were collected at 20 sites for paleomagnetic study from the Nanxin Formation around Yunlong (25.8°N, 99.4°E), western part of Yunnan province, China. After stepwise thermal demagnetization, high-temperature component magnetization with unblocking temperature of about 675°C is isolated. A positive fold test at the 99% confidence limit shows that the high-temperature component magnetization is primary for the middle Cretaceous Nanxin Formation. A tilt-corrected formation mean direction is D = 40.2°, I = 49.9° with α95 = 3.9°, corresponding to a paleopole at 54.6°N, 171.8°E with A95 = 4.4°. Comparison with previously reported Cretaceous paleomagnetic directions indicates that the Yunlong area has been subjected to southward displacement by 11.9±7.5° (corresponding to 1300±800 km) with respect to the stable Yangtze craton since Cretaceous time as well as clockwise rotation through 36.3±13.6°. This study elucidates that Indochina was squeezed out of the Asian continent due to collision of India accompanied with clockwise rotation.

Paleomagnetic study of Upper Jurassic rocks from the Sichuan basin: tectonic aspects for the collision between the Yangtze Block and the North China Block

Abstract A paleomagnetic study has been carried out on Late Jurassic redbeds in the Sichuan basin, at the northern part of the Yangtze Block. Upper Jurassic brownish-red sandstones and brownish-red siltstones were collected in the Penglaizhen Formation at 26 sampling sites around Jiangyang city (30.4°N, 104.5°E). Thermal demagnetization isolated a high temperature magnetization component with a maximum unblocking temperature of about 690°C. The primary nature of magnetization acquisition is ascertained through a 95% confidence level positive fold test, as well as a positive reversal test. The tilt-corrected overall mean direction of 24 sites is D=25.9°, I=26.7° (α95=4.7°), leading to a paleomagnetic pole situated at 61.3°N/222.7°E (A95=4.2°). This pole position agrees with two other Late Jurassic poles from the Sichuan basin, indicating that this basin has behaved as a rigid block since Late Jurassic. These three poles from the Sichuan basin therefore provide a characteristic Late Jurassic pole for the stable part of the Yangtze Block (64.3°N, 231.2°E, A95=6.7°). Comparison of the Late Jurassic pole of the Yangtze Block with the contemporaneous pole for the North China Block (NCB) identifies a significant difference, implying that the relative movement between the Yangtze Block and the NCB continued during, and probably after, the Late Jurassic. Paleomagnetic and geological evidence reveals that the Yangtze Block collided first with the NCB during the Early Permian, and continued to penetrate into it while undergoing a clockwise rotation until the Early Cretaceous.

Paleomagnetic study of Cretaceous rocks from the Yangtze block, central Yunnan, China: implications for the India-Asia collision

Abstract More than 200 samples for this study were collected at 30 sites from the Cretaceous Matoushan and Puchanghe Formations around the city of Chuxiong (25°N,101.5°E). Twenty-four sites have characteristic directions with a high temperature component above 350°C. Paleomagnetic directions from 21 sites of the Cretaceous formations reveal clockwise deflection in declination ( D = 44.6°, I = 41.3°, andα 95 = 10.7°). The reliability of the direction is ascertained through a positive fold test, and the presence of normal and reversed polarities. Comparison with paleomagnetic data from other areas indicates that the Chuxiong area was rotated clockwise through26° ± 17° with respect to the eastern part of the Yangtze block, whereas it was subjected to more than 44° of clockwise rotation with respect to the neighboring blocks of Lhasa (56° ± 21°) and western Sichuan (44° ± 16°) since Cretaceous time. The clockwise rotation of Chuxiong is a reflection of rotational motion of the large region including the southeastern Qiangtang Terrane and the Jinsha suture-Red River fault. Paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central Yunnan province, probably during the Paleogene. This large rotation is a spectacular example of the deformation in the southeastern part of the Asian continent due to the collision of India.