Masako Miki

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.

Opening mode of the Okinawa Trough: paleomagnetic evidence from the South Ryukyu arc

More than 250 samples of volcanics and sedimentary rocks were collected from the South Ryukyu Arc for paleomagnetic study and K-Ar dating. Stable primary components of magnetization were isolated from 18 sites after thermal demagnetization. The mean paleomagnetic direction for 15 Eocene volcanic sites (D = 30.1°, I = 40.3°, α95 = 10.3°) indicates a direction which is deflected 18.6° ± 11.6° clockwise from the expected field direction calculated from the 40 Ma pole of Eurasia. The clockwise deflection is observed also in the dike rock (D = −144.0°, I = −32.1°), the age of which is determined to be 9.6 ± 0.8 Ma by the conventional K-Ar dating method. These results indicate that, with respect to Eurasia, the southern part of the Ryukyu Arc has rotated clockwise 19° during the past 10 M.y. We attribute this rotation to the back-arc opening of the southern part of the Okinawa Trough, which occurred by means of the “wedge” mode later than 10 Ma.

GEOMAGNETIC PALEOSECULAR VARIATION IN EASTER ISLAND, THE SOUTHEAST PACIFIC

Abstract We collected more than 250 samples from 45 sites in lava flows of the three volcanoes on Easter Island. The K–Ar whole rock dating was attempted on 13 sites. All of the ages obtained are younger than 0.8 Ma. Reliable paleomagnetic directions were obtained from 34 sites. The mean VGP position is 89.2°N and 239.1°E with a 95% confidence value of 3.8°. The angular dispersion value of Easter Island VGPs was calculated to be 11.8° with an upper limit of 14.1° and a lower limit of 9.8°. The angular dispersion value is significantly lower than the values predicted by the theoretical secular variation models. The pacific dipole window appears to exist also in the southeast Pacific.