4 research outputs found
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Further Paleomagnetic Results from the Permian Emeishan Basalt in SW China
In recent years scientists at home and abroad have been conducting paleomagnetic investigations on the Permian Emeishan Basalt in SW China. The results reported, however, vary from one locality to another. We report here some new paleomagnetic results for the Emeishan Basalt from three localities in W. Sichuan and Yunnan provinces and compare them with relevant data published hitherto
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Paleomagnetism of upper Cretaceous rocks from South China
Paleomagnetic study of upper Cretaceous redbeds from two areas in South China was undertaken in an effort to constrain better the history of tectonic movements in southeastern Asia. From the Nanjing area (32°N, 119°E), high-temperature characteristic directions which pass a fold test were obtained from ten sites (43 samples) in the upper Cretaceous Puko and Yanzijing Formations. The paleomagnetic pole position (76.3°N, 172.6°E;A95 = 10.3°) agrees well with a late Cretaceous reference pole for Eurasia and thus confirms that this part of South China was fully sutured to Eurasia by the end of the Cretaceous with no paleomagnetically resolvable subsequent relative rotation. In western Sichuan (26.5°N, 102.3°E), high-temperature characteristic directions of normal and reverse polarity were obtained from twelve samples (48 specimens) in the upper Cretaceous to lower Paleocene Xiaoba and Leidashu Formations. A provisional paleomagnetic pole position (80.9°N, 296.8°E;A95 = 7.7°) differs significantly from a late Cretaceous Eurasia reference pole. In terms of apparent tectonic movement, the difference in paleopole positions would indicate a 13 ± 10.3° northward translation and 22 ± 11.2° counterclockwise rotation of western Sichuan relative to Eurasia. Confirmation of this reconnaisance result, which would suggest that western Sichuan was pushed ahead of an impinging India, is needed
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Equatorial paleomagnetic time-averaged field results from 0-5 Ma lavas from Kenya and the latitudinal variation of angular dispersion
Lavas of Pliocene-Pleistocene age were sampled in two regions in Kenya: Mount Kenya on the equator and the Loiyangalani region, east of Lake Turkana, at about 3°N. We sampled 100 sites distributed around the Mount Kenya Massif and to the northeast along the Nyambini Range. The equator bisects Mount Kenya, and all sites were sampled within 40′ of the equator. Thirty-two sites were sampled in the Loiyangalani area, making a total of 132 sites. Many sites from the Mount Kenya study were severely affected by lightning; however, after progressive AF demagnetization 69 sites yielded directions with α_95 equal to or less than 10°. Normal polarity sites dominate (N = 58 and a mean of declination (dec) = 1.2°, inclination (inc) = -0.7°, and α_95 = 3.6°) with only 11 reverse polarity sites (mean of dec = 182.3°, inc = 0.6°, and α_95 = 7.2°); no transitional directions were identified. Inverting the reverse sites yields a combined mean direction of dec = l.4°, inc = -0.7°, and α_95 = 3.2°. This result is not significantly different from what is expected from the geocentric axial dipole for the mean locality (dec = 0° and inc = 0°); a quadrupole component was not resolved. The samples from the Loiyangalani region were not seriously affected by lightning, and all 32 sites gave satisfactory data with α_95 less than 10° (17 reverse sites, dec = 183.4°, inc = 0.8°, and α_95 = 6.7°; 15 normal sites, dec = 358.6°, inc = -1.1°, and ° = 4.7°); after inverting the reverse sites the combined mean was dec = 1.1°, inc = -1.0°, and α_95 = 4.1°. Altogether, we had a total of 101 successful sites. A virtual geomagnetic pole (VGP) was calculated from each site mean; the VGP dispersion is low, with Sb = 10.9° for Mount Kenya and 9.8° for the Loiyangalani region. This dispersion agrees with updated Model G of McElhinny and McFadden (1997) and model TK03 of Tauxe and Kent (2004) that was tuned to the compilation of McElhinny and McFadden (1997) but disagrees with the higher dispersion near the equator and the smaller latitudinal gradient in dispersion estimated by Johnson et al. (2008). A new database is presented, and the included studies support a systematic decrease of dispersion from high to low latitudes