中国科学院南京地质古生物研究所机构知识库

The Tuoyun volcanics from the Tian Shan range (Central Asia) give an opportunity to investigate the variability of the Earth's magnetic field during the Cretaceous and Early Paleogene. In the paper we focus on Maastrichtian-Paleocene basalts and report new paleomagnetic results from 70 lava flows (respectively 45 directional groups) sampled near Tuoyun village (75.33 degrees E; 40.18 degrees N) from three distinct sections. Combined with previous results, our new data set yields a virtual geomagnetic pole at fS 1/4 180.2 degrees E,.S 1/4 49.5 degrees N with an angular standard deviation S 1/4 20:8 degrees j23:1 degrees 18:8 degrees (for N 1/4 93 directional groups). The mean inclination Is 1/4 36.2 degrees is indicative of an emplacement of the Tuoyun volcanics at a paleolatitude around 20 degrees N, consistent with the high dispersion k 1/4 14.6 of the directions. Such an inclination value is 10-15 degrees lower than the predictions from the apparent polar wander paths for stable Europe and East Asia. This observation can be partly explained by crustal shortening within the East Asian plate during the India-Asia convergence, but also needs to invoke a local field anomaly in Central Asia as previously proposed. Our absolute paleointensity experiments conducted on two lava flows and two baked sedimentary layers yield a mean virtual dipole moment of 58.9 +/- 6.9 ZA.m2, consistent with the values at 60-70 Ma in the global paleointensity database. Compared to relative paleointensity results during the Cretaceous Normal Superchron and during the Plio-Pleistocene, the relative variability in intensity-supposed to be a proxy for the geodynamo's activity-estimated here at 31 +/- 5% (N 1/4 15) from pseudo-Thellier experiments possibly corroborates a correlation between geomagnetic reversal frequency and paleosecular variation rate during the past 120 Myr.