Paleomagnetic results from the Early Cretaceous Zenong Group volcanic rocks, Cuoqin, Tibet, and their paleogeographic implications
Abstract
The India-Asia collision in the Cenozoic Era leads to not only intracontinental deformation in Asia, but also uplift of the Tibetan Plateau. Because the Lhasa terrane was the southern margin of Asia prior to the collision, Cretaceous paleomagnetic data from the Lhasa terrane are essential to understanding the India-Asian collision process and to estimating the magnitude of intracontinental deformation within Asia. Several paleomagnetic investigations for the Cretaceous rocks in the Lhasa terrane have been done, but they have not converged on a consensus regarding the paleolatitude of the southern margin of Asia. Besides, most of these Cretaceous paleomagnetic investigations are located in the Linzhou basin, eastern part of the Lhasa terrane. Our new paleomagnetic study was carried out on the Early Cretaceous Zenong Group volcanic rocks (ca. 110-130 Ma) near Cuoqin town, central Lhasa terrane. Remanent magnetization measurements were carried out with 2G-755R and 2G-755-4K cryogenic magnetometers, installed in the Institute of Geomechanics in Beijing and China University of Geosciences, Beijing, respectively. Paleomagnetic specimens were thermally demagnetized up to 580 °C or 680 °C in ASC-TD 48 furnaces. Acquisition of isothermal remanent magnetization (IRM), back-field demagnetization of saturation IRM, and thermal demagnetization of the three-axis IRM were performed on some representative specimens. The thermal demagnetization and rock-magnetic results reveal that the high-temperature component (HTC) of lava specimens is carried by magnetite and hematite while the HTC of tuff specimens is carried by magnetite. Stepwise thermal demagnetization up to 680°C isolated a stable high-temperature component HTC including antipodal dual polarities. The HTC directions passed both a fold test at 99% confidence and a reversal test at 95% confidence, suggesting a primary origin. The tilt-corrected average direction for 18 sites is D=327.0°, I=35.7°, κ=59.3°, α95=4.5°, which corresponds to a paleopole at 58.2° N, 341.9° E (A95 = 4.6°), yielding a paleolatitude of 19.8° ± 4.6° N for the study area. Our results, combined with previous paleomagnetic data, suggest that southern Tibet was at a mean paleolatitude of 20° N during the Cretaceous and Paleogene. However, when compared with the apparent polar wander paths of Asia, the difference suggests that a ca. 900 km north-south crustal shortening has accumulated between the Lhasa terrane and stable Asia since ca. 110 Ma. Therefore, the 900 km of crustal shortening must have been absorbed by fold and thrust belts and strike-slip faults which caused by the collision of India-Asian. Furthermore, comparing our paleomagnetic data with the India apparent polar wander path suggests the Neotethys was 6720 ± 690 km wide during the Early Cretaceous.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP11A1001C
- Keywords:
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- 1500 GEOMAGNETISM AND PALEOMAGNETISM