Jean Cogne

New paleomagnetic constraints on central Asian kinematics: Displacement along the Altyn Tagh fault and rotation of the Qaidam Basin

Received 23 July 2001; revised 9 January 2002; accepted 26 February 2002; published 5 October 2002. [1] In order to better understand the tectonic evolution of central Asia under the influence of the India-Asia collision, we carried out a paleomagnetic study of 1500 cores from 106 sites along the Altyn Tagh fault, in the Qaidam and Tarim basins, and on the Tibetan plateau. Samples were mainly collected from Jurassic to Neogene siltstones and sandstones. In most cases stepwise thermal demagnetization unblocks low and high temperature components carried by magnetite and hematite. Low temperature components are north and down directed and lie close to the recent geomagnetic field. High temperature components from 10 of 13 age/locality groups pass fold and/or reversal tests and likely represent primary remanent magnetizations. The ten overall mean directions display a complex pattern of vertical-axis block rotations that are compatible with a tectonic model of clockwise rotation of the Qaidam Basin and concomitant left-lateral slip on the Altyn Tagh fault. Two of the ten localities are rotated significantly counterclockwise; they lie adjacent to the Altyn Tagh fault zone, consistent with the idea that left-lateral strike-slip motion occurred along it. The age of counterclockwise rotation near the eastern extremity of the fault was dated as younger than 19 Ma. Three widely spread areas within the Qaidam Basin exhibit similar and significant clockwise rotations, on the order of 20� , with respect to the North China Block, Tarim and Eurasia. The mean of the three values is thought to represent the total rotation of Qaidam. Because the youngest rocks displaying clockwise rotations are Oligocene, the main phase of Qaidam Basin rotation, and hence shear on the Altyn Tagh fault, took place after or near the end of the Oligocene (24 Ma). Upper Neogene strata located on the Qaidam Basin are not significantly rotated, thus tectonic deformation acting since the Upper Neogene (5 Ma) is not resolvable by paleomagnetic methods. Given a 20� ±5 � clockwise rotation of the Qaidam Basin with respect to the Tarim Basin, the maximum left-lateral displacement on the Altyn Tagh fault since 24 Ma is 500 ± 130 km. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 1527 Geomagnetism and Paleomagnetism: Paleomagnetism applied to geologic processes; 8105 Tectonophysics: Continental margins and sedimentary basins;

LATE PERMIAN PALAEOMAGNETIC RESULTS FROM THE BRIVE BASIN (MASSIF CENTRAL, FRANCE)

Abstract A palaeomagnetic study has been carried out in the Brive basin in order to understand the continental deformation of the French Massif Central since the Permian. Fifteen sites of Permian-age red sandstones have been sampled in the Brive basin which is situated in the west of the Sillon Houiller fault (France). Laboratory analyses have revealed well-behaved and only reversed magnetic polarity for the higher-temperature component. The effects of positive folding test and Kiaman reversal chron indicate that the magnetic remanent is probably primary. The so-called ‘Autunian’ and ‘Saxo-Thuringian’ stages show very close palaeomagnetic directions, with an angular difference of 1.2 ± 6.9°. They likely correspond to the same geological period (i.e. Late Permian), and allow us to calculate one palaeomagnetic pole for this basin (48.8°N, 162.5°E, n = 15 and A95 = 2.6°). Comparing this result with previous palaeomagnetic studies from other Permian basins in the south Massif Central and Stable Europe, we may conclude that: (1) the Brive basin has been part of Stable Europe since, at least, the Late Permian, and no younger palaeomagnetically detectable movement of this basin has be found with respect to Stable Europe; (2) no significant movement along the Sillon Houiller fault has been detected since the Permian. The relative latitudinal movement of the Permian basins of the Massif Central (i.e. Brive, Saint-Affrique and Lodeve) is negligible. No significant rotation for each of these basins with respect to Stable Europe has been observed, except for the St. Affrique basin. This later basin experienced a counter-clockwise rotation of ∼7° with respect to Stable Europe which represents probably a local phenomenon.

Apparent synfolding magnetization as a result of overlap of pre‐ and post‐folding magnetizations

In most rocks, Natural Remanent Magnetization (NRM) can involve two or more superimposed components. Demagnetizations hopefully result in the separation of these components, which may then be resolved using least-squares regression methods. In a study of Cretaceous redbed sites from Qaidam (China), thermal demagnetization of most specimens revealed 3 rectilinear segments in orthogonal plots, giving the appearance of a 3-component magnetization. In order to help deciding if the intermediate straight segments could have resulted from overlap between post-folding low temperature (LTC) and pre-folding high temperature components (HTC), we modeled the overlap using a priori information on the directions and relative intensities of LTC and HTC derived from the actual observations. We constructed typical synthetic Zijderveld (1967) diagrams for each site, which could be compared with those obtained from the real samples. We conclude for the Qaidam samples that the intermediate temperature component (ITC) is most likely an artifact resulting from overlap between the LTC and HTC.