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Nature of remagnetization of Lower Triassic red beds in southwestern China



Widely distributed haematite-bearing red beds are an important source of palaeomagnetic field records. However, unresolved issues regarding remagnetization and inclination shallowing in red beds have questioned the reliability of the palaeomagnetic results obtained from such materials. In this study, we investigated the remagnetization mechanism in red beds from Lower Triassic sandstones in Yunnan Province, southwestern China. Our results indicate that the characteristic remanent magnetizations (ChRMs) of most samples (112/125) are dominated by only one component at temperatures 80–660 °C, with a mean direction of D/I= 0.9/46.6° (k= 440.2, α95= 2.1°). The corresponding geomagnetic pole is 89.1°N, 331.7°E (k= 338.2, A95= 2.4°). This coincides with the present geomagnetic field, which is a strong indication that these ChRMs are remagnetized. Combined rock magnetic and microscope investigations reveal that the remagnetization is due to the acquisition of a chemical remanent magnetization (CRM) carried by authigenic maghemite and haematite. Despite the widespread remagnetization, about 7 per cent of the studied samples still record a magnetization that we consider to be primary that was isolated at high temperatures 610–660 °C, with a mean direction of D/I= 213.3/18.6° (k= 16.0, α95= 11.7°). We propose that the CRM overprinting is controlled by the overlapping degree of the unblocking temperature between the CRM carried by the authigenic haematite and the primary remanent magnetization carried by the detrital haematite. Our results further suggest that microscope investigation, rock magnetic proxies for the haematite concentration and susceptibility–temperature curves are useful methods for pre-selecting samples suitable for isolating the primary remanence at this region. The linkage among the palaeomagnetic results, rock magnetic proxies and CRM remagnetization mechanism could be extended to other studies, although the detailed proxy would be different due to specific overprint process.