Remagnetization of Red Beds on the Tibetan Plateau: Mechanism and Diagnosis

Abstract

Red beds are important targets for paleomagnetic studies, yet discriminating secondary chemical remanent magnetization (CRM) from primary depositional remanent magnetization (DRM) in them remains challenging. Here we reanalyze the thermal demagnetization data of and conduct comprehensive rock magnetic, Mössbauer spectroscopic and petrographic studies on red beds from four Cenozoic basins on the eastern Tibetan Plateau (China): the Gongjue, Nangqian, Shanglaxiu, and Jinggu basins. The red beds in the latter two basins are remagnetized, as are most Nangqian red beds. The Gongjue red beds and some Nangqian red beds retain a DRM. Our results reveal that detrital (titano)magnetite and hematite are well preserved in red beds retaining the DRM, whereas remagnetized red beds contain large amounts of authigenic hematite and goethite with detrital Fe‐bearing minerals strongly altered. Postdepositional diagenetic alteration induced by heating and/or fluid circulation related to magmatism and/or crustal deformation is probably the main reason for the remagnetization. Hematite carrying the CRM in remagnetized red beds has wide distribution of grain size and unblocking temperature spectra, while hematite carrying the DRM is usually coarse and has confined unblocking temperature spectrum. This can be used as a criterion for diagnosing remagnetization. Nanoscale goethite appears to occur only in remagnetized red beds: another sensitive criterion for discriminating CRM from DRM. These property‐based criteria constrain the origin of the NRM in red beds better than the classic paleomagnetic field tests. Our research emphasizes that integrated rock magnetic, Mössbauer spectroscopic and petrographic studies provide robust tools to diagnose remagnetization in red beds.

Reply to Comment by Zhao et al. on “Paleomagnetism of the Late Cretaceous Red Beds From the Far Western Lhasa Terrane: Inclination Discrepancy and Tectonic Implications”

“…The AMS deformed characteristics of the samples from Bian et al's (2020) Jingzhushan Formation red beds should belong to stage 2 (incipient deformation) to stage 4 (weak cleavage), which is also consistent with weak and moderate deformed magnetic fabrics (Weil & Yonkee, 2009). Recently, the Late Cretaceous-Eocene red beds in the Gonjo basin of the Eastern Qiangtang terrane have been studied in detail for both paleomagnetism and rock magnetism (Huang et al, 2020;, Li, van Hinsbergen, Shen, et al, 2020Tong, Yang, Mao, et al, 2017;Zhang et al, 2018). AMS analyses indicate that the Late Cretaceous-Eocene red beds have subjected to northeast-southwestward compression (Li, van Hinsbergen, Shen, et al, 2020;Zhang et al, 2018).…”

“…According to the classification of AMS results proposed by Hrouda and Chadima (2020), their deformed characteristics should belong to stage 2 (incipient deformation) to stage 4 (weak cleavage). Noticeably, magnetostratigraphic results revealed multiple normal and reversed magnetozones (Bain et al., 2020; Li et al., 2020), which, combined with positive fold and reversal tests, suggests that the ChRM directions obtained from the Late Cretaceous‐Eocene red beds of the Gonjo basin are primary magnetization acquired before folding (Huang et al., 2020; Li, van Hinsbergen, Najman, et al., 2020; Tong, Yang, Mao, et al., 2017). These previous studies clearly show that the AMS deformed characteristics of the red beds belonging to stage 2 (incipient deformation) and stage 4 (weak cleavage) can record a primary magnetization or at least a biasing of magnetization that does not affect the interpretation of magnetostratigraphy.…”

“…Recently, the Late Cretaceous‐Eocene red beds in the Gonjo basin of the Eastern Qiangtang terrane have been studied in detail for both paleomagnetism and rock magnetism (Huang et al., 2020; Li, van Hinsbergen, Najman, et al., 2020, Li, van Hinsbergen, Shen, et al., 2020; Tong, Yang, Mao, et al., 2017; Zhang et al., 2018). AMS analyses indicate that the Late Cretaceous‐Eocene red beds have subjected to northeast‐southwestward compression (Li, van Hinsbergen, Shen, et al., 2020; Zhang et al., 2018).…”

The Magnetic and Color Reflectance Properties of Hematite: From Earth to Mars

“…Moreover, W. Huang et al. (2020) proposed that diagenetic alteration induced by heating and/or fluid circulation is probably the main cause of Tibetan Plateau red bed remagnetizations. They used secondary paleomagnetic components to provide information about coeval field and hydrothermal activity.…”

“…From thermomagnetic behavior, W. Huang et al (2019) also suggested that goethite can be used to diagnose red bed remagnetizations from integrated rock magnetic, Mössbauer spectroscopic, and petrographic methods. Moreover, W. Huang et al (2020) proposed that diagenetic alteration induced by heating and/or fluid circulation is probably the main cause of Tibetan Plateau red bed remagnetizations. They used secondary paleomagnetic components to provide information about coeval field and hydrothermal activity.…”