Search

The Gengma-Lancang-Menghai seismic belt, the southernmost part of the North-South seismic belt, is controlled by four active faults: the Nantinghe fault (NTHF), the Sanjianshan fault (SJSF), the Hanmuba-Lancang fault (HMB-LCF), and the Heihe fault (HHF), from northwest to southeast. However, the tectonic activity of the faults in the Gengma-Lancang-Menghai seismic belt has not been fully studied yet. In the present work, we conducted tectonic geomorphic indices to analyze the relative tectonic activity along the faults in the seismic belt based on the digital elevation model. We interpreted asymmetric factor, index of drainage basin shape, hypsometric integral, normalized stream-length gradient, valley floor-to-width to height ratio, and longitudinal profiles to show that the relative tectonic activity is higher in the north and lower in the south, and is strong in the region from Shanjia to Huimin in the center of the seismic belt. [ABSTRACT FROM AUTHOR]

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling. • Remagnetized carbonates often contain a large number of SP particles that were produced during burial diagenesis. • The hysteresis properties demonstrate their effectiveness in detecting remagnetization across various lithologies. • Day plot, Fabian diagram, thermal decay curves, IRM component analysis and End-member modelling are recommended. [ABSTRACT FROM AUTHOR]

Magnetic fabric analysis is a common technique to assess the strain regime during mountain building processes. Here, we use this approach to evaluate the tectonic evolution of the Tibetan Plateau and the Eastern Himalayan Syntaxis by analyzing the limestones of the Jurassic Buqu Formation in the Zaduo area, Eastern Qiangtang Terrane (China). These limestones were chemically remagnetized during the Cenozoic. For a proper assessment, it is relevant to understand how the mineralogy of the remagnetized limestones affects their magnetic fabric and how the magnetic fabric can improve our understanding of the tectonic strain and regional deformation. The role of the authigenic magnetite in the development of the magnetic fabric should thus be explored. Comparison of the bulk susceptibility (K m) with various natural and laboratory rock magnetic properties (K m versus natural remanent magnetization, K m versus saturation isothermal remanent magnetization, and K m versus saturation magnetization) indicates that susceptibility and remanences are both carried by authigenic magnetite. Most of the magnetite grains show axial ratios <1.3:1 according to the Néel diagram, and fall within the single-domain range based on the mass magnetic susceptibility (χ) and DC field-normalized anhysteretic remanent magnetization (χ ARM) ratio, giving rise to the inverse magnetic fabric observed. Twelve sites (120 specimens) are divided into four groups based on the magnetic fabric and rock magnetic behaviors. Overall, there is a clear trend of decreasing K m , natural remanent magnetization, saturation isothermal remanent magnetization, ferromagnetic percentage and shape parameter from Group I to IV. The K 1 axis of all four groups documents a NNE-SSW oriented compression during remagnetization, contrasting with the Eocene NE-SW compression in the Gongjue area farther east. This different compressional regime likely resulted in different rotations and structural trends surrounding the Eastern Himalayan Syntaxis. • Authigenic magnetite explains magnetic behavior of Zaduo remagnetized limestones. • Single domain magnetite particles cause the inverse magnetic fabrics. • These fabrics document NNE-SSW oriented compression during Eocene remagnetization. [ABSTRACT FROM AUTHOR]

Abstract: Plagioclase crystals with an anorthite content up to An99 have been discovered in mantle peridotites from the Yungbwa ophiolite, which crops out along the Indus–Yarlung Zangbo Suture, southwestern Tibetan Plateau. Amphiboles of pargasitic composition also occur in these plagioclase peridotites. Microtextures support the view that both plagioclase and amphibole were formed metasomatically by hydrous melts entering the Yungbwa peridotites, during which pyroxene was dissolved and olivine was precipitated. The extreme Ca-rich nature of the plagioclase indicates that the metasomatic melts had high Ca/Na ratios, whereas low Ti contents in accompanying spinel and amphibole indicate severe depletion of Ti in the hydrous melts. This suggests that the metasomatic melts were derived from a refractory mantle source. Clinopyroxene trace element data indicate that the metasomatic melts were also enriched in light rare earth elements relative to both middle and heavy rare earth elements. Therefore, the melts show geochemical characteristics similar to arc magmas. This implies that the mantle peridotites in the Yungbwa ophiolite experienced a two-stage evolution; they were formed at a mid-ocean ridge and subsequently entered a subduction zone setting. Their history thus records the opening and closing of the Neo-Tethys Ocean. [Copyright &y& Elsevier]