Chen L.,CAS Institute of Geology and Geophysics
Lithos | Year: 2010
This study presents an integrated study of the North China Craton (NCC) based on recent high-resolution seismic images combined with observations on surface geology, regional tectonics and mantle dynamics. Seismic images reveal markedly concordant and rapid variations in crustal and lithospheric structure and thickness, upper mantle anisotropy, and discontinuity structures and thickness of the mantle transition zone near the boundary between the eastern and central parts of the NCC. These rapid variations roughly coincide with the sudden change in both surface topography and gravity field as marked by the North-South Gravity Lineament (NSGL). Such a shallow-deep structural concordance may reflect different lithospheric tectonics and mantle processes in the two domains during the Phanerozoic reactivation of the craton. Sharp structural variations are particularly present to the west of the NSGL, especially between the Archean Ordos Plateau, which retains the characteristics of a typical craton, and the surrounding Cenozoic rift systems which are underlain by a significantly modified and thinned lithosphere. These observations provide deep structural evidence that the Phanerozoic reactivation was not confined to the eastern NCC as previously thought, but also affected areas in the central and western NCC, though to a much lesser degree. On both sides of the NSGL, lithospheric modification and thinning appear to be more pronounced along Paleoproterozoic belts suturing Archean blocks, demonstrating the importance of pre-existing lithosphere-scale structures in controlling the tectonic evolution of the NCC. It further indicates that craton reactivation probably is common given the fact that structural heterogeneities are always present in cratonic regions. The seismic structural images together with geological, petrological, geochemical and mineral physics data suggest that the fundamental destruction of the eastern NCC lithosphere may have been triggered largely by the deep subduction of the Pacific plate, especially during the Late Mesozoic. The complexity of deep structures and lithospheric properties in regions west of the NSGL may represent the relatively weak imprints of the Cenozoic India-Eurasia collision superposed upon that of the earlier tectonic events. © 2009 Elsevier B.V.
Paterson G.A.,CAS Institute of Geology and Geophysics
Geophysical Journal International | Year: 2013
Numerous non-ideal factors can influence paleointensity data, but the detection of these factors remains problematic and new approaches to understanding how paleointensity data behave are needed. In this study, a recently developed stochastic model of single domain (SD) paleointensity behaviour is expanded to investigate the effects that anisotropic and non-linear thermoremanent magnetizations (TRMs) have on the paleointensity results and the parameters used to select data. The model results indicate that before applying any form of correction these non-ideal factors can produce results that are self-consistent, but highly inaccurate. The methods that are currently used to correct for anisotropic and non-linear TRMs are effective and greatly increase the likelihood of obtaining accurate results. The corrections, however, do not restore the results to those of ideal SD samples measured with the same laboratory-to-ancient field ratio, but the data are restored to those of ideal SD samples with the equivalent laboratoryto-ancient magnetization ratios (MLab/MAnc). The simulations indicate that non-linear and anisotropic TRM have no or only a weak influence on the parameters commonly used to select paleointensity data, which means that these non-ideal factors are effectively undetectable. These new models suggest that the paleointensity behaviour of thermally/chemically stable SD samples, whether they are ideally behaved, anisotropy or non-linear TRM corrected, is near universal and depends only on MLab/MAnc and the choice of paleointensity protocol (i.e. Coe-type versus Thellier). Given the high self-consistency and highly inaccurate results that anisotropic and non-linear TRM can yield, it is essential to test for such effects and all Thellier-type paleointensity studies must include tests for anisotropic and non-linear TRM to assert the reliability of the data obtained. ©The Author 2013.Published by Oxford University Press on behalf of The Royal Astronomical Society.
He L.,CAS Institute of Geology and Geophysics
Journal of Geophysical Research: Solid Earth | Year: 2014
The deep subduction of the Pacific Plate underneath East Asia is thought to have played a key role in the destruction of the North China Craton (NCC). To test this hypothesis, this paper presents a new 2-D model that includes an initial stable equilibrated craton, the formation of a big mantle wedge (BMW), and erosion by vigorous mantle convection. The model shows that subduction alone cannot thin the cold solid craton, but it can form a low-viscosity BMW. The amount of convective erosion is directly proportional to viscosity within the BMW (η0bmw), and the rheological boundary layer thins linearly with decreasing log10(η0bmw), thereby contributing to an increase in heat flow at the lithospheric base. This model also differs from previous modeling in that the increase in heat flow decays linearly with t 1/2, meaning that the overall thinning closely follows a natural log relationship over time. Nevertheless, convection alone can only cause a limited thinning due to a minor increase in basal heat flow. The lowering of melting temperature by peridotite-melt interaction can accelerate thinning during the early stages of this convection. The two combined actions can thin the craton significantly over tens of Myr. This modeling, combined with magmatism and heat flow data, indicates that the NCC evolution has involved four distinct stages: modification in the Jurassic by Pacific Plate subduction and BMW formation, destruction during the Early Cretaceous under combined convective erosion and peridotite-melt interaction, extension in the Late Cretaceous, and cooling since the late Cenozoic. ©2014. American Geophysical Union. All Rights Reserved.
Paterson G.A.,CAS Institute of Geology and Geophysics
Journal of Geophysical Research: Solid Earth | Year: 2011
Detecting and excluding non-ideal behavior during paleointensity experiments is critical to asserting the reliability of data. Our knowledge of detecting non-ideal behavior, in particular the influence of multidomain (MD) grains, has expanded considerably over the past decade and experimental procedures now commonly incorporate checks to detect the effects of MD behavior. However, many older studies were carried out before these checks were devised and provide no quantifiable means of testing for the presence of MD grains. An estimated one third of all entries in the most recent paleointensity database do not include some form of check for MD behavior. The reliability of these results is therefore questionable and can only hinder efforts to understand the evolution of the geomagnetic field and the geodynamo. I propose a simple phenomenological check that can be applied to previous studies, provided that the raw data are available, that will allow the exclusion of MD behavior and provide a means of identifying reliable data. The check is a quantification of the curvature, k, of data points on an Arai plot, a feature commonly associated with MD behavior. Analysis of paleointensity data from samples with known grain size indicates that this new parameter is significantly correlated with grain size and with the accuracy of the paleointensity estimates made from both limbs of the curved data. Analysis of 181 samples from five historical data sets indicates that k is significantly correlated with experimentally obtained MD and alteration check parameters, and the accuracy of the paleointensity estimate. A threshold selection value of k ≤ 0.164 can be defined using the samples with known grain sizes. Applying this cut-off value, combined with a threshold on the quality of the circle fit and a commonly used alteration check, to the historical data yields an accurate result with low scatter. When compared with previously published selection criteria that incorporate experimental checks for non-ideal behavior, the result of applying the criteria proposed here is an improvement. The application of these three criteria rejects over 65% of all inaccurate results and has the highest concentration of accurate results when compared with the other criteria sets tested. Other selection criteria can be subsequently used to improve on this result. While modern studies should always include experimental checks to identify MD behavior, this new criterion will provide a useful tool for future studies and, importantly, a method to assess the reliability of previously published data. Copyright © 2011 by the American Geophysical Union.
Tan M.,CAS Institute of Geology and Geophysics
Climate Dynamics | Year: 2014
Inter-annual variation in the ratio of 18O to 16O of precipitation (δ18Op) in the monsoon regions of China (MRC, area approximately east of 100°E) has not yet been fully analyzed. Based on an analysis of the relationships between the time series of amount-weighted mean annual δ18O in precipitation (δ18Ow) and meteorological variables such as temperature, precipitation as well as atmospheric/oceanic circulation indices, it is recognized that the El Niño-Southern Oscillation (ENSO) cycle appears to be the dominant control on the inter-annual variation in δ18Op in the MRC. Further analysis shows that the trade wind plays a role in governing δ18Ow through affecting the intensity of the different summer monsoon circulations which are closely linked to the weakening (weaker than normal) and strengthening (stronger than normal) of the trade wind and gives the δ18Ow different values at or over inter-annual timescales. The southwest monsoon (SWM) drives long-distance transport of water vapor from Indian Ocean to the MRC, and along this pathway increasing rainout leads to more negative δ18Ow via Rayleigh distillation processes. In contrast, the southeast monsoon (SEM), which is consistent with the changes in the strength of the West Pacific subtropical high, drives short-distance water vapor transport from the West Pacific Ocean to the MRC and leads to less negative δ18Ow. Therefore, the δ18Ow value directly reflects the differences in influence between the SWM, which is strong when the SE trade wind is strong, and the SEM, which is strong when the SE trade wind is weak. In addition, the South China Sea Monsoon also transports local water vapor as well as plays a role in achieving the synchronization between the δ18Ow and ENSO. The author thus terms the δ18Op rhythm in the MRC the "circulation effect". In turn, the δ18Op variation in the MRC has the potential to provide information on atmospheric circulation and the signal of δ18Op recorded in natural archives can then be used to deduce a long-term behavior of the tropical climate system. © 2013 Springer-Verlag Berlin Heidelberg.