Lateral and radial viscosity structure beneath Fennoscandia inferred from seismic and magnetotelluric observations

IF 2.4 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Florence D.C. Ramirez , Kate Selway , Clinton P. Conrad , Valerie Maupin , Maxim Smirnov
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引用次数: 0

Abstract

Fennoscandia is continuously uplifting in response to past deglaciation, termed glacial isostatic adjustment or GIA, and its mantle viscosity is well constrained from ice sheet and sea level data. Here, we compare those GIA-constrained viscosities for the Fennoscandian upper mantle with geophysically-constrained viscosities. We construct the upper mantle viscosity structure of Fennoscandia by inferring temperature and water content from seismic and magnetotelluric (MT) data. Using a 1-D MT model for Fennoscandian cratons together with a global seismic model, we infer an upper mantle viscosity (below 250 km) of ∼1021±2 Pa·s, which encompasses the GIA-constrained viscosities of 1020 − 1021 Pa·s. The GIA viscosities are better matched if the Fennoscandian upper mantle is a wet harzburgite or a dry pyrolite, where pyrolite is ∼10 times more viscous than harzburgite. Using the average temperatures and water contents for harzburgitic upper mantle, the GIA viscosities require 1–4 mm grain sizes indicating a diffusion creep regime. In northwestern Fennoscandia, where a high-resolution 2-D resistivity model is available, greater inferred mantle water content implies viscosities that are 10–100 times lower than those for the Fennoscandian Craton. Our work suggests that the combination of seismic and MT observations can improve upper mantle viscosity estimates, especially for regions with laterally-varying viscosity structures or where GIA constraints are not available. Although our method represents an important step forward, viscosity uncertainty can be further reduced by incorporating additional constraints on rock composition, grain size and mantle stress, as well as more accurate geophysical data, into the viscosity calculation.

Abstract Image

从地震和磁强辐射观测推断芬诺斯坎迪亚地下的横向和径向粘度结构
芬诺斯坎迪亚因过去的冰川消融而不断隆起,这被称为冰川等静力调整(GIA),其地幔粘度可从冰盖和海平面数据中得到很好的约束。在这里,我们将冰川等静力调整约束下的芬诺斯坎迪亚上地幔粘度与地球物理约束下的粘度进行了比较。我们通过地震和磁法(MT)数据推断温度和含水量,构建了芬诺斯坎迪亚的上地幔粘度结构。利用芬诺斯坎迪亚陨石坑的一维MT模型和全球地震模型,我们推断上地幔(250公里以下)的粘度为Pa-s,包含了GIA约束的Pa-s粘度。如果芬诺斯坎迪亚上地幔是湿润的哈兹堡垒岩或干燥的辉绿岩(辉绿岩的粘度是哈兹堡垒岩的约 10 倍),则 GIA 粘度更匹配。利用哈兹堡岩上地幔的平均温度和含水量,GIA 粘度要求 1-4 毫米的晶粒大小,这表明了一种扩散蠕变机制。在有高分辨率二维电阻率模型的芬诺斯坎迪亚西北部,推断的地幔含水量更大,这意味着粘度比芬诺斯坎迪亚克拉通的粘度低 10-100 倍。我们的工作表明,地震观测和地幔测量相结合可以改进上地幔粘度的估算,特别是对于粘度结构横向变化的地区或没有全球地幔影响约束的地区。尽管我们的方法是向前迈出的重要一步,但通过在粘度计算中加入更多有关岩石成分、粒度和地幔应力的约束条件,以及更精确的地球物理数据,可以进一步降低粘度的不确定性。
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来源期刊
Physics of the Earth and Planetary Interiors
Physics of the Earth and Planetary Interiors 地学天文-地球化学与地球物理
CiteScore
5.00
自引率
4.30%
发文量
78
审稿时长
18.5 weeks
期刊介绍: Launched in 1968 to fill the need for an international journal in the field of planetary physics, geodesy and geophysics, Physics of the Earth and Planetary Interiors has now grown to become important reading matter for all geophysicists. It is the only journal to be entirely devoted to the physical and chemical processes of planetary interiors. Original research papers, review articles, short communications and book reviews are all published on a regular basis; and from time to time special issues of the journal are devoted to the publication of the proceedings of symposia and congresses which the editors feel will be of particular interest to the reader.
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