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Abstract This study provides a global assessment of the abundance of the major oxides in the deep continental crust. The combination of geochemistry and seismology better constrains the composition of the middle and lower continental crust better than either discipline can achieve alone. The inaccessible nature of the deep crust (typically >15 km) forces reliance on analog samples and modeling results to interpret its bulk composition, evolution, and physical properties. A common practice relates major oxide compositions of small‐ to medium‐scale samples (e.g., medium to high metamorphic grade terrains and xenoliths) to large scale measurements of seismic velocities (Vp, Vs, Vp/Vs) to determine the composition of the deep crust. We provide a framework for building crustal models with multidisciplinary constraints on composition. We present a global deep crustal model that documents compositional changes with depth and accounts for uncertainties in Moho depth, temperature, and physical and chemical properties. Our 3D compositional model of the deep crust uses the USGS Global Seismic Structure Catalog (Mooney, 2015) and a compilation of geochemical analyses on amphibolite and granulite facies lithologies (Sammon & McDonough, 2021,https://doi.org/10.1029/2021JB022791). We find a SiO2gradient from 61.2 ± 7.3 to 53.3 ± 4.8 wt.% from the middle to the base of the crust, with the equivalent lithological gradient ranging from quartz monzonite to gabbronorite. In addition, we calculate trace element abundances as a function of depth from their correlations with major oxides. From here, other lithospheric properties, such as Moho heat flux ( mW/m2), are derived.
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Inferring Crustal Viscosity from Seismic Wavespeeds: Applications to the Rheologic Structure of the Himalayas
We constrain the viscosity of the lower crust through a joint inversion of seismic P-wave (Vp) and S-wave (Vs) velocities. Previous research has demonstrated robust relationships between seismic velocity and crustal composition, as well as between the composition and viscosity of the lower crust. Here we extend these analyses, showing seismic velocity can be used as a robust indicator of crustal viscosity. First, we use the Gibbs free energy minimization routine Perple_X to calculate equilibrium mineral assemblages for a global compilation of crustal rocks at various pressures and temperatures. Second, we use a rheological mixing model that combines single-phase flow laws for major crust-forming minerals to calculate bulk viscosity from the predicted mineral assemblages incorporating the effects of strain rate, temperature, pressure, and water activity. We apply our method to regional seismic and heat flow data across East Asia in order to separate the relative variations in mid-crustal viscosity associated with composition and temperature. In some regions, temperature variations are the dominant influence on viscosity; e.g., we predict a 3 order of magnitude increase in viscosity between the low heat flow Sichuan Basin and higher heat flow surrounding regions. These viscosity variations are consistent with those previously inferred to produce the different topographic gradients in these areas [1]. However in constant heat flow regions, compositional variations exert the primary influence on viscosity; e.g., the North China Craton and the Yangtze Craton are predicted to have compositionally-controlled viscosities ranging from 1022–1023 Pa×s. Finally, the regional Vp/Vs ratios in the Tibetan Plateau cannot be explained by thermal and/or compositional variations alone, possibly indicating the presence of melt, which would lead to additional viscosity reductions. [1] Clark & Royden, Geology, 2000.
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- PAR ID:
- 10058030
- Date Published:
- Journal Name:
- Transactions - American Geophysical Union
- ISSN:
- 0002-8606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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