Department of Geology and Geophysics, Center for Tectonophysics, Texas A&M University, College Station, TX 77843.
Department of Geosciences, University of Akron, Akron, OH 44325.
Proc Natl Acad Sci U S A. 2023 May 23;120(21):e2219076120. doi: 10.1073/pnas.2219076120. Epub 2023 May 15.
A model for intermediate-depth earthquakes of subduction zones is evaluated based on shear localization, shear heating, and runaway creep within thin carbonate layers in an altered downgoing oceanic plate and the overlying mantle wedge. Thermal shear instabilities in carbonate lenses add to potential mechanisms for intermediate-depth seismicity, which are based on serpentine dehydration and embrittlement of altered slabs or viscous shear instabilities in narrow fine-grained olivine shear zones. Peridotites in subducting plates and the overlying mantle wedge may be altered by reactions with CO-bearing fluids sourced from seawater or the deep mantle, to form carbonate minerals, in addition to hydrous silicates. Effective viscosities of magnesian carbonates are higher than those for antigorite serpentine and they are markedly lower than those for HO-saturated olivine. However, magnesian carbonates may extend to greater mantle depths than hydrous silicates at temperatures and pressures of subduction zones. Strain rates within altered downgoing mantle peridotites may be localized within carbonated layers following slab dehydration. A simple model of shear heating and temperature-sensitive creep of carbonate horizons, based on experimentally determined creep laws, predicts conditions of stable and unstable shear with strain rates up to 10/s, comparable to seismic velocities of frictional fault surfaces. Applied to intermediate-depth earthquakes of the Tonga subduction zone and the double Wadati-Benioff zone of NE Japan, this mechanism provides an alternative to the generation of earthquakes by dehydration embrittlement, beyond the stability of antigorite serpentine in subduction zones.
基于剪切局部化、剪切加热和变质俯冲大洋板块和上覆地幔楔内薄碳酸盐层中的失控蠕动，评估俯冲带中深地震的模型。碳酸盐透镜体中的热剪切不稳定性增加了中深地震活动的潜在机制，这些机制基于蛇纹石脱水和变质板块的脆化或狭窄细粒橄榄石剪切带中的粘性剪切不稳定性。俯冲板块和上覆地幔楔中的橄榄岩可能与源自海水或深部地幔的含 CO 流体发生反应，形成碳酸盐矿物，以及含水硅酸盐。镁质碳酸盐的有效粘度高于蛇纹石中的纤蛇纹石，明显低于 HO 饱和橄榄石。然而，在俯冲带的温度和压力下，镁质碳酸盐可能比含水硅酸盐延伸到更大的地幔深度。变质俯冲地幔橄榄岩中的应变率可能在板块脱水后集中在碳酸盐层内。基于实验确定的蠕变规律的碳酸盐地层剪切加热和温度敏感蠕变的简单模型预测了具有高达 10/s 的应变速率的稳定和不稳定剪切的条件，与摩擦断层表面的地震速度相当。将该机制应用于汤加俯冲带和日本东北部双 Wadati-Benioff 带的中深地震，为地震的产生提供了一种替代脱水脆化的机制，超出了俯冲带中蛇纹石的稳定性。
