Collaborative Research: Microstructural and Modeling Constraints on Strain Localization, LPO Development and Rheology of the Upper Mantle

利用露头尺度关系对地幔剪切带的微观结构和建模进行了综合研究，测试和探讨了(A)实验流动规律的外推、晶格优选取向的发展和粒度演化；(b)预测应变局部化和粘性剪切热不稳定性的模型。研究的重点是约瑟芬橄榄岩（Klamath Mountains, Oregon）中暴露良好的剪切带，在那里剪切带边界可以很容易地识别，并且可以通过测量已有的富辉石带的挠度来量化有限应变。现场和实验室研究评估了晶格优先取向发展的模型，并校准了剪切感、粘性流动轨迹和橄榄岩微观结构中保存的有限应变的指标。数值模拟将利用粘弹性流变学再现约瑟芬橄榄岩剪切带周围的应变分布。橄榄石流动规律和晶粒尺寸演变的参数化作为应力、应变、应变速率和晶粒生长的函数将被纳入。受剪切区观测约束的流变特性与包含相同流变特性的数值模型的结果进行了比较。近似再现基本野外观测的正演模型将用于：(a)研究导致应变局部化的过程，并以此类推，研究构造板块边界；(b)评估粘性剪切热不稳定性导致俯冲带中深度地震的假设，以及其他可能发生在浅层地幔的地震，如沿海洋断裂带的假设。橄榄岩及其组成矿物的室内变形实验结果被广泛应用于上地幔地球动力学模型。然而，实验室研究使用的样品与上地幔尺寸相比非常小，并且在高于上地幔预期的应变速率下进行。这项研究弥补了实验室研究和地幔尺度过程之间在规模和时间上的差距，这对理解上地幔流变学至关重要。这不仅进一步限制了地球动力学建模，而且还将提高对控制中深度地震、震后变形、克拉通根保存和板块边界演化过程的理解。

A combined microstructural and modeling investigation of mantle shear zones using outcrop scale relationships, tests and explores (a) extrapolation of experimental flow laws, development of lattice preferred orientation, and grain size evolution; and (b) models that predict strain localization and viscous shear heating instabilities. Research focuses on well-exposed shear zones in the Josephine peridotite (Klamath Mountains, Oregon) where shear zone boundaries can be easily identified and finite strain can be quantified by measuring the deflection of pre-existing pyroxene-rich bands. Field and laboratory studies assess models for lattice preferred orientation development and calibrate indicators of shear sense, viscous flow trajectory, and finite strain preserved in peridotite microstructures. Numerical modeling will reproduce strain distribution around shear zones in the Josephine peridotite, using viscoelastic rheology. Olivine flow laws and parameterizations of grain size evolution as a function of stress, strain, strain rate, and grain growth will be incorporated. Rheological properties constrained by observation of the shear zones are compared to the results of the numerical models that incorporate the same rheology. Forward models that approximately reproduce basic field observations will be used to: (a) investigate processes responsible for strain localization and, by analogy, tectonic plate boundaries; and (b) evaluate the hypothesis that viscous shear heating instabilities cause intermediate depth earthquakes in subduction zones, and perhaps other earthquakes in the shallow mantle, such as along oceanic fracture zones.Results from laboratory deformation experiments of peridotite and its constituent minerals are widely used in geodynamical models of the upper mantle. Laboratory studies, however, use samples that are very small in comparison to upper mantle dimensions and are conducted at strain rates much higher than expected in the upper mantle. This study bridges the gap in size and time between laboratory studies and mantle-scale processes, which is essential for understanding upper mantle rheology. This not only further constrains geodynamical modeling, but will also improve understanding of processes controlling intermediate depth earthquakes, post-seismic deformation, preservation of cratonic roots, and the evolution of plate boundaries.