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、试验和探索(A)经验流动规律的外推、晶格择优取向的发展和颗粒尺寸演化；以及(B)预测应变局部化和粘性剪切加热不稳定性的模型，对地幔剪切带进行微结构和模拟研究。研究集中在约瑟芬橄榄岩(俄勒冈州克拉马斯山脉)中暴露良好的剪切带，那里的剪切带边界很容易识别，有限应变可以通过测量先前存在的富辉石带的偏转来量化。野外和实验室研究评估了晶格择优取向发展的模型，并校准了橄榄岩微结构中保存的剪切感、粘性流动轨迹和有限应变的指示器。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。将结合橄榄石流动定律和作为应力、应变、应变率和颗粒生长函数的颗粒尺寸演变的参数。将受剪切带观测约束的流变特性与包含相同流变学的数值模型的结果进行比较。近似再现基本野外观测的正演模型将用于：(A)研究应变局部化的过程，以此类推，研究构造板块边界；以及(B)评估粘性剪切加热不稳定性导致俯冲带中深部地震的假说，以及可能在地幔浅层地震，如沿大洋破裂带的其他地震。橄榄岩及其组成矿物的实验室变形实验结果被广泛用于上地幔的地球动力学模型。然而，实验室研究使用的样品与上地幔尺寸相比非常小，并且以比上地幔预期高得多的应变率进行。这项研究在实验室研究和地幔尺度过程之间的大小和时间上架起了桥梁，这对于理解上地幔流变学是必不可少的。这不仅进一步限制了地球动力学模拟，而且将提高对控制中深部地震、震后变形、克拉通根部的保存和板块边界演化的理解。

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.