Microstructure in Marble: Comparison of Dislocation and Grain Structure Produced in Natural and Laboratory Deformation

在这个项目中，研究人员将对自然样品(从瑞士阿尔卑斯山的逆冲断层Morcle Nappe采集)与在麻省理工学院实验室和德国波茨坦Helmholtz-Zentum德国地球科学研究中心GFZ变形的样品的位错和颗粒微观结构进行详细的比较。第一套卡拉拉大理石样品将在传统的三轴压缩下变形，应变速率为每秒0.001到0.000001，围压在100兆到300兆帕斯卡之间，温度在300K到700K之间，这些条件跨越了从碎裂破坏到位错蠕变的转变。第二组实验包括通过位错流动变形的人造和天然大理石，但扩展到也发生扩散蠕变的条件(温度在700到1000开尔文之间，颗粒尺寸在5到50微米之间，应变率和压力如上所述)。实验包括简单剪切、扭转和常规三轴加载。对于实验室应变速率、围压小于300兆帕斯卡、温度小于700开尔文的变形，孪晶界上的位错相互作用可能导致硬化，最终导致脆性破坏。在700开尔文以上，位错晶胞大小可能是本构关系中的一个重要状态变量。我们将使用光学显微镜、扫描和透射电子显微镜以及电子背散射衍射仪对这种组织及其演化进行详细的观察，以将结构与晶格择优取向的产生、动态再结晶和强度的演化联系起来。在沿断层倾角的几个不同位置，沿着垂直于莫尔克勒斯逆冲(瑞士海尔维推覆)的导线收集的天然钙质糜棱岩套件也将进行类似的显微结构观察。以前的野外研究提供了详细的微结构、地质测温和地球化学信息，可以用来限制这种大规模的机械史。这一背景提供了两个不同的机会：1)研究温度对莫尔克勒斯推覆体主推覆面应变局部化的影响，2)研究不同石英和白云石含量对碳酸盐中应变局部化的影响。天然岩石变形的局部化是普遍存在的，在所有观测尺度上，在大的自然温度和压力范围内。在碰撞山带中，碳酸盐岩中经常发现剪切带和断层，这种极端的局部化意味着这些岩石的力学性质对决定大范围岩体的强度至关重要。了解在自然条件下变形的岩石的力学性质是一个困难的问题，因为自然过程发生在极端压力和温度下，并且在非常慢的应变率下发生。要反算岩石在无法直接实验的条件下的力学行为，需要进行仔细的实验，彻底了解所涉及的过程的动力学，观察自然变形的岩石的微观结构，并比较自然和实验的微观结构。最终目标是了解自然构造过程，包括造山、断裂、地震力学和地壳变形周期的总体力学预算。

In this project, researchers will make a detailed comparison of the dislocation and grain microstructure of natural samples (collected from the Morcles Nappe, a thrust fault in the Swiss Alps) with those in samples deformed in laboratories at MIT and at the Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ in Potsdam, Germany. The first suite of Carrara marble samples will be deformed in conventional triaxial compression at strain rates of 0.001 per second to 0.000001 per second, confining pressures between 100 and 300 Megapascals, and temperatures between 300 to 700 Kelvin, conditions that span the transition from cataclastic failure to dislocation creep. The second set of experiments includes synthetic and natural marbles deformed by dislocation flow, but extends to conditions at which diffusion creep also occurs (temperatures between 700 and 1000 Kelvin, grain sizes between 5 and 50 micrometers, strain rates and pressures as above). Experiments include simple shear, torsion, and conventional triaxial loading. For deformation at laboratory strain rates, confining pressures less than 300 Megapascals, and temperature less than 700 Kelvin, dislocation interactions at twin boundaries may cause hardening that eventually lead to brittle failure. Above 700 Kelvin, dislocation cell size may be an important state variable in the constitutive law. Detailed observations of this microstructure and its evolution will be made using optical microscopes, scanning and transmission electron microscopes, and Electron Backscatter Diffraction to correlate structure with the production of lattice preferred orientation, dynamic recrystallization, and the evolution of strength. Similar microstructure observations will be made for suites of natural calc-mylonites collected along traverses perpendicular to the Morcles thrust (Swiss Helvetic Nappes) at several different locations along the dip of the fault. Previous field studies provide detailed information of microstructure, geothermometry, and geochemistry that can be used to constrain the large-scale mechanical history of this. This setting provides two distinct opportunities: 1) to investigate the influence of temperature on the localization of strain into the main thrust plane of the Morcles Nappe, and 2) to investigate the influence of varying quartz and dolomite content on strain localization within the carbonates.Localization of natural rock deformation is ubiquitous, at all scales of observation, under a broad range of natural temperatures and pressures. In collisional mountain belts, shear zones and faults are often found within carbonates, and the extreme localization implies that the mechanical properties of those rocks are critical in determining the strength of the large scale rock mass. Understanding the mechanical properties of rocks deforming under natural conditions is a difficult problem because natural processes occur at extreme pressure and temperatures and at very slow strain rates. To back-calculate the mechanical behavior of rocks under conditions that are not accessible to direct experimentation requires careful experimentation, a thorough knowledge of the kinetics of the processes involved, observations of the microstructure of naturally deformed rocks, and comparison of the natural and experimental microstructures. The ultimate goal is to understand natural tectonic processes including mountain building, faulting, earthquake mechanics, and the overall mechanical budget of the crustal deformation cycle.