Crustal Strength Profiles Across the Brittle-Ductile Transition

脆性-延性转变过程中的地壳强度分布

• 批准号: 0809443
• 负责人: John Platt
• 金额: $ 27万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-15 至 2012-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809443&HistoricalAwards=false
• 关键词: Crustal; Strength; Profiles; Across; Brittle

At about 20 km depth crustal rocks show a transition from the brittle behavior characteristic of the upper crust to ductile behavior characteristic of most of the Earths interior. The stress required to deform the ductile rock immediately below the brittle-ductile transition (BDT) is likely to approximate the bulk strength of the brittle crust immediately above the BDT. Rocks that have deformed by ductile processes can preserve microscale features in their crystalline structure that record the stress level during deformation. If these rocks have subsequently been brought to the surface (exhumed), we can use microstructural measurements to determine the strength of the crust around the BDT. The goal of this work is to obtain better estimates of the strength of the upper crust. Our measurements will allow us to reconstruct strength profiles of the continental crust. These will provide reliable data for assessing the strength of the tectonic plates in areas of active deformation. In particular, our results will improve mechanical models of the way continental crust responds to plate motions, with implications for mountain-building processes, the formation of sedimentary basins that host economically valuable reserves of petroleum and mineral resources, and the generation of earthquakes. The upper 20-25 km of the continental crust is the coldest and strongest part of the tectonic plates, and because cold rock is brittle, it generates most of the earthquakes. The strength and mechanical properties of the upper crust are fundamental to understanding the way the plates respond to forces acting on their boundaries and to forces generated internally by gravity. The bulk strength of the crust is difficult to measure directly, however; estimates vary by about a factor of ten, because of uncertainties about the strength of faults, the effect of fluids within the crust, and the temperature gradient through the crust. In areas of continental rifting and extension, such as the Basin & Range Province of the western USA, rocks have been exhumed from the ductile middle crust, cooling through the BDT as they rise to the surface. During exhumation and cooling, deformation becomes increasingly localized into narrow shear zones. As a result, different parts of the rock body record stress levels from different depths. We plan to make measurements of microstructural features, mineral chemistry, and isotopic composition, to determine the stress-temperature-time history of rocks from areas affected by recent extensional tectonics in SE California and in southern Spain. Electron backscatter diffraction will be used to determine the grain size, grain misorientation, and crystallographic preferred orientation of dynamically recrystallized quartz-bearing rocks for stress measurements. Element exchange between different minerals will be used to calculate chemical equilibria that are functions of pressure and temperature. Several different radiogenic isotopic systems will be used to constrain the temperature during exhumation and cooling. Earthscope has provided generous support for the geochronology component of the work.

在大约20公里深处，地壳岩石表现出从上地壳的脆性行为特征到地球大部分内部的延展性行为特征的转变。脆性-韧性转变（BDT）以下的韧性岩石变形所需的应力可能近似于BDT以上的脆性地壳的总体强度。经过延性变形的岩石可以在其晶体结构中保留微尺度特征，记录变形过程中的应力水平。如果这些岩石随后被带到地表（挖出），我们可以用微观结构测量来确定BDT周围地壳的强度。这项工作的目的是更好地估计上地壳的强度。我们的测量将使我们能够重建大陆地壳的强度分布图。这将为评估活跃变形地区构造板块的强度提供可靠的数据。特别是，我们的结果将改进大陆地壳对板块运动的反应方式的力学模型，对造山过程、沉积盆地的形成以及地震的产生产生影响。沉积盆地拥有经济上有价值的石油和矿产资源储备。大陆地壳的上部20-25公里是构造板块中最冷和最强的部分，由于冷岩石易碎，它产生了大多数地震。上地壳的强度和力学特性是理解板块对作用在其边界上的力和内部重力产生的力的反应方式的基础。然而，地壳的总体强度很难直接测量；由于断层的强度、地壳内流体的影响以及地壳的温度梯度等不确定因素，估计结果的差异约为10倍。在大陆裂谷和伸展的地区，如美国西部的盆地和山脉省，岩石从韧性的中地壳中被挖掘出来，当它们上升到地表时，通过BDT冷却。在挖掘和冷却过程中，变形越来越局限于狭窄的剪切区。因此，岩体的不同部位记录的应力水平来自不同的深度。我们计划测量微观结构特征、矿物化学和同位素组成，以确定来自加利福尼亚东南部和西班牙南部受近期伸展构造影响地区的岩石的应力-温度-时间历史。电子背散射衍射将用于确定动态再结晶石英岩石的晶粒尺寸、晶粒错取向和晶体学优选取向，用于应力测量。不同矿物之间的元素交换将用于计算压力和温度的化学平衡。将使用几种不同的放射性成因同位素系统来限制挖掘和冷却过程中的温度。Earthscope为这项工作的地质年代学部分提供了慷慨的支持。