Constraining Mantle Rheology at Lithospheric Conditions by Modeling Seamount Induced Deformation and Gravity Anomalies

通过模拟海山引起的变形和重力异常来约束岩石圈条件下的地幔流变

• 批准号: 1114168
• 负责人: Shijie Zhong
• 金额: $ 19.66万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-10-01 至 2015-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1114168&HistoricalAwards=false
• 关键词: Constraining; Mantle; Rheology; Lithospheric; Conditions

The surface layer of the solid Earth is called lithosphere that is on average about 100 km thick and includes the crust and mantle components. The lithosphere is relatively cold (i.e., temperature less than ~1200 C) and deforms in both brittle and plastic fashions under stresses. Mantle rheology at lithospheric conditions (i.e., temperature less than 1200 C) describes quantitatively the lithospheric deformation under stress, and is important for understanding long-term tectonic deformation, mountain building, volcanism, lithospheric thinning, and plate tectonics. Most studies on mantle lithospheric rheology employ a laboratory approach and investigate the response of rock samples or specimen to applied stress in laboratory settings. Although laboratory studies have provided many important insights on lithospheric rheology, it remains challenging to directly apply them in studies of lithospheric deformation processes. This project seeks to constrain lithospheric rheology by directly modeling observations of surface and Moho deflections and gravity anomalies associated with seamount and oceanic island loads at plate interiors and in subduction zones. Seamounts and oceanic islands are formed on oceanic lithosphere due to volcanic eruptions on a short time scale (~ 1 million years or less). They cause significant deformation on oceanic lithosphere that can be observed in deformation of sedimentary layers and gravity anomalies. The loading of seamount and oceanic islands (e.g., Hawaii) on lithosphere can therefore be viewed as a natural laboratory to study the lithospheric response to stress or loading. This project seeks to address the following questions: 1) What is the lithospheric strength (stress) and rheology at plate interiors and in subduction zones? 2) How does lithospheric viscosity vary with temperature (i.e., activation energy)? 3) Is the mantle rheology inferred from modeling field-based observations consistent with that inferred from recent laboratory studies? 4) What are the rheological effects of subduction processes such as faulting, bending and hydration, as seen by subduction zone seamounts? The specific tasks for the proposed two-year project include: 1) to formulate 2- and 3-D finite element viscoelastic loading models with realistic rheologies including viscoelastic, nonlinear creep, and frictional (i.e., Byerlee's law) rheology, 2) to use recent compilations of elastic plate thickness, plate age, load age and size for ~80 seamounts and oceanic islands to constrain mantle rheology at lithospheric conditions including the activation energy, 3) to use seismically imaged deflected crust data, load and lithospheric ages, gravity data for selected intraplate oceanic islands (Hawaii, La Reunion, Canary, Cape Verde, and Marquesas) and subduction zone seamounts (the Louisville Ridge seamount near the Kermadec trench and the Daiichi-Kashima seamount in the Japan Trench) in 3-D loading models to further constrain the rheology in different tectonic settings.This project is supported by the Geophysics and Marine Geology & Geophysics Programs.

固体地球的表层称为岩石圈，平均厚度约为100公里，包括地壳和地幔成分。岩石圈相对较冷（即，温度低于~1200 ℃），并在应力下以脆性和塑性方式变形。岩石圈条件下的地幔流变学（即，温度低于1200 ℃）定量描述了岩石圈在应力作用下的变形，对于理解长期构造变形、造山作用、火山作用、岩石圈减薄和板块构造具有重要意义。地幔岩石圈流变学的研究大多采用实验室方法，并在实验室环境中研究岩石样品或标本对外加应力的响应。尽管实验室研究为岩石圈流变学提供了许多重要的见解，但将其直接应用于岩石圈变形过程的研究仍然具有挑战性。该项目旨在通过直接模拟与板块内部和俯冲带海山和海洋岛屿负荷相关的地表和莫霍面偏转和重力异常观测来约束岩石圈流变学。海山和洋岛是在短时间尺度（约100万年或更短）的火山喷发作用下在大洋岩石圈上形成的。它们对海洋岩石圈造成显著的变形，这种变形可以在沉积层变形和重力异常中观察到。海山和大洋岛屿的负荷（例如，夏威夷）对岩石圈的作用可以看作是研究岩石圈对应力或载荷响应的天然实验室。该项目旨在解决以下问题：1）板块内部和俯冲带的岩石圈强度（应力）和流变性是什么？2)岩石圈粘度如何随温度变化（即，活化能）？3)从模拟现场观测中推断的地幔流变学与最近的实验室研究推断的一致吗？4)从俯冲带海山看，俯冲过程如断层、弯曲和水化作用的流变学效应是什么？ 拟议的两年项目的具体任务包括：1）制定二维和三维有限元粘弹性加载模型与现实的流变学，包括粘弹性，非线性蠕变和摩擦（即，Byerlee定律）流变学; 2）使用最近汇编的约80座海山和大洋岛屿的弹性板块厚度、板块年龄、载荷年龄和大小来约束岩石圈条件下的地幔流变学，包括活化能; 3）使用选定的板内大洋岛屿的地震成像偏转地壳数据、载荷和岩石圈年龄、重力数据（夏威夷、留尼汪岛、加那利群岛、佛得角、和马克萨斯）和俯冲带海山（克马德克海沟附近的路易斯维尔海岭和日本海沟的第一鹿岛海岭）三维加载模型，以进一步约束流变学在不同的构造环境&。