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

固体地球的表层被称为岩石圈，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℃），在应力作用下以脆性和塑性两种形式变形。岩石圈条件下（即温度低于1200℃）的地幔流变学定量描述了岩石圈在应力作用下的变形，对理解长期构造变形、造山、火山作用、岩石圈减薄和板块构造具有重要意义。大多数地幔岩石圈流变学研究采用实验室方法，在实验室环境下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观测到的与板块内部和俯冲带的海山和海洋岛屿载荷相关的地表和莫霍变形以及重力异常，来限制岩石圈流变。海山和海洋岛屿是火山喷发在短时间（~ 100万年或更短）内形成于海洋岩石圈上的。它们引起了海洋岩石圈的显著变形，这种变形可以在沉积层变形和重力异常中观察到。因此，海底山和海洋岛屿（如夏威夷）对岩石圈的负荷可以看作是研究岩石圈对应力或负荷反应的天然实验室。本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（即活化能）变化？3)从模拟现场观测推断的地幔流变学与最近实验室研究推断的地幔流变学一致吗？4)俯冲带海山的断裂、弯曲、水化等俯冲过程对流变学的影响是什么？建议的两年计划的具体任务包括：1)建立二维和三维有限元粘弹性加载模型，包括粘弹性、非线性蠕变和摩擦（即Byerlee定律）流变学；2)利用最近汇编的约80个海山和海洋岛屿的弹性板块厚度、板块年龄、载荷年龄和尺寸来约束岩石圈条件下的地幔流变学，包括活化能；3)利用地震成像的变形地壳数据、载荷和岩石圈年龄。选取板块内海洋岛屿（夏威夷、留尼汪岛、加那利岛、佛得角和马克萨斯岛）和俯冲带海山（Kermadec海沟附近的路易斯维尔海山和日本海沟的第一岛海山）的重力数据，在三维加载模型中进一步约束不同构造环境下的流变学。该项目由地球物理和海洋地质与地球物理项目支持。

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.