Collaborative Research: Understanding lithospheric structure and deformation in Alaska via integration of seismic imaging and geodynamic modeling

在阿拉斯加州，地球表面相对于北美板块的其他部分正以每年10毫米的速度移动。这种运动表明地球的岩石圈（构成构造板块的地球更坚硬的外层）正在变形。在阿拉斯加南部，太平洋板块岩石圈俯冲到北美板块之下，北美板块被压缩，例如形成了阿拉斯加山脉。然而，驱动阿拉斯加内部地表运动和北美板块变形的力量却不太清楚。这个项目的目标是测量北美板块的结构，特别是它的厚度和内部强度，并模拟作用在板块边缘和底部的力是如何传递到表面的。北美板块和底层地幔的性质将通过从遥远的地震中发出的地表波和体波来测量，并由阿拉斯加的传感器记录下来，特别是由美国国家科学基金会地球观测可移动阵列的监测站记录下来。测量到的北美板块特性将被纳入数值模型，以解释在阿拉斯加地表观测到的运动，以及其他板块对北美板块施加的力和更深地幔的运动。这项工作将提高对驱动迪纳里断层和其他断层上的运动和地震的力量的理解，这些力量创造了阿拉斯加的山脉。这个项目将有助于布朗大学和普渡大学的研究生和本科生的教育。阿拉斯加的变形表现出巨大的变化，从阿拉斯加南部的汇聚和隆起与俯冲有关，到阿拉斯加内陆更神秘的变形，包括向南的地表速度。这种变形的驱动因素及其与地壳和地幔结构的关系一直存在争议，但尚未被理解。该项目将通过获得北美岩石圈和阿拉斯加下方软流层地震结构的新模型（使用来自NSF EarthScope可移动阵列和其他网络的数据）来解决这些问题。这些约束条件将被纳入解释观测到的地表变形（受GPS、地质和地震活动数据的约束）的上板块变形驱动力的三维地球动力学建模中。为此，项目组将：(1)对转换体波（Sp和Ps）和瑞利面波进行单独和联合分析和反演，获得地壳和地幔剪切速度的详细模型，对地幔不连续面进行稳健成像。衰减和方位各向异性也将确定；(2)利用剪切速度和衰减来确定温度、体积组成、晶粒尺寸、含水量和部分熔体的边界，并利用这些参数范围来确定粘度和密度的边界，并结合观测到的表面运动动态建模的粘度约束；(3)将黏度、密度模型与观测地表变形的三维地球动力学模拟相结合，将瑞利波相速度和SKS分裂的方位各向异性观测相结合，对地球动力学模型进行验证。这项工作将提高对北美岩石圈和下伏软流圈地震结构的认识；它对壳幔流变学和密度的意义；以及上板块和软流圈密度和流变对上板块变形速率和动力学的影响。一系列关键问题将被解决，包括：(1)是什么过程导致了在阿拉斯加观测到的高速率变形和隆起，以及上板块流变学的作用是什么？岩石圈和软流圈是如何耦合的？上板块底部的软流圈流的牵引力有什么影响？(2)俯冲作用如何改变上板块岩石圈？雅库特地体俯冲的影响是什么？雅库特地幔是否具有独特的热或成分特性，使其在粘度或浮力方面异常？雅库塔地体俯冲作用与德纳里火山裂谷或弗兰格尔火山场有何关系？(3)上板块构造与俯冲带北部构造的关系如何？上板块性质的偏移发生在地界和走滑断层的什么地方？该项目将有助于布朗大学和普渡大学研究生的教育和职业发展。至少有一名本科生将通过领导力联盟（Leadership Alliance）在布朗大学参与这个项目，该项目招募来自STEM领域代表性不足群体的学生；一到两名布朗大学的本科生也将参加。该项目还将通过研究小组会议和课程接触到布朗大学和普渡大学更广泛的学生和博士后群体，并将在普罗维登斯的小学推广。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In the state of Alaska, the Earth's surface is moving at rates of 10's of millimeters per year with respect to the rest of the North American plate. This motion indicates that the Earth's lithosphere (the more rigid outer layer of the Earth that makes up the tectonic plates) is deforming. In southern Alaska, where the Pacific plate lithosphere is subducting beneath the North American plate, the North American plate is being compressed, for example creating the mountains of the Alaska Range. However, the forces that drive surface motion and North American plate deformation in the interior of Alaska are much less clear. The goal of this project is to measure the structure of the North American plate, in particular its thickness and its internal strength, and to model how forces acting on the edges and base of the plate are transmitted to the surface. The properties of the North American plate and the underlying mantle will be measured using surface and body waves that emanate from distant earthquakes and are recorded at sensors in Alaska, in particular the stations of the NSF Earthscope Transportable Array. The measured North American plate properties will be incorporated in numerical models that will explain motions observed at the surface in Alaska with the forces exerted on the North American plate by other plates and the motion of the deeper mantle. This work will improve understanding of the forces that drive motion and earthquakes on the Denali fault and other faults and that have created the mountains in Alaska. This project will contribute to the education of graduate and undergraduate students at Brown University and Purdue University.Deformation in Alaska manifests dramatic variations, from convergence and uplift in southern Alaska associated with subduction to more enigmatic deformation in interior Alaska that includes southward surface velocities. The drivers of this deformation and their relationship to underlying crust and mantle structure are debated and not yet understood. This project will address these questions by obtaining new models of the seismic structure of North American lithosphere and underlying asthenosphere beneath Alaska (using data from the NSF EarthScope Transportable Array and other networks). These constraints will be incorporated into 3D geodynamic modeling of the driving forces of upper plate deformation that explain observed surface deformation (as constrained by GPS, geologic and seismicity data). To accomplish this the project team will: (1) Carry out individual and joint analyses and inversions of converted body waves (Sp and Ps) and Rayleigh surface waves to obtain detailed models of crust and mantle shear velocity that robustly image mantle discontinuities. Attenuation and azimuthal anisotropy will also be determined; (2) Use shear velocity and attenuation to place bounds on temperature, bulk composition, grain size, water content and partial melt, and use these parameter ranges to place bounds on viscosity and density, incorporating viscosity constraints from dynamic modeling of observed surface motion; (3) Integrate models of viscosity and density with 3D geodynamic modeling of observed surface deformation and test geodynamic models with observations of azimuthal anisotropy in Rayleigh wave phase velocities and SKS splitting. The proposed work will improve understanding of the seismic structure of North American lithosphere and underlying asthenosphere; its implications for crust and mantle rheology and density; and the impact of upper plate and asthenosphere density and rheology on the rates and dynamics of upper plate deformation. A range of key questions will be addressed, including: (1) What processes enable the high rates of deformation and uplift observed in Alaska, and what is the role of upper plate rheology? How coupled are the lithosphere and asthenosphere and what are the impacts of tractions from asthenospheric flow at the base of the upper plate? (2) How has subduction modified the upper plate lithosphere? What are the effects of Yakutat terrane subduction, and does Yakutat mantle have distinct thermal or compositional properties that make it anomalous in terms of viscosity or buoyancy? What is the relationship of Yakutat terrane subduction and the Denali volcanic gap or the Wrangell volcanic field? (3) How do upper plate structure and tectonics relate north of the subduction zone? Where do offsets in upper plate properties occur across terrane boundaries and strike-slip faults? The project will contribute to the education and career development of graduate students at Brown and Purdue. At least one undergraduate will work on this project at Brown through the Leadership Alliance, a program that recruits students from groups underrepresented in STEM fields; one to two Brown undergraduates will also participate. The project will also reach a broader group of students and postdocs at Brown and Purdue through research group meetings and courses, and will be featured in outreach with elementary schools in Providence, RI.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.