Integrating Geophysical and Geochemical Data to Understand the Hydration and Thermal State of the Colorado Plateau Lithosphere

科罗拉多高原位于犹他州、亚利桑那州、新墨西哥州和科罗拉多州，是美国最壮观的风景和地质暴露的所在地。然而，该地区在地质上是一个谜，因为它的高海拔，在它的边缘出现了现代火山活动，岩石没有经历通常与造山有关的褶皱和断裂。这也是不寻常的，因为造山和火山活动通常发生在构造板块的边缘，而科罗拉多高原远离北美板块的边缘。先前的研究表明，科罗拉多高原边缘下的地幔正在经历小规模的对流，这使得深处的热物质更接近地表。这种热的地幔物质然后加热下地壳，由于吸收了早期经过该地区的俯冲海洋板块释放的水，下地壳被认为含有不同寻常的富含水的矿物质。水降低了岩石的融化温度，加热可能导致该地区的隆起和融化。该项目将使用地球物理和地球化学数据以及热力学和温度模型来更好地了解科罗拉多高原边缘下的地下结构、水量和温度。这项工作的目的是提高我们对科罗拉多高原构造演化的认识，更好地了解大峡谷西部和旧金山火山带等地沿其边缘火山活动的原因。该项目将对学生进行现代地球化学和地球物理技术方面的培训，并将支持一个研讨会，将研究这些问题和类似问题的研究人员聚集在一起。最终，这项工作将提供对全球构造过程的洞察，这些构造过程建立和支持高海拔，并产生远离板块边缘的火山活动。北美西部的构造长期以来一直受到沿大陆西缘发生的板块边界作用的影响。这些过程具有复杂的历史，板块辐合方向的变化、板块倾角的变化以及从辐合边缘向转换边缘的转变，极大地改变了岩石圈和软流圈的状态，并导致了整个地区的多次变形。科罗拉多高原位于该板内变形带内，具有抗变形能力强、地表海拔高、无明显隆升驱动机制的特点。尽管科罗拉多高原长期稳定，如古生代平坦岩石的暴露所证明，但它最近被确定为一个受边缘小规模地幔对流影响的地区，有人认为高原正在屈服于火山活动和向其内部扩张。由于这些原因，科罗拉多高原提供了一个理想的地点来研究稳定的大陆岩石圈如何随着时间的推移而演变，以及“克拉通”岩石圈如何不稳定。为了更好地理解这些过程，我们将对科罗拉多高原上地幔进行一项综合研究，将汇编现有的地震和地球化学数据，分析一小组目标地球化学样本以解决知识空白，反演地震和地球化学数据以约束熔融条件，并利用热力学建模来确定稳定的矿物组合、熔体百分比、密度和潜在的水化状态。综合结果将用于验证以下假设：岩石圈水合作用是科罗拉多高原隆升的重要控制因素；小规模对流驱动融化和火山活动，并促进高原边缘的隆升和变形。我们的地球物理、地球化学和建模工作要解决的具体问题是：科罗拉多高原岩石圈的现代密度结构是什么？法拉龙平板俯冲作用是如何使上覆岩石圈水合物化的？大陆岩石圈密度是如何随时间演变的？科罗拉多高原下面是否发生了岩石圈下沉？研究结果将对该地区的构造演化和大陆岩石圈的密度演化具有全球性的意义，无论是对活动变形带还是稳定变形带。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。

The Colorado Plateau, located in the states of Utah, Arizona, New Mexico, and Colorado, is home to some of the most spectacular scenery and geologic exposures within the US. However, the region is enigmatic geologically for its high elevation, the occurrence of modern volcanism along its margins, and rocks that have not experienced the folding and faulting normally associated with mountain building. It is also unusual because mountain building and volcanism typically occur at the edges of tectonic plates, whereas the Colorado Plateau is located far from the margins of the North American plate. Previous work has suggested that the mantle beneath the margins of the Colorado Plateau is undergoing small-scale convection, which brings hot material at depth closer to the surface. This hot mantle material then heats the lower crust, which is believed to contain an unusual amount of water-rich minerals due to the absorption of water that was released from a subducting oceanic plate passing beneath the area earlier in its history. Water lowers the melting temperature of rocks, and heating may contribute to uplift and melting within the region. This project will use geophysical and geochemical data along with thermodynamic and temperature modeling to better understand the subsurface structure, the amount of water, and the temperatures beneath the margins of the Colorado Plateau. The goal of this work is to improve our understanding of the tectonic evolution of the Colorado Plateau and better understand the causes of volcanism along its margins in places such as the western Grand Canyon and the San Francisco Volcanic field. This project will train students in modern geochemical and geophysical techniques and will support a workshop to bring together researchers who address these and similar problems. Ultimately, this work will provide insight into the global tectonic processes that build and support high elevations and produce volcanic activity far from plate margins.The tectonics of western North America have long been influenced by plate boundary processes occurring along the continent's western margin. These processes have a complex history in which variations in plate convergence direction, slab dip, and the transition from a convergent to a transform margin have significantly altered the state of the lithosphere and asthenosphere and led to multiple episodes of deformation across the region. The Colorado Plateau is located within this intraplate deformation zone and is notable for its resistance to deformation and high surface elevations with no obvious driving mechanism for uplift. Despite its long-term stability, as evidenced by exposures of flat-lying Paleozoic rocks, the Colorado Plateau has recently been identified as a region influenced by small-scale mantle convection along its margins and it is argued that the plateau is succumbing to volcanism and extension encroaching into its interior. For these reasons, the Colorado Plateau provides an ideal location to study how stable continental lithosphere evolves with time and how "cratonic" lithosphere may destabilize. To develop a better understanding of these processes, we will conduct an integrative study focused on the Colorado Plateau's upper mantle that will compile existing seismic and geochemical data, analyze a small targeted set of geochemical samples to address knowledge gaps, inverse model seismic and geochemical data to constrain melt conditions, and utilize thermodynamic modeling to determine stable mineral assemblages, melt percentages, density, and potential hydration states. The synthesized results will be used to test the hypotheses that lithospheric hydration is an important control on the uplift of the Colorado Plateau and that small-scale convection is driving both melting and volcanism, and contributing to uplift and deformation on the edges of the plateau. The specific questions to be addressed with our geophysical, geochemical, and modelling work are: What is the modern density structure of the Colorado Plateau lithosphere, how did Farallon flat-slab subduction hydrate the overriding lithosphere, how does continental lithospheric density evolve through time, and have lithospheric downwellings occurred beneath the Colorado Plateau? The findings from this work will have global implications for the tectonic evolution of the region and for the density evolution of continental lithosphere in both active deformational belts and stable regions.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.