Deciphering the Origin of Intra-Plate Volcanism in the Pacific Northwest Using Geodynamic Models with Data Assimilation

我们建议研究太平洋西北部板块内火山省的起源，包括中新世中期Steens-Columbia河洪水玄武岩和随后的Yellowstone和Newberry热点轨迹。我们将测试的可能机制包括源自核心-地幔边界的地幔柱，以及来自浅地幔过程（如板块驱动的上升流）的火山作用。除了地质观测外，我们还强调更好地约束深部地幔动力学，包括地幔流动模式和太平洋西北地区缓慢地震异常的性质。我们的方法是采用数据同化的四维地球动力学建模，包括高分辨率正向俯冲模拟和伴随对流计算，重建过去的地幔状态。重要的模式约束来自于USArray揭示的改进的地幔结构和对北美西部新生代俯冲史的充分了解。这一提议试图解决科学界长期争论的问题：远离活动板块边界的火山活动的形成。太平洋西北地区是一个理想的研究区域，因为那里存在火山活动，如1600万年前哥伦比亚河洪水玄武岩的大量喷发，以及正在进行的黄石-纽伯里热点火山。这些火山事件的起源仍然难以捉摸，因为我们对这些火山活动起源的深部地幔的构造知之甚少。现在，随着USArray地震实验最终描绘出次大陆地幔的三维图像，更好地了解太平洋西北火山作用的形成成为可能。为了解决这个看似复杂的问题，我们将依靠基于强大超级计算机的复杂数值模型。这些模型不仅要考虑地表地质记录，还要考虑地下的地幔结构，最重要的是考虑该系统自远古以来的时间演化。从科学上讲，这项工作几乎影响到固体地球研究的每一个领域。这项研究的另一个重要意义是提高对自然灾害形成及其对气候和环境影响的认识。

We propose to investigate the origin of the intra-plate volcanic province in Pacific Northwest, with features including the mid-Miocene Steens-Columbia River flood basalt and subsequent Yellowstone and Newberry hotspot tracks. Possible mechanisms we will test include a mantle plume originating from the core-mantle boundary, and volcanism from shallow mantle processes such as slab-driven upwelling flow. Besides geological observations, we emphasize on better constraining deep mantle dynamics including the pattern of mantle flow and the nature of slow seismic anomalies beneath the Pacific Northwest. Our approach is 4-D geodynamic modeling with data assimilation, which involves both high-resolution forward subduction simulations and adjoint convection calculations reconstructing past mantle states. Important model constraints come from the much improved mantle structure revealed by USArray and the well understood Cenozoic subduction history beneath western North America. This proposal attempts to solve for a long-debated issue in the scientific community: formation of volcanisms far away from active tectonic plate boundaries. The Pacific Northwest represents an ideal study region due to the existence of such volcanisms as the voluminous eruption of Columbia River flood basalt ~16 million years ago and the ongoing Yellowstone-Newberry hotspot volcanoes. Origin of these volcanic events has remained elusive because of our poor understanding on the configuration of deep mantle where these volcanisms originate. Now with the USArray seismic experiment finally delineating a 3-dimentional picture of the sub-continental mantle, a better understanding on the formation the Pacific Northwest volcanism becomes possible. To solve this apparently complex problem, we will rely on sophisticated numerical models based on powerful supercomputers. These models will take into account not only surface geologic records, but also the mantle structures underneath, and most importantly the time-evolution of the system since the remote past. Scientifically, the proposed work has impacts on almost every field of solid Earth research. Another important implication of this research is to improve knowledge on the formation of natural hazards and their effects on climate and environment.