Collaborative Research: Is the Isabella anomaly a fossil slab or the foundered lithospheric root of the Sierra Nevada batholith?

一项对岩石圈结构的详细地震调查将检验关于伊莎贝拉高地震速度异常的构造起源的两种假设。伊莎贝拉高地震速度异常位于加州南部大峡谷的上地幔。基于现有的地震成像，这两种假设都是可行的，这些地震成像使用的数据来自相隔约70公里的台站，但它们对伴随俯冲终止和大陆弧岩石圈演化的过程有着截然不同的含义。一种假说将伊莎贝拉异常归因于内华达山脉南部岩基下沉的基性根。另一种解释将伊莎贝拉异常归因于一块化石板块，该板块是蒙特雷微板块的延续，因为它与太平洋板块机械耦合，因此在大峡谷下方连贯地平移。重要的是，后一种假设将化石板块置于圣安德烈亚斯断层的沿走向范围之下，该断层主要由地震蠕变变形，并承载深部地壳构造震颤，这可能是由板块的流体引起的。被动震源成像采用密集宽带阵列，站距约7公里，从海岸延伸到内华达山脉山麓，将通过岩石圈界面的详细映射和确定它们是否连续穿过具有300公里累积右侧位移的板块边界断层，有力地验证这两个假设。散射波偏移和层析成像方法将协同使用，以约束密集阵列下的岩石圈结构。地震研究将促进对俯冲终止的结构遗产和力学的理解，内华达山脉弧岩石圈的俯冲后演化，以及现今圣安德烈亚斯断层爬行部分的基底边界条件。科学进步的明显机会不仅在于对基本构造过程的潜在影响，而且在于存在两个定义明确的岩石圈尺度结构假设，这些假设可以用现代被动震源成像方法进行有力的测试。该项目的其他影响还包括通过支持一名初级PI、一名研究生和一名本科生研究员，帮助新墨西哥大学（UNM）建立一个新的地球物理小组。扩大新墨西哥州的地球物理学研究和教学？美国最大的机构在吸引代表性不足的少数民族获得地球科学机会方面具有突出的潜力。加州理工学院将支持一名研究生和一名本科生研究员。该项目将在新墨西哥大学和加州理工学院之间开展新的合作。

A detailed seismic investigation of lithospheric structure will test two hypotheses for the tectonic origin of the Isabella high seismic velocity anomaly in the upper mantle of California's southern Great Valley. Both hypotheses are viable based on existing seismic imaging that uses data from stations spaced about 70 km apart, but they have dramatically different implications for the processes that accompany subduction termination and the evolution of continental arc lithosphere. One hypothesis attributes the Isabella Anomaly to the sinking mafic root of the southern Sierra Nevada batholith. The other attributes the Isabella Anomaly to a fossil slab that is a continuation of the Monterey microplate coherently translating beneath the Great Valley because it is mechanically coupled to the Pacific plate. Importantly, the latter hypothesis places the fossil slab beneath the along-strike extent of the section of the San Andreas fault that dominantly deforms by aseismic creep and hosts deep crustal tectonic tremor, which might be caused by fluids from the slab. Passive source seismic imaging using a dense broadband array with ~7 km station spacing extending from the coast to the Sierra Nevada foothills will robustly test the two hypotheses with detailed mapping of lithospheric interfaces and identification of whether or not they are continuous across a plate bounding fault with 300 km of cumulative right lateral displacement. Scattered wave migration and tomographic imaging methods will be used in concert to constrain lithospheric structure beneath the dense array. The seismic study will advance understanding of the structural legacy and mechanics of subduction termination, post-subduction evolution of the Sierra Nevada arc lithosphere, and present day basal boundary conditions on the creeping section of the San Andreas fault. A clear opportunity for scientific advance is identified not only by the potential implications for fundamental tectonic processes, but also by the existence of two well-defined hypotheses for lithospheric-scale structure that can be robustly tested with modern passive source seismic imaging methods. Additional impacts of the project include aiding in the development of a new geophysics group at the University of New Mexico (UNM) by supporting a beginning-career PI, a graduate student, and an undergraduate researcher. Expansion of geophysics research and teaching at the state of New Mexico?s largest institution has outstanding potential to attract under-represented minorities to opportunities in the geosciences. A graduate student and undergraduate researcher will be supported at Caltech. The project will develop a new collaboration between UNM and Caltech.