Evolution of a Long-Lived, Supra-Subduction Accretionary Complex (Baker Terrane, NE Oregon): A Study of Structural Development and Crustal Growth During an Arc-Arc Collision

与俯冲有关的增生杂岩是造山带的基本构造块，被认为是较新的板块构造过程(显生界)的标志。活动的吸积楔可以反映跨越5000万年或更长时间的长寿历史，而古老的吸积杂岩甚至可以反映更长的历史，可能涉及碰撞变形事件以及剥离、逆冲和底侵。增生杂岩通常包括不可能在单一构造环境中形成的多种岩石类型的混合物。通过海洋地球物理调查研究了与活动俯冲有关的增生楔体的结构，产生了复杂的构造史，涉及大洋岩石圈的叠置作用、弧前伸展和(或)收缩、超俯冲冲断带和(或)宽阔的混杂岩带的圈定以及构造楔入。尽管这些海洋地球物理研究对于广泛了解各种构造环境中的增长楔体的发展具有重要意义，但对长期存在的增长复合体的变形历史的许多方面仍然知之甚少，需要直接研究暴露在陆地上的例子来探索这些过程。位于俄勒冈州东北部的复合贝克地体提供了一个天然的实验室，可以在一个暴露良好的古代增生杂岩中研究结构发展和地壳生长的本质。在这个项目中，研究人员将评估一个构造模型，在该模型中，贝克地体是一个复合的超俯冲增生杂岩，在北美边缘发展，是与蓝山省碰撞造山有关的岩石圈变形的主要焦点。拟议的研究对蓝山的地壳结构做出了具体的预测，将通过详细的地质和构造填图、放射虫生物地层学、断层相关岩石的微观结构研究、地球化学和Nd锶同位素研究、碎屑锆石研究和U-Pb放射性测年来进行测试。特别是，提出的模型是贝克地体的北缘是将远行的瓦洛瓦岛弧地体与贝克吸积杂岩地体分开的基本岩石圈边界。这项研究将涉及怀俄明大学研究生的培训。本科生还将被聘为地质野外助理。此外，本研究收集的样本和数据将在课堂上广泛使用。例如，样品将用于本科生的地球系统科学、岩石学和构造地质学课程，以及微观结构分析的研究生课程。最后，贝克地体是蓝山省的一个主要组成部分，彻底了解这些聚积岩(即与该区域广泛分布的年轻岩石有关的构造基底)对于通过蓝山省进行的拟议地球物理调查的成功至关重要(例如，地球范围项目内的GeoSwath举措)。

Subduction-related accretionary complexes are fundamental building blocks in orogenic belts and are considered to be a hallmark of more recent plate-tectonic processes (Phanerozoic). Active accretionary wedges can reflect a long-lived history that may span 50 or more millions of years, whereas ancient accretionary complexes can even reflect a longer history that may involve collisional deformation events as well as offscraping, underthrusting, and underplating. Accretionary complexes commonly include a mixture of rock types that could not have formed in a single tectonic setting. The architecture of active subduction-related accretionary wedges has been studied by marine geophysical surveys that have yielded complex structural histories involving imbrication of oceanic lithosphere, fore-arc extension and/or contraction, delineation of supra-subduction thrust belts and/or broad zones of mélange, and tectonic wedging. Despite the significance of these marine geophysical studies in developing a broad understanding of the development of accretionary wedges in a variety of tectonic settings, many aspects of the deformational history of long-lived accretionary complexes are still poorly understood and require the direct examination of exposed, on-land, examples to explore these processes. The composite Baker terrane of northeastern Oregon, provides a natural laboratory to study the nature of structural development and crustal growth in a well-exposed, ancient accretionary complex. In this project, the investigators will evaluate a tectonic model in which the Baker terrane is a composite supra-subduction accretionary complex that developed marginal to North America and was the principal focus of lithospheric deformation related to a collisional orogeny in the Blue Mountains province. The proposed study makes specific predictions about the crustal structure of the Blue Mountains that will be tested through detailed geologic and structural mapping, radiolarian biostratigraphy, microstructural studies of fault-related rocks, geochemical and Nd and Sr isotope studies, detrital zircon studies, and U-Pb radiometric dating. In particular, the model proposed is that the northern margin of the Baker terrane is a fundamental lithospheric boundary that separates the far-traveled Wallowa island-arc terrane from the Baker accretionary complex terrane.This research will involve the training of graduate students at the University of Wyoming. Undergraduate students will also be employed as geologic field assistants. In addition, samples and data collected from this research will be used extensively in the classroom. For example, samples will be used in undergraduate Earth systems science, petrology, and structural geology classes, and in a graduate course in microstructural analysis. Finally, the Baker terrane is a major component of the Blue Mountains province, and a thorough knowledge of these accreted rocks (i.e., the structural basement in regard to the widespread younger rocks of the region) is essential to the success of proposed geophysical surveys through the Blue Mountains province (e.g., the GeoSwath initiative within the EarthScope project).