Collaborative Research: Volcanic Material in Triassic Sedimentary Units of Southwestern North America: Understanding the Initiation of Arc Magmatism

认识北美西南部构造演化的一个突出问题是二叠纪-三叠纪岩浆活动弧后记录的范围和性质。 三叠纪沉积序列记录了沿科迪勒拉大陆边缘沿着的弧火山活动的最早阶段，这一记录在被大量中生代中晚期深成岩活动肢解的残余深成岩弧中保存得很差。本研究利用先前发表的关于科罗拉多高原上钦勒和门科皮地层的丰富资料，扩大了对高原三叠纪岩石中发现的火山物质的了解，并对亚利桑那州南部和西部以及加州东部的科罗拉多高原部分提出的或可能的等同物进行了表征。 为解决这些问题而采用的各种方法包括：对灰层和凝灰质砂岩的碎屑锆石进行U-Pb测年，以提供Chinle组的完整年龄谱;分析Chinle组火山碎屑的年龄和完整化学成分;研究亚利桑那州南部和西部已知或疑似三叠纪碎屑岩中的碎屑锆石，以提供物源数据;以及墨西哥索诺拉这些单元和El Antimonio群中火成岩碎屑的完整地球化学特征。 这些数据将与从加州东南部莫哈韦沙漠中类似年龄的岩体中获得的越来越多的信息相吻合。 通过比较，可以检验以下假设：残余深成弧及其火山覆盖层是空气传播的火山灰和河流携带的碎屑物质的来源，这些物质与当地产生的碎屑混合，形成存款层，并在亚利桑那州西部的近弧后区域、加德纳峡谷和亚利桑那州南部的其他未命名的弧近端单元中形成Buckskin地层和相关单元。以及弧后较远地区的Moenkopi组和Chinle组。该研究将为沿着北美西海岸中生代板块运动提供新的视角，包括对大陆西部边缘向东南平移到墨西哥北方的假设的更好理解。这项研究还将为北美西部的古代地形提供线索，包括主要火山的位置和主要河流的流向。作为该项目的一部分，与弗拉格斯塔夫联合学区高中地球科学班合作，将年轻学生带入积极研究的世界，希望能强烈鼓励他们从事科学研究和职业。

An outstanding problem in understanding the tectonic evolution of southwestern North America is the extent and nature of the retroarc record of Permo-Triassic magmatism. Triassic sedimentary sequences hold a record of the earliest stages of arc volcanism along the Cordilleran continental margin, a record poorly preserved in the remnant plutonic arc dismembered by voluminous Middle to Late Mesozoic plutonism. Drawing on the rich base of previously published information about the stratigraphy of the Chinle and Moenkopi formations on the Colorado Plateau, this study is expanding the understanding of volcanic material found in the Triassic rocks of the Plateau, and characterizing proposed or likely equivalents to the Colorado Plateau section in southern and western Arizona and in eastern California. A variety of methods used to address these issues includes U-Pb dating of detrital zircons from ash beds and tuffaceous sandstones to provide a complete age spectrum for the Chinle Formation; analysis of volcanic clasts in the Chinle Formation for age and complete chemistry; study of detrital zircons in known or suspected Triassic clastic rocks in southern and western Arizona for provenance data; and complete geochemical characterization of igneous clasts in these units and in the El Antimonio Group of Sonora, Mexico. These data will mesh with the growing body of information being obtained from similar-age plutons in the Mojave Desert of southeastern California. The comparison will allow testing of the hypothesis that the remnant plutonic arc and its volcanic cover were the source of airborne ash and stream-borne clastic material that mixed with locally derived detritus to deposit the Buckskin Formation and related units in the proximal retroarc area of western Arizona, the Gardner Canyon and other, unnamed arc-proximal units in southern Arizona, and the Moenkopi and Chinle formations in the relatively distal retroarc region.The research will provide a new view of plate movement along the west coast of North America during Mesozoic time, including an improved understanding of the hypothesis that the western margin of the continent was translated southeastward into northern Mexico. The study will also give clues as to the ancient topography of western North America, including where major volcanoes stood, and where major rivers flowed. As part of this project, collaborations with Flagstaff Unified School District high-school Earth Sciences classes to bring young students into the world of active research, in hopes of strongly encouraging them to follow into scientific studies and careers.