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

了解北美西南部构造演化的一个突出问题是二叠纪-三叠纪弧后岩浆活动记录的范围和性质。三叠纪沉积层序记录了科迪勒大陆边缘最早的弧火山作用阶段，这一记录在被大量的中生代至晚中生代深成作用肢解的残余深成弧中保存得很差。这项研究基于之前发表的关于科罗拉多高原钦勒和门科皮地层的丰富信息，扩大了对高原三叠纪岩石中火山物质的理解，并描述了亚利桑那州南部和西部以及加利福尼亚州东部科罗拉多高原剖面的拟议或可能的等价物。用于解决这些问题的各种方法包括：对火山灰床和凝灰质砂岩中的碎屑锆石进行U-Pb测年，以提供Chinle组的完整年龄谱；分析Chinle组中的火山碎屑岩的年龄和完整的化学成分；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的来源数据；以及对这些单元和墨西哥索诺拉的El Antimonio群的火成岩碎屑进行完整的地球化学特征描述。这些数据将与越来越多的从加利福尼亚州东南部莫哈韦沙漠的类似年龄的深成岩体中获得的信息相吻合。这一对比将允许检验这一假设，即残留的深成岩弧及其火山盖层是气传火山灰和河流来源的碎屑物质的来源，这些物质与当地衍生的碎屑混合在一起，在亚利桑那州西部弧后区域的近端沉积了鹿皮建造和相关单元，在亚利桑那州南部的加德纳峡谷和其他未命名的弧后区域，以及在相对较远的弧后区域的门科皮和钦勒建造。研究将为中生代时期北美西海岸的板块运动提供一个新的视角，包括对大陆西缘向东南转移到墨西哥北部的假设的更好的理解。这项研究还将提供北美西部古代地形的线索，包括主要火山所在的位置，以及主要河流的流向。作为这一项目的一部分，与弗拉格斯塔夫联合学区高中合作开设了地球科学课程，将年轻学生带入活跃的研究世界，希望有力地鼓励他们从事科学研究和职业生涯。

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.