EAGER: Testing the Hypothesized Grand Canyon-Appalachian Connection

乔治·格瑞尔斯和威廉·托马斯亚利桑那大学这个项目有一个具体的目标，就是验证这样一个假设，即构成大峡谷砂岩的一些沙子主要来自阿巴拉契亚山脉，还有一个更普遍的目标，那就是重建古生代晚期（3.5 - 2.45亿年前）沉积物在美国各地的扩散路径。由于山区建设过程和气候的复杂相互作用，对这一时期进行源区研究是具有挑战性的。在这一时期，瓦奇塔-阿巴拉契亚-加里多尼亚山脉是泛大陆组装过程中碰撞作用的结果，科迪勒兰山脉经历了弧型岩浆活动和几期会聚构造运动，祖先落基山脉隆升。随着北美穿过古赤道向北移动，以及南半球冰川作用造成了海平面的巨大变化，区域和全球古气候也发生了巨大变化。因此，北美作为一个奇妙的实验室，可以研究这些不同过程之间的相互作用，有可能作为我们现代世界的古代模拟物。假设的大峡谷-阿巴拉契亚连接是基于~720-280 Ma的碎屑锆石颗粒，这些颗粒在大峡谷的二叠系至二叠系地层中越来越多地出现，Gehrels及其同事在2011年6月的《岩石圈》杂志上报道。根据与阿巴拉契亚前陆盆地上古生界岩体结晶年龄和碎屑锆石年龄的相似性，这些年轻颗粒被解释为来自阿巴拉契亚造山带。这一解释在2011年8月出版的《岩石圈》杂志上受到了托马斯的质疑，因为几乎没有证据表明碎屑碎屑从阿巴拉契亚前陆盆地向西移动，而且年轻的沉积物可能来自其他造山带（如北部的富兰克林系），部分来自当地的火成岩。Thomas（2011）还提出，通过将额外的DZ物源数据与上古生界地层的相关系、厚度模式和岩石学特征相结合来验证这些假设，可以重建晚古生代沉积在整个北美的扩散路径。我们建议将~60个样品的U-Pb年龄和Hf同位素测定与传统的地层信息结合起来，专门验证大峡谷-阿巴拉契亚假说，并更广泛地重建北美南部晚古生代的扩散路径。除了上面提到的地质贡献之外，还有三个影响：(1)为研究区域附近学术机构的教职员工和本科生提供参与样本收集、分析和解释的机会。(2)将U-Pb年代学、Hf同位素分析与传统的砂岩地层/沉积学分析相结合，推进物源分析领域，重建扩散路径，研究构造与气候的联系。(3)通过向我们研究区域附近的州立和国家公园的经营者提供我们的工作成果，与公众分享信息，遵循大峡谷“时间之路”展览的模式。

EAGER: TESTING THE HYPOTHESIZED GRAND CANYON-APPALACHIAN CONNECTIONGeorge Gehrels and William ThomasUniversity of ArizonaThis project has a specific goal of testing the hypothesis that some of the sand that make up the sandstones of the Grand Canyon were supplied primarily from the Appalachian mountains, and a more general goal of reconstructing dispersal pathways for sediment across the US during late Paleozoic (350-245 million years ago) time. Conducting source area studies for this time period is challenging because of the complex interplay of mountian building processes and climate. During this time the Ouachita-Appalachian-Caledonian mountain range formed as result of collisional interactions during assembly of the supercontinent Pangea, the Cordilleran mountain range experienced arc-type magmatism and several phases of convergent tectonism, and the Ancestral Rocky Mountains were uplifted. There were also dramatic changes in regional and global paleoclimate as North America moved northward across the paleo-equator and as southern hemisphere glaciation created large changes in sea level. North America accordingly serves as a fabulous laboratory in which to examine the interplay between these disparate processes, potentially serving as an ancient analogue of our modern world. The hypothesized Grand Canyon-Appalachian connection is based on detrital zircon grains of ~720-280 Ma that appear in increasing abundance in Mississippian through Permian strata of the Grand Canyon, as reported in the June 2011 issue of Lithosphere by Gehrels and colleagues. These young grains were interpreted to have been shed from the Appalachian orogen primarily on the basis of similarities with crystallization ages of Appalachian plutons and detrital zircon ages reported from upper Paleozoic strata of the Appalachian foreland basin. This interpretation was challenged in the August 2011 issue of Lithosphere by Thomas because there is little evidence that clastic detritus was transported westward out of the Appalachian foreland basins, and because the young sediment could have been shed from other orogens (e.g. the Fanklinian system to the north) and in part from local igneous rocks. Thomas (2011) also proposed that testing these hypotheses by combining additional DZ provenance data with facies relations, thickness patterns, and petrographic characteristics of upper Paleozoic strata would enable reconstruction of the dispersal pathways of sediment transport across North America during late Paleozoic time. We propose to combine U-Pb age and Hf isotope determinations of ~60 samples with traditional stratigraphic information to specifically test the Grand Canyon-Appalachian hypothesis and more generally reconstruct late Paleozoic dispersal pathways of southern North America.Three impacts beyond the geologic contributions noted above include:(1) Providing opportunities for faculty members and undergraduate students at academic institutions near study areas to become involved in sample collection, analysis, and interpretation.(2) Advancing the field of provenance analysis by integrating U-Pb geochronology, Hf isotope analysis, and traditional stratigraphic/sedimentologic analysis of sandstones in an effort to reconstruct dispersal pathways and to examine linkages between tectonics and climate.(3) Sharing information with the public by providing operators of State and National Parks near our study areas with the results of our work, following the model of the 'Trail of Time' exhibits at the Grand Canyon.