Collaborative Research: Acadian vs. Taconian Tectonism in the Southern Appalachian Western Blue Ridge - Implications for Models of Terrane Accretion

[0309284]将在阿巴拉契亚西部蓝岭（WBR）南部测试和完善大陆生长的地体增积模型，合作内容包括：(a)对假定的Laurentian变质沉积盆地进行详细的地层和构造分析，(b)对变质和地层年龄进行U-Th-Pb调查，以及(c)对WBR、NC和TN的低品位和高品位变质沉积进行40 Ar/39 Ar分析。通过与沿走向（NE和SW）、前陆向西北和向外地的一系列构造事件的比较，得出了西寒带构造史。WBR变形和变质作用一直被认为是影响东劳伦纪拖缘的一系列古生代增生中的第一次，通常被认为是Taconian （~450 Ma）弧-陆碰撞的结果。随后的吸积事件被认为是在阿卡迪亚期（~400 Ma）和阿勒卡尼期（~300 Ma）造山运动期间逐渐发生在西滩外。因此，传统上，阿巴拉契亚山脉支持常见的“堆积”增生模式，即弧和大陆碎片一个接一个地增生到正在生长的大陆的边缘。在阿巴拉契亚山脉北部的劳伦森边缘和西蓝岭外的东蓝岭（EBR），塔克尼亚时代的变质作用和变形作用都有很好的记录。然而，最近的几项研究现在表明，南劳伦边缘的阿卡迪亚变质作用而不是塔科尼亚变质作用。这些发现包括：a)在一个重要的WBR地层单元中发现了中古生代化石；b)西南地区的中下古生代Talladega带与WBR地层单元的序列对比；c)在已报道的奥陶系40 Ar/39 Ar年龄的地区，发现了稀疏的晚志留世-早泥盆世U-Th-Pb年龄（这是Taconian变质作用的主要证据）。地层学和构造学研究表明，前奥陶世（? ?）是由一个单一的解理形成事件通过志留系-泥盆系岩石影响的。因此，奥陶系40 Ar/39 Ar年龄可能是不正确的，变形和变质作用完全是阿卡迪亚的。如果得到证实，Laurentia远东南边缘的这些观测结果将对阿巴拉契亚山脉的增生模型产生重大影响，因为它们表明Taconian弧并没有沿着大陆的长度完全增生。相反，在阿卡迪亚期间，南弧可能已经在外部停滞，直到后来受到接近的大陆碎片的“推动”。将在大烟山、墨菲向斜和山麓带的两个样带对WBR岩石进行三方面的地层、构造和地质年代学调查，以确定原始年龄、岩石与已知劳伦纪沉积物的相关性、任何前和后塔古纪岩石之间的结构相容性、变质作用年龄和冷却历史，特别是确定塔古纪造山运动、阿卡迪亚造山运动或两者是否影响该地区。地层学和构造研究将集中在沿US64从Murphy向斜到Foothills带的样带上，并将显示经典的变质沉积WBR层序是否实际上是劳伦层序，以及它们是否以及如何与沿走向到SW （Talledega带）的古生物学上确定年代的劳伦层序相关联。变质岩和年代学研究将包括两个样带，一个沿着US64（即，与地层样带相同），另一个穿过Smokies。离子探针U-Pb年龄将收集(a)来自互层和横切火成岩中的锆石，以帮助确定地层年龄分配；(b)来自元古代（格伦弗利亚）片岩，以查看它们是否记录了Taconian扰动的任何证据。高品位岩石基质中变质独居石颗粒微样的TIMS U-Pb年龄以及锆石和蓝晶石包裹体的TIMS U-Pb年龄将提供最精确和准确的进阶变质年龄估计。最后，通过低品位岩石和高品位角闪岩的40 Ar/39 Ar分析，我们将通过更详细的Ar提取和激光显微取样，重新研究奥陶系40 Ar/39 Ar年龄，以确定年龄是否受到过量40 Ar（角闪岩）或未重置碎屑云母（低品位岩石）的影响。所有的数据将结合起来，以显示沉积是否真正属于劳伦森，以及哪些造山运动影响了该地区。观察到的构造演化将被用于将这部分西太平洋逆冲带置于阿巴拉契亚地体增长的整体框架中，包括阿巴拉契亚山脉南部的横走向和北部的顺走向。最终了解阿巴拉契亚地体组合的历史将为大陆形成过程中的一般地体增生过程以及下古生代的板块重建假设提供信息。拟议的研究将在几个领域产生广泛的影响。(a)研究-教育联系。本工作将为3名研究生和1名本科生提供研究培训。正如在哈梅斯和科恩的简历中所描述的那样，研究样本和当地研究领域在他们的本科教学和实地项目中占有重要地位。(b)研究/教育基础设施。提案中描述的合作大部分是第一次（即PI以前没有合作过）。这项研究将加强东南地区的构造社区网络，正如Kohn和Coleman （UNC-CH）之间拟议的合作工作一样。(c)教育传播。所有的pi都以及时和适当的方式向其他地球科学家发表和展示他们的结果。同样重要的是，一个研究地点位于大烟山国家公园。科恩与公园工作人员保持着密切的联系。新的结果将迅速传播给口译员，并最终传播给每年约1000万公园游客。

0309284TullTerrane accretion models of continental growth will be tested and refined in the southern Appalachian western Blue Ridge (WBR) in a collaboration involving (a) detailed stratigraphic and structural analysis of putative Laurentian metasedimentary basins, (b) U-Th-Pb investigation of metamorphic and stratigraphic ages, and (c) 40 Ar/39 Ar analysis of low-grade and high-grade metasediments in the WBR, NC and TN, via comparison of the resulting WBR tectonic history to the series of tectonic events recorded along strike (NE and SW), in the foreland to the NW, and to outboard terranes. Long viewed as the first event in a series of Paleozoic accretions affecting the eastern Laurentian trailing margin, WBR deformation and metamorphism have been commonly ascribed to Taconian (~450 Ma) arc-continent collision. Subsequent accretion events are thought to have occurred progressively outboard of the WBR during the Acadian (~400 Ma) and Alleghanian (~300 Ma) orogenies. Thus, traditionally the Appalachians support common "pile-up" accretion models whereby arcs and continental fragments accrete one after another to the edge of the growing continent. Taconian-age metamorphism and deformation are well documented on the Laurentian margin in the northern Appalachians, and in the Eastern Blue Ridge (EBR) immediately outboard of the WBR. However, several recent studies now suggest Acadian rather than Taconian metamorphism for the southern Laurentian margin. These include: a) the discovery of middle Paleozoic fossils from a key WBR stratigraphic unit, b) sequence correlations between the lower-middle Paleozoic Talladega belt to the southwest and WBR units, and c) sparse Late Silurian-Early Devonian U-Th-Pb ages from areas with reported Ordovician 40 Ar/39 Ar ages (which are the main line of evidence for Taconian metamorphism). Stratigraphic and structural studies suggest that a single cleavage-forming event affected pre-Ordovician (?) through Siluro-Devonian rocks. Thus, the Ordovician 40 Ar/39 Ar ages may be incorrect, and deformation and metamorphism were exclusively Acadian. If verified, these observations for the distal SE edge of Laurentia have a major impact on accretion models for the Appalachians because they imply that the Taconian arc(s) did not fully accrete along the length of the continent. Instead, the southern arc(s) may have stalled outboard until receiving a later "push" from an approaching continental fragment during the Acadian. A tri-partite stratigraphic, structural, and geochronologic investigation of WBR rocks will be conducted in two transects through the Great Smoky Mountains, Murphy syncline, and Foothills belt, to identify original ages, correlatability of rocks with known Laurentian sediments, structural compatibility between any pre- and post- Taconian rocks, age(s) of metamorphism, and cooling history, especially to identify whether the Taconian orogeny, Acadian orogeny, or both affected the area. Stratigraphic and structural research will focus on a transect along US64 from the Murphy syncline to the Foothills belt, and will show whether the classic metasedimentary WBR sequences are in fact Laurentian, and if and how they correlate with paleontologically well-dated Laurentian sequences along strike to the SW (Talledega belt). Metamorphic and chronologic research will include two transects, one along US64 (i.e., identical to the stratigraphic transect) and a second through the Smokies. Ion probe U-Pb ages will be collected (a) from zircons in interbedded and cross-cutting igneous rocks to help define stratigraphic age assignments, and (b) from Proterozoic (Grenvillian) gneisses to see whether they record any evidence for a Taconian disturbance. TIMS U-Pb ages of microsampled metamorphic monazite grains in the matrix of high-grade rocks and inclusions in staurolite and kyanite will provide the most precise and accurate age estimate of prograde metamorphism. Finally, 40 Ar/39 Ar analyses of low-grade rocks and high-grade amphiboles that previously indicated Ordovician 40 Ar/39 Ar ages will be restudied with more detailed Ar-extraction and laser microsampling to see whether ages are biased by excess 40 Ar (amphiboles), or unreset detrital micas (low-grade rocks). All data will be combined to show whether metasediments are truly Laurentian, and which orogenies affected the area. The observed tectonic evolution will then be used to place this part of the WBR into an overall framework of Appalachian terrane accretion, both across strike in the southern Appalachians and along strike to the northern Appalachians. Ultimately understanding the history of Appalachian terrane assembly will inform general terrane accretion processes during formation of continents, as well as plate reconstruction hypotheses for the lower Paleozoic.The proposed research will have broad impact in several areas. (a) Research-education links. This work will provide research training for 3 graduate students and one undergraduate. As described in Hames' and Kohn's vitas, research samples and local research areas figure heavily in their undergraduate teaching and field programs.(b) Research/education infrastructure. The collaborations described in the proposal are largely first time (i.e., the PI's have not worked together before). This research will strengthen tectonics community networks within the southeast, as will the proposed collaborative work between Kohn and Coleman (UNC-CH). (c) Educational dissemination. All PIs publish and present their results in a timely and appropriate fashion to other geoscientists. It is also important that one study site is in Great Smoky Mountains National Park. Kohn maintains close ties to park personnel. New results will be rapidly disseminated to the interpreters and ultimately the ~10 million park visitors each year.