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

合作研究：南阿巴拉契亚西部蓝岭的阿卡迪亚与塔科尼亚构造运动 - 对地体增生模型的影响

• 批准号: 0309284
• 负责人: James Tull
• 金额: $ 9.4万
• 依托单位: Florida State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-07-01 至 2006-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0309284&HistoricalAwards=false
• 关键词: Collaborative; Research; Acadian; vs.; Taconian

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.

0309284TullTerrane大陆增长模型将在阿巴拉契亚西部蓝脊(WBR)南部进行测试和完善，合作内容包括：(A)对可能的劳伦特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的低品位和高级变质沉积进行40Ar/39Ar分析，方法是将得到的WBR构造历史与沿走向(NE和SW)、前陆到NW和外陆带记录的一系列构造事件进行比较。长期以来，WBR变形和变质作用一直被认为是影响劳伦斯东部后缘的一系列古生代吸积作用中的第一次事件，通常被归因于塔克期(~450 Ma)弧-陆碰撞。随后的吸积事件被认为是在阿卡迪期(~400 Ma)和阿勒格尼亚期(~300 Ma)造山过程中逐渐发生在WBR外侧的。因此，传统上，阿巴拉契亚山脉支持共同的“堆积”模式，即弧形和大陆碎片一个接一个地聚集在不断增长的大陆边缘。在阿巴拉契亚山脉北部的劳伦斯边缘和紧邻WBR外侧的东部蓝岭(EBR)上，塔康纪期的变质和变形得到了很好的记录。然而，最近的几项研究表明，南劳伦萨边缘的是阿卡迪亚变质作用，而不是塔科纪变质作用。它们包括：a)在一个关键的WBR地层单位中发现了中古生代化石，b)西南方向的下-中古生界Talladea带与WBR单位之间的层序对比，以及c)奥陶纪40Ar/39Ar年龄地区稀疏的晚志留世-早泥盆世U-Th-Pb值(这是塔科纪变质作用的主要证据)。地层和构造研究表明，单一的劈理形成事件影响了前奥陶纪(？)穿过志罗-泥盆纪的岩石。因此，奥陶系40Ar/39Ar年龄可能是错误的，变形和变质作用完全是阿卡迪期的。如果得到证实，这些对劳伦蒂亚东南端边缘的观测结果将对阿巴拉契亚山脉的吸积模型产生重大影响，因为它们暗示塔科尼亚弧(S)并没有完全沿着大陆的长度生长。相反，南方弧线(S)可能已经停滞不前，直到在阿卡迪亚时期受到接近的大陆碎片的后来的“推动”。将在大烟山、墨菲向斜和山麓地带的两个断面上对世界核反应堆岩石进行三部分的地层、构造和年代学调查，以确定原始年龄、岩石与已知劳伦特沉积物的可比性、任何前和后塔克世岩石的结构兼容性、变质作用的年龄(S)和冷却历史，特别是为了确定塔克纪造山作用和阿卡迪亚造山作用，还是两者都影响了该地区。地层学和构造研究将集中在沿US64从墨菲向斜到山脚带的一条断面上，并将显示经典的变质沉积WBR层序是否实际上是劳伦斯层序，以及它们是否以及如何与沿走向到西南方向(塔列德加带)的古生物测年良好的劳伦斯层序相关联。变质和年代学研究将包括两个断面，一个沿着US64(即，与地层断面相同)，另一个穿过Smokies。离子探针U-Pb年龄将收集(A)从夹层和横贯的火成岩中的锆石中收集，以帮助确定地层年龄分配，以及(B)从元古界(格伦维利亚)片麻岩中收集，以了解它们是否记录了塔科纪扰动的任何证据。高品位岩石基质中变质独居石颗粒和球柱石、蓝晶石中包裹体的TIMS U-Pb年龄将提供最准确和最准确的进退变质作用年龄估算。最后，对以前表明奥陶系40Ar/39Ar年龄的低品位岩石和高级角闪岩的40Ar/39Ar分析将进行更详细的Ar提取和激光显微取样，以确定年龄是否受到超过40Ar(角闪石)或未重置的碎屑云母(低等级岩石)的影响。所有的数据将被结合起来，以显示变质沉积是否真的是劳伦斯式的，以及哪些造山运动影响了该地区。然后，观测到的构造演化将被用来将WBR的这一部分置于阿巴拉契亚地体增生的整体框架中，既穿过阿巴拉契亚山脉南部的走向，又沿着阿巴拉契亚山脉北部的走向。最终了解阿巴拉契亚地体拼装的历史将为大陆形成过程中一般的地体增长过程以及下古生界的板块重建假说提供信息。(A)研究与教育的联系。这项工作将为3名研究生和1名本科生提供研究性培训。正如哈姆斯和科恩的《维塔斯》中所描述的那样，研究样本和地方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。提案中描述的协作很大程度上是第一次(即，PI以前没有合作过)。这项研究将加强东南部的构造学社区网络，科恩和科尔曼之间拟议的合作工作(UNC-CH)也将如此。(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公布和展示他们的结果。同样重要的是，有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翻译人员，并最终传播给每年约1000万公园游客。