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: 0310194
• 负责人: Willis Hames
• 金额: $ 8.64万
• 依托单位: Auburn 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=0310194&HistoricalAwards=false
• 关键词: Collaborative; Research; Acadian; vs.; Taconian

0310194HamesTerrane 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.

0310194将在阿巴拉契亚山脉南部的蓝岭西部（WBR）测试和完善大陆生长的Hames地体增生模型，合作内容包括：（a）对假定的劳伦阶变质沉积盆地进行详细的地层和结构分析，（B）变质和地层年龄的U-Th-Pb调查，以及（c）对WBR的低级和高级变质沉积物进行40 Ar/39 Ar分析，NC和TN，通过将由此产生的WBR构造历史与沿着走向（NE和SW）记录的一系列构造事件进行比较，在前陆到NW，以及到外侧斜坡。WBR变形和变质作用长期以来被认为是影响东劳伦特后缘的一系列古生代增生的第一个事件，通常被归因于塔昆（~450 Ma）弧-陆碰撞。随后的加积事件被认为是在西印度群岛（~400 Ma）和阿勒格尼（~300 Ma）造山作用期间逐渐发生在西印度群岛的外侧。因此，传统上阿巴拉契亚山脉支持共同的"堆积"增生模型，即弧和大陆碎片一个接一个地增生到生长大陆的边缘。塔康期变质和变形在阿巴拉契亚山脉北方的劳伦阶边缘和东部蓝岭（EBR）的WBR外侧有很好的记录。然而，最近的几项研究表明，南劳伦阶边缘的变质作用发生在西印度群岛而不是塔康阶。其中包括：a）在一个关键的WBR地层单元中发现了中古生代化石，B）西南部的下-中古生代塔拉德加带与WBR单元之间的层序对比，以及c）在有奥陶纪40 Ar/39 Ar年龄报告的地区获得了稀疏的晚志留世-早泥盆世U-Th-Pb年龄（这是塔昆变质作用的主要证据）。地层和构造研究表明，一个单一的劈理形成事件影响前奥陶纪（？）穿过志留纪-泥盆纪的岩石因此，奥陶纪40 Ar/39 Ar年龄可能是不正确的，变形和变质作用完全是奥陶系。如果得到证实，这些意见的远东南边缘劳伦有一个重大影响的吸积模型阿巴拉契亚山脉，因为它们意味着塔康弧（S）没有完全吸积沿着大陆的长度。相反，南部的弧可能已经在舷外停滞，直到后来在南极洲期间接受来自接近大陆碎片的“推动”。将在大烟山、墨菲向斜和山麓带的两个断面中对WBR岩石进行三方地层、结构和地质年代学调查，以确定原始年龄、岩石与已知劳伦特沉积物的相容性、任何塔昆前和塔昆后岩石之间的结构相容性、变质作用的年龄和冷却历史，特别是确定塔昆造山带，或者两者都影响了该地区。地层学和构造研究将集中在沿美国64从墨菲向斜到山麓地带的一个断面上，并将显示经典的变质沉积WBR序列是否实际上是劳伦阶，以及它们是否以及如何与沿走向SW（塔勒德加带）的古生物学上年代久远的劳伦阶序列相关。变质和年代学研究将包括两个断面，一个是沿沿着US 64（即，第二条穿过大烟山。离子探针U-Pb年龄将收集（a）从互层和横切火成岩中的锆石中，以帮助确定地层年龄分配，（B）从元古代（格伦维尔）片麻岩中，以查看它们是否记录了塔康期扰动的任何证据。高级岩石基质中的变质独居石颗粒和十字石、蓝晶石中的包裹体的TIMS U-Pb年龄将提供最精确的变质作用年龄估计。最后，将通过更详细的Ar提取和激光显微取样来重新研究之前表明奥陶纪40 Ar/39 Ar年龄的低级岩石和高级角闪石的40 Ar/39 Ar分析，以了解年龄是否因过量40 Ar（角闪石）或未重置的碎屑云母（低级岩石）而产生偏差。所有的数据将结合起来，以显示是否变质沉积物是真正的劳伦，以及哪些造山作用影响了该地区。观察到的构造演化，然后将被用来放置这部分的WBR到一个整体框架的阿巴拉契亚山脉加积，无论是跨罢工在阿巴拉契亚山脉南部和沿着罢工的北方阿巴拉契亚山脉。最终了解阿巴拉契亚板块组装的历史将为大陆形成过程中的一般板块增生过程提供信息，以及下古生代的板块重建假说。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。这项工作将为3名研究生和1名本科生提供研究培训。正如Hames和Kohn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。提案中描述的合作主要是第一次（即，以前没有一起工作过）。这项研究将加强东南部的构造社区网络，科恩和科尔曼（UNC-CH）之间的拟议合作也将如此。(c)教育传播。所有PI都会及时、适当地向其他地球科学家发布并展示他们的结果。还有一点很重要，那就是大烟山国家公园有一个研究点。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1000万的公园游客。