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

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0309470KohnTerrane 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.