February 2002-August 2005
Level of Access
Much of Earth's continental crust comprises Precambrian through Cenozoic orogens, where preexisting rocks have been variably modified by metamorphism and deformation. Owing to the association between collisional tectonic events and mountain building, this long record of orogenesis provides the most accessible means of reconstructing paleotectonic settings and events that have shaped our planet. Although the Appalachian mountains are widely used to
illustrate Earth's tectonic evolution and associated orogenesis, collisional processes and events that drove the Ordovician Taconian orogeny in the Quebec Maine segment of the northern Appalachians remain enigmatic. Deformational features in this region cannot be satisfactorily related to a specific convergent setting or collisional event(s) because the nature of the eastern (present coordinates) Laurentian margin during the early Paleozoic is poorly understood.
Interpretations are divided as to whether the orogeny was caused by ophiolite emplacement, island-arc collision, or microcontinent collision. Because the Taconian orogeny initiated the Appalachian mountain-building cycle, understanding its architecture and evolution is essential for understanding the Appalachians as a whole.
The PI proposes to investigate structural, metamorphic, and temporal relationships among the rock units that make up the Chain Lakes massif and Boil Mountain ophiolite in order to test competing models and hypotheses for the Taconian orogeny in the northern Appalachians. Their multidisciplinary approach incorporates mapping, structural, microstructural, metamorphic, and geochronological studies. Integrating field and laboratory studies, the PI will establish: (a) the relative and absolute timing of metamorphic and deformational events within the Chain Lakes massif, (b) the crystallization age of the mafic portion of the Boil Mountain ophiolite, (c) the age and kinematics of ophiolite emplacement, (d) whether mafic volcanic rocks near the ophiolite represent a genetic part of the ophiolite, and (e) the age of possibly rift-related mafic dikes in the Chain Lakes massif. The geochronology portion of the project will include in-situ U-Pb analyses of monazite to provide high temporal resolution for metamorphic events, small-number multigrain U-Pb analyses of zircon or baddeleyite from gabbro to determine the crystallization age of the ophiolite, U-Pb analyses of zircon to establish the age of the dikes, and laser step- heating 40 Ar- 39 Ar analysis of muscovite to determine the minimum ophiolite emplacement age. The PI will compare his results with existing interpretations of the Thetford Mines ophiolite and overlying units in southern Quebec, the Maquereau Dome in the Gaspe Peninsula, and the Dashwoods Subzone in Newfoundland in order to evaluate temporal and spatial relationships among these potentially correlative units. Because the extents and boundaries of terranes are fundamental components of the tectonic models, testing the terrane relationships will test the models. The work will clarify the nature of the Taconian orogeny in the northern Appalachians, and will bear on conceptual models describing Iapetan paleogeographic reconstruction and collisional accretionary processes.
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Johnson, Scott E., "Architecture and Evolution of an Accretionary Orogen" (2005). University of Maine Office of Research and Sponsored Programs: Grant Reports. 196.