No Rock Is Accidental! Stratigraphy, Structure, and Tectonics in the Wilson Cycle

Abstract In this upper level exercise, students analyzed and interpreted a suite of rock specimens collected from a variety of tectonic terranes in the Piedmont, Blue Ridge, and Valley and Ridge provinces of western Virginia and eastern West Virginia. Many of the locations were eventually visited during five field trips that comprised the “Integrating Structural and Stratigraphic Field Data” exercise (Whitmeyer and Fichter, this volume). Students worked in groups to observe, describe, and identify key sedimentary, metamorphic, and/or igneous features of each specimen. This extended into applying a variety of theoretical models (e.g., P/T, flow regime, stress/strain, geochemical, etc.) to determine the environmental conditions under which each rock formed. With some specimens, this involved multiple and distinctive historical/evolutionary stages; for example, a plutonic charnockite, that is now a protomylonite, with an overprint of greenschist facies. After collectively reviewing their interpretations, students determined the ideal environmental regime—whether it was igneous, sedimentary, or metamorphic—for each specimen on a set of tectonic diagrams illustrating the sequential stages in a Wilson Cycle ( Wilson, 1966 ). Accepting the dictum that “no rock is accidental”—that all rocks must form in specific places for specific reasons—this exercise gave students a heuristic model for solving in the field and lab problems of local and regional tectonic evolution in the Mid-Atlantic Appalachian orogenic cycles.