Does the Chopawamsic Fault Represent the Main Iapetan Suture in the Southern Appalachian Orogen? Geology, Geochemistry, and Geochronology of the Western Piedmont of Virginia and Insight into Local Intraplate Seismicity.

摘要

The western Piedmont of north-central Virginia includes two main lithotectonic units: the early Paleozoic Potomac terrane and the Ordovician Chopawamsic terrane. The Potomac terrane is mostly an assemblage of multiply deformed phyllite, metasandstone, and metagraywacke. Previous workers have interpreted the sediments that now make up the Potomac terrane to have been deposited offshore of, and derived from, the Laurentian craton. The Chopawamsic terrane consists mostly of bimodal volcanic and associated epiclastic rocks. Previous work has shown that the Chopawamsic arc was built above currently unexposed Mesoproterozoic crust; however, the cratonic affinity of this crust has not been identified. The suture between the Potomac and Chopawamsic terranes is the Chopawamsic fault. If the Chopawamsic terrane is peri-Gondwanan (exotic to Laurentia), then the Chopawamsic fault would mark the closure of the early Paleozoic Iapetus Ocean in the southern Appalachian orogen---where it has not been previously identified. Chapters 1, 2, and 4 address this main hypothesis of the possible Iapetan suture and chapter 3 addresses the connection of deformed Paleozoic rocks and modern intraplate seismicity in the Central Virginia Seismic Zone.;Chapter 1 focuses specifically on the relationship between the Chopawamsic fault and the latest Ordovician -- earliest Silurian Ellisville granodiorite pluton. The pluton had previously been shown to intrude the fau however, no detailed mapping had been done to prove this relationship. My new mapping, in conjunction with previous mapping to the north, as well as new geochemical and geochronological sampling throughout the geographic areas and textural phases of the pluton, confirms the stitching nature of the Ellisville pluton. The age of the early and most volumetrically significant phase of the pluton, reported here, is 444 +/- 4 Ma. Because no sign of reactivation along the fault has been previously reported or discovered in this study, we interpret the age of the Ellisville pluton to represent a date of latest possible motion on the Chopawamsic fault. Limited kinematic information for the Chopawamsic fault is also presented here.;In chapter 2, the results of 1,289 detrital zircon U-Pb, LA-ICPMS analyses are reported. Samples in the composite Potomac terrane include mostly Mesoproterozoic zircons that are consistent with a Laurentian source. Samples from the Chopawamsic terrane include---almost exclusively---Ordovician zircons likely derived from co-eval volcanic rocks in the Chopawamsic terrane. Limited older zircons in these Chopawamsic metasedimentary rocks may hint at a possible peri-Gondwanan source, but are not numerous enough to be conclusive. The Arvonia successor basin, which overlies the Chopawamsic terrane, includes populations of both Mesoproterozoic and Ordovician-Silurian detrital zircons. These dates, with supporting field relationships, indicate that the Arvonia basin was deposited in the Late Ordovician -- Early Silurian only after the accretion of the Chopawamsic arc to the Potomac terrane.;Chapter 3 was not a planned part of the original study but came about after a M5.8 earthquake, centered in central Virginia, shook the majority of the eastern U.S. Aftershock analysis and field observations in the epicentral area are presented here and support a connection between the attitude of previously deformed rocks and the orientation of modern seismogenic surfaces.;Chapter 4 is a field trip guidebook prepared for the 2014, Blacksburg, VA southeast section meeting of the Geological Society of America. This guide highlights some of the most important and accessible outcrops that have been sampled in this study for geochemical and/or geochronological analyses. Many of the field trip stops correspond to rocks described in the first 3 chapters, and others highlight geochemical and geochronological analyses yet to be formally prepared for publication in a non-field guide format.