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Structural Analysis of the Shores Mélange at the Seven Islands, central Virginia Piedmont
Zach, Terri
Zach, Terri
Abstract
Roughly 450 million years ago, the Chopawamsic volcanic island arc approached the Laurentian margin signaling the closing of the Iapetus Ocean and the start of the Taconian orogeny. Today, we look towards the central Virginia Piedmont for evidence of the suturing event that welded these two geologic bodies. Using the James River to our advantage, we can analyze the geologic outcrops to investigate this potential suture zone. This study aims to understand better the structural and geologic history of the Shores Mélange, located in eastern Seven Islands, Virginia. We created a highly detailed map that focuses on the bedrock geology of this vital area. We identified five major geologic units that make up the Shores Mélange. The most abundant is the metagraywacke unit. The foliation of this unit strikes to the northeast and dips moderately steeply northwest. The fold axis of this unit trends north-northeast with a gentle plunge. Quartz boudins in this area show top-to-the-right (dextral) sense of shear, with some quartz veins showing enough deformation to become rootless folds. The schistose gneiss unit is the next largest unit with foliation that strikes north to northeast and dips from moderately to steeply southeast. However, at some locations, we can identify synforms and antiforms. The leucogranite highlights the folding in this unit, which we interpreted as an intrusive, felsic rock. This leucogranite appears to be a dike, with limbs and clasts found in the schistose gneiss unit and the greenstone unit. The greenstone unit has similar foliation strikes and dip angles. However, it dips to the west and east, revealing an antiform in one location. The other greenstone unit also has foliation that strikes dominantly northeast and dips steeply to near vertically northwest. The youngest unit we identified is the Jurassic diabase dike, which appears only once on the eastern side of the study area. Much of this study area reveals a complex history, including evidence for a second post-Taconian orogeny deformation event.
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2020-05-01
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Geology