STRUCTURAL ANALYSIS OF THE SHORES MÉLANGE AT THE SEVEN ISLANDS, CENTRAL VIRGINIA PIEDMONT: POLYPHASE DEFORMATION OF THE TACONIAN SUTURE

The Shores Mélange is an enigmatic unit in the central Virginia Piedmont that previous workers have posited to be the suture between Laurentia and the exotic Chopawamsic Terrane, an Ordovician volcanic arc. The Shores Mélange is characterized by a suite of metasedimentary rocks, some of which contain mafic/ultramafic blocks, and layered metavolcanic rocks. Our research aims to better understand the structural geometry and deformation history of the Shores Mélange along a 3-km stretch of the James River known as the Seven Islands. In the Seven Islands area, the James River drops over a significant knickzone with multiple rapids and large bedrock outcrops. We combine drone imagery and field observations to create detailed outcrop-scale maps of the Seven Islands area.

Five units occur in the Shores Mélange. In the west, the bedrock consists of micaceous meta-graywacke with thin quartzite layers. Epidote-rich greenstone/schist separates the meta-graywacke from a coarse-grained schistose gneiss that contains mafic/ultramafic blocks that range from ~10 cm to several meters long. The schistose gneiss is intruded by thin (1-5 m wide) leucogranite dikes, previously interpreted to be tonalite/trondhjemite blocks in the mélange. These dikes were subsequently folded as the rocks experienced NW-SE shortening and concomitant dextral shearing under greenschist facies conditions. The penetrative foliation within the sequence strikes NE-SW and is thoroughly folded. When present, elongation lineations plunge gently to the NE-SW. Boudinaged veins are typically deformed into rootless folds, consistent with polyphase deformation. The deformed and metamorphosed units are intruded by NE-striking diabase dikes of Mesozoic age.

Based on regional geological considerations and local structure, the Shores Mélange likely represents an accretionary wedge formed along the Taconian suture. However, the structural geometry of the Shores Mélange was strongly modified by later transpressional deformation likely associated with the Alleghanian orogeny. Ongoing geochronological work will determine the intrusive age of the leucogranite dikes and thermochronology of metamorphic minerals in the Shores Mélange.