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