A NEW BEDROCK GEOLOGIC MAP OF THE HORSE SHOE QUADRANGLE, HENDERSON AND TRANSYLVANIA COUNTIES, NC: MORE DATA FROM THE BREVARD ZONE

The North Carolina Geological Survey, in conjunction with the United States Geological Survey STATEMAP program, has produced a new 1:24,000-scale bedrock geologic map of the Horse Shoe quadrangle in Henderson and Transylvania counties, NC. Bedrock geology of the Horse Shoe quadrangle comprises elements of the Ashe Metamorphic Suite-Tallulah Falls Formation (AMS-TFF), Brevard Zone (BZ), Chauga River Formation (CRF), Henderson Gneiss (HG) and other Ordovician granitoid bodies.

Field data were collected at over 2,400 stations with 40 samples being selected for thin-section and whole rock ICP analyses. Sixty stream sediment heavy mineral samples were processed and analyzed for mineral content. Foliation and mylonitic foliation generally strike northeast and dip southeast. Dips are less steeply inclined at observation points southeast of the BZ. A prominent, steeply-dipping fracture set of azimuth 315°-135° along with a lesser fracture set of azimuth 225°-45° are identified from joint data.

Neoproterozoic to Cambrian metasediments of the AMS-TFF underlie the northwestern corner of the map and are the oldest rocks in the area. AMS-TFF rocks have been metamorphosed to upper amphibolite facies and are locally migmatitic.

Immediately southeast and structurally above the AMS-TFF is the BZ. BZ rocks consist mainly of sericite/graphitic phyllite and mylonitic granitic gneiss/phyllonite. The protolith of much of the mylonitic granitic gneiss/phyllonite is interpreted to be HG. As is reported elsewhere along the BZ, retrogressive greenschist facies metamorphism is widespread in BZ rocks on the Horse Shoe quadrangle. Several zones of younger brecciation were identified that may be related to brittle faulting along the Rosman fault reported by Horton (1980).

Southeast of and structurally above the BZ is a large volume of mostly typical HG (strongly lineated granitoid gneiss with conspicuous k-feldspar augen). Several subdivisions were identified within the HG based on mineralogy (biotite and amphibole esp.), compositional layering and degree of deformation.

CRF metasediments were identified in the southeastern corner of the map. They are interpreted to be the continuation of the Eastatoe fault block mapped on the Standingstone Mountain quadrangle by Garihan and Ranson (2007).