18 Geologic Maps and Digital Geologic Maps (Posters)

Friday, 14 March 2003
8:00 AM - 12:00 PM

DIGITAL GEOLOGIC MAPS OF THE WEAVERVILLE AND SOUTHERN MARSHALL QUADRANGLES IN THE NORTH CAROLINA BLUE RIDGE

Paper number: 18-9
Program: THEME SESSIONS
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Bedrock maps at 1:24,000 scale in the Weaverville and southern Marshall, NC 7.5’ quadrangles include rocks of the Ashe Metamorphic Suite (AMS) to the SE in contact with Middle Proterozoic basement rocks of the Mars Hill terrane. The basement rocks comprise a very heterogeneous package dominated by biotite granitic gneiss and migmatitic biotite hornblende gneiss with thin discontinuous lenses of altered ultramafic, calc-silicate/marble, amphibolite, and porphyroclastic gneisses. The dominant gneissic fabric preserves strain heterogeneity and both structural and lithologic heterogeneity are the norm at both outcrop and map scales of observation. Localized mylonitic zones preserve ductile fabrics, typically with thrust-sense kinematics, commonly contain very fine to coarse retrograde muscovite, and are interpreted as Alleghenian deformation zones. The most extensive of these, characterized by en echelon zones or lenses of mylonite to ultramylonite with little apparent offset, extends NE-SW across the southern Marshall quadrangle. West of this tectonite zone, the rock types are more varied and hornblende-bearing rocks are more abundant. East of this tectonite zone, basement structural trends define a broad folded pattern with gentle SSE-plunging axes.

The AMS rocks are dominantly interlayered aluminous schist and meta-graywacke map units with minor amphibolite and ultramafic rocks, including a previously unrecognized body of interlayered altered meta-dunite and meta-gabbroic rock. The slivered AMS-basement contact is continuous with the Holland Mountain thrust to the southwest. A broad high-strain zone with localized mylonite overprints rocks on either side of the contact and has been previously interpreted as the Burnsville shear zone. Within this zone dextral kinematic indicators are associated with gentle NE-SW elongation lineations and mylonites in the AMS include peak Ky-Sill zone metamorphic mineral assemblages. NNW-SSE basement structural trends appear deflected or truncated in the vicinity of this contact.

The maps were compiled in AutoCAD using Fieldlog, a Geological Survey of Canada database program. Final map production was completed using Adobe Illustrator. Mapping was supported by USGS State Map funds to the NC Geological Survey.