THE BEDROCK GEOLOGIC MAP OF THE WESTERN HALF OF THE ASHEVILLE 100K: A NEW SYNTHESIS

The North Carolina Geological Survey presents the Bedrock Geologic Map of the Western Half of the Asheville 1:100,000 scale Quadrangle (100K). This map is a compilation of new work and sixteen 1:24,000 scale detailed geologic maps created by fifteen authors during a span of thirty-five years. Funds for compilation work and for much of the detailed geologic mapping were provided by the STATEMAP component of the National Cooperative Geologic Mapping Program.

First order elements include Mesoproterozoic basement, the Ashe Metamorphic Suite/Tallulah Falls Formation (AMS/TFF), the Cartoogechaye terrane, the Hot Springs window, and Ocoee and Chilhowee metasedimentary and sedimentary rocks.

The 100K provides an updated model for the Mesoproterozoic basement. Based on age dates and map relationships, distinct igneous bodies were intruded by subsequent magmatism during the protracted Grenville orogeny. Crystallization ages for basement gneisses span from 1360 Ma to 1020 Ma (Officer et al., 2003; Berquist et al., 2005). All were affected by high temperature metamorphism and deformation with the oldest bodies being subjected to multiple events. One of the most striking examples is of granoblastic granulitic gneisses transformed to S-tectonites by high temperature mylonitization. Previously, these gneisses were used in part to define the Mars Hill terrane (MHT). Recognition that they are protoliths of a more widespread map unit may require a redefinition of the MHT.

Basement rocks are crosscut by undeformed 734 Ma (Goldberg et al., 1986) Bakersville metagabbro dikes, thus precluding extensive Paleozoic metamorphism and deformation. Undeformed 417-402 Ma (Miller et al., 2000; Mapes, 2002) trondhjemite dikes crosscut both the basement and AMS/TFF rocks, suggesting Taconic emplacement of the AMS/TFF along the Holland Mountain-Chattahoochee fault.

A major shear zone overprints the basement in the central and northeastern portion of the map. To the west, the shear zone comprises the boundary between basement and Neoproterozoic metasedimentary rocks. The zone narrows from over five km wide in the northeast corner of the map to less than one km along the western edge. Muscovite age dates in the shear zone range from 355 to 330 Ma.

Previously unrecognized ultramylonites were mapped within basement rocks.