DETAILED GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS OF THE ASHE METAMORPHIC SUITE AT ELK KNOB STATE PARK, NC

Elk Knob State Park, located 15.3 km north of Boone, NC, is one of the highest peaks (1682 m) of the eastern Blue Ridge Belt and is underlain by NeoProterozoic Ashe Metamorphic Suite rocks. Detailed geologic mapping of Elk Knob at a 1:12,000 scale reveals four map-scale lithologic units. The structurally lowest unit is a garnet-free hornblende gneiss unit that is overlain successively by kyanite-bearing pelitic schist, biotite-quartz-feldspar semischist, and garnet-bearing honblende gneiss. The dominant S2 foliation in all units is defined by alignment of hornblende, feldspar, micas and/or kyanite formed during peak metamorphism at upper amphibolite facies conditions (Abbott and Raymond, 1984). The S2 foliation strikes predominantly ~N15E and dips gently to steeply SE (30º-80º), but the dip changes locally to gentle-to-moderate NW (25º-50º) on the SE side of the knob. The dominant ~N15E orientation gives way to EW-striking, S-dipping foliations within the schist on the peak’s north side. Truncation of S2 foliations against folded and variably trending lithologic contacts and mylonitic textures in rocks along contacts indicates that all rock units are bounded by faults. Shear sense indicators suggest top-to-the NNW movement.

Cross sections, stereograms, and structure contour analyses show a complex history of faulting and at least three folding events. Evidence of D1 is limited to ultramafic rocks within the Ashe Metamorphic Suite and was not observed in the study area. The dominant S2 foliation is axial planar to early, tight-to-isoclinal, gently plunging ~NS F2 folds (20º-30º, 20ºNE and 190º-200º, 10ºSW). Truncation of S2 foliations and F2 folds against faults implies post-metamorphic faulting, however transposition of fault contacts with S2 foliations on the southeast side of the mountain suggests possible syn-metamorphic faulting. Although the dominant foliation is S2, the map pattern of units and large-scale structures is the result of later F3 and F4 folding. F3 is characterized by inclined, tight-to-open EW folds and may be refolded by inclined-to-upright, tight-to-open NS F4 folds. Kyanite lineations trending NS and EW support high temperature conditions during F2, F3, and possibly F4.