THE LINVILLE FALLS FAULT: MICROTEXTURAL EVIDENCE FOR OSCILLATION BETWEEN DUCTILE AND BRITTLE CONDITIONS

The Linville Falls Fault (LFF) frames the Grandfather Mountain window in northwestern North Carolina, and represents the base of the Blue Ridge-Piedmont megathrust sheet (BRTS). This study presents new structural, geochemical, and ⁴⁰Ar/³⁹Ar data from the type locality of the LFF. Here the LFF is a 0.3–0.5 m thick fault zone of light greenish gray, porphyroclastic ultramylonite with pink and white bands (3–5 mm thick) of quartz (Qz) and K-feldspar (Ksp) and dips gently NW, (210°–230°, 14°–20°). The footwall consists of upper Chilhowee Group quartzite of the Table Rock thrust sheet. The hanging wall, the BRTS, consists of coarse-grained, porphyroclastic to mylonitic granitoid. Several strain gradients can be recognized in the granitoid gneiss, and strain generally decreases away from the LFF. Locally pseudotachylite veins occur in the granitoid gneiss. A regional NW-trending mineral lineation occurs in the footwall and hanging wall, and indicates top-to-NW direction of transport. The lineation is collinear with recumbent fold axes in quartzite immediately below the LFF. An upright, NE–SW-trending crenulation and axial planar cleavage transects the LFF.

Optical microscopy shows that white/pink bands in the ultramylonite are dominated by foam-textured Qz. Green bands are comprised almost entirely of phengitic Ms (6.6 Si pfu). Locally Ksp porphyroclasts form boudin trains that cross these bands at a low angle. Nearly pure Ksp veins cut the Qz ribbons at a high angle. These veins are variably folded within the Qz ribbons and are truncated at phengite-rich folia. It appears that new Ksp precipitated in the veins during fracture. The veins were then transposed into the mylonitic foliation and, along with the porphyroclastic feldspars, were replaced by phengite. This process produced the banding in the mylonite. The microtextures indicate that the LFF was active between 300° C and 450° C and that the LFF oscillated between fracture and creep. The approximate foliation-forming (softening) reaction that facilitated creep is:

3Ksp + 2H^+→Ms + 6SiO₂(aq) +2K⁺

Preliminary ⁴⁰Ar/³⁹Ar data suggest retentive domains in porphyroclastic Ksp retain pre-mylonite ages (350 Ma)—suggestive of deformation at <400°C. Phengite separates yield spectra that climb to ~310 Ma likely a minimum age for most phengite growth and motion on the LFF.