THE SOUTHERN APPALACHIAN BLUE RIDGE: A NATURAL LABORATORY TO EXAMINE CONCEPTUAL MODELS OF FAULT ZONES AT DIFFERENT CRUSTAL LEVELS

Conceptual models of faulting predict broad distributed zones of deformation at depth that transition to discrete fault zones in the upper crust. The southern Appalachian Blue Ridge, like many ancient orogens, provides the opportunity to examine fault zones at different crustal levels. In the Blue Ridge of NC, VA and TN, faults within and bounding the Blue Ridge–Piedmont megathrust sheet (BRTS) provide such an opportunity. The Fries fault zone consists of an ~11 km-wide anastomosing network of greenschist- to amphibolite-facies shear zones that overprint basement rocks. The shear zones strike NE–SW and dip SE, and the thickness and lateral extent of the zones vary from meters to several kilometers. Mylonitic to phyllonitic rocks in the shear zones contain muscovite (Ms), quartz (Qz) and K-feldspar (Ksp), and may surround domains of lower-strain rocks. Kinematic indicators are dominantly top-to-the-NW and the down-dip lineation plunges 40°/110°. To the west, the Catface and Stone Mountain faults mark the base of the BRTS and frame the Mountain City window on the east. Here, high-strain zones associated with the faults are narrow zones up to ~240 m wide. Microstructures from sheared Mount Rogers Fm. metarhyolite include deformation lamellae and undulose extinction in Qz, and fractured Ksp variably replaced by phengitic Ms. The mineral stretching lineation plunges 47°/138°, and kinematics are top-to-the-NW. The Iron Mountain and Holston Mountain faults, which bound the western part of the window and base of the BRTS, are discrete, bedding-parallel faults.

Muscovite ⁴⁰Ar/³⁹Ar ages across the western BRTS (n = 8) yield ages of 347–335Ma and Ksp spectra climb to ages older than coexisting Ms. This age relationship and microstructures indicate that some Ms in all samples grew below closure temperature. In the Fries fault zone, multiple generations of Ms are present complicating age interpretation, however Catface fault mylonite contains one textural population of Ms and yields an ⁴⁰Ar/³⁹Ar age of ~340 Ma. Collectively, the age data indicate all faults and fault fabrics were active in the earliest Alleghanian orogeny and correspond to the time of initial emplacement of the BRTS at ~340 Ma. Although now disconnected, these faults may represent a temporal and depth-dependent continuum of faults during the Alleghanian.