REACTION SOFTENING AND THE AGE OF THE BLUE RIDGE THRUST: #STRUCTURALPETROLOGY

The Blue Ridge thrust (BRT) in the Southern Appalachians is an orogen-scale thrust system and an example of a basement-involved, but thin-skinned tectonic structure. Field, microstructural, and argon isotopic data from BRT mylonites exposed in two windows constrain the timing of deformation and indicate that reaction softening helped facilitate motion on this shallow thrust system.

The Linville Falls fault (LFF) frames the Grandfather Mountain window in NW North Carolina and exposes lower-grade rocks beneath the BRT. At its type locality the LFF is a 0.3-0.5 m thick and banded ultramylonite. Petrography shows that white-pink bands are dominated by foam-textured quartz (Qz) and unrecovered fine-grained K-feldspar (Ksp), and that green bands are comprised of crenulated phengitic muscovite (Ms). Locally Ksp porphyroclasts form boudin trains that climb across Ms bands. Ksp veins truncate the white-pink bands at a high angle and are variably folded before being truncated at their contact with phengite-rich folia. These textures suggest that the LFF oscillated between fracture (vein production) and ductile creep, with creep aided by consumption of Ksp veins and porphyroclasts via the softening reaction 3Ksp + 2H+ --> Ms + 6SiO2 (aq) + 2K+. Ms from these phengitic folia yield ⁴⁰Ar/³⁹Ar age spectra that climb to ~310 Ma, but linking this isotopic age to the age of the BRT is complicated by the textural evidence in Qz for heating following mylonitization and by crenulation of the Ms folia.

To the west, the BRT is exposed at the margin of the Mountain City window, and juxtaposes phyllonitic meta-rhyolites of the Mount Rogers Formation (MRF) over lowermost greenschist-facies metasediments. Away from the thrust MRF rhyolites are nearly pristine, but incipient replacement of Ksp by Ms is apparent in thin section. In phyllonites, the replacement of feldspars by Ms is nearly complete, the new Ms forms contiguous folia, and aggregates of Ms define a SE-plunging lineation. Ms from these folia yield flat ⁴⁰Ar/³⁹Ar age spectra with ages of ~330-340 Ma. Coexisting Ksp yield ages that climb to >400 Ma, and combined with sub-chlorite grade footwall rocks, indicate the Ms folia that define the BRT record a crystallization age. Here the ⁴⁰Ar/³⁹Ar age dates the reaction that facilitated ductile creep on the BRT.