DEFORMATION KINEMATICS AND STRAIN PARTITIONING IN THE CATOCTIN FORMATION, BLUE RIDGE VIRGINIA: IMPLICATIONS FOR THICK-SKINNED TECTONICS IN THE APPALACHIANS

The Ediacaran Catoctin Formation forms a distinctive marker unit in the Blue Ridge cover sequence from southern Pennsylvania to central Virginia. On the western limb of the Blue Ridge anticlinorium in central Virginia, the Catoctin Formation comprises a 400 to 900 m thick sequence of metabasaltic greenstone with thin (1-5 m) interlayers of meta-arkose, phyllite and meta-conglomerate. We studied exceptional three-dimensional exposures of the Catoctin Formation exposed in the historic 19^th century Blue Ridge Tunnel in order to characterize the kinematic history of deformation in the western Blue Ridge.

Meso- and micro-scale strain is strongly partitioned in the Catoctin Formation. The greenstone is well-foliated, and where present, elongation lineations plunge down-dip to the SE. The 3D shape of the chlorite-rich amygdules reveals predominantly flattening strains with XZ strain ratios of >3:1, and kinematic indicators consistently record top-to-the-NW-directed shearing. The main foliation is cut by narrow low-angle, top-to-the-west shear zones. Meta-arkose and meta-conglomerate form an array of boudins with varying levels of geometric complexity at multiple scales. Boudins are best developed on overturned limbs of tight NW-verging folds, and also record top-to-the-NW-directed shearing. Zones of epidote-rich breccia, as well as quartz and K-feldspar veins are common within the boudins and at their margins. Previous workers have interpreted sedimentary units in the Catoctin Formation as fluvial channel or braid-plain sheet deposits, however these ‘sedimentary elements’ are more representative of asymmetric boudins whose current complicated geometry reveals little about their depositional setting. The clastic framework of feldspar and quartz deformed primarily from dissolution mass transfer, not by crystal plastic processes. Mineral assemblages and microstructures in both the greenstone and metasedimentary rocks indicate deformation temperatures of 350˚ - 400º C. Under greenschist facies conditions, the mechanical differences between contrasting lithologies is significant and controlled material flow in the cover sequence above the thick-skinned Blue Ridge basement complex.