Dextral and Extensional Faults in the Iron Mountains, Southwest Virginia; Strain Variation in an Over-thickened Salient Wedge During Late Stage Alleghanian Collision

Abstract

Along strike strain variation related to the Alleghanian Orogeny are observed in the Iron Mountains, southwest Virginia, located in the transition between the Virginia salient and Tennessee recess. A regional scale dextral transpression fault—the E-W striking Byllesby-Falls fault system (BFFS)—is present across the Iron Mountains. Tectonic convergence direction during the later stages of the Alleghanian orogeny transitioned from an initial NW-directed transport phase to a WNW-directed transport phase (e.g. Wise, 2004). This change in tectonic transport direction introduced a lateral kinematic component into the structural corner of the orogen and was accommodated by formation of the BFFS during the later Alleghanian stage. Additionally, there are 2 populations of mesoscale normal faults observed in Iron Mountains: 1) faults orthogonal to BFFS with purely normal slip motion, and 2) faults parallel to the BFFS with either oblique normal slip or alternating between normal and dextral slip motion. The first normal slip fault population is appropriately oriented to accommodate tangential extension along the BFFS during oblique convergence in the structural corner. The second normal fault population may have formed to accommodate extension in response to an over-steepening orogenic wedge as it exceeded critical taper angle. However, this normal fault population also accommodated dextral motion within the salient wedge. These fault populations in the Iron Mountains indicate that extension and dextral transpression motion were simultaneously active components and record three-dimensional structural processes in the salient wedge during the last stage of Alleghanian collision.