STRUCTURAL GEOMETRY OF THE MECHUM RIVER BELT, BLUE RIDGE PROVINCE, VIRGINIA

Metasedimentary rocks of the Neoproterozoic Mechum River Formation crop out in an elongate ~100 km long northeast-southwest trending belt near the center of the Blue Ridge anticlinorium in central and north-central Virginia. Previous workers have interpreted this belt as a graben, half graben, thrust-bounded inlier and a structural infold. Along the northwestern contact Mechum River metasedimentary rocks unconformably overlie Grenville-age basement. The southeastern contact is a steeply dipping reverse fault in which Grenville-age basement is juxtaposed against Mechum River metasedimentary units. This fault cuts pre-existing folds and foliations and is interpreted to be an out-of-sequence fault that may be related to Alleghanian ramping of the master Blue Ridge thrust sheet. The structural geometry of the Mechum River belt changes from the southwest to the northeast. In central Albemarle County, the structure is a sequence of moderately plunging northwest-verging asymmetric folds. In northern Albemarle County the reverse fault at the southeastern contact has cut through fold hinges and the belt forms a southeast dipping homoclinal sequence. In Madison County, the belt is characterized by asymmetric folds that plunge steeply to the south-southeast and are compatible with an overall triclinic deformation symmetry. In Rappahannock County, the Mechum River belt is up to 5 km wide and characterized by relatively open folds that plunge gently northeast. A narrow belt of steeply-dipping Mechum River meta-arkose, phyllite, metarhyolite, and interlayered volcaniclastic rock is exposed 2 km east of the main belt near Castleton in Culpeper County. In its present geometry the Mechum River belt is not a graben formed during Neoproterozoic rifting, rather it is a structural inlier developed during Paleozoic contractional deformation.