Strain in Ordovician and Devonian shales from the Massanutten Synclinorium: implications for fold development and tectonic history

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Virginia Polytechnic Institute and State University

Syntectonic chlorite pressure fringes on framboidal pyrites record strain in Ordovician and Devonian shales from the Massanutten synclinorium. Total strains display no systematic relation to minor or major structures. Strain histories are related to the development of minor, rather than regional, structures. A two-component model is proposed for minor folding involving both flexural slip and pure shear. Curved fringes, straight fringe segments separated by sharp curvatures and multiple sets of fringes indicate that increments of pure shear were interspersed with continuous to episodic limb rotations accommodated by flexural slip.

Similarities of deformation histories of Ordovician and Devonian shales indicate that in this area cover rocks experienced no penetrative deformation during the Taconic Orogeny. Fold geometries and cleavage fans of major structures are consistent with a ramp-fold origin for the Massanutten Synclinorium/Blue Ridge Anticlinorium fold couple. The deformation sequence is characterized by 1) a significant component of lateral shortening associated with minor folds, cleavage and penetrative strain, 2) amplification of the anticlinorium by asymmetric growth over a subsurface ramp. Rotation of the northwest limb to near-vertical during continued compression and transport resulted in local crenulation of the s₁ fabric.