Tectonic implications drawn from differences in the surface morphology on two joint sets in the Appalachian Valley and Ridge, Virginia

Surface morphology distinguishes two joint sets in the Virginia-West Virginia Appalachians and points to a difference in rupture velocity between the two sets. The earlier joint set, J₁, propagated prior to or during fault-related folding and was subsequently tilted as folds grew to have steeper limb dips. J₁ is characterized by a rougher surface displaying a symmetrical plumose pattern created during the propagation of a simple rupture front. In ceramics, such morphology is the product of a faster rupture under relatively high stress intensity (K₁ > K_1c, where K_1c indicates critical stress intensity). This relatively fast joint growth is consistent with an effective tensile stress sustained by oblique plate convergence at the onset of the Alleghanian orogeny. The later joint set, J₂, propagated normal to the Allegheny front in a subvertical orientation independent of local bed dip. A multilobed rupture leaves a smooth surface on J₂ joints. By analogy with ceramics, such a surface is indicative of slower propagation by subcritical crack growth (K₁ < K_1c). This slow joint growth is consistent with the slow generation of effective tensile stress arising from hydrocarbon maturation postdating fold growth during the waning stages of the Alleghanian orogeny.