DIFFERENCES IN JOINT RUPTURE VELOCITY AS INDICATED BY DISSIMILARITIES IN THE SURFACE MORPHOLOGY ON JOINTS IN THE APPALACHIAN VALLEY AND RIDGE, VIRGINIA: IMPLICATIONS FOR TIMING OF HYDROCARBON GENERATION IN THE SOUTHEASTERN PORTION OF THE APPALACHIAN BASIN

Differences in surface morphology distinguish two joint sets in the Va-W.Va Appalachians north of the Roanoke Recess and point to dissimilar growth rates during propagation of each set. The earlier, partially mineralized 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 and regular 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 (K_I ® K_Ic). This early joint set is best developed in the upper portion of the Catskill Delta complex where tectonic compaction may have played a role in generating joint-driving pore pressure. The implication is that this relatively fast joint growth was by natural hydraulic fracturing (NHF) with pore fluid pressure accumulating at the rate of fold growth during active plate convergence in the Alleghanian orogeny. Crack growth was by flow-limited propagation (i.e., Lacazette and Engelder, 1992). The later joint set, J₂, propagated normal to the Allegheny front in a sub-vertical orientation independent of local bed dip. A multi-lobed rupture that leaves a very planar, smooth surface characterizes J₂. By analogy with ceramics such a surface is indicative of slower propagation by subcritical crack growth (K_I < K_Ic). This slow joint growth is consistent with NHF by infiltration-limited propagation accompanying slow infiltration of pore fluids. Because these joints are best developed in Devonian black shale and deeper in the Paleozoic section, maturation of hydrocarbon is a likely source for the joint-driving pore fluid. This maturation took place after the growth of major structures by thrust emplacement in the Valley and Ridge.