q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; May 2004; v. 32; no. 5; p. 413–416; doi: 10.1130/G20216.1; 5 figures. 413

Tectonic implications drawn from differences in the surfacemorphology on two joint sets in the Appalachian Valley

and Ridge, VirginiaTerry Engelder

Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA

ABSTRACTSurface morphology distinguishes two joint sets in the Virginia–West Virginia Appa-

lachians and points to a difference in rupture velocity between the two sets. The earlierjoint set, J1, propagated prior to or during fault-related folding and was subsequentlytilted as folds grew to have steeper limb dips. J1 is characterized by a rougher surfacedisplaying a symmetrical plumose pattern created during the propagation of a simplerupture front. In ceramics, such morphology is the product of a faster rupture underrelatively high stress intensity (KI . KIc, where KIc indicates critical stress intensity). Thisrelatively fast joint growth is consistent with an effective tensile stress sustained by obliqueplate convergence at the onset of the Alleghanian orogeny. The later joint set, J2, propa-gated normal to the Allegheny front in a subvertical orientation independent of local beddip. 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 (KI , KIc).This slow joint growth is consistent with the slow generation of effective tensile stressarising from hydrocarbon maturation postdating fold growth during the waning stages ofthe Alleghanian orogeny.

Keywords: joints, Alleghanian orogeny, subcritical crack propagation, rupture velocity.

INTRODUCTIONWhen lithospheric plates converge during

continental-continental collision, a number offactors control the orientation of the stressfield at various locations along the active mar-gin, including the shape of the plate marginsand the direction of convergence between thetwo plates. The stress field can change withtime as a function of evolving margin geom-etry as convergence moves toward final su-ture. One record of an evolving stress field isthe overprinting of joint sets (e.g., Engelderand Geiser, 1980; Gray and Mitra, 1993; Zhaoand Jacobi, 1997; Younes and Engelder,1999). However, a signature indicating theslowing of convergence and concomitant de-formation rate toward final suture has yet tobe reported. This paper describes an evolutionof joint-surface morphology that reflects aswitch in joint-driving mechanism leading toa decrease in rupture velocity toward the finalstage of Alleghanian deformation in the Ap-palachian foreland.

The Allegheny structural front from north-ern Virginia through southern Pennsylvania isstraight (0298 6 18) for .300 km (Fig. 1).This structural front provides a boundaryagainst which a blind duplex of Cambrian–Ordovician carbonates was thrust during the

Alleghanian orogeny (Perry, 1978). A linearsegment of a mountain belt presents an op-portunity to test whether and when cross-foldjoints can develop in a roof sequence withoutthe benefit of regional stretching associatedwith oroclinal bends in mountain belts (e.g.,Macedo and Marshak, 1999).

REGIONAL JOINT SETSJoint development in the roof sequence of

the Appalachian Valley and Ridge between thelatitudes of 388 and 388459N includes twowell-developed joint sets (Fig. 2). The region-al distribution of these two joint sets was as-sessed by measuring the orientations of be-tween 5 and 50 of the most prominent jointsin each of 76 outcrops along 3 traverses of themountain belt. When appearing together in thesame bed, the two joint sets usually crosscut,meaning that joints of the earlier set wereclosed when the joints of the later set propa-gated (Fig. 2A). Vein infilling, a property ofthe earlier of two joint sets when present,coats some J1 surfaces (cf. Evans and Battles,1999). The earlier joint set, J1, has a regionaltrend of 0828, thus transecting folds at ;508

(Fig. 3B). The later set, J2, has a regional trendat 3138, and this is the cross-fold orientation

approximately orthogonal to the Alleghenyfront.

The earlier set, J1, propagated subnormal tobedding but was tilted to a shallower dip asbeds of the roof sequence were folded by theprogressive thrusting in the blind duplex be-low (Fig. 2B). The later set, J2, propagatedand remained subvertical, regardless of theorientation of bedding even when appearingon the nose of folds that have been tilted todip along strike of the belt (Fig. 2B). Neitherset propagated strictly normal to bedding, asindicated by the position of the mean pole todata taken from each set at each outcrop (Figs.3B, 3D). Yet, the plunge of the mean pole tooutcrop-mean poles for all cross-fold (J2)joints across the region is exactly 08 (Fig. 3D).The plunge of the mean pole to outcrop-meanpoles for early (J1) joints across the region is68S, a consequence of the asymmetry of fault-related folding. Even when beds are restoredto horizontal, first about plunge and thenabout dip, many J1 joints dip , 808, suggest-ing that this joint set was not predestined tooccupy a position normal to bedding, as is socommon for joint propagation during regionalextension in gently folded terrain.

Joint sets J1 and J2 have a distinguishingsurface morphology (Fig. 4). While joints of

414 GEOLOGY, May 2004

Figure 1. Geologic map of central Appalachian Mountains of Virginia and West Virginia.

both sets are planar, J1 joints tend to displaya rougher, more pronounced morphology,whereas J2 joints are much smoother (Fig.2C). Joints in siltstone and fine-grained sand-stone beds of at least four formations of theroof sequence (i.e., Oswego, Juniata, Brallier,and Chemung Formations) carry these dis-tinctly different morphologies. J1 joints rup-ture through a bed and leave plumes emergingup and down from one major plume axisroughly halfway between bedding surfaces(Figs. 2C and 4). Such ruptures are well or-ganized with one rupture front (Savalli andEngelder, 2004). J2 joints are more likely torupture into several lobes with minor plumeaxes curving into almost any position relativeto bedding (Figs. 2C and 4). These latter rup-tures are poorly organized and split into mul-tiple rupture fronts. Other J2 joints high in theroof sequence display the classic cyclic fanpattern of natural hydraulic fractures, wherearrest after one cycle is marked by a suddendecrease in surface roughness (Fig. 2D). Fi-

nally, J2 joints in black shale propagate up tobut not through concretions, whereas J1 jointsare found to cut concretions (McConaughyand Engelder, 1999).

The distribution of the two joint sets in theroof sequence is noteworthy. With the excep-tion of the Juniata, J1 joints are most commonat the top of the roof sequence, where theywere found in 28 of 29 outcrops of the Che-mung and Hampshire Formations (Fig. 5). J2

joints are common deeper in the roof se-quence, where they are best developed in theblack shale of the Harrell Formation and sand-stone beds of the Tuscarora and JuniataFormations.

DISCUSSIONJoint Morphology

Experiments on ceramics show that rough-ness of fracture surfaces increases with in-creasing dynamic stress intensity (Kd . KIc

where KIc is the critical stress intensity) andrupture velocity (v) (Hull, 1999). Because sur-

face roughness varies in the same manner dur-ing stable joint propagation, Savalli and En-gelder (2004) apply these results to ananalysis of surface morphology in the subcrit-ical crack-propagation regime (Fig. 4). Byanalogy, joints with well-organized rupturefronts and rougher surfaces are characteristicof the upper portion of subcritical region I andinto region II. Joints of the J1 set in the roofsequence of Virginia display a surface rough-ness consistent with propagation under a driv-ing stress able to sustain a stress intensity (KI)where KI , KIc.

Joint driving stress (Ds) is an effective ten-sile stress that may arise when pore pressuresuperimposes on compressive stress fromburial, s3 (5 Sh), that is partially relieved bya combination of regional, convergent-normalextension or local stretching associated withfolding. Favorable conditions for generatingan extension-related Ds are most likely atshallow depths, where Sh is low. The predom-inance of J1 at the top of the roof sequence isconsistent with a scenario where rapid infiltra-tion of hydrostatic pore pressure will providea loading mechanism that can sustain ruptureat v 5 1024 to 1021 m/s, as predicted for rup-ture in sandstone (Segall, 1984).

Lobate, disorganized rupture fronts onsmoother surfaces are consistent for propaga-tion in subcritical regime I, where v , 1026

m/s (Savalli and Engelder, 2004). In region I,small DKI can lead to a large Dv and, hence,chaotic advance of individual rupture lodes,giving the pattern observed on J2 joints (Fig.4). A scenario for slow rupture in verticalplanes occurs during the waning period oforogenic deformation after folding of the roofsequence is nearly complete.

By the Late Carboniferous, burial wouldhave carried the Harrell Formation, a blackshale, to thermal maturation, where high fluidpressure evolved slowly. J2 joints containcharacteristics of hydraulic fractures propa-gating under a fluid-drive mechanism (i.e.,McConaughy and Engelder, 1999). The highdensity of J2 joints in the Harrell Formationreinforces the fluid drive scenario. Maturationof hydrocarbons led to fluid-driven jointing inblack shale elsewhere in the Appalachian ba-sin (Lash et al., 2004). Higher fluid pressureswould have been more common in the deeperparts of the sedimentary pile below a seal rocklike the shale of the Brallier Formation(Fig. 5).

Regional TectonicsDating the propagation of the regional

joints in the roof sequence is possible becauseof their structural position. Generally, prefold-

GEOLOGY, May 2004 415

Figure 2. J1 and J2 joint sets. A: Chemung Formation. B: Brallier Formation. C: Juniata Formation displaying rough J1 surfaces andsmooth J2 surfaces; inset shows propagating morphology of one J2 joint. D: Chemung Formation with arrows pointing to three arrestlines of natural hydraulic fracture on J2 surface.

Figure 3. Dip vs. frequency histograms for bedding and two joint sets (J1, J2) and lower-hemisphere stereonet projections of poles to meanorientations for joints at each outcrop. Rose diagrams are based on rotated data for J1 and unrotated data for J2. Number of outcrops inwhich each joint set appears is indicated in parenthesis.

ing joints are orthogonal to bedding, whereasJ1 is not strictly orthogonal to bedding, evenwhen sampled in the gently folded rocks ofthe Appalachian plateau. This finding suggeststhat folding was under way at the time J1

propagated.The strike of J1 is remarkably uniform

(;0828) throughout the roof sequence despitethe evidence that it did not propagate normal

to bedding in many instances. The uniformlyoriented strike for J1 indicates a well-organized Sh in the roof sequence that is con-sistent with an oblique convergence betweenAfrica and North America during early stagesof the Alleghanian orogeny. The Alleghanianorogeny is characterized by major dextralfaulting eastward from the Brevard fault zoneand in southeastern New England (Hatcher et

al., 1989). Such dextral strike-slip faulting inthe southern Appalachians is consistent witha regional stress field at Sh 5 0828.

Although joints of the later J2 set are oftentilted several degrees relative to vertical, theirpreferred regional attitude is vertical (Fig.3D). Such predominately vertical joints indi-cate that major fold growth was waning by thetime of J2 propagation. By this time oblique

416 GEOLOGY, May 2004

Figure 4. Hypothetical joint-propagation-velocity curve showing three subcritical re-gimes (adapted from Atkinson and Meredith,1987). Insets show rough surface morphol-ogy for J1 joints where stress intensity, KI,approaches critical stress intensity, KIc, andsmooth morphology for J2, where KI K KIc.

Figure 5. Stratigraphy of roof sequence list-ing number of outcrops sampled (in paren-theses) and percentage of those outcropswith J1 and J2 joints.

convergence had ceased to control the region-al stress field, which was then homogeneous,with s3 (5 Sh) nearly coaxial with a straightAllegheny front (0338 versus 0298).

In conclusion, the orientation and smoothmorphology on J2 joints indicate the cessationof oblique convergence during the waningphase of the Alleghanian orogeny. In the ab-sence of regional stretching around an oro-clinal bend, a pore-pressure–based drivingmechanism allows late joint developmentalong the straight 300 km segment of theVirginia–West Virginia Valley and Ridge.

ACKNOWLEDGMENTSThis work was supported by the Pennsylvania

State University Seal Evaluation Consortium. Ithank Atilla Aydin and Bob Jacobi for constructivereviews.

REFERENCES CITEDAtkinson, B.K., and Meredith, P.G., 1987, Experi-

mental fracture mechanics data for rocks andminerals, in Atkinson, B.K., ed., Fracture me-chanics of rock: Orlando, Academic Press,p. 111–166.

Engelder, T., and Geiser, P.A., 1980, On the use ofregional joint sets as trajectories of paleostressfields during the development of the Appala-chian plateau, New York: Journal of Geo-physical Research, v. 85, p. 6319–6341.

Evans, M.A., and Battles, D.A., 1999, Fluid inclu-sion and stable isotope analyses of veins fromthe central Appalachian Valley and Ridgeprovince: Implications for regional synorogen-ic hydrologic structure and fluid migration:Geological Society of America Bulletin,v. 111, p. 1441–1460.

Gray, M.B., and Mitra, G., 1993, Migration of de-formation fronts during progressive deforma-tion: Evidence from detailed structural studiesin the Pennsylvania anthracite region, U.S.A.:

Journal of Structural Geology, v. 15,p. 435–450.

Hatcher, R.D., Thomas, W.A., Geiser, P.A., Snoke,A.W., Mosher, S., and Wiltschko, D.V., 1989,Alleghanian orogen, in Hatcher, R.D., et al.,eds., The Appalachian-Ouachita orogen in theUnited States: Boulder, Colorado, GeologicalSociety of America, Geology of North Amer-ica, v. F-2, p. 233–318.

Hull, D., 1999, Fractography observing, measuringand interpreting fracture surface topography:Cambridge, Cambridge University Press,366 p.

Lash, G.G., Loewy, S., and Engelder, T., 2004, Pref-erential jointing of Upper Devonian blackshale, Appalachian plateau, USA: Sorting outthe role of organic carbon content, in Cos-grove, J., and Engelder, T., eds., The initiation,propagation, and arrest of joints and otherfractures: Geological Society [London] Spe-cial Publication (in press).

Macedo, J., and Marshak, S., 1999, Controls on thegeometry of fold-thrust belt salients: Geolog-ical Society of America Bulletin, v. 111,p. 1808–1822.

McConaughy, D.T., and Engelder, T., 1999, Joint in-teraction with embedded concretions: Jointloading configurations inferred from propaga-tion paths: Journal of Structural Geology,v. 21, p. 1049–1055.

Perry, W.J., 1978, Sequential deformation in thecentral Appalachians: American Journal ofScience, v. 278, p. 518–542.

Savalli, L., and Engelder, T., 2004, Mechanismscontrolling the shape of the rupture front asjoints cut through layered clastic sediments:Geological Society of America Bulletin (inpress).

Segall, 1984, Rate-dependent extensional deforma-tion resulting from crack-growth in rock: Jour-nal of Geophysical Research, v. 89,p. 4185–4195.

Younes, A., and Engelder, T., 1999, Fringe cracks:Key structures for the interpretation of pro-gressive Alleghanian deformation of the Ap-palachian plateau: Geological Society ofAmerica Bulletin, v. 111, p. 219–239.

Zhao, M., and Jacobi, R.D., 1997, Formation of re-gional cross-fold joints in the northern Appa-lachian plateau: Journal of Structural Geology,v. 19, p. 817–834.

Manuscript received 22 September 2003Revised manuscript received 16 January 2004Manuscript accepted 21 January 2004

Printed in USA

520 GEOLOGY, June 2004

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

Terry Engelder

Geology, v. 32, p. 413–416 (May 2004)

An error was found in the following paragraph in the section titled ‘‘Regional Tectonics.’’ In both instances, the subscript to S should havebeen a capital H. The correct version appears here:

The strike of J1 is remarkably uniform (ø 0828) throughout the roof sequence despite the evidence that it did not propagate normal to beddingin many instances. The uniformly oriented strike for J1 indicates a well-organized SH in the roof sequence that is consistent with an obliqueconvergence between Africa and North America during early stages of the Alleghanian orogeny. In fact, the Alleghanian orogeny is characterizedby major dextral faulting eastward from the Brevard fault zone and in southeastern New England (Hatcher et al., 1989). Such dextral strike-slipfaulting in the southern Appalachians is consistent with a regional stress field at SH 5 0828.