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Early Jointing in Coal and Black Shale: Evidence for an Appalachian-Wide Stress field As a Prelude to the Alleghanian Orogeny

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Early Jointing in Coal and Black Shale: Evidence for an Appalachian-Wide Stress field As a Prelude to the Alleghanian Orogeny Early jointing in coal and black shale: Evidence for an Appalachian-wide stress field as a prelude to the Alleghanian orogeny Terry Engelder* ⎤ ⎥ Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Amy Whitaker† ⎦ ABSTRACT means of stratigraphically controlled decolle- Early ENE-striking joints (present coordinates) within both Pennsylvanian coal and ment tectonics (Gates et al., 1988; Wise, Devonian black shale of the Central and Southern Appalachians reflect an approximately 2004). rectilinear stress field with a dimension Ͼ1500 km. This Appalachian-wide stress field (AWSF) dates from the time of joint propagation, when both the coal and shale were FRACTURE EVIDENCE FOR THE buried to the oil window during the 10–15 m.y. period straddling the Pennsylvanian- AWSF Permian boundary. The AWSF was generated during the final assembly of Pangea as a Along the Appalachian Mountains an ENE consequence of plate-boundary tractions arising from late-stage oblique convergence, joint set is the first to propagate in many out- where maximum horizontal stress, SH, of the AWSF was parallel to the direction of closure crops of Devonian through Pennsylvanian between Gondwana and Laurentia. After closure, the AWSF persisted during dextral slip rocks (e.g., Nickelsen and Hough, 1967; Nick- of peri-Gondwanan microcontinents, when SH appears to have crosscut plate-scale trans- elsen, 1979; Kulander and Dean, 1993; Pashin current faults at ϳ30؇. Following Ͼ10 m.y. of dextral slip during tightening of Gondwana and Hinkle, 1997). This early joint set strikes against Laurentia, the AWSF was disrupted by local stress fields associated with thrusting parallel to the orientation of the maximum on master basement decollements to produce the local orocline-shaped Alleghanian map horizontal stress, SH, in a stress field that was pattern seen today. a prelude to the Alleghanian orogeny. In total, these joint sets appear as one megaset record- Keywords: joints, coal cleat, Alleghanian orogeny, lithosphere stress field, fault strength, trans- ing a rectilinear stress field extending for pressional tectonics. Ͼ1500 km across three promontories separat- ed by oroclinal embayments of the Central INTRODUCTION mountain belt and its predecessor, the post- and Southern Appalachians (Fig. 1). This rec- The map pattern of the Appalachian Valley Rodinian margin of Laurentia, it is an tilinear stress field is the AWSF. and Ridge gives the distinct impression that Appalachian-wide stress field (AWSF). The the Alleghanian orogeny was a collision of AWSF, a prelude to the Alleghanian orogeny, New Data Gondwana against serrations arising from Late was disrupted when dextral transpression re- Joints in the orientation of the AWSF are Proterozoic rift-related offsets on the Lauren- turned the Appalachian foreland to the shape found in Late Mississippian through Early tian margin. Major teeth on the serrated mar- of the Rodinian margin of Laurentia, by Pennsylvanian sandstones near the Virginia gin include the Alabama, Virginia, and New York promontories (i.e., Rankin, 1976). How- ever, late Paleozoic strike-slip tectonics (e.g., Hylas fault zone [Gates and Glover, 1989], Modoc fault zone [Snoke et al., 1980], and the Brevard fault zone [Hatcher, 2001]) and coe- val basins (e.g., the Narragansett of Rhode Is- land; Mosher, 1983) reflect dextral transcur- rent plate motion during the amassing of Pangea (Fig. 1). Large segments of these transcurrent fault systems are more or less straight, indicating that tectonic processes such as the welding of the Taconic and Potom- ic deformed wedges to irregular basement (i.e., Faill, 1997) acted to smooth the serrated margin of post-Rodinian Laurentia. The purpose of this paper is to describe ev- idence for a more or less rectilinear stress field that was transmitted into the smoothed margin of the Laurentian foreland for a period ex- ceeding 10 m.y. during the assembly of Pan- gea. Because this late Paleozoic stress field was unaffected by the obvious serrations (i.e., promontories and embayments) of the present *E-mail: [email protected]. †Present address: Chevron Energy Technology Figure 1. Distribution of ENE joint sets along Appalachian Mountains. References to insets Company, 1500 Louisiana Street, Houston, Texas and their map locations (dashed rectangles) are in text. Pennsylvanian isopachs (m) are 77002, USA. dashed within insets 7 and 8 (after Colton, 1970). ᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; July 2006; v. 34; no. 7; p. 581–584; doi: 10.1130/G22367.1; 3 figures. 581 Figure 2. Time line for coal deposition and propagation of Appalachian cleat and joints. Ages are consistent with International Commission on Stratigraphy (www.stratigraphy.org; stage names are North American). AWSF—Appalachian-wide stress field. promontory, the transition between the Central 1997). A coal rank of high volatile A bitu- The weakly developed ENE face cleat in bi- and Southern Appalachians (inset 5 in Fig. 1). minous is generated by pressure-temperature tuminous coal of Pennsylvania is found only These ENE joints trace continuously from flat- conditions equivalent to burial just below the near the Allegheny Front. Weakly developed lying beds of the Appalachian Plateau into the top of the oil window. Depending on heat flow ENE face cleat along the Allegheny Front is folded rocks of the Glen Lyn syncline, the this may occur at a depth of 2–3 km (Stach et also found in Kentucky and West Virginia, but syncline marking the Allegheny Front in Vir- al., 1982). Propagation of face cleat in the Ap- only in Morrowan coal (Long, 1979; Kulander ginia. Within several hundred meters of the palachians is dated using burial curves (e.g., and Dean, 1993). Glen Lyn syncline on the plateau side of the Evans, 1995; Pitman et al., 2003). A regional When most of the Pennsylvania Plateau first front, these joints have slip fibers indicating stress field during coalification controls the reached subbituminous rank as late as ca. 290 reactivation during folding. Furthermore, orientation of face cleat (Laubach et al., Ma (i.e., the Evans [1995] burial curve) an when tilted by folding, ENE joints strike 1998). orocline-shaped Alleghanian stress field (i.e., obliquely to the fold axes (mode ϭ 080Њ for ϭ A strong ENE face cleat first developed SH NNW) had supplanted the rectilinear the preferred strike of joints at 47 outcrops). when ca. 315 Ma coal of the Black Warrior AWSF. Virgilian (304–299 Ma) coals of Penn- The joints strike obliquely to folds and are Basin (inset 6 in Fig. 1) reached the oil win- sylvania record this latter stress field (Nick- present in flat-lying rocks, ruling out outer arc dow ca. 300 Ma (Pitman et al., 2003). A weak elsen and Hough, 1967). In West Virginia and stretching as a mechanism controlling propa- face cleat developed when these beds meta- Kentucky, both Desmoinesian and Virgilian gation. As such, because they are present in morphosed from lignite to subbituminous coal coals farther into the foreland carry face cleat its folded limb, these joints predate the Al- ϳ4 m.y. earlier (Fig. 2). As shallower Mor- in the cross-fold Alleghanian orientation (Fig. leghanian Glen Lyn syncline. rowan (315–311 Ma) coals of the Black War- 1). Burial to the oil window in the Central rior Basin were metamorphosed within the oil Appalachians was slower and later than in the DISCUSSION window, they developed a strong face cleat of Black Warrior Basin (e.g., Colton, 1970). The AWSF is dated using the time of joint the same orientation, which means that the Thus the initiation of ENE face cleat was de- propagation. In this regard, joints in unfolded Early Pennsylvanian sandstone (inset 5 in Fig. AWSF in the Southern Appalachians persisted layed and in the outer foreland cleat devel- 1) correlate in time with the transition from for several million years starting ca. 304 Ma. opment was not initiated until after the onset late-stage Gondwana-Laurentia convergence A pervasive ENE face cleat throughout the of a NNW SH (Fig. 2). and dextral transpression along the smoothed- Black Warrior Basin indicates that coalifica- The earliest joint set in Devonian black margin of Laurentia to large-scale folding. tion took place entirely under the influence of shale (inset 3 in Fig. 1) of the Appalachian However, the propagation of face cleat (i.e., the AWSF. Plateau, New York, also trends ENE (Engen- joints in coal) gives the tightest constraint. Face cleat in two different geological prov- der et al., 2001). The presence of these ENE inces in Pennsylvania also record an ENE SH: joints only within or just above black shale Dating the AWSF Using Organic-Rich the anthracite district (inset 9 in Fig. 1) of the suggests that they were driven as a conse- Rocks eastern Pennsylvania Valley and Ridge (Nick- quence of maturation of hydrocarbons after A widely spaced (i.e., weak) face cleat ini- elsen, 1979) and the bituminous district (inset burial to the oil window (Lash et al., 2004). tiates in lignite or subbituminous coal during 10) of the western Pennsylvania Plateau Although black shale approached the top of shrinkage of the coal structure by devolatiza- (Nickelsen and Hough, 1967). Coal in both the oil window by Late Mississippian time, tion (Ting, 1977). Closely spaced (i.e., strong) districts is Desmoinesian (307–305 Ma) (Fig. significant maturation was delayed until ca. cleat develops later when coal reaches a rank 2). Like examples from the Black Warrior Ba- 300 Ma or later for two reasons. First, Mor- of high volatile A bituminous with a vitrinite sin, the anthracite coal reached the oil window rowan-Atokan (317–307 Ma) erosion removed reflectance (Rmo) of 0.8 (Pashin and Hinkle, early enough to record the AWSF throughout.
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