The Appalachian-Ouachita rifted margin of southeastern North America WILLIAM A. THOMAS* Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 ABSTRACT component of extension propagated north- rocks of Early and Middle Cambrian age along eastward to form the intracratonic fault the Southern Oklahoma fault system are over- Promontories and embayments along the systems northeast of the transform fault, but stepped by post-rift strata of Late Cambrian age late Precambrian-early Paleozoic Appala- most of the extension of the Ouachita rift was (Ham and others, 1964). The purposes of this chian-Ouachita continental margin of south- transformed along the Alabama-Oklahoma article are to synthesize available data into an eastern North America are framed by a transform fault to the Mid-Iapetus Ridge interpretation of the mechanisms controlling the northeast-striking rift system offset by outboard from the Blue Ridge passive shape of the rifted margin and to consider the northwest-striking transform faults. Inboard margin. implications of differences in age of rifting. from the continental margin, basement fault INTRODUCTION systems have two sets of orientation; one is RIFT-RELATED ROCKS AND northeast parallel with rift segments, and the Late Precambrian-early Paleozoic rifting and STRUCTURES other is northwest parallel with transform opening of the Iapetus (proto-Atlantic) Ocean faults. produced a North American continental margin Blue Ridge Late Precambrian clastic and volcanic syn- along which the late Paleozoic Appalachian- rift rocks overlie Precambrian basement Ouachita orogenic belt subsequently formed General Setting. The Blue Ridge is an elon- rocks along the Appalachian Blue Ridge. (Figs. 1, 2). Several interpretations have con- gate external basement massif (Fig. 1) along Lower Cambrian sandstone at the base of a verged on the conclusion that a zigzag trace of which late Precambrian syn-rift sedimentary and transgressive passive-margin succession over- the Appalachian-Ouachita rifted margin out- volcanic rocks, as well as older basement rocks, steps the rift-fill successions and basement lines large-scale promontories and embayments have been translated and deformed by younger rocks, defining the time of transition from an in the edge of North American continental crust Appalachian compressional structures, espe- active rift to a passive margin along the Blue (for example, Hoffman and others, 1974; Cebull cially large-scale Alleghanian (late Paleozoic) Ridge. Locally thick Early Late Cambrian and others, 1976; Rankin, 1976; Thomas, 1976, thrust faults. Westward-directed thrust faults of and older sedimentary rocks fill downthrown 1977, 1985a; Lowe, 1985); however, these in- large displacement characterize the southern blocks of the intracratonic Mississippi Val- terpretations differ in detail and in mechanisms part of the Blue Ridge. Toward the northeast ley-Rough Creek-Rome graben system and of rifting. Among the more significant differ- along strike, the surface structure is a northeast- Birmingham basement fault system. These ences, each of the large-scale embayments in the plunging anticlinorium above a blind detach- basement fault systems, which indicate north- continental margin is interpreted (1) as framed ment. Although rift-related rocks are in several west-southeast extension like the Blue Ridge by an intersection of the rift with a transform separate thrust sheets, along-strike distribution is rift, are overstepped by Upper Cambrian fault (Thomas, 1976, 1977) or (2) as formed at defined by mapping, and across-strike distribu- strata. The northwest-striking Southern Ok- the intersection of two "successful" arms of a tion can be inferred from restored cross sections lahoma fault system is interpreted to be a three-armed radial rift (rift-rift-rift triple junction) (for example, see Rast and Kohles, 1986). transform fault that propagated into the con- (Burke and Dewey, 1973; Hoffman and others, The tectonic framework of accumulation of tinent from the Ouachita rift. Early and Mid- 1974; Rankin, 1976). The trace and nature of late Precambrian sedimentary and volcanic dle Cambrian rift-related igneous rocks along intracratonic fault systems that extend from the rocks along the Blue Ridge is generally inter- the fault system and adjacent Precambrian Appalachian-Ouachita orogen into the craton preted in the context of fault-bounded basins basement are overstepped by Upper Cam- ("aulacogens" as defined by N. S. Shatski; see along an Atlantic-type rifted continental margin brian sandstone. discussion in Hoffman and others, 1974) are (for example, Hatcher, 1972, 1978; Rankin, The differences in age of rift-related rocks critical to discrimination between these alterna- 1975, 1976; Thomas, 1976, 1977; Wehr and suggest a spreading-center shift at the begin- tives, because syn-rift intracratonic fault systems Glover, 1985; Rast and Kohles, 1986; Schwab, ning of the Cambrian Period from the Blue must be (1) intracratonic projections of either 1986; Simpson and Eriksson, 1989). In the Ridge rift to the Ouachita rift southwest of transform faults or rift segments or (2) the failed northwestern part of the Blue Ridge, rift-related the Alabama-Oklahoma transform fault. arms of three-armed radial rifts. Components of rocks are in laterally discontinuous and variable From Early to Early Late Cambrian, a small the Appalachian-Ouachita rift are diachronous. accumulations that overlie Precambrian (-1.0 For example, rift-related rocks in the Appala- Ga and older) crystalline basement rocks and chian Blue Ridge are overstepped by post-rift are overlain by post-rift strata in the Lower •Present address: Department of Geological Sci- strata of Early Cambrian age (Simpson and ences, University of Kentucky, Lexington, Kentucky Cambrian Chilhowee Group (Fig. 3). Along the 40506. Eriksson, 1989), whereas rift-related igneous southeastern side of the Blue Ridge, rift-related Geological Society of America Bulletin, v. 103, p. 415-431, 6 figs., 1 table, March 1991. 415 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/3/415/3381149/i0016-7606-103-3-415.pdf by guest on 26 September 2021 J- y & t ^r northeastern limit \PENNy of Catoctin outcrop ^ along Blue Ridge southwestern limit of ^ Swift Run-Catoctin along SJ y ^ ,N/ northwest limb of Blue Ridge J\ //7s of EXPLANATION —v— cratonward limit of Appalachian-Ouachita detachment —»— thrust fault anticline - M> cratonward limit of Appalachian accreted terranes intracratonic basement fault margin of Gulf and Atlantic Coastal Plains outline of Altamaha magnetic anomaly Marathon outcrop Figure 1. Outline map of Appalachian-Ouachita orogenic belt and intracratonic fault systems. Locations of rift-related rocks are shown in present structural position. Map and locations of structures and rocks compiled from references cited in text. End points of cross sections of Figures 3,4, and 5 indicated by letters. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/3/415/3381149/i0016-7606-103-3-415.pdf by guest on 26 September 2021 EXPLANATION rifted margin of continental crust O— • transform fault Intracratonlc basement fault crustal-scale southeast-dipping seismic reflectors palinspastically restored width of passive-margin shelf fades OKLA foc, %Q abrupt margin of \ i continental crust ARK. °°c, (PASSCAL data) cK ... % V TEXAS ST^S. \ %/> ^ MARATHON %>CPROMONTORY - SCALE EMBAYMENT^ ll 0 100 km Figure 2. Outline map of interpreted late Precambrian-eariy Paleozoic continental margin as bounded by rift segments and transform faults. Map includes locations of observations that provide control for the reconstruction of the continental margin and intracratonic fault systems (compiled from references cited in text). Intersections between rift segments and transform faults are drawn orthogonally as a simplifying generalization. End points of cross sections of Figures 3,4, and 5 indicated by letters. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/3/415/3381149/i0016-7606-103-3-415.pdf by guest on 26 September 2021 A A' ROME TROUGH BLUE RIDGE RIFT # Knox # Knox • Mount Simon # Elbrook # Rome Base of transgressive Sauk sequence: # Shady Late Cambrian # Chilhowee EXPLANATION Base of transgressive Sauk sequence: DOMINANT ROCK TYPES Early Cambrian OF LITHQSTRATIGRAPHIC UNITS • siliciclastic rocks # Grove # carbonate rocks # Frederick v volcanic rocks Araby LOCATIONS OF KEY STRUCTURES present leading edge • palinspastic leading edge Rome trough sedimentary 10 km -, Early Late Cambrian and older SCALE (3.2 km) •v Mechum River (0.7 km) • Fauquier Goochland terrane • Blue Ridge 50 km B' ROME TROUGH BLUE RIDGE RIFT • Knox • Conasauga • Rome • Shady • Chilhowee Shady (Lower Cambrian) shelf edge Rome trough sedimentary Early Late Cambrian and older (1.9 km) •v Grandfather Mountain Ocoee (9 km) (12 km) Kings Mountain belt Blue Ridge I C Great Smoky Mountains Blue Ridge Corbin-Salem Church basement massif northern end of Catoctin outcrop DATUM: TOP OF CHILHOWEE Chilhowee Precambrian basement • Ocoee v Catoctin v Mount Rogers c • Swift Run srr,b. ^ (12 km) (3 km) ar> b, (1 km) basalt in Unicoi (lower part of Chilhowee) Figure 3. Palinspastic cross sections of the Blue Ridge rift and Rome trough. Cross sections A-A' and B-B' are perpendicular to strike; cross section C-C' is parallel with strike of the present Blue Ridge structures. Names of lithostratigraphic units in rift-related successions (maximum thickness in parentheses)