The Iapetan rifted margin of southern Laurentia William A. Thomas* Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky, 40506-0053, USA ABSTRACT Geophysical modeling supports a steep conti- and of Atlantic-Gulf rift-stage extension and nental margin along the Alabama-Oklahoma passive-margin subsidence. The objective of The Iapetan rifted margin of southern transform, and a similar structure can be this article is to summarize the relevant data for Laurentia includes the northeast-striking inferred for the Texas transform. The Blue interpretation of the trace, structure, and age of Blue Ridge, Ouachita, and Marathon rifts, Ridge rift north of the Alabama promontory the Iapetan rifted margin of southern Laurentia, which are offset by the northwest-striking is dated by synrift volcanic rocks as young using the large-scale elements of the rifted mar- Alabama-Oklahoma and Texas transform as 564 Ma, and passive-margin transgres- gin as an outline. faults, framing the Alabama and Texas sion beginning in earliest Cambrian is docu- The large-scale framework of the Iapetan promontories and the Ouachita and Mara- mented along the Alabama promontory and rifted margin of Laurentia is interpreted in the thon embayments of the continental margin. farther north. The age of the Ouachita rift context of northeast-trending rift segments Interpretations of the original trace, struc- is documented by the 530–539 Ma synrift offset by northwest-trending transform faults tural style, and age of the rifted margin rest volcanics of the transform-parallel intra- (Fig. 1) (e.g., Thomas, 1976, 1977, 1991, on identifi cation of synrift rocks and struc- cratonic Southern Oklahoma fault system, 2006). In southern Laurentia, intersections of tures, as well as continental-shelf and off- by Early Cambrian synrift sediment along two large-scale transform faults (Alabama- shelf sedimentary deposits on the passive the conjugate rift margin in the Argentine Oklahoma and Texas transforms) with rift seg- margin. Both late Paleozoic Ouachita- Precordillera, and by late synrift graben-fi ll ments (Blue Ridge, Ouachita, Marathon) outline Appalachian allochthons and post-orogenic of Early to early Late Cambrian age in the two promontories (Alabama and Texas) and two Atlantic-Gulf passive-margin deposits cover rift-parallel intracratonic Mississippi Valley em bayments (Ouachita and Marathon) of the the Iapetan rift margin, necessitating the and Birmingham graben systems, as well as rifted margin (Fig. 1). Inboard from the rifted use of data from deep wells and geophysi- by subsidence history of the passive margin margin, late synrift intracratonic fault systems cal surveys along with geologic maps of the on the Texas promontory. The diachroniety include rift-parallel extensional faults (Missis- exposed Ouachita-Appalachian thrust belts of rifting refl ects an inboard shift from the sippi Valley and Birmingham graben systems) to characterize the synrift and post-rift rocks Blue Ridge rift to the Ouachita rift along and transform-parallel faults (Southern Okla- and structures. The continental margin and the Alabama-Oklahoma transform and rift- homa fault system). In addition, the rift and passive-margin shelf strata are primarily ing of the Argentine Precordillera from the transform margins of the Ouachita embayment in the footwall of the Ouachita allochthon; Ouachita embayment. are conjugate to the Iapetan rift margin of the however, some Ouachita thrust faults dis- Argentine Precordillera microcontinent (Fig. 2) placed shelf-margin basement and cover. INTRODUCTION (Thomas and Astini, 1996), and the rift history Appalachian thrust faults imbricate synrift of the Precordillera is complementary to that of fi ll of the intracratonic Birmingham graben The late Precambrian–Cambrian Iapetan rifted southern Laurentia (Thomas and Astini, 1999). and the passive-margin shelf. Palinspastic margin, as well as the subsequent Cambrian- restoration of thrust-belt structures uses bal- Ordovician passive margin, of southern Lau- CORNER OF ALABAMA anced cross sections to locate the original rentia is covered by late Paleozoic Ouachita- PROMONTORY trace of the Iapetan margin. Thickness and Appalachian allochthons (emplaced during the subsidence history of the passive-margin assembly of supercontinent Pangaea) and by In the northeast-striking, northwest-verging successions, as well as a general lack of pre- Mesozoic-Cenozoic synrift and passive-margin Appalachian thrust belt in Alabama and Geor- served synrift deposits, indicate an upper- strata of the Gulf Coastal Plain (deposited dur- gia, the décollement is near the base of the plate structure along the Blue Ridge rift ing opening of the Atlantic Ocean and Gulf of Paleozoic sedimentary succession above Pre- on the Alabama promontory and along the Mexico) (summary in Thomas, 2006). Because cambrian crystalline basement rocks, and a Ouachita rift on the Texas promontory. The of the younger tectonic and sedimentary cover, Cambrian-Ordovician passive-margin (Iapetan upper plate on the Texas promontory is con- interpretations of the geometry and tectonic ele- post-rift) succession with upward transition jugate to a lower-plate rift structure on the ments of the Iapetan margin are based on data from clastic to carbonate rocks is imbricated Argentine Precordillera. Although data are from deep wells and geophysical surveys. Reso- in the allochthon (Figs. 1, 3, and 4C) (Thomas limited, the evolution of the passive margin lution of the Iapetan margin in the subsurface and Bayona, 2005). Along the trailing (south- along the Marathon rift in the Marathon requires palinspastic reconstruction of the early eastern) edge of the sedimentary thrust belt, the embayment suggests a lower-plate structure. post-rift passive margin to remove the effects lower-greenschist Talladega slate belt includes of Ouachita-Appalachian orogenesis, includ- similar passive-margin stratigraphy (Tull et al., *[email protected]. ing subsidence of synorogenic foreland basins, 1988). In the Appalachian Piedmont, southeast Geosphere; February 2011; v. 7; no. 1; p. 97–120; doi:10.1130/GES00574.1; 12 fi gures. For permission to copy, contact [email protected] 97 © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/1/97/3342031/97.pdf by guest on 01 October 2021 Thomas 95°W Rough to the Pine Mountain internal basement massif, Creek Rome and a basement-rooted thrust fault beneath the Ozark 37°N massif apparently merges into the Appalachian dome décollement above the extensive shallow base- Nashville ment (Fig. 3) (McBride et al., 2005). Mississippi Arkoma Valley dome Southern Oklahoma B D Mylonite zones (Goat Rock and Bartletts Black Ferry fault zones) along the southeast side of the Bhm Bt Warrior BB A Pine Mountain internal basement massif mark ALABAMA-OKLAHOMA the leading edge of a relatively wide Suwannee- Ouachita PM 32°N Wiggins suture zone between Laurentian crust embayment C BLUE RIDGE Fort E GEORGI on the northwest and African crust and sedi- Worth A mentary cover of the Suwannee terrane on the Val Verde Llano Wa G SuwanneeAlabama terrane southeast (Figs. 1 and 3) (summary in Thomas, F uplift palinspastic site of OUACHITA promontory 2010). To the south beneath the Gulf Coastal Lu PRECORDILLERA DR Plain, the suture zone is imaged seismically as Texas MarathonTEXAS a wide band of southeast-dipping refl ectors that promontory embaymen extend down to the Moho (Nelson et al., 1985; 85°W McBride et al., 2005), suggesting that the suture MARATHON 0 200 400 600 km t zone consists of highly tectonized lithons inter- laced with mylonite zones. Crystallization ages APPALACHIAN-OUACHITA OROGEN IAPETAN RIFTED MARGIN of the most southerly exposed metamorphic leading edge of Appalachian rocks (Uchee belt, southeast of the exposed TRANSFORM and Ouachita thrust belts mylonite zones) indicate a peri-Gondwanan arc RIFT leading edge of Appalachian terrane, suggesting comparisons with the peri- accreted metamorphic terranes Gondwanan Carolinia terrane to the northeast along Appalachian Piedmont strike and with shear zones in Appalachian the Suwannee terrane across strike to the south intracratonic fault Piedmont metamorphic terranes (Fig. 1) (Steltenpohl et al., 2008). The footwall leading edge of Appalachian of the leading edge of the Suwannee-Wiggins GULF AND ATLANTIC COASTAL PLAINS external basement massifs suture zone forms the present limit of Lauren- tian crust at the corner of the Alabama promon- edge of Gulf and Atlantic possible limits of tory (Figs. 1 and 3). Coastal Plains Suwannee-Wiggins suture zone Palinspastic restoration (minimum resto- Figure 1. Outline map of palinspastically restored Iapetan rifted margin of southern Lau- ration, using line-length and area balancing) rentia, synrift intracratonic basement faults, and palinspastic site of Argentine Precordillera of thrust sheets in the sedimentary thrust belt terrane (modifi ed from Thomas, 1991, 2006); of leading edge of Ouachita and Appalachian places the trailing thrust sheets approximately thrust belts, which overlie and/or deform the Iapetan rifted margin; and of the edge of Gulf at the present location of the Pine Mountain and Atlantic Coastal Plains, which cover pre-Mesozoic rocks and structures. Map shows internal basement massif, showing that the the restored trace of the Blue Ridge rift along the Alabama promontory, where Laurentian early Paleozoic passive-margin carbonate-shelf crust has been truncated at the leading