Making the Southern Margin of Laurentia themed issue Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental margin Audrey D. Huerta1 and Dennis L. Harry2 1Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926-7418, USA 2Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523, USA ABSTRACT beneath the coastal plain and continental strength of the lithosphere by replacing strong shelf, are direct consequences of the prerift ultramafi c mantle with relatively weak felsic The tectonic evolution of the North Amer- structure of the margin. crust (Fig. 1) (Braun and Beaumont, 1987; ican Gulf of Mexico continental margin is Dunbar and Sawyer, 1989; Chery et al., 1990; characterized by two Wilson cycles, i.e., INTRODUCTION Krabbendam , 2001). repeated episodes of opening and closing The Gulf of Mexico continental margin is of ocean basins along the same structural The spatial association between continen- similar to the U.S. Atlantic margin in that the trend. This evolution includes (1) the Pre- tal breakup and preexisting orogens is often axis of Mesozoic continental breakup trended cambrian Grenville orogeny; (2) formation described within the context of a Wilson cycle, subparallel to the buried middle Paleozoic of a rift-transform margin during late Pre- wherein orogenic belts formed by continen- Ouachita fold-and-thrust belt (Pindell and cambrian opening of the Iapetus Ocean; tal collision during closure of ancient ocean Dewey, 1982; Salvador, 1991a; Thomas, 1976, (3) the late Paleozoic Ouachita orogeny dur- basins are reactivated during subsequent rifting 1991). However, extension on the central North ing assembly of Pangea; and (4) Mesozoic episodes (Wilson, 1966; Vauchez et al., 1997). American Gulf of Mexico margin is restricted rifting during opening of the Gulf of Mexico. A classic example is the U.S. Atlantic margin, to regions south of the Ouachita fold-and-thrust Unlike the Atlantic margins, where Wilson where opening of the North Atlantic Ocean belt and terminates abruptly on the southern cycles were fi rst recognized, breakup in the began with a continental rifting episode within (oceanward) fl ank of the orogen (Ewing, 1991). Gulf of Mexico did not initially focus within the late Paleozoic Appalachian-Caledonian oro- The Ouachita fold-and-thrust belt has under- the orogen, but was instead accommodated gen (Ziegler, 1989). The association between gone very little extensional deformation. Thus, within a diffuse region adjacent to the oro- the positions of continental breakup and older the two margins differ in that the Ouachita oro- gen. This variation in location of rifting is orogenic belts is usually attributed to weakening gen appears to have acted as a strong region a consequence of variations in the prerift of the lithosphere due to faulting in the brittle during continental rifting rather than a zone of architecture of the orogens. The Appala- upper crust and the presence of a crustal root. weakness like the Appalachian orogen. In the chian-Caledonian orogeny involved substan- The presence of the crustal root reduces the Gulf of Mexico, the weakest lithosphere must tial crustal shortening and formation of a thick crustal root. In contrast, the Ouachita orogeny resulted in minimal crustal short- A) B) C) ening and thickening. In addition, rather Yield Strength (MPa) Yield Strength (MPa) Temperature (°C) than a crustal root, the Ouachita orogen was 0 100 200 300 0 100 200 300 0 500 1000 1500 underlain by the lower plate of a relatively 0 pristine Paleozoic subduction system that is 20 characterized by a shallow mantle. A fi nite element model simulating extension on the 40 margin demonstrates that this preexisting 60 structure exerted fundamental controls on 80 the style of Mesozoic rifting. The shallow Depth (km) 30 km Crust 35 km Crust 100 mantle created a strong lithosphere beneath 7.37 × 1012 N-m 5.68 × 1012 N-m the orogen, causing extension to initiate 120 (net strength) (net strength) adjacent to, rather than within, the orogen. 140 On the Atlantic margins, the thick crustal root resulted in a weak lithosphere and ini- Figure 1. The role of crust thickness on lithosphere strength. (A) Sche- tiation of extension within the interior of the matic illustration of the rheology of the lithosphere showing refer- orogen. Major features of the modern Gulf ence model with 30-km-thick crust. Net strength of the lithosphere, of Mexico margin, including the Interior Salt obtained by integrating the yield stress over depth, is indicated at the Basin, outboard unextended Wiggins arch, bottom. (B) A weak model with a 35-km-thick crust. (C) Geotherm and an unusually broad region of extension used for yield strength calculations. Geosphere; April 2012; v. 8; no. 2; p. 1–12; doi:10.1130/GES00725.1; 13 fi gures; 3 tables; 2 animations. For permission to copy, contact [email protected] 1 © 2012 Geological Society of America Huerta and Harry have been outboard of the orogenic belt, within collision between Laurentia and Gondwana 1989; Keller et al., 1989; Mickus and Keller, the alloch thonous terrane that was accreted during assembly of Pangea (Fig. 2B) (Thomas, 1992; Harry et al., 2003; Harry and Londono, to the southern North American continent dur- 1976; Arbenz et al., 1989; Thomas et al., 1989; 2004). Following collision, the shallow mantle ing the Ouachita orogeny. Viele, 1989; Viele and Thomas, 1989). Defor- beneath the Ouachita suture would have ther- We propose that the differences in the style of mation in the orogenic belt was buffered by its mally reequilibrated during the ~50 m.y. that extension of the two Mesozoic rifts can be attrib- position within the Ouachita Embayment of the elapsed between collision and rifting, result- uted to differences in the preceding Paleozoic early to middle Paleozoic Laurentian rift margin, ing in strong lithosphere, whereas the accreted orogens. The Appalachian orogeny was a “hard” with most compressional deformation occur- arc would have had a relatively thick crust and continent-continent collision that produced sub- ring to the east on the Alabama Promontory and weak lithosphere that was susceptible to exten- stantial shortening in both the hinterland and to the west on the Texas recess (Thomas, 1976, sional deformation. internides, with signifi cant crustal thickening in 1991; Pindell, 1985; Houseknecht, 1986; Hale- A two-dimensional fi nite element model of the central part of the orogenic belt and exhu- Erlich and Coleman, 1993). As a consequence, continental rifting is used to test the hypothesis mation of a deep metamorphic core (Fig. 2A) shortening within the Ouachita orogen in the that lateral strength variations in the lithosphere (Thomas, 1976; Pratt et al., 1988; Thomas et al., central North American Gulf of Mexico coast inherited from Paleozoic tectonic events exerted 1989; Hatcher et al., 1989; Sheridan et al., 1993). is much less pronounced than in the Appala- a primary control on the distribution and nature The ensuing Mesozoic rifting initiated within the chian system, with no evidence of a crustal root of Mesozoic extensional deformation on the interior of the orogen, where the crust was thick- or exposure of high-grade metamorphic rocks Gulf of Mexico continental margin. The model est and the lithosphere weakest. Remnants of the (Arbenz et al., 1989; Viele, 1989; Thomas, begins with a lithospheric thermal and rheologi- crustal root are still present beneath the south- 1991). Instead, the Ouachita orogen in the cal structure that is based on reconstructions of ern Appalachian fold-and-thrust belt and in New central Gulf of Mexico region is underlain southern North America after accretion of the England (Pratt et al., 1988; Taylor, 1989). by a relatively pristine Paleozoic subduction allochthonous terrane during middle Paleozoic The Ouachita orogen is considered to be a system with thin crystalline crust and a rela- time. The model allows for thermal reequilibra- “soft” collision, resulting from arc-continent tively shallow mantle (Chang and McMechan, tion during a tectonically quiescent period from A) Central North Atlantic B) Eastern Gulf of Mexico 488.3 Ma Taconic orogeny North Western Carolina Theic-Rheic North Ouachita Wiggins America Piedmont Arc Ocean Africa America Trough Arc Gondwana Ordovician Silurian 443.7 Ma Devonian Central Piedmont Suture 359.2 Ma Ouachita orogeny Black Warrior Ouachita Mississippian Basin Suture 318.1 Ma Penn- sylvanian Alleghanian orogeny Blue Ridge Brevard Carolina Alleghanian 299.0 Ma Thrust Fault Terrane Suture Ouachita Suture Permian Outboard Terranes 251.0 Ma Rifting Rifting Ouachita Valley & Blue Atlantic Black Warrior Wiggins Gulf of Orogen Triassic Ridge Ridge Piedmont Ocean Africa Basin Arch Mexico Gondwana Interior Salt Basin Rift Basins Jr 199.6 Ma Figure 2. (A) Tectonic evolution of the eastern New Jersey continental margin and Appalachian orogen. (B) Tectonic evolution of the Mississippi Gulf of Mexico margin and Ouachita orogen. 2 Geosphere, April 2012 Rifting of the Gulf of Mexico late Paleozoic through Early Triassic time, prior 35° to the onset of extensional deformation that led Ouachita Front to opening of the Gulf of Mexico during the Appalachian Front Early Jurassic. The model results are consistent Salt with the geologic and geophysical features of Limit System the region, including the distribution of major recambrian P Rift extensional structures, the amount and location Ouachita Front of crustal thinning, and the duration of rifting on 30° the central Gulf of Mexico margin. Edge Wiggins Cretaceous Shelf Arch Interior Salt REGIONAL GEOLOGY Basin Escarpment Florida The coastal plain of the North American Gulf of Mexico continental margin is covered almost 25° entirely with mostly conformable Late Jurassic through Quaternary sedimentary strata. North of Salt Limit he the Ouachita orogen, the basement is generally Campec thought to consist of Grenville age (ca. 1.2 Ga) Escarpment granitic crust that formed the southern edge of the Proterozoic Laurentian craton (Taylor, 1989; 20° Culotta et al., 1992; James and Henry, 1993; Mosher et al., 2008).