Introduction: Making the Southern Margin of Laurentia

Robert Stern1, William R. Dickinson2, and Timothy Lawton3 1Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, USA 2Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th Street, Tucson, Arizona 85721, USA 3Department of Geological Sciences, New Mexico State University, MSC 3AB, P.O. Box 30001, Las Cruces, New Mexico 88003, USA

Continental crust is very long-lived but conti- nent. At the other end of the spectrum, Africa supercontinent Rodinia and the Ouachita orogeny nents themselves are ephemeral. Earth’s inven- is the only shrinking continent, riven with rifts ~300 Ma leading to the formation of Pangea) tory of continental crust changes slowly but and surrounded by spreading ridges and con- and two episodes of rifting to form new ocean how this material is partitioned into separate tinental fragments, geomemories of the much basins (Cambrian rifting ~530 Ma to form the aggregations is always changing. We call these larger continental assembly, Gondwana. Other Rheic ocean and Jurassic rifting ~165 Ma to distinct aggregations of buoyant, felsic crust processes also change continental size and form the Gulf of Mexico). Southern Laurentia “continents” and now there are 6 of them: North shape, such as strike-slip (transform) fault- faces the Pacifi c on its west, and subduction America, South America, Antarctica, Eurasia, ing, although such geo-whittling doesn’t affect of Pacifi c seafl oor has been an intermittently Africa, and Australia. There are many smaller continental confi gurations as profoundly as col- important aspect of its tectonic evolution since minicontinents like Arabia, Madagascar, and liding and rifting do. Interactions with mantle Pangea formed ~300 Ma. To the east, southern Greenland. From the human perspective, these melting anomalies (plumes) or subduction of a Laurentia felt the extensional stresses of Tethys, names have great signifi cance as the sites of spreading ridge beneath a continent also act as opening within Pangea. All these episodes our nations, civilizations, and cultures, but the modest re arrangers of continental crust. played roles in shaping the southern margin names of continents are not useful in the long “The Supercontinent Cycle” (Nance et al., of Laurentia, and left scraps of crust that are run because continents gain and lose mass, such 1988) captures the most important aspects of part of North America, but not part of Laurentia. that over time their sizes and shapes change continental reconfi guration, albeit greatly sim- Many of these exotic terranes in the SW are greatly. Heraclitus noted that you cannot step plifi ed. The splitting apart and coming together accreted fragments of Pacifi c arcs or plateaux, twice into the same river, for new waters are of continental fragments is stochastic and can- whereas exotic terranes in the SE are orphans left ever fl owing around you. In this sense, a conti- not be characterized as truly cyclic. Continents by Gondwana when Pangea broke up. nent is like a river, forever changing. split up and come together because of stresses Where does Laurentia terminate in the south? Of course, continents do not change as fast imposed by plate interactions in the vicinity, not Some show Laurentia ending near the U.S.- as a river fl ows, but their changes are far less because of global pulsations. With this under- Mexico border, but this is an oversimplifi cation predictable. Rivers fl ow downstream, generally stood, the supercontinent cycle is useful for (Fig. 1). A recent estimate of lithospheric thick- getting larger as they move to the sea, but conti- emphasizing the transitory nature of continents ness based on seismic-wave receiver function nents are just as likely to shrink as to grow with and how reconfi gurations cause changes in sea data indicate that the “stable” part of the North time. This is because continental shape-shifting level, climate, and macroevolution. American continent is bounded to the west by manifests plate tectonic processes, which sepa- This themed issue of Geosphere, “Making the the Laramide deformation/Rocky Mountain rate as well as amalgamate crust. Continents Southern Margin of Laurentia,” looks at how the front and the Ouachita and Appalachian fronts mostly change by rifting and colliding. These margins of part of our continent were shaped. A to the south and east (Yuan and Romanowicz, processes refl ect the approach of a continent couple of questions come immediately to mind: 2010). However, Grenville-aged crust of Lau- to a subduction zone (collision) or the forma- What is Laurentia, and how is it different than the rentia extends southward in Mexico, far beyond tion of a new ocean (rifting) in the midst of a North American continent? Where is its southern the limit of the seismically defi ned stable conti- continent. These changes affect the continents margin? Laurentia is named after the Laurentian nent. The suture between Laurentia and Gond- in many ways. Continental aggrandizement craton, another name for the North American wana is the Ouachita orogen, so Coahuila is a and diminishment change surface elevations as craton. The Laurentian craton is named after Gondwanan orphan, as is stretched basement well as continental outline, forming mountains the St. Lawrence River, which fl ows over it. below coastal Texas, Louisiana, and Missis- and basins. Drainage networks must constantly This craton is exclusively Precambrian crust, sippi (Sabine, Wiggins, and Monroe uplifts). adjust, not only to changing relief due to inter- distinct in this regard from the North American Florida is also an accreted peri-Gondwanan ter- actions on the margins (and formation of new continent, which includes signifi cant tracts of rane. Along strike to the southwest in Mexico, margins), but also to the drift of the continents accreted Phanerozoic crust (Fig. 1). In contrast the crust is all accreted subduction complexes into different climate regions. to the more ancient crust of northern Laurentia, or arcs, which originated in the Pacifi c realm. Continents mostly grow by colliding and which mostly formed >1.8 Ga, that of southern West of the Ouachita front, Laurentian base- shrink by rifting. Eurasia today is the larg- Laurentia formed 1.8–1.3 Ga (Karlstrom et al., ment exists in northern Sonora and Chihua- est continent and is growing rapidly. With the 2001). Southern Laurentian crust was modifi ed hua but it does not extend past the California- recent addition of India, imminent addition by a wide range of processes, including two Coahuila transform (Mojave-Sonora megashear of Arabia and then Australia, Eurasia is well episodes of continental collision (the Grenville overprinted by the Mesozoic magmatic arc). In on its way to becoming the next superconti- orogeny ~1.1 Ga accompanying formation of this region, the Laurentia-Gondwana join (at the

Geosphere; December 2010; v. 6; no. 6; p. 737–738; doi: 10.1130/GES00642.1; 1 fi gure.

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Figure 1. Geotectonic elements associated with the southern margin of Laurentia. Gulf of California and Gulf of Mexico closed (prior to Cenozoic and Mesozoic seafl oor spreading, respectively). Restored Yucatan-Campeche block (Y-C) positioned against Texas coastal magnetic anomaly of Mickus et al. (2009). Present displaced position of Y-C (cross-hatched) was attained by Jurassic seafl oor spreading to open the Gulf of Mexico and coordinate slip on the Tehuantepec Transform (TT) to the west. Can–Canada; Mex–Mexico.

Ouachita orogen) reaches south through Chi- mometers moves slowly across the region, tia, its extensions to Australia and Baltica, and impli- huahua but butts against Phanerozoic accreted which is providing an unparalleled look into the cations for refi ning Rodinia: Precambrian Research, v. 111, p. 5–30, doi: 10.1016/S0301-9268(01)00154-1. terranes somewhere near the southern border of lithospheric and mantle structure of the region. Mickus, K., Stern, R.J., Keller, G.R., and Anthony, E.Y., Chihuahua, at the southern edge of the Meso- This themed issue draws attention to this region, 2009, Potential fi eld evidence for a volcanic rifted margin along the Texas Gulf Coast: Geology, v. 37, zoic arc. Unfortunately, critical relationships are in the expectation that the next few years will p. 387–390, doi: 10.1130/G25465A.1. hidden beneath Cretaceous and Tertiary cover, provide useful new information about the litho- Nance, R.D., Worsley, T.R., and Moody, J.B., 1988, The so no one knows exactly where the triple join spheric and upper mantle structure of the U.S. supercontinent cycle: Scientifi c American, v. 259, p. 72–79, doi: 10.1038/scientifi camerican0788-72. is between Laurentia (in the north), Gondwana part of Laurentia. Yuan, H., and Romanowicz, B., 2010, Lithospheric layering in the North American craton: Nature, v. 466, p. 1063– (eastern Mexico), and post-Ouachita accreted REFERENCES CITED terranes (to the southwest). 1068, doi: 10.1038/nature09332. This special issue is launched as Earthscope’s Karlstrom, K.E., Ahäii, K.I., Harlan, S.S., Williams, M.L., McLelland, J., and Geissman, J.W., 2001, Long-lived MANUSCRIPT RECEIVED 9 SEPTEMBER 2010 Magnetotelluric Transportable Array of seis- (1.8–1.0 Ga) convergent orogen in southern Lauren- MANUSCRIPT ACCEPTED 19 SEPTEMBER 2010

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