The Gulf of Mexico and the southern margin of Laurentia William R. Dickinson* Department of Geosciences, University of Arizona, Tucson, Arizona 87721, USA

Mickus et al. (2009; p. 387 in this issue of the concept of a volcanic rifted margin into the Ouachita Grenville Geology) call fresh attention to the origin of the current picture of the Gulf of Mexico seems a orogen Grenville Wiggins Gulf of Mexico, and particularly to the geody- crucial step, provided it is correct, toward under- suture namic processes that formed its rifted margin standing Gulf geodynamics. Mickus et al. draw EM north of the oceanic crust forming its central out some of the implications of the idea. L M W S F expanse. I cannot hide my sympathy for the per- From knowledge that the crust beneath the l o F r spectives of the paper, for my co-authorship of interior of the Gulf has an oceanic profi le and i l o d C a S a m p r recent abstracts would betray me (Stern et al., lies beneath a water depth appropriate for Meso- e c h id B a h e e n k l a 2008; Dickinson et al., 2009; Stern and Dick- zoic seafl oor, and that the Gulf is bounded on f inson, 2009). all sides by passive continental margins lacking Y ú c a The nature of the Gulf of Mexico (Fig. 1) is subduction zones, we know that the Gulf is still t a n important to understand for several reasons: (1) surrounded by the continental blocks that sepa- Yúcatan Suwannee it is a prime exemplar of a so-called small ocean rated to allow the oceanic center of the Gulf to Campeche terrane block A basin fl oored by oceanic crust that is not contig- form. The kinematics of Gulf opening were nev- uous with the oceanic crust beneath the world’s ertheless uncertain for several decades after the Ouachita 90° W orogen principal oceans (Atlantic, Indian, and Pacifi c); advent of the theory of plate tectonics. Grenville Grenville Wiggins (2) its formation was the last geotectonic step The central issue was the geometry of motion suture in delineating the southern margin of Laurentia, of the Yúcatan-Campeche block (Fig. 1) in pull- EM the Precambrian core of North America; and (3) ing away from Texas. When one restores the L S M W F l sediment prisms fl anking it are host to one of the pre-drift confi guration of the Atlantic realm f o ul r G t i world’s great petroleum provinces. by backtracking Atlantic seafl oor magnetic oas F d C l a S o The history of the Gulf of Mexico as an oce- anomalies, Venezuela is shoved up quite close r h i e d anic basin is a challenge to reconstruct because to Texas (Dickinson and Lawton, 2001a), and u st l a 25° c r f c its fl anks are so well masked by the thick sedi- the Yúcatan-Campeche block must somehow N n i e a o c ment prisms that make it such a rich hydrocarbon fi t between the restored margins of North and N h e e c resource, and there are no decipherable seafl oor South America. One outdated approach to resto- p n k Suwannee a m a magnetic anomalies preserved within its interior ration was to assume that the Yúcatan-Campeche C B terrane to indicate directly the geometry of its opening. block pulled away from North America with the An important message of Mickus et al. (2009) same motion vector as adjacent South America, 250 km a n is that modern geophysical techniques and ana- and was simply left stranded behind when the a t ú c Yúcatan lytical methods now give us potential leverage two continents fully separated, but this assump- Y Campeche Tehuantepec block on the anatomy of the Gulf of Mexico that we tion yields a highly unlikely pre-drift position transform 90° W B could not gain in the past. for the Yúcatan-Campeche block spanning the The prime focus of Mickus et al. is on the pro- Rio Grande embayment of southwest Texas and Figure 1. Pre-rift (A) and post-drift (B) con- vocative interpretation that the northern fl ank of northeast Mexico. A more satisfactory initial fi gurations of the Gulf of Mexico region. the Gulf of Mexico in Texas is a volcanic rifted position for the Yucatán-Campeche block, with a EM—Eagle Mills rift basins. Uplifts (surface and subsurface): L—Llano; M—Monroe; margin, a geotectonic setting that has spawned re-entrant in its margin draped around the Llano S—Sabine; W—Wiggins. whole books of commentary (Menzies et al., projection of Laurentia in central Texas (Fig. 1), 2002) but has not previously been envisioned is achieved by understanding that the Yúcatan- for the Gulf Coast. Tagging rifted margins as Campeche block rotated 42° clockwise while either volcanic or amagmatic is just a conve- pulling away from Texas, leaving the oceanic of Mexico was in a backarc paleogeographic nient way of labeling two end members of what Gulf of Mexico in its wake (Pindell and Dewey, position during Gulf seafl oor spreading. This is actually a spectrum of geotectonic behavior 1982; Marton and Buffl er, 1994). This geometry geotectonic setting opens the door to regarding during continental rifting. Indeed, Mickus et al. of opening is supported by paleomagnetic data the Gulf geodynamically as a backarc basin as suggest from their geophysical analysis that the from the Chiapas massif near the southwest cor- well (Stern and Dickinson, 2009). As noted by Gulf margin grades eastward from a volcanic ner of Yúcatan (Molina-Garza et al., 1992). Mickus et al. (2009), the northwestern corner rifted margin in central Texas to an amagmatic Once the motion of the Yúcatan-Campeche of the Gulf at the north end of the Tehuantepec rifted margin in Louisiana and beyond. They block is divorced from the motion of South paleotransform connected seamlessly with the note, moreover, that the Gulf margin south- America, the geodynamic impulse for opening backarc Border rift system (Dickinson and Law- ward from Texas along the east coast of Mexico the Gulf of Mexico is no longer fully apparent. ton, 2001b) extending in aulacogen-like fashion is a transform margin, yet another variant of a Opening of the Gulf during the interval 160–140 for ~1500 km into the continental block. continent-ocean interface. Even so, introducing Ma ago (Bird et al., 2005) coincided with Nazas Mesozoic continental rifting during the arc magmatism in northeastern Mexico (Bar- breakup of Pangea was just the last phase in the *E-mail: [email protected]. boza-Gudino et al., 1999), meaning that the Gulf delineation of the southern margin of Laurentia.

©2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2009; 2009 v. 37; no. 5; p. 479–480; doi: 10.1130/focus052009.1. 479

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/37/5/479/3537702/i0091-7613-37-5-479.pdf by guest on 01 October 2021 Earlier geotectonic events of signal importance REFERENCES CITED Molina-Garza, R.S., van der Voo, R., and Urrutia- included: (1) Proterozoic collisional orogeny Barboza-Gudino, J.R., Tristán-González, M., and Fucuguachi, J., 1992, Paleomagnetism of the along the Grenville belt during the assembly Torres-Hernández, J.R., 1999, Tectonic setting Chiapas massif, southern Mexico: Evidence for of pre-Oxfordian units from central and north- rotation of the Maya block and implications for of Rodinia (Mosher et al., 2008), (2) the rifting eastern Mexico: A review, in Bartolini, C., Wil- the opening of the Gulf of Mexico: Geological away of the Argentine Precordillera (Thomas son, J.L., and Lawton, T.F., eds., Mesozoic sedi- Society of America Bulletin, v. 104, p. 1156– and Astini, 1996) in early Paleozoic time to mentary and tectonic history of north-central 1168, doi: 10.1130/0016-7606(1992)104<1156: form the passive pre-Ouachita margin of south- Mexico: Geological Society of America Special POTCMS>2.3.CO;2. ern Laurentia (Thomas, 1991), and (3) late Paper 340, p. 197–210. 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Pindell, J.L., and Dewey, J.F., 1982, Permo-Triassic with Mesozoic rifting in the Gulf of Mexico, Dickinson, W.R., and Lawton, T.F., 2001a, Carbon- reconstruction of western Pangea and the evo- iferous to Cretaceous assembly and fragmenta- lution of the Gulf of Mexico/Caribbean region: most of those precursor geotectonic elements tion of Mexico: Geological Society of America Tectonics, v. 1, p. 179–211, doi: 10.1029/ of the southern Laurentian continental margin Bulletin, v. 113, p. 1142–1160, doi: 10.1130/00 TC001i002p00179. are buried under Mesozoic–Cenozoic sediment 16-7606(2001)113<1142:CTCAAF>2.0.CO;2. Stern, R.J., Anthony, E.Y., Dickinson, W.R., Grif- cover (e.g., Keller and Hatcher, 1999). 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