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Plate Tectonic Constraints on the Biogeography of Middle America and the Caribbean Region Author(s): Peter J. Coney Reviewed work(s): Source: Annals of the Missouri Botanical Garden, Vol. 69, No. 3 (1982), pp. 432-443 Published by: Missouri Botanical Garden Press Stable URL: http://www.jstor.org/stable/2399080 . Accessed: 30/05/2012 01:02 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Missouri Botanical Garden Press is collaborating with JSTOR to digitize, preserve and extend access to Annals of the Missouri Botanical Garden. http://www.jstor.org PLATE TECTONIC CONSTRAINTS ON THE BIOGEOGRAPHY OF MIDDLE AMERICA AND THE CARIBBEAN REGION PETER J. CONEY' ABSTRACT NorthAmerica-Eurasiaand South America-Africawere certainlyjoined in the classic reconstruction of Pangaea by Middle Triassic time. The line of collision and suture included the Appalachian Quachita-Marathonorogenic trendin the United States extendingsouthwestwardinto what is now prevailed and northeasternand southeasternMexico and into Guatemala. Widespread continentality there was no Gulf of Mexico or Caribbean Sea. In Late Triassic time and continuinginto Early Jurassic time this constructbegan to founderby initialriftingbetween South America-Africaand NorthAmerica. No oceanic crust was formed,however, thus Africa-SouthAmerica were still completelyconnected by land or shallow sea to NorthAmerica untilmid-Jurassictime. Duringthis same uppermostTriassic to Middle Jurassicperiod a largelycontinentalmagmaticarc was draped across the Pacific marginof southwesternNorth America and apparentlycontinuedunbrokeninto northwesternSouth America. Sometimein the Middle Jurassicoceanic crustbegan to formby seafloorspreadingin the central Atlanticand Gulfof Mexico as separationof South America-AfricafromNorthAmerica accelerated. Once this dense crust began to formthe trailingmarginsof the continentssubsided below sea-level and constructionofthe Atlanticand Gulfcoast continentalshelves began. Evidence is quite conclusive fromthe souththatthis ocean floorspreadingdid not reach the Pacific Ocean, but was transformed western corner of the newly opened Gulf of Mexico northwestwardacross Mexico via a complex faultsystemthatreached the Pacific marginnear Los Angeles. left-sliptransform In Early Cretaceous time spreadingcontinued in the central Atlantic but extended southward into the southernAtlantic.As the main axis of spreadingextended into the south Atlantic,spreading ceased in the Gulf of Mexico. The south Atlantic spreadinginitiatedseparation of South America faultsuntilLate fromAfrica,but theyprobablyremainedin partialcontact via ridge-ridgetransform Cretaceous time. South America musthave finallycompletelyseparatedfromNorthAmericain Early Cretaceous time,probablyvia a riftalong the eastern edge of Yucatan and the Nicaraguan rise. By Late Jurassic time the Pacific continentalmarginarc had waned and was replaced by a complex, largelyoceanic, magmaticarc whose positionrelativeto southwesternNorthAmerica and northwesternSouth America is not known. What we do know is that by Late Cretaceous-Early Tertiarytimeit had accreted againstthe Pacificmarginsof both. Connectionsbetween the continents are also not known but could have included a largelysubmarinemagmaticarc, parts of which may have subsequentlydispersed eastward as the GreaterAntilles.Much of what is now Middle America is apparentlyunderlainby oceanic crustat least as young as Late Cretaceous in age. By Late Cretaceous time the Greater Antilles magmaticarc seems to have fullyformedand subductionsystemduringLate Cretaceoussubsequentlymoved northeastwardas a northeast-facing Early TertiaryLaramide time. The Greater Antilles arc-trenchsystemceased activityin Late Eocene time as it collided with Florida and the Bahama platformand as Laramide orogenywaned throughoutwesternNorthAmerica. This was followedby a major plate reorganizationin the Caribbean-MiddleAmerica regionnearly40 m.y. B.P. which established the Caribbean plate more or less as we know it today. The principal change was initiationof the Lesser Antillesmagmaticarc as an east-facingsubductionsystemthat began to consume Atlantic ocean floor. Also, a west-facingsubduction system may have formed about this time along a proto-CentralAmerican westernmarginof the Caribbean plate. However, much of what is now Central America may have initiallybeen offsouthernMexico. The northern and southernmarginsof the Caribbean plate evolved into complex transformand transpressivesystems as Northand South America moved westwardpast a nearlystationaryCaribbean plate. These motions significantly fragmentedthe Greater Antillesinto theirpresent array. There is no evidence forany complete land connectionbetween North and South America via the Greaterand/orLesser Antilles throughoutlater Mesozoic or Tertiarytime. Nor is there any evidence for complete land connectionvia CentralAmerica and the Isthmusof Panama beforeNeogene time. 1 Departmentof Geosciences, Universityof Arizona, Tucson, Arizona 85721. ANN. MISSOURI BOT. GARD. 69: 432-443. 1982. .25/0 0026-6493/82/0432-0443/$01 1982] CONEY-PLATE 433 TECTONIC CONSTRAINTS PRESENT North America AMERICAS P A CFI C PLATE PLTE( to 600 _: South t AmericaX KMS. / NAZCA P LA TE i \ The six majorplates and theirmotionsare shown. Heavy FIGURE 1. Presentplate configuration. black lines are spreadingcenters or rise crests; heavy, barbed lines are subductionzones; dashes patternshows distributionof magmaticarcs. Dotted line is presentcoastline of the continents,solid line seaward is edge of continentalcrust. Opposed arrows on transformfaultsconnectingrise crest segmentsshow sense of relativemotionon fault. INTRODUCTION Platetectonics(see Cox, 1973)has providedforthefirsttimea quantitative meansto makereasonablyaccurateglobalpaleogeographic reconstructions for Mesozoic-Cenozoictime.This new-found abilityhas enabledgreatinsightinto regionaltectonicevolutionofboththeoceansandcontinents, andtheimplications forpaleoclimatology, and paleobiogeography are onlyjust paleo-oceanography, to be fullyexplored.The objectiveof thispaper is to outlineplate beginning tectonicarguments thepaleogeographic regarding evolutionof MiddleAmerica and theCaribbeanregionand adjacentNorthand SouthAmerica(see also Dickinson& Coney, 1980).This hopefully can providea paleotectonicbackground can be compared.It shouldbe realized againstwhichbiogeographic arguments levelofpaleotecby thebiogeographic community, however,thattheresolution tonicreconstruction is frequently coarserthanthatdesiredby biogeographers. In thatsense it is possiblethatpaleobiogeographic data maybe as helpfulto as is thereverse,whichmakessymposiasuch as thisverypropaleotectonics pitious. THE PROBLEM OF THE MIDDLE AMERICA-CARIBBEAN REGION The presentplatetectonicsettingoftheMiddleAmerica-Caribbean regionis are not simpleand in theircomplexity generalizedin Fig. 1. The relationships 434 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 69 in lies in parttheproblemofthisregion.Six platesare foundhereall interacting a complexway. northThe Pacificplate,whichfloorsmostof thePacificOcean, is moving2 Baja Caliwestwardas it spreadsawayfromtheEast Pacificrise. It is carrying forniaand the State of Californiawest of the San Andreasfaultwithit as it awayfromtheEast Pacificrise moves.The Cocos plateis movingnortheastward Mexicoand Middle and Galapagosriseand is beingsubductedbeneathsouthern AmericaalongtheMiddleAmericatrench.TheNazca platemoveseastwardaway beneathSouth fromthe East Pacificrise and Galapagosrise and is subducting America.Northand SouthAmerica,probablyas separateplates,movegenerally rise. Caughtin themiddleof all thesemowesterlyawayfromtheMid-Atlantic tionsis the Caribbeanplate. Its motionis not well known,but it is probably The Caribbeanplatesubmovingveryslowlyif it is notessentiallystationary. Antilles on beneath the Lesser theeast, and Cocos and ductsAtlanticsea-floor west. Its northern boundMiddle America on the beneath the Nazca plate region is an even fault and its southern a transform boundary system ary is complex faultsystem.Bothofthesefaultsystems morecomplextransform-transpressive accommodatethe motionsof Northand SouthAmericaas theymovegenerally one converging westwardpast the Caribbeanplate. They are probablyslightly upontheotherand upontheCaribbeanplateas theymove. The problemin theCaribbeanregionis thatthetwo "Atlantic"platesofthe Americasandthethree"Pacific"platesall spreadfromrisecrestsandthusleave as theyspread.Thisallows,bybackanomaliespaintedon thesea-floor magnetic to therisecrest,ratheraccuratereconstrucing-updatedanomaliessequentially no suchdataexistsfortheCaribbean tionsofrelativeplatepositions.In contrast, plate,since,withtheexceptionof a smallrisecrestin theCaymantrough,it is faults.As a result,no complete surrounded by subductionzones or transform thehistory recordofmotionwithrespectto otherplatesis preserved.Therefore of the Caribbeanplate mustbe deducedfromcontrolledmotionsof adjoining (see Martin& Case, 1975;and platescombinedwiththegeologicandgeophysical otherpapersin Nairu& Stehli,1975)recordpreservedon the Caribbeanplate is at bestalwayssomewhatequivocaland subject itself.Thiskindofinformation The modelproposedhereis perhapsone ofseveralperto variedinterpretation. missibleinterpretations (forexample,see Malfait& Dinkelman,1972).Certainly, andinsight ina regionas poorlyknownandcomplexas thisone,newinformation 2 Plate motionscan only be describedas motionswithrespectto some frameof reference.There are two general methods. In purelyrelativemotionschemes the motionof a plate is described with respect to an adjacent plate, which for computationalpurposes is assumed to be fixed (see Ladd, 1976). A second approach is the so-called "absolute" motion scheme. Here plate motions are describedwithrespectto some referenceframeoutsidethe plates themselves.Usually the chosen frame is the assumed rotationalaxis of the earth which is derived frompaleomagnetic data and fromthe positions of assumed--semi-fixed thermalanomalies or "hot spots" in the mantle. These "hot spots" produce intra-platevolcanism such as the Hawaiian Islands. As a plate moves over a "hot spot" a trailof volcanic edificessuch as the Hawaii-Emperorseamountchain can be produced. If the edifices can be dated an "absolute" motionof the plate is derived. Then, using relativemotionsbetween this plate and otherplates, the motionsof all the plates withrespect to the fixedreferenceframecan be attained.The motionsdiscussed in thisreportare thissecond, or "absolute," type(see Coney, 1978). 1982] CONEY-PLATE TECTONIC CONSTRAINTS TRIASSIC /No th America Africa / 600 o 435 ~ X A'"'eri- KMS./ FIGuRE 2. Triassic plate configuration. Dashed lightlines forminggridare generalizedpresentday lines of latitudeand longitude.Deformationassociated withthe Appalachian-Marathon-Quachita orogenyshown by wavy lines. Dot patternis Cordilleranmiogeocline. could forcesignificant revisionof today's conceptions,includingwhatis presentedhere. TRIASSIC The reconstruction forTriassictimeis shownin Fig. 2. It is theclassicconfiguration of Pangaea (see Irving,1977)as manifested in the Caribbean-Middle Americaregion.Thisreconstruction derivesfromclosureofthecentralAtlantic Ocean controlledby the magneticanomaliesthere,whichplaces Africaclose againstthe marginof easternNorthAmerica.Since South Americawas still stitchedto Africaat thistime,whatone does withAfricaone mustalso do with SouthAmerica.This predetermines thepositionof SouthAmericawithrespect to NorthAmerica.SouthAmericais shownoverlapping all ofwhattodayis the CaribbeanSea, MiddleAmerica,and almostone-halfof Mexico. This is a geometricfactfromwhichone mustconcludethatall therocksand landsincluded withintheoverlappedregioneitherdid notexistat thistimeor theyweresomewhereelse (Coney,1978). The assumption is thatthelineof suturebetweenAfrica-South Americaand NorthAmericaresultedfromLate Paleozoic collisionand possibleright-lateral strike-slip betweenthetwo continental faulting massesto producethe Appalachian-Quachita mountainsystem(Grahamet al., 1975).The finalconsolidation, certainly completedby Late Triassictime,is thereconstruction shownin Fig. 436 ANNALS OF THE MISSOURI BOTANICAL GARDEN LATE North X .> KMS. JURASSIC America > 600 [VOL. 69 South S America ~~~~~~~~Afrc ) \A FIGURE 3. Late Jurassic plate configuration.On this and subsequent figuresthe position of Quito, Equador is shown with respect to North America assumed fixed. The arrow indicates the amountof movementthispositionmade duringthe period followingthe previous figure.The MojaveSonora "megashear" is shown by heavy line withopposed arrows indicatingsense of movement. 2. Widespread continentality prevailed throughoutthe region and there was no Gulfof Mexico or Caribbean Sea, nor any AtlanticOcean. In short,Africa-South America-NorthAmerica were a singlelandmass and the regionwas everywhere above sea-level. LATE JURASSIC The reconstructionforLate Jurassic(Fig. 3) shows the centralAtlanticOcean and Gulfof Mexico opened as Africa-SouthAmerica separatedfromNorthAmerica. Paleomagnetic data suggeststhat North America moved rapidlynorthwestward relative to a near-stationaryAfrica-SouthAmerica. The process of rifting and separation probably began in Late Triassic time by inceptionof extension between the two major blocks. There is considerable geologic and geophysical evidence for widespread developmentof riftvalleys and ridges as continental crust began to extend. Continentalityprevailed as the riftedbasins filledwith debris shed offadjacent highlandsand upliftedblocks. By Middle Jurassictime extensionhad reached sufficient amountssuch thatinter-connecteddown-faulted and subsided areas allowed distal incursionsof shallow marinewater, probably fromthe TethysOcean northof Africaand southof Europe. Evaporationof these incursionsproduced widespread salt and other evaporite deposits that are well known throughoutthe Gulf of Mexico regiontoday. Once oceanic crustbegan to formin Middle Jurassictime the marginsof the newlyriftedcontinentalmasses began to subside rapidlydue to higherdensityof new oceanic materialand thermaldecay of the continentalmargins.This allowed 1982] CONEY-PLATE TECTONIC CONSTRAINTS 437 widespreadmarinetransgressionupon thenewlyformedcontinentalmarginsfrom the Atlanticand Gulf of Mexico oceanic basins. The transgressionsinitiateddevelopmentof the continentalshelves as sedimentswere draped across the bared edges of the continents.This deceleratingprocess of shelfdevelopmenthas continuedto the presenttime. There is some evidence to suggestthat the young riftbetween Africa-South America and North America did not reach the Pacific Ocean. It appears to have across Mexico as a major left-slipfaultsystem. been transformednorthwesterly This complex faultmay have extendedfromthe southwesterncornerof the newly opening Gulf of Mexico northwestwardto the Pacific coast near Los Angeles in southernCalifornia.This importantfeaturehas been termedthe Mojave-Sonora "megashear" and estimatesare that up to 800 kilometersof left-slipmovement took place along it between Late Triassic and Late Jurassictime (Silver & Anderson, 1974). During this same period, between Late Triassic and Late Jurassic time, a volcanic-magmaticarc developed along the Pacific marginof North and South America. The feature seems to have been almost everywhereabove sea level standingon continentalcrust and extendingunbrokenbetween the two continents.The implicationis thatin spiteof over 800 kilometersof relativeseparation between Africa-SouthAmerica and NorthAmerica, and in spite of the formation of the centralAtlanticOcean and Gulfof Mexico as deep marinebasins between them, the two supercontinentsremained in above or near sea-level contact via Mexico untilLate Jurassic-EarlyCretaceous time. That Africaand South America were stilljoined, thus both in contact with North America via Mexico, has implicationsforJurassicreptilianinterchangebetween East Africaand significant westernUnited States (Galton, 1977). EARLY CRETACEOUS The reconstructionfor Early Cretaceous timeis the least reliable and subject to considerableuncertainty.Based on the spreadinghistoryin the centralAtlantic,we know thatAfricacontinuedto separatefromNorthAmerica,but spreading in the Gulfof Mexico apparentlyended by Late Jurassictime. We also know that spreadingbetween Africa and South America began in Early Cretaceous time. These geometricfactsdemandcontinuedseparationof South AmericafromNorth America but withoutspreadingand transformmovementsin the Gulf of Mexico and along the Mojave-Sonora megashear. This necessitates spreadingbetween the two continentsin what mightbe termeda proto-Caribbeansea (Fig. 4). It must be recalled, however, that no record of this spreadinghas been positively to date. It is simplya constructapparentlydemandedby plate geometry. identified marginmagmaticarc had By Late Jurassictimethe Pacificmargin-continental waned and was replaced by a complex, largelysubmarineoceanic magmaticarc whose position was apparentlysomewhere off-shoreof the Jurassiccontinental margin(Campa & Coney, in press). The hypotheticalspreadingcenterbetween Northand South America mayhave extendedsouthwestwardtojoin withvarious Pacific spreading centers. If correct, a deep ocean basin may have separated NorthAmerica and South America duringpart of Early Cretaceous time. 438 ANNALS OF THE MISSOURI BOTANICAL GARDEN EARLY North [VOL. 69 CRETACEOUS America Africa t 600 KMS. ~ >~ ~South America \) FIGURE4. Early Cretaceous plate configuration. Symbols as on previous figures.The assumed spreadingcenterbetween Northand South America is shown projectinginto the Pacific Ocean. LATE CRETACEOUS The reconstructionforLate Cretaceous (Fig. 5) is nearlyas equivocal as that for Early Cretaceous and it is certainlyplagued by uncertaintiesthat may be years fromresolution. Spreading had started in the south Atlantic separating AfricafromSouth America, but these continentswere perhaps in close contact across transformfaultsbetween northeasternBrazil and the west Africancoast. The principalenigma,however,is thegeologybetweenNorthand South America. We do know that on several islands of the GreaterAntillesrocks at least as old as Middle Cretaceous have the aspect of a submarinevolcanic arc (see Khudoley & Meyerhoff,1971). This suggeststhe proto-GreaterAntillesformedas a submarinevolcanic chain thatpresumablystood above a subductionzone. The mostlikelylocation of thissubductionzone was off-shoreof the Pacific-American marginfarto the southwestof thepresentpositionof the GreaterAntilles.It may have been part of the off-shoreCretaceous arc discussed earlierthatstood southwest of Mexico. This arc apparentlyhad accreted againstNorthAmericaby Late Cretaceous-Early Tertiarytime and may have simply extended southeastward across the growinggap between North and South America. It was subsequently swept northeastwardthroughthe wideninggap thatwas to become the Caribbean Sea. The Cretaceous arc is of interest,ifit existed as portrayed,since it could have harboredfaunas and florasupon its far-flung volcanic islands, which were swept eastward as the arc migratedtoward its present position. It could have also provided "stepping stones" for dispersals between the two continentsduring 1982] 439 CONEY-PLATE TECTONIC CONSTRAINTS LATE CRETACEOUS North America Africa i4 600 K<MS.\ South Quto X ,( America t \ FIGURE 5. Late Cretaceous plate configuration. Symbols as on previousfigures.The Cretaceous volcanic arc is shown accretingagainst Northand South America and connectingthe two continents by scatteredvolcanic islands. Middle and Late Cretaceous time. The linkage was, however, no more than scatteredislands and fringing reefs,probably much like present-dayLesser Antilles. EOCENE The Eocene reconstruction(Fig. 6) representsevents that culminatedfrom Late Cretaceous to Early TertiaryLaramide orogenyas manifestedin the Middle America-Caribbeanregion. This importantorogeny affectedthe entire North American Cordilleraand was importantin the Andes of South America as well. It seems to have resultedin partfromaccelerated convergencerates between one or more oceanic plates in the Pacific realm and the North and South America plates. It may have also been influencedby large scale accretionsto the Cordilleran marginof "suspect" (Coney et al., 1980) submarinearcs and lesser lithospheric scraps duringthis time. The principalresult of Laramide orogenyin the Middle America-Caribbean region was the developmentof the Greater Antilles as a northeastfacing subduction systemcomposed of a largelysubmarinemagmaticarc witha trenchon its northeastside (Dickinson & Coney, 1980). The interpretation presentedhere is that the arc-trenchsystemmigratednortheastwardrelative to North America subductingproto-Caribbeansea-floorbeneath it as it moved. This motioneventuallycaused the systemto collide withthe Florida-Bahama platformin Eocene 440 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 69 EOCENE North America ~~~~~~'(~~~~~~~~~~~~A ,, 600 , d KMS. ~~~South America / FIGuRE 6. Eocene plate configuration. Symbols as on previous figures.The GreaterAntillesis shown sweeping into the gap between North and South America and collidingwith Florida and the Bahama platform.Soon afterthe time of this figureplate geometrychanged to produce the features shown on Fig. 1. time. It is assumed that the western end of the system slid northwardalong a transformfaultpast the eastern edge of the Yucatan Peninsula. The eastern end of the systemis somewhatconjectural,but may have includedthe now submarine Aves ridge and perhaps part of the Venezuelan Coast Ranges. Paleomagnetic data fromseveral parts of the GreaterAntilles-VenezuelanCoast Ranges system translationsand rotations(see MacDonald & Opdyke, 1972). suggestsignificant If the model presentedabove is correct,it is assumed thatpartsof the Greater Antillesmay have evolved fromthe arc systemshown on Figs. 3 and 4 forEarly and Late Cretaceous time. This arc would then have subsequentlyswept northeastward and fragmentedinto the wideninggap between Northand South America. At all times the systemwas apparentlylargelycomposed of submarinevolcanic edificesthat occasionally rose above sea-level to formscatteredvolcanic islands and fringingreefs. These constructionsmay have partiallycoalesced by Eocene time. The contact withSouth America via the eastern end of the system was probablyless completethanthe contactat the westernend withNorthAmerica. This was because the arc-trenchsystemwas forced to extend itselfsoutheastward with time as the two continentscontinuedto separate, and since the arc was actually movingobliquely away fromSouth America. MIDDLE AND LATE TERTIARY The Greater Antilles arc-trenchsystemceased activityin Late Eocene time as it collided with Florida and the Bahama platform.This waningof arc activity 1982] CONEY-PLATE TECTONIC CONSTRAINTS 441 was coincidentin timewiththe waningof Laramide orogenythroughoutwestern North America. What followed was a major plate reorganizationin the Middle America-Caribbeanregion, which established the Caribbean plate more or less as we know it today (Fig. 1). The Caribbean plate was formedby initiationof the Lesser Antillesmagmatic arc as an east-facingsubductionsystemthat began to consume AtlanticOcean sea-floor.There is no evidence of the existence of the Lesser Antilles prior to Late Eocene-Early Oligocene time. The arc-trenchsystemseems to have formed just east of the by then extinctAves ridge. The westernedge of the Caribbean plate apparentlywas a southwestfacing arc-trenchsystem along what today is the west side of Middle America. The northernand southernmarginsof the Caribbean plate evolved into complex transformand transpressivesystems as Northand South America moved westwardand northwestward respectivelypast a nearlystationaryCaribbean plate. If this model is correct,the Caribbean plate must have been almost 700 kilometerswest of its presentposition with respect to North America and South America. This means proto-MiddleAmerica lay south of southernMexico and the Lesser Antillessoutheastof the Bahama platform.The motionsof Northand South America swept the Caribbean plate relativelyeastward and fragmentedthe GreaterAntillesintotheirpresentarraywithCuba transferred to the NorthAmerica plate as the Cayman transform evolved. The southernmarginofthe Caribbean plate was more transpressivesince South America's motionwas more obliquely northwestward.This has sheared and compressed the Venezuelan Coast Ranges and thrustthem southeastwardonto South America. The northwesterly motion of South America also served to shortenthe distance between North and South America,causing South America to compress and overthrustthe Caribbean plate at its western end. This may have flexed the Middle America region into the presenttwistnow seen in Panama and along withcollision served to aid in raising it above sea-level in Late Tertiarytime to produce the all importantland-bridge between the continents(see Case, 1974). In contrastto the Laramide Greater Antilles,the Cenozoic Lesser Antilles seem to have been muchmoreclosely tied at theirsouthernend to South America as they migratedeastward along the Venezuelan coast. Also, the growthand developmentof a sub-aerialMiddle America south of the transform faultzone in Guatemala seems to have progressed southward and eastward toward South America withthe finallink being collision between Panama and South America in Pliocene time. SOME SUMMARY AND BROAD BIOGEOGRAPHIC IMPLICATIONS 1. During Triassic and Early Jurassic time, widespread continentalityprevailed across the entireregion and North America-SouthAmerica-Africawere one landmass part of the super-continent of Pangaea. 2. By Middle Jurassictime, the centralAtlanticand Gulf of Mexico opened and by Late Jurassictime both were flooredby deep marine oceanic crust. A land connectionbetween Africa-SouthAmerica and NorthAmerica persistedvia Mexico, however, untilLate Jurassictime. 442 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 69 3. During a briefperiod in Early Cretaceous time, North and South America may have been separated by a spreadingcenterand a growingdeep marinebasin. Spreadingstartedbetween Africaand South America in Early Cretaceous time, but land contact may have persistedlocally via transformfaultmarginsoffpresent-daynortheasternBrazil untilLate Cretaceous time. 4. Perhaps as early as Aptian-Albiantimea submarinevolcanic arc may have provided a connection via scatteredvolcanic islands between southernMexico and Colombia-Venezuela. Parts of this arc may have subsequentlyswept northeastward as the GreaterAntilles. 5. The GreaterAntilleswere perhaps most closely tied to NorthAmerica via Yucatan and eventuallythroughcollision withFlorida. 6. The Lesser Antilles were (as they are today) more closely tied to South America duringtheirdevelopmentfromLate Eocene to the presenttime. 7. Middle America grew sub-aeriallysouthwardfrom North America as a complex volcanic arc-trenchsystem,eventuallyformingthe finalland bridgeby collision south of Panama in Pliocene time. 8. If overland dispersals between North and South America occurred, they would have been most favoredduringTriassic and Early Jurassiccontinentality, then again in Late Tertiarytimevia the land bridgein Panama. They could have persistedintoLate Jurassictimevia Mexico. From Late Jurassicto Late Tertiary time only inter-islandpassages were available. could have been strandedby isolation 9. Permo-Triassicnon-marinelife-forms on NorthAmerica and South America fromfragmentation of Pangaea. Any landin the GreaterAntillesshould be less thanLate Jurassic-earliest locked life-forms Cretaceous in age and survival would depend on ability to cope with widely separated and shiftingisland habitats. Jamaica and Middle America seem more closely tied to Mexico than the GreaterAntilles. Finally, therehas been considof the ancestralGreaterAntillesthroughpost-Eocene strikeerable fragmentation slip faultingbetween the NorthAmericanand Caribbean plates. EPILOGUE The biogeographiccommunityshould be aware that recent developmentsin the geological sciences stronglysuggestthat even withinthe mobilisticconcept of plate tectonics there has apparentlybeen considerablymore mobilityin the evolution of continentalmarginmountainsystems, such as the American Cordilleras,thanwas recognizedonlyseveral years ago. The recordin westernNorth America,forexample (Coney et al., 1980; Coney, 1981), shows thatall of Alaska, two-thirdsof the Cordillera in western Canada, about 50% of the Cordillera in western United States, and as much as 80% of Mexico (Campa & Coney, in press) are made up of what are termed "suspect terranes." These terranesare suspect because paleogeographiccontinuitywith the cratonic old part of North America cannot be proven throughlarge periods of post-Precambriantime. Paleomagneticdata, stratigraphicand structuraldata, and distributionsof faunas 1982] CONEY-PLATE TECTONIC CONSTRAINTS 443 support the concept that many of the terranesare fragmentsof various sorts reaches of the Pacific Ocean. swept against North America, some fromfar-flung Most of the accretions onto NorthAmerica occurredsince Middle Jurassictime. The biogeographicimplicationsof all this have hardlybegun to be explored. LITERATURECITED terranesand mineralresource distributionsin CAMPA,M. F. & P. J. CONEY. Tectono-stratigraphic Mexico. Can. Jour.Earth Sci. (in press). CASE, J. E. 1974. Oceanic crustformsbasement of easternPanama. Geol. Soc. America. Bull. 85: 645-652.. CONEY, P. J. 1978. Mesozoic-Cenozoic Cordilleraplate tectonics. Geol. Soc. America Mem. 152. . 1981. Accretionarytectonics in western North America. Ariz. Geol. Soc. Digest 14: 23-37. D. L. JONES & J. W. H. MONGER. 1980. Cordilleransuspect terranes. Nature 288: 329333. Cox, ALLAN. 1973. Plate Tectonics and Geomagnetic Reversals. W. H. Freeman and Co., San Francisco. 702 pp. DICKINSON,W. R. & P. J. CONEY. 1980. Plate tectonic constraintson the originof the Gulf of Mexico. Pp. 27-36 in R. H. Pilger (editor), The Origin of the Gulf of Mexico and the Early Opening of the Central NorthAtlanticOcean. Louisiana State UniversitySymposium. GALTON,P. M. 1977. The ornithopoddinosaurbryosaurusand a Laurasic-Gondwanalandconnection in the upper Jurassic.Nature 268: 230-232. GRAHAM,S. A., W. R. DICKINSON& R. V. INGERSOLL. 1975. Himalayan-Bengalmodel forflysch dispersal in the Appalachian-Quachitasystem. Geol. Soc. America Bull. 86: 273-286. IRVING,E. 1977. Driftof the major continentalblocks since the Devonian. Nature 270: 304-309. KHUDOLEY, K. M. & A. A. MEYERHOFF. 1971. Paleogeographyand geologic historyof the Greater Antilles.Geol. Soc. America Mem. 129: 1-199. LADD, J. W. 1976. Relative motionof South Americawithrespectto NorthAmerica and Caribbean tectonics. Geol. Soc. America Bull. 87: 969-976. 1972. Tectonic rotationssuggestedby paleomagneticresults W. D. & N. D. OPDYKE. MAcDONALD, fromnorthernColumbia, South America. Jour.Geophysical Research 77: 5720-5730. 1972. Circum-Caribbeantectonicand igneous activityand B. T. & M. G. DINKELMAN. MALFAIT, the evolutionof the Caribbean plate. Geol. Soc. America Bull. 83: 251-272. MARTIN,R. G. & J. E. CASE. 1975. Geophysical studies in the Gulf of Mexico. Pp. 65-106 in A. E. M. Nairu & F. G. Stehli(editors),The Ocean Basins and TheirMargins,Volume 3: The Gulfof Mexico and the Caribbean. Plenum Press, New York. NAIRU, A. E. M. & F. G. STEHLI (editors). 1975. The Ocean Basins and Their Margins, Volume 3: The Gulf of Mexico and the Caribbean. Plenum Press, New York. 706 pp. 1974. Possible left-lateralEarly to Middle Mesozoic disruption SILVER, L. T. & T. H. ANDERSON. of the southwesternNorthAmericancratonmargin.Geol. Soc. America, Abs. withPrograms6: 955-956.
