Lower Mesozoic detrital zircon geochronology of Sonora, México 301

ThelowerMesozoicrecordofdetritalzirconU-PbgeochronologyofSonora,México,anditspaleogeographicimplications

Carlos M. González-León1,*, Victor A. Valencia2, Timothy F. Lawton3, Jeffrey M. Amato3, George E. Gehrels2, William J. Leggett3,

Oscar Montijo-Contreras4, and Miguel A. Fernández5

1 Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, 83000 Hermosillo, Sonora, Mexico.

2 Geosciences Department, University of Arizona, Tucson, AZ, 85721, USA.3 Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico, 88003, USA.

4 Posgrado en Ciencias de la Tierra, Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, 83000 Hermosillo, Sonora, Mexico.

5 Servicios Industriales Peñoles S.A. de C.V., Blvd. Navarrete 277, Colonia Racquet Club, 83200 Hermosillo, Sonora, Mexico.

* cmgleon@servidor.unam.mx

ABSTRACT

Detrital zircon U-Pb geochronology from each of the formations of the Triassic-Lower Jurassic Barranca and El Antimonio groups of central and northwestern Sonora and from the Lower Jurassic Basomari and Middle Jurassic Lily formations of northern Sonora indicate they contain distinctive zircon populations. A Proterozoic population has peak ages near 1.8, 1.7, 1.6, 1.4, and 1.1 Ga. A population of Permo-Triassic grains with important peak ages near 269, 254, 245, 234 and 227 Ma. A third population of Early Jurassic age (~190 Ma) is only present in the middle member of the Lower Jurassic Sierra de Santa Rosa Formation and in the Basomari Formation. The fourth population of Middle Jurassic zircons with age peaks near 168 and 162 Ma is only present in the Lily Formation. A fifth population of Neoproterozoic and Paleozoic zircons, present only in the Basomari and Lily Formations, has Silurian and Devonian grains (~430 – 380 Ma) and Neoproterozoic grains (590 – 547 Ma). Possible source areas for these populations are Proterozoic igneous and metamorphic basement and/or Neoproterozoic and Paleozoic sandstones of southwestern USA and Sonora, the mostly Triassic magmatic arc of the Mojave Desert in California and the Permo-Triassic arc of northern Mexico, the Jurassic continental magmatic arc of southwestern North America, and the Jurassic eolian sand seas in Arizona.

Regional lithofacies, fossils, and paleocurrents indicate that the Barranca Group records a large fluvio-deltaic system on the margin of the El Antimonio marine basin, and Proterozoic and Permo-Triassic zircon populations common to the Barranca and El Antimonio Groups indicate that detritus was derived from the same source areas to the north. Lithofacies, age, and detrital zircon populations of the Basomari and Lily Formations indicate that they were deposited within the Jurassic magmatic arc of North America. The Basomari and Lilly Formations contain abundant Early Jurassic zircon grains, as does the middle member of the Sierra de Santa Rosa Formation; however, the Sierra de Santa Rosa Formation lacks a Neoproterozoic and Paleozoic grain population present in the Basomari and Lily Formations. The Basomari Formation, which is located north of the proposed trace of the Mojave-Sonora megashear, contains Proterozoic granitic clasts derived from Caborcan basement, which suggests that the Caborca block must have been located close to the Basomari basin by Early Jurassic time, a relation that contradicts the existence of the Mojave-Sonora megashear. The new data also indicate a maximum Early Triassic

Revista Mexicana de Ciencias Geológicas, v. 26, núm. 2, 2009, p. 301-314

González-León,C.M.,Valencia,V.A.,Lawton.T.F.,Amato,J.M.,Gehrels,G.E.,Leggett,W.J.,Montijo-Contreras,O.,Fernández,M.A.,2009,ThelowerMesozoicrecordofdetritalzirconU-PbgeochronologyofSonora,México,anditspaleogeographicimplications:RevistaMexicanadeCienciasGeológicas,v.26,núm.2,p.301-314.

González-León et al.302

depositional age for the previously undated Arrayanes Formation and correlation with the Antimonio Formation on the basis of a shared young detrital zircon peak age at ~254 Ma. A Coyotes Formation sample fails to confirm its supposed Early Jurassic age as it only yielded Proterozoic grains. Young zircon grain ages in the Lily Formation indicate a maximum Middle Jurassic age for that formation.

Key words: detrital zircon, Barranca Group, El Antimonio Group, Triassic-Jurassic, Sonora, Mexico.

RESUMEN

Nuevas edades obtenidas del fechamiento U-Pb de circones detríticos de cada una de las formaciones de los grupos El Antimonio y Barranca (Triásico-Jurásico), así como de las formaciones jurásicas Basomari y Lily, de Sonora, permiten distinguir las siguientes poblaciones de circones detríticos. Una población proterozoica que da grupos de edad cercanos a 1.8, 1.7, 1.6, 1.4 y 1.1 Ga. Una población permo-triásica que da grupos de edades cercanas a 269, 254, 245, 234 y 227 Ma. Una tercera población de edad jurásica temprana (~190 Ma) que sólo se presenta en el miembro medio de la Formación Sierra de Santa Rosa y en la Formación Basomari. La cuarta población sólo se encuentra en la Formación Lily y consiste en grupos de circones que dan edades de 168 y 162 Ma. La quinta población es de circones neoproterozoicos (edades entre 590 y 547 Ma) y paleozoicos (silúricos y devónicos; edades entre 430 y 380 Ma) y está presente en las formaciones Basomari y Lily. Las posibles áreas fuente para estas poblaciones de circones detríticos son los basamentos proterozoicos y/o las secuencias sedimentarias neoproterozoicas y paleozoicas del suroeste de los Estados Unidos y Sonora; el arco magmático de edad principalmente triásica del Desierto Mojave en California y el arco permo-triásico del norte de México; el arco magmático continental jurásico del suroeste de los Estados Unidos y el norte de México, y los campos eólicos jurásicos del centro-norte de Arizona. Las litofacies, fósiles y datos de paleocorrientes conocidos del Grupo Barranca indican que su deposito se debió a un importante sistema fluvio-deltáico que desembocaba en las áreas marginales de la cuenca marina donde se depositó el Grupo El Antimonio y sus poblaciones de circones proterozoicos y permo-triásicos indican que sus detritos fueron derivados de áreas localizadas hacia el norte de Sonora.

Las litofacies, edad y poblaciones de circones detríticos de las Formaciones Basomari y Lily muestran que estas unidades fueron depositadas dentro del arco magmático jurásico y comparten su población de circones de ~190 Ma con el miembro medio de la Formación Sierra de Santa Rosa, la cual, sin embargo, no contiene los circones neoproterozoicos que sí tienen esas dos unidades. A pesar de estas diferencias, la Formación Basomari que se localiza al norte de la traza propuesta de la megacizalla Mojave-Sonora contiene clastos ígneos del basamento del bloque Caborca, lo cual sugiere que ambas entidades deben haber estado localizadas en posición cercana durante el Jurásico Temprano, no apoyando de esa manera la idea de la existencia de la mencionada falla. Los nuevos datos proveen además una edad máxima del Triásico Temprano para la Formación Arrayanes, ya que la edad más joven de un grupo de sus circones detríticos es de 254 Ma, el cual también está presente en la Formación Antimonio. Los circones datados de la Formación Coyotes no probaron su supuesta edad jurásica temprana ya que sólo proporcionaron edades proterozoicas, mientras que los grupos más jóvenes de circones detríticos de la Formación Lily proporcionaron una edad máxima de Jurásico Medio para esa unidad.

Palabras clave: circones detríticos, Grupo Barranca, Grupo El Antimonio, Triásico-Jurásico, Sonora, México.

INTRODUCTION

Recentadvancesinlaserablationinductivelycoupledmassspectrometrypermitanalysisoflargenumbersofdetritalzirconagesinsedimentaryunits.Determinationofzirconages,combinedwithknowledgeofstratigraphy,sedimentologyandbiostratigraphy,providesusefulinfor-mationthatcontributestotheunderstandingofdifferenttectonicproblems,includingevolutionofbasins,prov-enanceandpaleogeography.Forinstance,adetritalzirconreferenceframehasbeenadvancedintherecentyearsforthe

NeoproterozoictoTriassicstrataofwesternNorthAmericawiththeobjectiveofconstrainingtheterraneevolutionofthatregion(Gehrels,2000);theresultingdatasetprovidesabasisofcomparisonfordatafromawidearea.

AprovenancedatasetfortheNeoproterozoicandPaleozoicmarinesandstonesofSonora,basedonU-Pbgeo-chronologyofdetritalzircons,hasrecentlyemergedastheresultofanumberofseparatestudies.QuartzarenitesfromtheNeoproterozoicLasViboras,ElAguilaandCerroLasBolasgroups(Stewartet al.,2002;IzaguirreandIriondo,2007)thatextendfromnorthwesterntosouth-centralSonora

Lower Mesozoic detrital zircon geochronology of Sonora, México 303

ofSonoramaybetheProterozoicbasementofsouthwesternUSAandSonora(e.g.,Karlstromet al.,1990,2004;GehrelsandStewart,1998;Stewartet al.,2001;Iriondoet al.,2004;AndersonandSilver,2005;Farmeret al.,2005;Nourseet al.,2005;Amatoet al.,2008a,Pooleet al.,2008),althoughPooleet al.(2008)havesuggestedthatsomedetritusinthePermianforedeepsequencemaybederivedfromanevolv-ingaccretionarywedgetothesouth.

PreviouslypublisheddetritalzircongeochronologyforlowerMesozoicstrataofSonoraisrestrictedtoafewsandstonesamplesof theTriassicandLowerJurassicBarrancaandElAntimoniogroups(Figure1).ResultsofdetritalzirconanalysisfromthreesandstonesamplesoftheElAntimonioGroupwerereportedbyGonzález-Leónet al.(2005)andtheresultsofonesamplefromtheSantaClaraFormationoftheBarrancaGroupwerepreviouslyreportedbyGehrelsandStewart(1998).Inthispaper,wereportnewdataondetritalzircongeochronologyfromeachofthesixrepresentativeformationsthatcomposetheTriassic-LowerJurassicBarrancaandElAntimoniogroupsofcentraland

haveyieldeddistinctiveProterozoicpopulationswithpeaksonrelativeprobabilitydistributiondiagrams(Ludwig,2003)near1.7,1.4and1.1–1.0Ga,whereastheNeoproterozoictoearlyPaleozoicCordilleranmiogeoclinalsuccessionofnorthwesternSonorayieldeddetritalzirconpopulationpeakswithagesaround1.1and0.68Ga(Stewartet al.,2001).TheCambrianProveedoraandBolsaQuartziteshaveadominantpopulationwithapeakaround1.1Gaandminorpeaksat1.7,1.4and1.2Ga(GehrelsandStewart,1998;Grosset al.,2000;Stewartet al.,2001).OtherPaleozoicsandstonesfromtheeugeoclinalandmiogeoclinalsucces-sionsofSonoraalsoyieldeddistinctivezirconpopulations.Ordovicianmiogeoclinalandeugeoclinalstratayieldedzirconswithagesthatclusterat~1.8Ga,~1.1GaandanArcheanpeakat~2.7Ga(GehrelsandStewart,1998);theDevonianandPermianmiogeoclinalandeugeoclinalstratayieldedmainagegroupsat~1.9,~1.7-1.6,~1.4and1.1–1.0Ga(GehrelsandStewart,1998;Pooleet al.,2008).ThisreferencedatasetindicatesthatsourceareasformostofthedetritalsedimentsoftheNeoproterozoicandPaleozoicstrata

Figure1.MapofSonorashowinglocalitiesmentionedandmainoutcropsofrockunitsdescribedinthiswork.Msm:proposedtraceoftheMojave-Sonoramegashear(fromAndersonandSilver,1981).

González-León et al.304

northwesternSonora,respectively.WealsoreportnewdataobtainedfromonesamplecollectedfromtheLowerJurassicBasomariFormationandfromtwosamplesoftheMiddleJurassicLilyFormation.ThesenewanalysescontributetoamorecompletedatasetondetritalzircongeochronologyfortheMesozoicstratigraphyofSonora,whichinturnshedslightontheinterpretationofdetritalprovenanceoftheseunits.OurresultsprovideabetterunderstandingoftheTriassicandJurassicpaleogeographyofSonoraandhaveimplicationsfortheexistenceofmajorMesozoicstrike-slipfaultinginSonora,especiallyinviewofthefactthattheBarrancaandElAntimoniogroupsarelocatedsouth,andtheBasomariandLilyformationsarelocatednorthofthehypotheticaltraceoftheMojave-Sonoramegashear(SilverandAnderson,1974).

TRIASSIC AND JURASSIC STRATA

Triassic and Lower Jurassic strata present innorthwesternandcentralSonoraincludetheBarranca(Alencáster,1961a)andElAntimonio(González-Leónet al.,2005)groups(Figure1).TheBarrancaGroupisdividedintotheArrayanes,SantaClaraandCoyotesformations(Alencáster,1961a)andtheElAntimonioGroupconsistsoftheAntimonio,RíoAsunciónandSierradeSantaRosaformations(Hardy,1981;LucasandEstep,1999;González-Leónet al.,2005)(Figure2).StrataoftheBarrancaGroupareestimatedtobe~3.5kmthickandrepresentmostlyalluvial, fluvial, and transitional deltaic to shallow-marine deposits. The lowermost Arrayanes Formation records flu-vialdeposition;MarzolfandAnderson(2005)assignedita

Figure2.StratigraphiccolumnsandcorrelationoftheElAntimonioandBarrancagroups,andtheRanchoBasomariandLilyformations.Starsandnumbersindicatestratigraphicpositionofdatedsamples.

Lower Mesozoic detrital zircon geochronology of Sonora, México 305

MiddletoLateTriassicagebasedontectono-sequencecor-relations.TheSantaClaraFormationistheonlyfossiliferousunit of the group and includes fluvial, deltaic, and shallow-marineterrigenousstratathathaveanabundantLateTriassic(Carnian-Norian?) paleoflora (Weber, 1997, and references therein)anduncommonmarinefossils(Alencáster,1961b).AlluvialconglomerateoftheCoyotesFormationuncon-formablyoverlietheSantaClaraFormationanditsageisinferredtobeEarlyJurassic(Alencáster,1961a;StewartandRoldán-Quintana,1991).

TheElAntimonioGroup(3.5kmthick)isdividedinto14marinesequencesintheSierradelÁlamo(Figures1and2),thelowermostofwhichisLatePermianinage(González-León,1997).TheTriassicAntimonioFormationcomprisessequencesIItoVIandspanstheDienerian(?),Spathian,Anisian,Ladinian(?)andCarnianstagesaccordingtopaleontologicandpaleomagneticdata(summarizedinGonzález-Leónet al.,2005;Steineret al.,2005).TheRíoAsunciónFormationincludessequencesVIItoIX(NoriantoRhaetian).Anerosionaldisconformitythatoverliesse-quenceIXseparatestheLowerJurassicSierradeSantaRosaFormation(sequencesXtoXIV)fromtheTriassicstrata.PartialoutcropsoftheSierradeSantaRosaFormationoccuratseveralotherlocalitiesinnorthwesternSonora,includingthehillssouthofSierraCaracahui,SierraLópez(GameñoFormationofPooleet al.,2000)andPozodeSerna(Figure1),althoughthemostcompletesection,besidesSierradelÁlamo,isatSierradeSantaRosa(Figures1and2),wherethe formation was defined and divided into lower, middle, anduppermembers(Hardy,1981).

OtherJurassicunitsinSonorathatpertaintothisstudyaretheBasomariandLilyformations,whichcropoutnearRanchoBasomari,southwestofCucurpe,andatSierraCopercuin,westofNacozari,respectively(Figures1and2).TheBasomariFormationis~750mthick;itslowermost140mcomprisesdarkpurplemassivesiltstonewithinterbeddedmatrix-supported,poorlysorted,sub-roundedtosubangular,granule-topebble-conglomeratewithlocallargeolistolithicblocksofPaleozoic(?)limestone.Upsection,a340mthickintervalconsistsofgranulesand-stonewithinterbedded,lenticular,poorlysorted,subangu-lar,massive,matrix-toclast-supportedpebbletocobbleconglomerateinbedsupto10mthick.Theupperpartofthisformationconsistsofthickmassivesandstone(upto50mthick)withlensesofgranuleconglomerate,intervals(upto30mthick)ofmassivesiltstoneandrhyolitictuffsintheuppermostpartofthisunit.Clastsintheconglomer-atearequartzite,porphyrygranite,metamorphicrocks,andesite,sandstone,basaltandchert.ItsEarlyJurassicageisindicatedbyarhyolitetuffinitsupperpartthatwasdatedat189.2±1.1Ma(U/Pb;Leggettet al.,2007).Thisformation,whichunderliestheRanchoSanMartinFormation(Leggettet al., 2005), is here interpreted as a flu-vialandalluvialcontinentaldepositbasedonlithologyandabsenceoffossils.

TheLilyFormation(McAnulty,1970)isa500m

thicksuccessioncomposedinitslowerpartofinterbeddedrhyolitic tuff and welded tuff, quartz-rich and arkosic fluvial sandstoneandpolymicticpebbleconglomerate.Itsupperpartconsistsofquartzarenite,conglomerate(withclastsofquartzarenite,andesite,rhyoliteandgraniticrocks)andsubordinate,lacustrinelimestonebeds.Thisunitcropsoutin the western flanks of Sierras Copercuin and Cobriza in northeasternSonora(about10kmwestofNacozari,Figures1and2)whereitisinfaultcontactwiththeCambrianBolsaQuarziteandintrudedbyLaramideplutons.TheageofthisunitisMiddleJurassicaccordingtoaU/Pbageof174MaobtainedbyAndersonet al.(2005)fromatuffcollectedatSierraCopercuin.ThisunitisconsideredcorrelativewiththeRanchoSanMartinFormationwhereLeggettet al.(2007)reportedaU/Pbageof168.4±1.6Mafromarhyolitic ash flow tuff.

DETRITAL ZIRCON GEOCHRONOLOGY

Mineralseparations,andU-Pbgeochronologyonin-dividualdetritalzircongrainsofthesampleswasconductedbylaser-ablation-multicollectorinductivelycoupledplas-ma-massspectrometry(LA-MC-ICP-MS)attheArizonaLaserChronCenterintheDepartmentofGeosciences,UniversityofArizonausingtheproceduresdescribedbyGehrelset al.(2006).AnalyticaldataofthestudiedsamplesarereportedintheTableA1(electronicsupplement)andtheirageprobabilitycurvesarepresentedinrelativeprob-abilitydistributiondiagrams(Ludwig,2003)inFigure3.ThePb/UconcordiaplotsarepresentedinFigure4.Allageerrors are quoted at 1σ (Ma) level, and errors for isotopic ratios are quoted at 1σ (%). All reported UTM sample loca-tionsutilizetheWorldGeodeticSystem1984.

Samples2000-1,2000-0and2000-2fromtheElAntimonioGroup(Figure1)werefirstanalyzedusingisotopedilutionandthermalionizationmassspectrometry(Gehrels,2000)andresultsfrom20grainspersamplewerereportedinGonzález-Leónet al.(2005).Forthepresentstudywereanalyzedca.100grainsfromsamples2000-1and2000-0ofTriassicageandcollectedandanalyzedanewsample(4-5-08-1)fromtheLowerJurassicsequenceXIintheSierradelÁlamo.

Sample2000-1(UTM12R:347580E,3402960N)isfromamedium-grainedsandstoneinthelowerpartofSpathiansequence3oftheAntimonioFormation.EightypercentofthetotalzircongrainsareProterozoicwithagesnear1.7,1.4and1.1Gaandapeakageat1.7Ga.EightgrainsyieldedPermo-Triassicagesbetween267and240Mawith a peak at ~254 Ma, five grains have ages between 640 and420Ma,andthreegrainsyielded,althoughdiscordant,agesbetween2.7and2.6Ga(Figures3aand4).Sample2000-0,collectedfromamedium-grained,shallowmarinesandstoneoftheNoriansequenceVIIoftheRíoAsunciónFormation(UTM12R:351461E,3399957N),yieldedmostly grains with Proterozoic ages (~90%) with a major

González-León et al.306

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1778

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1101

12241467

1764

2624

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f Ana

lysi

s Relative

probabilityARRAYANES FORMATION

SANTA CLARA FORMATION

COYOTES FORMATION1428

1637

231

245

1095

1417 17181849

248-254

1090

1436

16371702

BASOMARI FORMATION190

243-257

378-433

1059-11281414-1482

1742

LILY FORMATION 2

LILY FORMATION 1

412-422

547-5901024-1199

1421 1889

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7-26-07-1 n=98

Lily 1 n=93

WL01405Z n=95

7-28-07-1 n=92

peakat1.7Ga,andasmallpopulationofTriassicgrains(~10%) with a peak at 227 Ma (Figure 3a). Sample 4-5-08-1 wascollected(UTM12R:351169E,3399907N)fromabedof medium-grained, fluvial sandstone that overlies a 20-m-thickclastsupportedconglomerate(lowerpartofunit17inGonzález-León,1997)locatedatthebaseofsequenceXIoflateSinemurianage(PálfyandGonzález-León,2000;Tayloret al.,2001).Thissampleyieldedazirconpopulationof Proterozoic grains (78% of the grains) with peaks at 1.7 and 1.1 Ga and a Triassic population (22% of the grains) withapeakat234Ma(Figure3a).Sample2000-2fromtheyounger(lowerPliensbachian,PálfyandGonzález-León,2000)middlememberoftheSierradeSantaRosaFormationatSierradeSantaRosa(Figure2)wasreportedbyGonzález-Leónet al.(2005)tocontainLowerJurassicgrainswithapeak at 193 Ma (~55% of the grains) and Proterozoic grains withapeakat~1.1Ga(Figure3a).

ZirconswereseparatedfromsandstonesamplesoftheArrayanes,SantaClaraandCoyotesformationsoftheBarrancaGroup,yieldingthenextresults(Figure3b).Anarkosic,medium-grainedsandstonefromtheLowerMemberoftheArrayanesFormation(sample7-

26-07-1)collectedintheSanJavier-LaBarrancaregion(Figures1and2)(UTM12R:634347E,3167294N)yieldedProterozoic grains (~60%) with age peaks near 1.7, 1.6, 1.4 and 1.1 Ga, the largest peak at 1.4 Ga. 33% of the grains are TriassicandPermianwithagesbetween288and239Maandapeakat250-254Ma.Threegrains(althoughtwoofthemarediscordant)haveagesbetween~465and~300Maandtwootherswithagesbetween2.8and2.6Ga(Figure3b).Onequartzarenitic,coarse-grainedsandstonefromtheSantaClaraFormationcollectedinthelocalityofminaLaLourdesarea(UTM12R:549022E,3163600N;MontijoContreras,2007)yieldedzircongrainswithagesbetween297 and 205 Ma (50% of the grains) with peak ages at 269, 245and231Ma,andProterozoicgrainswithapeakat1.1Gaandsubordinatepeakagesat1.7and1.4Ga(Figure3b).Onepebble-sandstonesamplefromtheCoyotesFormation(sample7-28-07-1)collectedabout2.5kmsouthofthetownofSanJavier(Figure1),alongthemainroad(UTM12R:622512E,3162331N),yieldedonlyProterozoicgrainsintwo populations with peak ages near 1.4 Ga (~60% of the grains) and 1.6 Ga (~30% of the grains) (Figure 3b).

Asandstonesample(WL01405Z)collectedfromthe

Figure3.AgeprobabilitydiagramsforTriassicandJurassicsamples.a:samplesfromtheAntimonio,RíoAsunciónandSierradeSantaRosaforma-tionsoftheElAntimonioGroup,includingsample2000-2(thick,grayline)fromthemiddlememberoftheSierradeSantaRosaFormationreportedinGonzález-Leónet al.(2005).b:SamplesfromtheArrayanes,SantaClaraandCoyotesFormationsoftheBarrancaGroup.c)SamplesfromtheBasomariandLilyFormations.

Lower Mesozoic detrital zircon geochronology of Sonora, México 307

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lowerpartoftheBasomariFormation(Figure2)yieldedEarly Jurassic grains (17%) with a major peak age near 190 Ma, Triassic and Permian grains (13%) with peaks near 243–257 Ma, Paleozoic and Neoproterozoic grains (20%) withagesbetween645to310Ma,andProterozoicgrains(~50%) with peaks near 1.7, 1.4, 1.1 and 1.0 Ga (Figure 3c). ZirconsfromtwodifferentsandstonesamplesoftheLilyFormation collected from the southwestern flank of Sierra Copercuin(sampleLily1locatedatUTM12R622131E,3361530N;andsampleLily2at621809E;3362217N)yieldedamaximumlateMiddleJurassicageforthisunit.Both samples yielded Jurassic zircons (25–43% of the total grains)withagesbetween195and160Maandpeakages~164and160Ma,respectively,TriassicandPermiangrainswith ages between 300 and 200 Ma (<10% of the grains) thatshowsubordinateconcentrationofagesat214and217Ma, Paleozoic and Neoproterozoic grains (11–22% of the grains)withagesbetween954and385Maandconcentra-tionsat590–547and412–422Ma,andProterozoicgrains(27% of the grains) with concentration of ages near 1.8, 1.4 and1.0-1.2Ga(Figure3c).

DISCUSSION AND CONCLUSIONS

Zircon populations

ThenewdetritalzircongeochronologydatareportedhereinfromninesamplesofTriassicandLowerandMiddleJurassicstratapermitnewinferencesregardingtheearlyMesozoicprovenanceandpaleogeographyofSonora.ThesedataalsoprovideamaximumEarlyTriassic(IUGS-ICSchart,2007)depositionalageforthepreviouslyundatedArrayanesFormation,whichaccordingtoitsyoungerzirconpeakageitmustbeyoungerthanabout250Ma.ThisdataalsoindicatethattheArrayanesmaybeaboutthesameageastheAntimonioFormation,asbothunitsshareacommondetritalzirconpeakageat~254Ma.Ontheotherhand,allofthezirconsanalyzedfromtheCoyotesFormationareProterozoic,andthusinconclusivewithregardtoitsinferredEarlyJurassicage.Thezirconpopulationofthissamplemaybebiasedtoolderagebecauseitcontainsabundantquartzarenitepebblesrecycledfromolderstrata.

Thefollowingdistinctivezirconpopulationsidenti-fied in the analyzed samples are described and discussed according to their decreasing abundance. The first popu-lation(Figure3d)belongstoProterozoiczirconswhoseabundance varies between 30 and 100% in the analyzed samples,andhaspeakagesaround1.8,1.7,1.6,1.4,and1.1Ga.ProterozoiczirconsaremoreabundantintheTriassicsamples where they compose 50–90% of the analyzed grains butdecreaseintheJurassicsampleswheretheycompose30–50 % of the analyzed grains. The exception to this trend istheCoyotesFormationwhich,asnoted,maybebiasedbytheabundanceofquarzareniteclaststhatarerecycledfromProterozoicand/orPaleozoicsedimentaryunitsin

Figure4.U-Pbconcordiaplotsfordetritalzirconsfrom(a)ElAntimonioGroup,(b)BarrancaGroup,and(c)BasomariandLilyformations.Errorsareshownatthe2-sigmalevel.

González-León et al.308

Sonora,whichhavesimilardetrital-zirconagepopulations(GehrelsandStewart,1998;Stewartet al.,2002)tothisunit.ProterozoicgrainsintheElAntimonioGroupsampleshavedominantpeakagesnear1.7,1.4and1.1Ga,whereasthoseoftheBarrancaGrouphaveadominantpeakagenear1.4Gaandsubordinatepeaksat~1.7and1.1Ga.TheJurassicBasomariandLilyformationshavedispersedProterozoicpeaksnear1.8,1.7,1.2,and1.0Ga.

AseconddistinctiveagepopulationconsistsofPermian and Triassic zircons that make up between 8% and 50% of the analyzed samples. This population is prominent intheTriassicformationsanditsabundancedecreasesintheJurassicformations.ThePermo-TriassicgrainsaremoreabundantintheBarrancaGroup,inwhichtheycomposeup to 50% of the analyzed grains, than in the El Antimonio Group in which they compose no more than 22% of the analyzedgrains.PermiangrainsdominateovertheTriassicgrainsintheArrayanesandSantaClarasamples,andarelargelyabsentfromtheformationsoftheElAntimonioGroupexceptforuncommongrains in theAntimonioFormation.TriassicgrainsintheElAntimonioGroupin-crease in abundance upsection from 3% in the Antimonio Formation, 10% in the Río Asunción Formation and 22% inthelowerpartoftheSierradeSantaRosaFormation,butarenotpresentintheMiddleMemberoftheSierradeSantaRosaFormation(sample2000-2inGonzález-Leónet al.,2005;Figure3a).ThePermo-TriassicgrainsintheBasomari Formation make up nearly 13% of the analyzed grains,butTriassicgrainsarescarce.Thissamepopulationcomposes about 10% of the total analyzed grains in the LilyFormation.

ThePermo-TriassiczircongrainsoftheBarrancaGrouphavepeakagesnear284,269,254,~246and231Ma,peakagesnear254,239,234and227MaintheElAntimonioGroup,andpeakagesnear257–243MaintheBasomariFormation.Aminorpeakageat~217MaispresentintheLilyFormation(Figure3).

ThethirdzirconpopulationofEarlyJurassicageispresentinthemiddlememberoftheSierradeSantaRosaFormationandintheBasomariFormation.Themid-dlememberoftheSierradeSantaRosaFormationhasaprominent Jurassic age population (55%) with a peak age near190Ma,similartotheBasomariFormationthathasa population (17%) with a peak age at 190 Ma (Figure 3). It is also present, but subordinate (<10%), in the Lily Formation and was not identified in the lower part of the SierradeSantaRosaFormation(sample4-5-08-1),whereabiostratigraphiclateSinemurianageisthesameasthegeochronologicage(189.2±1.1Ma)ofatuffintheup-perpartoftheBasomariFormation(Leggettet al.,2007;Figure2).ThefourthpopulationisonlypresentintheLilyFormationandconsistsofMiddleJurassiczirconswithagepeaksat168(sampleLily1,Figure3)and162Ma(sampleLily 2, Figure 3) and has an abundance of ~30% of the total analyzedgrains.

A fifth population that consists of Neoproterozoic and

PaleozoiczircongrainsispresentintheBasomariandLilyformations.ThePaleozoicgrainsareSilurianandDevonianin age and compose ~13% of the analyzed grains in the Basomari Formation and ~10% of the grains in the Lily Formation.TheBasomarihaspeakagesat433–378MaandtheLilyshowsconcentrationsofagesat422–412Ma.The Neoproterozoic zircons compose up to 10% in the Lily Formation,inwhichtheyshowconcentrationsat590–547Ma and they compose ~6% in the Basomari Formation, inwhichtheyshowasubordinatepeakat612Ma(notindicatedinFigure3c).Veryscarcegrainswithscatteredagesbetween~400and~1000MaoccurintheArrayanes(n=1),SantaClara(n=2),Asunción(n=1)andAntimonioformations(n=6).AsmanyasthreegrainswithArcheanagesbetween2.8and2.6GaarepresentineachoftheLily,ArrayanesandtheAntimonioformations.

Provenance

TheprominentpopulationofdetritalzirconswithProterozoicageswithpeaksnear1.7,1.6,1.4,and1.1GawasprobablyderivedfromProterozoicigneousandmeta-morphicbasementofthesouthwesternUSAandSonora.ThisbasementincludestheYavapaiandMazatzalprovincesand~1.1and~1.4Gagraniticplutonsthatintrudethem(seereferencescitedabove).ThegrainsalsomayhavebeenrecycledfromNeoproterozoicandPaleozoicsandstonesthatcontainsimilargrain-agepopulations(referencescitedabove).Thenearabsenceof1.8Gazirconsthatcouldreflect provenance of the Mojave province, however, is noteworthy;uncommongrainsofthatageoccuronlyintheSantaClaraandLilyformations.Thiscouldindicatethat theProterozoicgrainsweremostlyrecycledfromNeoproterozoicandPaleozoicsedimentaryrocksofSonoraandthesouthwesternUSA,inwhichgrainsolderthan~1.7Gaarenearlyabsent(GehrelsandStewart,1998;Grosset al.,2000;Stewartet al.,2001).

PossiblesourceareasforthePermo-TriassiczirconpopulationintheBarrancaandElAntimoniogroupsaremorecontroversialanduncertain,buttheyincludethemost-lyTriassicmagmaticarcoftheMojaveDesertinCalifornia(BarthandWooden,2006)andthePermo-TriassicarcofnorthernMexico(Torreset al.,1999).ThesecondregionwasconsideredbyGehrelsandStewart(1998)tohavebeenthesourceofPermo-TriassicdetritalzirconsintheBarrancaGroup and the first region was considered by González-Leónet al.(2005)asasourceareafortheElAntimoniostratazircons.Permo-TriassicrocksoftheMojaveDesertaregraniticplutonsthatrangeinagefrom~210Mato~260Ma(Barthet al.,1990,Milleret al.,1995,Barthet al.,1997,Barthet al.,2001;Walkeret al.,2002;BarthandWooden,2006),whereasthePermo-TriassicmagmaticarcofnorthernMexicointhenearbystatesofChihuahuaandCoahuilaincludesmostlygraniticintrusivesandrhyoliticrockswithagesbetween270to220Ma(thepublished

Lower Mesozoic detrital zircon geochronology of Sonora, México 309

dataarepresentedinTable1,whilefourotherK/Aragesbetween~266and~250MaofgraniticintrusivesfromthatregionwerereportedbyTorreset al.,1999,astakenfromPEMEXinternalreports).Inaddition,graniticclastswithintheSanMarcosFormationinnorth-centralMexicoaredatedbetween225and213Maandwereprobablyderivedfromthatarc(inMcKeeet al.,1990,McKeeet al.,1999).ThePermo-TriassiczirconpopulationfromtheBarrancaandElAntimoniogroupshaspeakagesnear269,~250,~240,~234and227Ma,someofwhicharecommontobothgroups.TheJurassicBasomariandLilyformationsshowclosesimilarpeakagesat257,243,and~217Ma.

UpperTriassicstratainthewesterninteriorUSA,forexampletheChinleFormationinArizonaandequivalentOsobbFormationinNevada,andDockumGroupandSantaRosaFormationinNewMexicoandTexas,respectively,yieldsimilarPermo-Triassicpopulationsofdetritalzircons(GehrelsandDickinson;1995;Riggset al.,1996;Foxet al.,2005;Foxet al.,2006).TheChinleandDockumgroupsinNewMexicoandTexashaveagepeaksat280–220Ma(FoxandLehman,2005)and295–225Ma(Foxet al.,2006);thesourcesofdetritusinthesestratahavebeeninterpretedasthemagmaticarcofnortheasternMexicoandtheCordilleranmagmaticarcinsouthwesternUSA.Similarly,TriassicdetritalzirconpopulationsintheLateTriassicformationsoftheLuningandPineNutassemblagesofsouthernNevadayieldzirconswithagesbetween243and218Maandpeakprobabilityagesat231Maand223Ma(Manuszaket al.,2000),whoseprovenancewasassignedtoarc-typerelatedmagmatismoftheSierra-KlamathregioninsouthwesternUSA(Manuszaket al.,2000).

TheEarlyJurassiczirconpopulation(agepeaknear190Ma)iswellrepresentedintheBasomariandmiddlememberoftheSierradeSantaRosaFormation(Figure3a

and3c).ThispopulationispoorlyrepresentedintheLilyFormationandisabsentfromthelowermostpartoftheSierradeSantaRosaFormation(sample4-5-08-1).ThispopulationprobablyrecordstheinceptionoftheJurassiccontinentalmagmaticarcofsouthwesternNorthAmerica,whoseoldest,althoughscarce, igneousrecordoccursinsouthernArizonaandintheMojaveDesertregions(Asmeromet al.,1990;RiggsandBusby-Spera,1990;Riggset al.,1993;Milleret al.,1995).TheMiddleJurassiczirconpopulationisabundantintheLilyFormationandisalsoconsideredtobederivedfromtheJurassicconti-nentalmagmaticarc,whichbyMiddleJurassictimewaswelldevelopedinsouthwesternUSAandnorthernSonora(Andersonet al.,2005).

TheNeoproteozoicandPaleozoiczirconpopulationrepresentedintheBasomariandLilyformationsisnearlyabsentinformationsoftheElAntimonioandBarrancagroups.ZirconsoftheseageswereprobablytransportedbywindintotheJurassicmagmaticarc(Riggset al.,1993)ofSonora from Jurassic eolian sand fields in Arizona (Wingate, NavajoandEntradasandstones),wheregrainsofpopula-tionBofDickinsonandGehrels(2003),withanagerangebetween747and309Ma,arewellrepresented.TheLilyFormation records fluvial, alluvial and lacustrine deposition concomitantwithrhyoliticvolcanismoftheCordilleranvolcanic arc, but the fluvial quartz-rich beds likely record reworkingofeoliandeposits.JurassiceolianitedepositswithinthisarcarepresentinnorthernSonorawithintheRanchoSanMartinFormation(whoseyoungestdetritalzirconshaveagesnear169±6Ma;Leggettet al.,2005),intheElenitaFormation(Valentine,1936)nearCananeaandinotherlocalitieslikeSierraSanAntonio,eastofNogales(C.G-L.unpublisheddata),andatPlanchasdePlata,westofNogales(RiggsandBusby-Spera,1991).

Locality Rock Age (Ma) Reference

SanMarcos(Ch) GneissclastinSMF 230±3(Rb/Sr) McKeeet al.(1990)Aldama(Ch) Granite 250±20(K/Ar) inTorreset al.(1999)Carrizalillo(Ch) Granite 267±21(K/Ar) inTorreset al.(1999)Plomosas(Ch) Rhyolite 270±30 (Pb-α) deCsernaet al.(1970)SierraMojada(Co) QmdclastinSMF 213±14(Rb/Sr) McKeeet al.(1990)SierraMojada(Co) QmdclastinSMF 215±2(U/Pb) inMcKeeet al.(1999)S.LosRemedios(Co) Tonalite 216(Ar/Ar) MolinaGarza(2005)

ca.220(U/Pb) Joneset al.,1995(inMolinaGarza,2005)ElCoyote(Co) Granodiorite 220±4(U/Pb) inMcKeeet al.(1999)PozoMayran1(Co) Rhyolite 222±2(Rb/Sr) inGrajales-Nishimuraet al.(1992)SierraMojada(Co) QmdclastinSMF 225±4(Rb/Sr) McKeeet al.(1990)PozoPaila1(Co) Ignimbrite 236±39(Rb/Sr) inGrajales-Nishimuraet al.(1992)PotreroMula(Co) Granite 238±2(U/Pb) inMcKeeet al.(1999)LasDelicias(Co) Granodiorite 256±20(K/Ar) inTorreset al.(1999)LasDelicias(Co) DaciteclastinLDF 268±6(U/Pb) inMcKeeet al.(1999)PozoTarahumara(Co) Andesite 272±22(?) Morenoet al.(2000)

Ch:Chihuahua,Co:Coahuila,Za:Zacatecas,Ta:Tamaulipas,SMF:SanMarcosFormation(LowerCretaceous),Qmd:Quartzmonzodiorite,LDF:LasDeliciasFormation(UpperPermianinMolina-Garza,2005).

Table1.PermianandTriassicigneousrocksreportedfromnorthernMexico.

González-León et al.310

29° 00’

Gu l f

o f

Ca l i f o r n i a

CucurpeCaborca

Alamos

Antimoniopaleobay

S O N O R A

Outcrop and inferred extension of the fluvial, deltaicand marginal marine strata of the Barranca Group

Outcrops and inferred extension of marine depositionof the Triassic and Lower Jurassic El Antimonio Group

Bivalves and ammonites in Triassic marine strata

Proposed trace for the Mojave-Sonora megashear

Speculative Triassic drainage systemfor the Barranca Group

Fossil plants and coal in Triassic strata

100 km

USA

Pacific Ocean

Gul

fofM

exic

o

MEXICO

Sonora

ColoradoPlateau

MS m

A R I Z O N A

Permo-TriassicCordilleranmagmatic

arc

Permo-Triassicmagmatic arcof northern

Mexico

MS m

San Marcialbasin

HERMOSILLO

27° 00’

31° 00’

113° 00’ 111° 00’

Figure5.InferredpaleogeographyofSonoraduringTriassicandEarlyJurassictimeifdisplacementbytheMojave-Sonoramegashearisnotsupported.ArrowsininsetmapindicateinferredprovenancefordetritalzirconsintheTriassicandJurassicformationsofSonora,asdiscussedinthemanuscript.

Detrital zircon ages and paleogeography

Alencáster (1961a) first proposed that the Barranca GroupincentralSonorawasdepositedintheSanMarcialbasinthatconnectedtothenorthwestwiththeElAntimoniopaleo-bay(“paleobahíadelAntimonio”),wheretheElAntimonioGroupwasdeposited(Figure5).StewartandRoldán-Quintana(1991)proposedthatdepositionoftheBarrancaGroupoccurredbylargefluvialsystemsinariftbasinadjacenttohigh-reliefupliftsincentralSonora.Stewartet al.(1997)alsoproposedthattheembaymentcon-nectionbetweentheSanMarcialbasinandElAntimoniopaleo-baywasrecordedbytheUpperTriassic–LowerJurassic(?)successionoftheSierraSantaTeresa(Figures1and5)thatlithologicallyresemblesstrataoftheBarrancaGroupandcontainsstrataandfossilsofsequenceVIIoftheRíoAsunciónFormation(Goodwin,1999).BasedonnewlydescribedsectionsoftheBarrancaGroupincentralSonoraandoncorrelationwiththeElAntimoniostrata,Montijo-Contreras(2007)proposedthatstrataoftheBarranca Group were deposited in a large fluvio-deltaic and shallowmarinesettingthatgradedsouthwardandwestwardintoopenmarineenvironments(Figure5).Previously,StanleyandGonzález-León(1995)consideredthattheElAntimoniosuccessionwasdepositedadjacenttosouthernCalifornia,intheUSA,laterthrustedoveritsCordilleranmiogeoclinalbasementoftheCaborcablock,andsubse-

quentlytranslatedsoutheastwardtoitspresentpositionbytheMojave-SonoramegashearinMiddleJurassictime(SilverandAnderson,1974;AndersonandSilver,2005).StanleyandGonzález-León(1995)alsosuggestedthattheElAntimoniostrataweredepositedcloseto,andcorrelatedwithTriassicstrataof,southwesternUSA,liketheLuningFormationofNevada,basedonpaleontologicsimilarities.Otherauthors,basedonpaleontologicandlithologiccriteriaalsocorrelatedtheTriassicstratabetweenthesetworegions(Lucaset al.,1997;BlodgettandFrýda,2001;González-Leónet al.,2005;MarzolfandAnderson,2005;BlodgettandStanley,2006;Scholzet al.,2008).

FollowingtheMojave-Sonoramegashearhypothesis,theElAntimonioandBarrancagroupsarelocatedsouthofthetraceofthisfault,whiletheLowerandMiddleJurassicBasomariandLilyformations[alongwithotherunitsinnorthernSonoraliketheRanchoSanMartin(Leggettet al.,2005)andtheElenitaformations(Valentine,1936)]that record fluvial and eolian deposition associated with silicicmagmatismoftheNorthAmericaJurassicvolcanicarcarelocatednorthofthefault(Figure5).Alternatively,iftheMojave-Sonoramegasheardidnotexist,theBarrancaandElAntimoniogroupsrecordmarginalmarinetodistalmarinefaciesofabasinthatdevelopedclosetoitspresentposition,southofwheretheJurassicarcdeveloped(Lucaset al.,1999),althoughtectonicaccretionoftheElAntimoniostrataduringMiddletoLateJurassictimeissuggestedby

Lower Mesozoic detrital zircon geochronology of Sonora, México 311

paleomagneticdata(Molina-GarzaandGeissman,1996;1999).

DetritalzircondatareportedhereindicatethattheTriassicandLowerJurassicbasinsofSonorawerefedabundantlywithdetritusderivedfromexposedProterozoicbasementand/orfromtheNeoproterozoicandPaleozoicstratasuccessionofSonoraandsouthwesternUSA.Thepresentgeographicdistributionofthesebasementprov-inces,indicatethatthesedimentsourceareaswerelocatedtothenorthofthebasins,aninferencealsosupportedbypredominantlySW-directedpaleocurrentdirectionsavail-ableonlyfortheBarrancaGroup(StewartandRoldán-Quintana,1991).Ifa12to50ºJurassicclockwiserotationoftheCaborcablock(Molina-GarzaandGeissman,1996;1999;MolinaGarzaandIriondo,2005)isrestored,paleo-currentsindicatethatthelargeriversystemthatdepositedthe Barranca Group would have flowed southward into theSanMarcialbasin.Also,ifmarginalmarinetoshallowmarinedepositiononapassivemarginisconsideredfortheTriassicformationsofSonora(González-Leónet al.,2005),then the San Marcial fluvial system was contemporaneous with a large fluvial system that fed the Potosí submarine fan incentralMexico(Centeno-García,2005).Bothofthesefluvial systems drained interior North America. However, thepassivemarginmodeliscontradictedbyevidenceforthedevelopmentofanearbyTriassiccontinentalmagmaticarcinsoutheasternCaliforniaandsouthwesternArizona(BarthandWooden,2006)thatextendedintonorthwest-ernSonora,asindicatedbythepresenceofa~233Ma(U/Pb)granite inSierraLosTanques innorthwesternSonora(Figure1;CampbellandAnderson,2003).Indeed,conglomerateandsandstoneoftheElAntimonioGroup’sTriassicformationscontainabundant igneousdetritus(González-Leónet al.,2005)thatmaybederivedfromtheactiveTriassicarc.

Accordingtodifferencesinsandstonepetrography,conglomerateclastcompositionandPermianzirconsoftheBarrancaandElAntimoniogroups,differentsourceareasforsedimentsofthetwosuccessionsarepossible.Commonigneousgrainsandclasts influvial toshallowmarinesandstoneandconglomerateoftheElAntimonioGroupformations(González-Leónet al.,2005)areabsentfromtheBarrancaGroup,wheresandstonecompositionrangesfromsubarkosictoquartz-richandclastsintheconglomeratebedsareexclusivelyquartzarenite,chertandquartz(StewartandRoldán-Quintana,1991;CojanandPotter,1991).Similarly,PermiandetritalzirconsthatareabundantintheBarrancaGroup formations (up to 23% of the total grains) are absent intheElAntimonioGroupformations,althoughTriassicgrainsinbothgroupsoccurinsimilarproportions.ThePermiandetritalzirconsintheBarrancaGroupmorecloselyresembleagesofthecontinentalmagmaticarcofnorthernMexico,wherePermianages(Table1)areolderthanintheMojavearc.Forthisreason,thePermian-Triassicmag-maticarcofnorthernMexicoisconsideredastheprobablesourceareafortheBarrancaPermianzircons.TheTriassic

zirconpopulationoftheElAntimonioandBarrancagroupshowever,couldhavebeenderivedfromeitherthenorthernMexicoortheCalifornia-Arizonamagmaticarcs,whoselatestPermian-Triassicagesoverlap.Althoughbotharcsareconsideredseparateentitiesbecauseigneousrocksof Permian and Triassic ages have not been identified in northernSonoraandsouthernArizona,itcouldhavebeenacontinuousarcthatwasobliteratedbyyoungereventslikethedevelopmentoftheJurassicmagmaticarc,orbecauseitiscoveredbyyoungerrocks,orsimplybecauseitsrockshavenotyetbeendated.

Theinformationprovidedbythenewdetritalzircondata,however,donotconclusivelysupportorrejectleft-lateraldisplacementoftheTriassic-LowerJurassicforma-tionsoftheBarrancaandElAntimoniogroups,eitherbytheMiddleJurassicMojave-Sonoramegashear(AndersonandSilver,2005)orthePermian-TriassicCalifornia-Coahuilatransformfault(DickinsonandLawton,2001),bothmodelsrequiringsoutheastwarddisplacementoftheCaborcablockbynearly1,000km.However,strikingdifferencesinzirconpopulationsarepresentintheLowerJurassicformationslocatedonoppositesidesoftheproposedfaults.WhiletheLowerJurassicBasomariFormationlocatedtothenorthof the Mojave-Sonora megashear contains a significant PaleozoicandNeoproterozoicdetritalzirconpopulationthatisalsopresentintheMiddleJurassicLilyFormation,thatpopulationisnotpresentintheLowerJurassicSierradeSantaRosaFormationlocatedtothesouthoftheinferredstrike-slipfault.IftheNeoproterozoicandearlyPaleozoicagepopulationrecordseoliantransportintotheJurassicarcofnorthernSonorafromsandseasintheColoradoPlateauashereininferred,thesesandsdidnotreachtheElAntimoniopaleo-bay,possiblybecauseofaninterveninggeographicbarrierorbecausethemarinefaciesweredepositedoffshore,far from the influence of those wind systems. In addition, theEarlyJurassiczirconpopulation(peakat~190Ma)thatispresentinthelowerpartoftheBasomariFormationandMiddleMember(lowerPliensbachian)oftheSierradeSantaRosaFormationisnotpresentinthelowerstrataoftheSierradeSantaRosaFormation(sample4-5-08-1,lateSinemurian),whichcouldindicatethatJurassicmagmaticdetritusdidnotreachthebasinbythattime,butratherar-rivedsomewhatlaterduringdepositionofthemiddlepartoftheformation.

FurtherevidencethatmayindicatetheElAntimonioandBarrancagroupswerenotdisplacedbytheMojave-SonoramegashearcomesfromtheBasomariFormation.Thisunitislocated40kmnorthoftheproposedtraceofthisfaultandU-PbagesandNdisotopicvaluesobtainedform its granitic clasts indicate they share affinity with the basementoftheCaborcablock(Amatoet al.,2009).TheageandcompositionoftheseProterozoicclastssuggestedtoAmatoet al.(2009)thattheCaborcablockmusthavebeenlocatedclosetotheBasomaribasininSonorabyEarlyJurassictime,alocationnotpredictedbythepreviousmodel(SilverandAnderson,1974;AndersonandSilver,

González-León et al.312

2005)postulatingMiddleJurassicdisplacementalongtheMojave-Sonoramegashear.

ACKNOWLEDGMENTS

González-LeónacknowledgesinternalsupportfromInstitutodeGeología,UNAM,andIndustriasPeñoles,S.A.thatfundedforgeochronologyanalysisofsamplesfromtheLilyFormation.ThisstudywaspartiallysupportedbyNSFEAR-0229565toLawtonandAmato.AZLaserChronispartiallysupportedbyNSF-EAR0443387and0732436.WeappreciatecriticalreviewsbyNancyRiggs,AlexanderIriondoandEditorSusanaAlanizthatgreatlyimprovedthemanuscript.

APPENDIX A. SUPPLEMENTARY DATA

TableA1canbefoundatthejournalwebsite<http://satori.geociencias.unam.mx/>,inthetableofcontentsofthisissue(electronicsupplement26-2-01).

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Manuscriptreceived:September30,2008Correctedmanuscriptreceived:February15,2009Manuscriptaccepted:February17,2009