Aeromagnetic anomaly modeling, Chicxulub crater 185

ABSTRACT

A structural model of the Chicxulub crater is derived from aeromagnetic anomaly modeling, borehole information and magnetic mineral data. Magnetic susceptibility measurements from borehole cores and samples in the crater show that suevite-like breccias have a variable strong magnetic signature, which is related to basement and melt clasts. The crystalline component estimated from clast analyses in the suevite-like breccias has on average higher magnetic susceptibilities (up to 1200×10-5 SI) than that of impact melt (~500×10-5 SI) and crystalline basement (400×10-5 SI). Reduction to the pole and downward analytical continuations show the discrete composite character of the anomaly, with inverse dipolar anomalies. The second-derivative of magnetic anomaly depicts five concentric rings, with the external ring correlating with the cenote ring and marking the surface expression of crater rim. The analytical signal and the radially averaged spectrum yield an estimate of the averaged depth to the magnetic sources, ranging from 1000 to 6000 m. There are three major magnetic sources within the Chicxulub crater: 1) the melt unit, 2) the suevite-like breccia, and 3) the central uplift. Using all these data, including new 2-D magnetic models, a new structural model is proposed. It reveals a system of regional vertical faults that explain the magnetic signal over the southern sector of the crater, whereas a 2.5 km deep central uplift and highly magnetized breccia sequences and melt sheet might be the sources of the main magnetic anomalies.

Key words: Chicxulub crater, magnetic susceptibility, aeromagnetic anomaly, structural model, uplift, Mexico.

RESUMEN

En este trabajo presentamos un modelo actualizado de la estructura de impacto de Chicxulub, utilizando nuevos modelos de la anomalía aeromagnética. Estudios de la variación de la susceptibilidad magnética a lo largo de la columna litológica al interior del cráter revelan que las brechas de tipo suevita tienen una firma magnética más fuerte que la unidad fundida (melt). La componente cristalina, estimada a partir del análisis de clastos encontrados en las brechas de tipo suevita, tiene una susceptibilidad

AeromagneticanomaliesandstructuralmodeloftheChicxulubmultiringimpactcrater,Yucatan,Mexico

Mario Rebolledo-Vieyra1, Jaime Urrutia-Fucugauchi2,*, and Héctor López-Loera3

1 Centro de Investigaciones Científicas de Yucatán, CICY, Centro para el Estudio del Agua, Calle 43 No. 130, Colonia Chuburná de Hidalgo, 97200 Mérida, Yucatán, Mexico.

2 Universidad Nacional Autónoma de México, Instituto de Geofísica, Proyecto Universitario de Perforaciones en Océanos y Continentes, Circuito Exterior s/n, Cd. Universitaria, Del. Coyoacán, 04510 Mexico D.F., Mexico.

3 Instituto Potosino de Investigación Científica y Tecnológica, IPICYT, Camino a la Presa San José 2055,Col. Lomas 4a sección, 78216 San Luis Potosí, S.L.P., Mexico.

* juf@geofisica.unam.mx

Revista Mexicana de Ciencias Geológicas, v. 27, núm. 1, 2010, p. 185-195

Rebolledo-Vieyra,M.,Urrutia-Fucugauchi,J.,HéctorLópez-Loera,H.,2010,AeromagneticanomaliesandstructuralmodeloftheChicxulubmultiringimpactcrater,Yucatan,Mexico:RevistaMexicanadeCienciasGeológicas,v.27,núm.1,p.185-195.

Rebolledo-Vieyra et al.186

magnética más alta (hasta 1200×10-5 SI), que el melt (~500×10-5 SI) y los clastos del basamento cristalino (400×10-5 SI). La reducción al polo y la continuación hacia abajo, documentan el carácter fragmentado de la anomalía. La segunda derivada de la anomalía aeromagnética delinea cinco anillos concéntricos al interior del cráter; el último anillo se correlaciona con el anillo de cenotes, lo cual apoya la interpretación de que el origen del anillo de cenotes está ligado con el cráter. La señal analítica y el espectro radialmente promediado arrojan una profundidad estimada a las fuentes magnéticas que va de los 1000 m a los 6000 m. Utilizando estos datos desarrollamos nuevos modelos magnéticos en 2-D, los cuales indican que el carácter fraccionado en la porción norte del cráter está controlado por un sistema de fallas verticales. La principal anomalía central es producto de un levantamiento estructural, cuya cima se encuentra a ~2,500 m de profundidad a partir del fondo marino, en el área central del cráter.

Palabras clave: cráter Chicxulub, magnetometría, anomalía aeromagnética, modelo estructural, levantamiento, México.

INTRODUCTION

Impactcrateringisoneofthemajorprocessesshap-ingplanetarysurfacesintheSolarsystem.Impactcratersarecharacteristicfeaturesinallplanets,satellitesandasteroids,exceptforthegaseousgiantplanets.OnEarth,thegeologicalrecordofimpactcratershasbeenerasedbythetectonic,magmaticanderosionalprocesses(Melosh,1989).LargecomplexmultiringcratersthatareproductsofmajorimpactsarerareonEarth,withonlythreecratersdocumented:Vredefort(SouthAfrica),Sudbury(Canada)andChicxulub(Mexico)(GrieveandTherriault,2000;Urrutia-FucugauchiandPerez-Cruz,2009).Chicxulubistheyoungestandbestpreserved.Theimpacteventisalsowellknownsinceithasbeenlinkedtoglobaleffectsonclimateandenvironment,resultinginoneofthemajorlifeturnoversmarkingthetransitionfromtheMesozoictotheCenozoicattheCretaceous/Paleogene(K/Pg)boundary(Hildebrandet al.,1991,1998;Sharptonet al.,1992).

Geophysicalmethodshavelongbeenusedtoidentifyandinvestigateimpactcraters,whichoftenshowchar-acteristiccircularanomalypatterns(e.g.,PilkingtonandGrieve,1992;Sharptonet al.,1993).Aeromagneticandgroundmagneticsurveyshavebeenparticularlysuccess-fulinstudyingthestructureandstratigraphyofcraters.Thechangesinphysicalpropertiesinducedbyshockandpressure/thermalprocesseshavebeenalsostudiedandrelatedtopetrographicandgeochemicaldataandtotheimpactandcrateringprocesses.RecentinvestigationsonMarslargeimpactbasins,whicharecharacterizedbylowamplitudemagneticanomalies,indicateapparentmajordemagnetizationeffectsontargetrocksinducedbylargeimpacts(e.g.,Acuñaet al.,1999).ThesefeatureshavebeendocumentedbypaleomagneticstudiesofbasementrocksfromtheVredefortcrater(Muundjuaet al.,2007).Ingeneral,investigationsonterrestrialimpactstructuresandlaboratoryexperimentsdocumentwiderangesinmagneticproperties(i.e.,magneticsusceptibilityrangesfromdia-magneticrange,0.00S.I.to1200×10-5SI).Ingeneral,itisconsideredthattheshockproducesadropinthemagneticsusceptibilityandoftenintheremanentmagnetizations

(Pladoet al.,1999).Lithologiesliketheimpactmeltthatcooldownslowlymayacquireathermoremanentmagneti-zation (TRM) in the direction of the present magnetic field (e.g.,ManicouaganandChicxulub).Inothercases,likeinlargecomplexcraters,theimpactlithologiesmayacquireanewremanencebyreheatingandtransientstresses;i.e.,thethermoremanentandshockremanence(SRM)magnetiza-tions along the direction of the Earth’s magnetic field at the timeofimpact.

BuriedintheYucatancarbonateplatforminsoutheast-ernMexico,theChicxulubcraterwasunveiledbygeophysi-calsurveysconductedaspartofoilexplorationprogramsof Pemex (Penfield and Camargo, 1981; Hildebrand et al.,1991;Sharptonet al.,1992).Inparticular,theburiedcratershowsacircularconcentricBouguergravityanomalypat-tern,whichisclearlymarkedinthelowamplituderegionalgravitypattern.Thecentralzoneofthegravityanomalyismarkedbyhighamplitudemagneticanomalies.ThemagneticanomalyofChicxulubcraterisanexampleofacomplexcratergeophysicalsignature,andingeophysicalmodels,ithasasemi-circularshapewithalargeamplitudecentralanomalysurroundedbysmallerinversedipolaranomalies(Figure1).Inthepasttwodecades,thecraterhasbeen intensively investigated using potential field, electro-magneticandseismicsurveys.Althoughnumerousstudiesandgeophysicalmodelinghavebeenusedtoinvestigatethecraterstructure(e.g.,Hildebrandet al.,1991,1998;Sharptonet al.,1992,1993;Connorset al.,1996;Morganet al.,1997;MorganandWarner,1999;Delgado-Rodríguezet al.,2001;Gulicket al.,2008;VeermeschandMorgan,2008),inverseandforwardmodelsderivedfromthesedataarelimitedbythelackofdirectdataonthenatureoftargetlithologies.DataonphysicalpropertiesoftargetlithologiesandspeciallyofthedeepYucatanbasementremainpoorlyconstrained.Stratigraphicdatacamemainlyfromoilexplorationbore-holesandregionalmodels(e.g.,MurrayandWeide,1962;López-Ramos,1973,1983;Marshall,1974;Weidie,1985,Wardet al.,1995).Recentdrillingprogramsthatincludedcontinuouscoringhaveprovidednewinformationonthesubsurfacestratigraphyandconstraintsonthebasement(Urrutia-Fucugauchiet al.,1996,2004a,2008;Rebolledo-

Aeromagnetic anomaly modeling, Chicxulub crater 187

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Aeromagnetic Anomaly of the Chicxulub Crater

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arepresented.Inthefollowingsection,theprocessingandanalysisoftheaeromagneticdataareanalyzed.Thenthemagneticmodelingmethodisdescribed,andresultsarelinkedwiththepaleomagneticdata.Finally,wediscussthecombinedresultswithregardstothepreviousmodelsandthegeologicalconstraintsofthecraterbasin.

PETROPHYSICAL AND GEOPHYSICAL DATA

Paleomagnetic and rock magnetic measurements

Natural remanent magnetization (NRM) and low-field magneticsusceptibilitymeasurementswereperformedoncore samples from the UNAM Scientific Drilling Program. TheNRMintensityanddirection(referredtoarbitraryazi-muth)weredeterminedusingaspinnerJR-5magnetometer.The low-field magnetic susceptibility was measured with theBartingtonsusceptibilitysystem,usingthelaboratorysensor.Wethusobtainedhigh-resolutionrecordsofthemag-neticsusceptibilitybehaviorfromthreeboreholesdrilledinthesouthernsectorofthestructure(Rebolledo-VieyraandUrrutia-Fucugauchi,1999).Fromthesestudiesweobtainedlogdataofmagneticsusceptibilityforthesueviticbreccias,whichhavemeanmagneticsusceptibilityof229×10-5SI,butwithvaluesashighas1200×10-5SI,forbuntebrecciawithasusceptibilitymeanwithinthediamagneticrange,forindividualclastsofimpactmelt,500×10-5SI,andcrys-tallinebasement,400×10-5SI,andalsoforthecarbonatesandanhydrites,whichmagneticsusceptibilityisalsowithinthediamagneticrange.The40Ar/39ArdatesreportedbySharptonet al.(1992)onmeltsamplesrecoveredfromthePEMEXboreholeChicxulub-1giveanagefortheimpactof~65.5Ma.Theisotopicdatesagreewiththepaleomagneticdataofthemeltsamples,whichplacetheimpactwithinreversepolaritygeomagneticchronC29r.Themagneticpolarityofmelt,investigatedbyUrrutia-Fucugauchiet al.(1994)inPEMEXboreholeYucatan-6,Rebolledo-VieyraandUrrutia-Fucugauchi(2004)inYaxcopoil-1borehole,andRebolledo-VieyraandUrrutia-Fucugauchi(2006)inUNAMboreholesgivereversepolarity,withmeanupwardinclinationsof40o–42o.Consideringthisdata,weassumeda-41oinclinationandadeclinationof163ousingtheNorthAmericapolarwandercurvefortheLateCretaceous-earlyPaleogene.

Aeromagnetic anomaly data

The aeromagnetic data come from a 450 m flight-alti-tudesurvey.Dataforthemodelinghavebeenreducedtoaregularanomalygrid171kmby171km,withdigitizeddatapointsevery1km.Dataprocessingconsistedinreductiontothepole(Figure2a),secondverticalderivative(Figure2b),pseudo-gravity(Figure2c)andanalyticaldownwardcontinuations(Figure2d);wealsocalculatedtheradial

Vieyraet al.,2000).Laboratorymeasurementsonphysicalpropertiesprovidedataneededtoconstraintgeophysicalmodeling.SamplesfromtheChicxulubsueviticbrecciasandfromPEMEXboreholesthatprovideinformationonthenatureoftheYucatanbasementarealsoconsideredtoconstrainthestructuralcratermodel.

Inthiswork,wepresentnewstructuralmodelsoftheimpactcraterderivedfrommodelingoftheaeromagneticanomaly field (Figure 1). In the analysis, we consider previ-ousmodelingoftheaeromagneticanomalies(Pilkingtonet al.,1994;Ortiz-Alemánet al.,2001),boreholeandgeophysi-callogginginformation(Urrutia-Fucugauchiet al.,1996,2004a,2008;Rebolledo-Vieyraet al.,2000),paleomagnetic,rockmagneticandgeochemicaldata(Urrutia-Fucugauchiet al.,1994,1996,2004b;Urrutia-FucugauchiandPérez-Cruz,2008).First,themagneticmeasurementsonrocksamples

Figure 1. a: Aeromagnetic anomaly field over the Chicxulub crater in the Yucatan peninsula and Gulf of Mexico. Survey flight altitude is 450 m above sea level. Location of regional profiles are shown: Lines A-A’, B-B’, C-C’andD-D’usedfor2-Dmodeling(seeFigures3and4).b:Radiallyaveragedspectraoftheaeromagneticanomaly,unitsintheverticalaxisaredepthinkm.

Rebolledo-Vieyra et al.188

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averagespectrumtoestimatethedepthtosources(Figure2e)(e.g.,Macleodet al.,1993;PilkingtonandHidebrand,2000). Using this data, we modeled four regional profiles, orientedN-S,E-W,N45oEandN45oWcrossingtheentirebasin(Figure1).DataprocessingandmodelingwasmadewiththeOasisMontajsoftwarepackage.Fromthisprocess,weconstrainedtheprincipalstructuresofthecraterasbeingseparatedintoseveralmagneticdomains.Intheanalysis,weconsideredamagneticdomainasaregionwithsimilarmagneticcharacteristicsintermsofwavelengthsandam-plitudes.Insomecases,lowamplitudemagneticanomalies,characterizedbysubduedsignals,andnullorweakmagneticsusceptibilitycontrastshavealsobeenseparatedintodistinctdomains.Whennoticeablecontrastsinanomalyamplitudes,possiblyassociatedwithmagneticsusceptibilitycontrasts,

areobservablewithinadomain,wecallthemmagneticsub-domains(Figure2).

Analyses of the aeromagnetic anomaly field (Figure 1a)suggestseparationofanomalypatternsintothreedo-mains,withcharacteristicamplitudesandwavelengths.Thefirst is characterized by large amplitude dipolar magnetic anomalieslocatedatthecentralpartofthecrater,extend-ingonshoreandoffshore.Ithas,atleast,threemagnetichighs,oneofthemhavinganisolatedmaximumandtheothertwohavingdipolarhightolowdistancesbetween7and10km.Twomagneticsub-domainscanbeseparated:Asub-domainassociatedwithmagneticresponsesoflongwavelengthsandlargeamplitudes,andasecondmag-netic sub-domain located SE from the first sub-domain and completely onshore. The magnetic configuration of this

Figure 2. a) Aeromagnetic anomaly field of the Chicxulub crater reduced to magnetic pole. Local geomagnetic field inclination is 45oanddeclinationis5o. b) Second derivative of the aeromagnetic anomaly field. c) Pseudo-gravimetric analysis of aeromagnetic anomaly field. Density contrast is 1.8 g/cm3.d)Downwardcontinuationtoareferencesurfaceof450m.

Aeromagnetic anomaly modeling, Chicxulub crater 189

anomalyhasthreeanomalouszones,withnormaldipolarbehavior;thedipolardistancesrangefrom7to8km.Thismagneticsub-domainischaracterizedbyanomalieswithlongwavelengthsandintermediateamplitudes.Inthetotalmagnetic field anomaly map, this domain has an elongated trendorientedN80oW,withamaximumlengthof70kmand40kmwide.Thesecondaeromagneticdomainislo-catedtotheSW,andischaracterizedbyanisolatedhighanomaly with an oval shape striking NW, defined by long wavelengthsandlargeamplitudes;itsminimumdimensionsare64km,strikingNW,and50km,strikingNE.Thethirdmagneticsub-domain,locatedtotheNE,hasanelongatedshapewithmorethan50kmlengthineitherdirection.Theconfiguration reflects presence of major lineaments striking NE-SWandNW-SE.Withinthemagneticdomainzones,secondarylineamentscanbeobserved,andanE-Wlinea-mentthatapparentlylimitsthemagneticanomalyformingthe first magnetic sub-domain; its fragmented character can beassociatedtofaultsand/orfractures.

Toestimateaveragestatisticaldepthstomagneticsources,wecalculatedtheradiallyaveragedspectrum(Figure1b).Inthedepthestimations,itisassumedthattheejectablanketwithinthecraterbehavesasacoherentandcontinuousdeposit,allowingustoestimatethedepthtothesourcesatabout~6,000minthecenterofthestructureto~500mtowardstheedgeofthecentralanomaly.

AEROMAGNETIC DATA PROCESSING AND MODELING

Reduction to magnetic pole

The configuration of the magnetic field reduced to the polepermitsseparationofthreemagneticdomains(Figure2a). The first magnetic domain is located in the central part,anditisformedbyfourmagneticanomalieswithdipolar configurations. The first anomaly is located in the NWwithinthedomain;itiselongatedstrikingNNEandhasapolardistanceof17kmandisassociatedtoamag-neticresponseofshort-longwavelengthsandintermediateamplitude.ThesecondanomalyislocatedwithintheNWcentralportionofthemagneticdomainandisformedbyanormaldipolaranomaly,elongatedandstrikingENEwithhigh-to-lowdistanceof~17km.Theanomalyconsistsoftwopositivehighsthatinthenorth,asinthesouth,presentdipolar configuration; to the north it has normal polarity andtothesouthithasreversepolarity.AENE-strikinglineamentthatborderstheanomalydomaindividesthetwolobes.Thesesub-domainspresentmagneticanomaliesoflongwavelengthsandlargeamplitudes.Thethirdzoneofthismagneticdomainischaracterizedbyanormaldipolaranomaly,elongated,strikingN-S,withapolarhigh-to-lowdistanceofabout9km,associatedtoamagneticresponseoflongwavelengthsandintermediateamplitudes.Thefourthmagneticsub-domainisformedbyatriangularshaped

anomalythatinitscentralpartshowsanormaldipolewithapolarhigh-to-lowdistanceofabout17km.Thissub-domainischaracterizedbyintermediatewavelengthsandamplitudes.

Thesecondmagneticdomain,locatedSWfromthecentralzone,isformedbyananomalywithlongwavelengthandlargeamplitude.TheanomalyhasanovaltrendwiththelongaxisstrikingNE-SW,andcoversalargearealocatedonshoreandoffshore.

ThethirdmagneticdomainislocatedNEfromthecentralzoneandischaracterizedbyananomalyassociatedtoalongwavelengthsandlargeamplitudesignal.Tobet-terconstrainthemagneticcharacteristicsofthezone,itisnecessarytohaveamoreextensivemagneticcoverage.

Lineaments identified in analyses of magnetic anom-alytrendssupporttheoccurrenceoffaultingandfractur-ingatdepth.LineamentsstrikingNW-SE,NE-SWandN-Sbordertheanomalydomainzones.AmajorlineamentstrikingNNE-SSWcrossesthecraterbasin,separatingtheanomaly domain zones one and two of the first domain. The dimensionsestimatedinthisanalysisforthestructureareamaximumlongitudeof167kmstrikingNW76SEandaperpendiculardistanceof106km.

Second vertical derivative

Thesecondverticalderivativeyieldsamagneticconfiguration that emphasizes the circular trends in the magnetic anomaly field (Figure 2b). Three circular trends are unveiled. The first is formed by an interior ring of 71.5 kmstrikingNWand65.5kmstrikingNE.Thesecondisassociatedtoacircularintermediateringwithalongitudeof137kmstrikingNWand113kmintheNEdirection.The third trend is poorly defined.

Pseudo-gravimetry

Thepseudo-gravimetricanomaly,whichistheverti-calderivativeofthepole-reducedmagneticpotential,wascalculated.Withthisanalysis(Figure2c)fourdomainsareidentified: the central larger domain may correspond to a broadlow,separatedbyahighinthemiddle.Theotherthreepseudo-gravimetric domains are located on the flanks of the broadlow,andarerepresentedbythepseudo-gravimetrichighsatW,SW,SE,EandNE.Withinthispseudo-gravi-metric configuration, the main lineaments are observed strikingNNW-SSEandENE-WSW.

Downward analytical continuations

Downwardanalyticalcontinuationsoftheaeromag-neticanomalywerecomputeddowntoareferencesurface450mdepth(Figure2d).Resultsofthisanalysisshowed

Rebolledo-Vieyra et al.190

that the reduced anomaly field is composed by a series of dipolarandisolatedhigh/lowmagneticanomalies,whichcan be grouped in three magnetic domains. The first and main domain is located in the central part of the configura-tionandisconformedbytwomagneticsub-domains.Thefirst presents a magnetic anomaly characterized by three positivelobeswithtwoofnormalpolarityassociatedtomagneticresponseswithlongwavelengthsandlargeam-plitudes.Thesecondsub-domainislocatedtotheSEandischaracterizedbyintermediatetoshortlongwavelengthwithmediumamplitudes.Thesecondlobeislocatedtothe SE from the first sub-domain and is characterized by intermediatelongitudeandamplitudewavelengths.TheseconddomainislocatedtotheSWfromthecenterandcharacterizedbyadipolaranomalyoflargedimensionselongatedwithaNW-SEtrend.Thislobe,thoughpoorlydefined is formed by long wavelengths and large amplitudes. ThethirdmagneticdomainislocatedtotheN,NWandNEfrom the center of the configuration, it has an inverted-U shapeandlongwavelengthsandlargeamplitudes.AtitsWlimit,ithasalargemagnetichighwithisolatedhighsandlows.ThisdomainhaslargedimensionsanditsurroundsthecentralmagneticdomaintowardsitslimitsNW,NandNE. Analysis of the downward continuation field shows an anomalouszoneof161kmlong,strikingNWand143kmintheNEdirection.

Two-dimensional modeling

Tofurtherconstrainthegeometryandrelativelocationofsourcebodies,forwardtwo-dimensionalmodelsalongregional profiles crossing the basin were constructed. The profiles are oriented N-S (A-A’ profile; Figure 1a), E-W (B-B’ profile; Figure 1a), N45oE (C-C’ profile; Figure 1a) andN45oW (D-D’ profile; Figure 1a).

N-S profile(Figure1a,A-A’;Figure3a)showsamag-netic high towards the center of the profile, ~20 km in width; the“picktopick”magneticrangeis~700nT.Fromtheprofile, and the aeromagnetic anomaly, the central magnetic anomalyisfragmentedanditmightbeassociatedtoafaultsystem.Thiscentralmagnetichighisassociatedwiththecentralbasementupliftincomplexcratermodels(Melosh,1989;O´KeefeandAhrens,1999).Thebasementmaterialishighlymagnetic.Thedepthtothetopofthecentralupliftisestimatedfromourmodelstobefrom2000to2900m.The top of the central uplift is tilted rather than flat, capped bya~400mthickcoverofimpactlithologies.Thecentralupliftisboundedbytwomayorlineamentsanditshowsanimportantlineamentatthetopofthecentralanomalyandaseriesofsmallerlineamentstowardsthenorthernlimit,whichaccountfortheirregularbehaviorofthemagneticlow.Weinferredthattheselineamentsmightbeassociatedto faulting or fractures. Towards the south of the profile, wheretheCSDPYaxcopoil-1boreholeislocatedatabout62kmfromthecratercenter,wefoundamagnetichigh

thatisprobablyassociatedwithimpactlithologiesat~1000depthandanapproximatethicknessof1000m.

E-W profile(Figure1a,B-B’;Figure3b).Inthismodel,asintheN-Smodel,thedepthtothetopofthecentralupliftisestimatedbetween2000and3000m.Thecentralupliftiscappedby~400thickcoverofimpactlithologies.Astrikingfeatureinthismodelisthehighfrequencycontentsalong the magnetic profile. The short wavelengths could be associatedtoshallowermagneticsourcesortoeffectsofafaultsystemlocatedpreferentiallytowardtheeastofthecentraluplift.Thefaultsystemischaracterizedbynormalfaultingbasedinthehighangleandthedistributionofthehangingwalls.Asinthepriormodels,weconsideredthesuevite-likebrecciaasthemajormagneticsource,besidesbeingtheshallowersource.

N45ºW profile(Figure1a,C-C’;Figure4a).Faultingtowards the northern sector of the profile is incorporated intothemodel,withamajorverticalfaultdelimitingthecentraluplift,followedbyaseriesofsmallerlengthverticalfaults.Inthismodel,averticalfaultwithinthetopofthecentralupliftisadded.Similartothecharacteristicsalongthe N-S profile, there is a large amplitude magnetic high towards the southern portion of the profile (>320 nT). The centrallargeamplitudeanomalyismodeledwithabodyaradiusof~45km.

N45ºE profile(Figure1a,D-D’;Figure4b).Modelforthis profile delineates a major fault zone located towards thenorthernportionofthecrater.Themodelincludesanextensivefaultsystemofhighangledippingnormalfaults.Inthemodel,averticalfaultisincludedwithinthetoppartofthecentraluplift.Topofthecentralupliftisestimatedtoliebetween2000and3000m.

DISCUSSION

EarlygeophysicalmodelsoftheChicxulubcraterhavebeenlimitedbylackofdirectlaboratorydataonbasement, target, impact and crater in-fill Tertiary lithologies. PreliminarydatafromPEMEXboreholeswereincorporatedinpreviousstudies(Pilkingtonet al.,1994,2000;Urrutia-Fucugauchiet al.,1996;Ortíz-Alemánet al.,2001).TherearethreemajormagneticsourceswithintheChicxulubcrater:1)themeltunit,2)thesuevite-likebreccia,and3)thecentraluplift.Inthiswork,magneticpropertiesofthetargetandimpactlithologiescomingfromcoresamplesinthe UNAM and Chicxulub Scientific Drilling Programs were usedtoconstructstructuralmodelsandinterpretationsoftheaeromagneticanomalies.ThemodelspresentedincorporatethemagneticdataobtainedfromtheUNAMboreholesandalsotheinformationfrompriormodels.Intheradiallyaveragedspectrum,weestimateadepthof~1000mtothetopoftheimpactlithologiesinthecentralportionoftheanomalyand~2500mtothetopofthecentraluplift.Fromthemodels,weestimatedthethicknessanddistributionoftheimpactlithologies.Thedataobtainedfromthese

Aeromagnetic anomaly modeling, Chicxulub crater 191

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-0.6

Thecircularshapeofthecentralaeromagneticanomalyisemphasizedinthesecondderivativeanalysis.Thebunte-likebrecciaandthemega-brecciadonotyieldamagneticsignalbecauseofthenatureoftheircomponents,mainlycarbonatesandevaporites,whicharewithinthediamagneticrange.Thesecharacteristicspreventedusfromhavingmorerealisticgeologicalmodels,eventhoughwehaveestimated

modelsdocumentthepresenceofafaultsystemtowardsthe central portion of the crater, which is reflected in the magnetichighandlowsoftheanomaly;thisfaultsystemispredictedinthemodelsofformationandevolutionofcomplex-craters.Thesecharacteristicsarealsoevidentbytheanalysesperformedtotheaeromagneticanomaly,likethedownwardcontinuationandthereductiontopole.

Figure 3. Magnetic models for Chicxulub crater. a) 2-D model of the profile A-A’ in Figure 1. b) 2-D model of the profile B-B’ in Figure 1.

Rebolledo-Vieyra et al.192

Profile C-C’

Profile D-D’

Mag

net

ics

(nT

)D

epth

(k

m)

Mag

neti

cs (

nT)

Dep

th (

km)

_•= Observed, = Calculated, = Error_

V.E.= 6.11Scale=1193807

V.E.= 5.7Scale=1139389

400

0

-400

-800

-1.6

2.4

6.4

Distance (km)72 13212

280

180

80

-20

-120

-220

0

4

8

12

Distance (km)

12 72 132

b)

a)

_•= Observed, = Calculated, = Error_

thedepthsandthicknessofthemagneticsourcesthat,asexpected,arethickertowardsthecenterofthestructure;however, in places such as in southern portion of the profile A-A’,apparentpatchesofimpactitelithologiesasthickas1000mmaybepresent.Frompriordrillingexperience,weknowthatinthesequenceofimpactbrecciathesuevite-likebrecciaoverliesthebunte-likebreccia.Weestimatethethicknessofthesuevite-likebrecciatobesomewhere

between400mand1000m,butsince thebunte-likebrecciadoesnotyieldamagneticsignal,weassumedthatthethicknessproposedforthesuevite-likeincludesthebunte-likebreccia.FurtherstudiesfromtheCSDPwillhelpsolvetheproblemofthespatialvariationofbrecciasequencethickness.RecentdatafromYaxcopoil-1boreholedoes not fit with this interpretation; the Yaxcopoil-1 column documenteda100mthickimpactsequenceandnobunte-

Figure 4. Magnetic models for Chicxulub crater. A) 2-D model of the profile C-C’ in Figure 1. B) 2-D model of the profile D-D’ in Figure 1.

Aeromagnetic anomaly modeling, Chicxulub crater 193

a) b)

c) d)

e) f)

g)

-10

-5

0

5

10

15

-40

-20

0

20

40

60

-4

-2

0

2

4

6

-4

-2

0

2

4

-2

0

2

0

2

1

-1

0

2

1

-1

30 km

likebrecciaunderlyingthesuevite-likebreccia(Kringet al.,2004;Urrutia-Fucugauchiet al.2004a,2004b).However,Yaxcopoil-1boreholemayhavebeendrilledintheterracezoneclosetoafaultwhereimpactlithologiespinch-out,withtheresultofthinnerimpactsequencethanexpectedandthusthelithologicalsequencerecoveredmightnotbe representative of the southern crater sector. Total field magneticanomaliesarecalculatedatvariouslevelsusingasimplestructuralmodelforthecentralcraterstructure,assumingthemajormagneticsourcebodies.ResultsaresummarizedinFigure5,fromgroundsurfacetoa6kmlevelreferencesurface.Takingintoconsiderationthespatialandfrequencycharacteristicsoftheaeromagneticanomalyandtheboreholeinformationwesubmit thatthegeophysicalmodelspresentedinthisstudyarewell

supportedandrepresentthestructureinthecentral-southerncratersector.

CONCLUSIONS

TherearethreemajormagneticsourceswithintheChicxulubcrater:1)themeltunit,2)thesuevite-likebreccia,and3)thecentraluplift.Reductiontothepoleanddown-wardanalyticalcontinuationsshowthediscretecompositecharacter of the anomaly field, with large amplitude inverse dipolaranomaliesinthecentralsector.Thesecond-deriva-tive of the magnetic anomaly depicts five concentric rings withtheexternalringcorrelatingwiththecenotering.Theanalyticalsignalandtheradiallyaveragedspectrumyield

Figure5Schematicplanviewofthemagneticanomalies(totalintensity,nT)of(a)earlypost-impact,atvariouserosionlevels:(b)1km;(c)2km;(d)3km;(e)4km;(f)5km;and(g)6km.Theimpacttookplaceatcenterofthearea.Notethattheamplitudescalevaries.

Rebolledo-Vieyra et al.194

anestimateofthedepthtothemagneticsourcesthatrangesfrom1000to6000m.Fromtheradiallyaveragedspectrumweestimateadepthof~1000mtothetopoftheimpactlithologiesinthecentralportionoftheanomalyand~2500mtothetopofthecentralbasementuplift.Fromthemodelsweestimatethethicknessanddistributionoftheimpactbrecciasequenceandmeltsheet.The2-Dmodelsdocumentthepresenceofafaultsystemtowardsthecentralportionofthe crater, which is reflected in the magnetic high and lows oftheaeromagneticanomalies;thisfaultsystemispredictedinstructuralmodelsofformationandevolutionofcomplexmultiringcraters.Thesecharacteristicsarealsoevidentbytheanalysesperformedtotheaeromagneticanomaly,likethedownwardcontinuationandthereductiontopole.Thesecondderivativemapsemphasizethecircularshapeoftheaeromagnetichighamplitudeanomaly.

Usingallthesedata,includingnew2-Dmagneticmodels,anewstructuralmodelisproposed,whichrevealsasystemofregionalhigh-anglefaultsthatexplainthemag-neticsignaloverthesouthernsectorofthecrater,whereasa2.5kmdeepcentralupliftandhighlymagnetizedbrecciasequencesandthemeltsheetmightbethesourcesofthemainmagneticanomalies.

ACKNOWLEDGMENTS

Thisstudy formspartof theResearchProgramon Chicxulub Crater and the UNAM Scientific Drilling ProgramonContinentsandOceans.Commentsbytwojournalreviewersonanearlyversionofthepaperaregreatlyacknowledged.ThankstoAdrienLeCossecforassistancewith preparation of the manuscript figures. Partial financial supportforthestudyhasbeenprovidedbyDGAPAPAPIITProjectIN-114709,CONACYT224932andtheIxtliDigitalObservatoryProject.

REFERENCES

Acuña,M.,Connernney,J.,Ness,N.F.,Lin,R.,Mitchell,D.,Carlson,C.,McFadden,J.,Anderson,K.,Reme,H.,Mazelle,C.,Vignes,D.,Wasilewski,P.,Clouthier,P.,1999,GlobaldistributionofcrustalmagnetizationdiscoveredbytheMarsGlobalSurveyorMAG/ERexperiment:Science,284,790-793.

Connors,M.,Hildebrand,A.R.,Pilkington,M.,Ortiz-Aleman,C.,Chavez,R.E.,Urrutia-Fucugauchi,J.,Graniel-Castro,E.,Camara-Zi,A.,Vasquez,J.,Halpenny,J.F.,1996,YucatankarstfeaturesandthesizeofChicxulubcrater:GeophysicalJournalInternational,127,F11-F14.

Delgado-Rodríguez,O.,Campos-Enríquez,J.O.,Urrutia-Fucugauchi,J.,Arzate,A.,2001,OccamandBostick1-Dinversionofmagnetotel-luricsoundingsintheChicxulubimpactcrater,Yucatan,Mexico:Geofisica Internacional, 40, 271-283.

Grieve,R.A.F.,Therriault,A.,2000,Vredefort,Sudbury,Chicxulub:Threeofakind:AnnualReviewsEarthPlanetarySciences,28,305-338.

Gulick,S.,Barton,P.,Christeson,G.,Morgan,J.,MacDonald,M.,Mendoza-Cervantes,K.,Urrutia-Fucugauchi,J.,Vermeesch,P.,Warner,M.,2008,Importanceofpre-impactcrustalstruc-

turefortheasymmetryoftheChicxulubimpactcrater:NatureGeoscience,1,131-135

Hildebrand, A.R., Penfield, G.T., Kring, D.A., Pilkington, M., Camargo, A.,Jacobsen,S.B.,Boyton,W.B.,1991,ChicxulubCrater:Apos-sibleCretaceous/TertiaryboundaryimpactcraterontheYucatanPeninsula,Mexico:Geology,19,867-871.

Hildebrand,A.R.,Pilkington,M.,Ortiz-Aleman,C.,Chavez,R.E.,Urrutia-Fucugauchi,J.,Connors,M.,Graniel-Castro,E.,Niehaus,D.,1998,MappingChicxulubcraterstructurewithgravityandseismic reflection data inM.M.Graddy,R.Hutchinson,G.J.H.McCall,D.A.Rotherby(eds),Meteorites:FluxwithTimeandImpactEffects:GeologicalSocietySpecialPublication,140,153-173.

Kring,D.A.,Horz,L.,Zurcher,L.,Urrutia-Fucugauchi,J.,2004,ImpactlithologiesandtheiremplacementintheChicxulubimpactcra-ter: Initial results from the Chicxulub scientific drilling project, Yaxcopoil,Mexico:MeteoriticsandPlanetaryScience,39,879-897.

López-Ramos,E.,1973,EstudiogeológicodelapenínsuladeYucatán:BoletíndelaAsociaciónMexicanadeGeólogosPetroleros,25:23-76.

López-Ramos,E.,1983,GeologíadeMéxico:México,D.F.,UniversidadNacionalAutónomadeMéxico,3a.ed.,453pp.

MacLeod,I.N.,Vieira,S.,Chaves,A.C.,1993,Analyticsignalandreduc-tion-to-the-pole in the interpretation of total magnetic field data atlowmagneticlatitudes,inProceedingsofthe3rdInternationalCongressoftheBrazilianSocietyofGeophysics,830-835.

Marshall,R.H.,1974,PetrologyofsubsurfaceMesozoicrocksoftheYucatanPlatform,Mexico:NewOrleans,Lousiana,UniversityofNewOrleans,M.S.Thesis,97pp.

Melosh,H.J.,1989,ImpactCratering:AGeologicProcess:NewYork,U.S.A.,OxfordUniversityPress,254pp.

Morgan,J.,Warner,M.,1999,Chicxulub:Thethirddimensionofamulti-ringbasin:Geology,27,407-410.

Morgan,J.,Warner,M.,theChicxulubWorkingGroup,1997,Sizeandmor-phologyoftheChicxulubimpactcrater:Nature,390,472-476.

Murray,G.E.,Weidie,A.E.,1962,RegionalgeologicsummaryofYucatanPeninsula,inFieldtriptopeninsulaofYucatan:NewOrleans,Louisiana,NewOrleansGeologicalSociety,142pp.

Muundjua,M.,Hart,R.J.,Gilder,S.A.,Carporzen,L.,Galdeano,A.,2007,MagneticimagingoftheVredfortimpactcrater,SouthAfrica:EarthPlanetaryScienceLetters,261,456-468.

O’Keefe,J.D.,Ahrens,T.J.,1999,Complexcraters:Relationshipofstratig-raphyandringstoimpactconditions:JournalofGeophysisicalResearch,104,27,091-27,104.

Ortiz-Alemán,C.,Urrutia-Fucugauchi,J.Pilkington,M.,2001,Three-dimensionalmodelingofaeromagneticanomaliesover theChicxulubcrater,inLunarPlanetaryScienceConference,CDExpandedAbstractVolume:Houston,LunarandPlanetaryInstitute(LPI)Contribution,abstractno.1962.

Penfield, G.T., Camargo, A., 1981, Definition of a major igneous zone inthecentralYucatanplatformwithaeromagneticsandgravity(abstract),in51stSocietyExplorationGeophysicistsAnnualMeeting,TechnicalProgramAbstracts,p.37.

Pilkington,M.,Grieve,R.A.F.,1992,Thegeophysicalsignatureofterrestrialimpactcraters:ReviewsofGeophysics,30,161-181.

Pilkington,M.,Hildebrand,A.,2000,Three-dimensionalmagneticimagingoftheChicxulubCrater:JournalofGeophysicalResearch,105:23,479-23,491.

Pilkington,M.,Hildebrand,A.R.,Ortiz-Aleman,C.,1994,Gravityandmagnetic field modeling and structure of the Chicxulub crater, Mexico:JournalofGeophysicalResearch,99,13,147-13,162.

Plado,J.,Pesonen,L.J.Puura,V.,1999,Effectoferosionongravityandmagneticsignaturesofcompleximpactstructures:Geophysicalmodelingandapplications,inDressler,B.O.,Sharpton,V.L.(eds.),LargeMeteoriteImpactsandPlanetaryEvolutionII:GeologicalSocietyofAmerica,SpecialPaper339,229-240.

Rebolledo-Vieyra,M.,Urrutia-Fucugauchi,J.,1999,High-resolutionmagneticsusceptibilityrecordoftheimpactlithologiesoftheChicxulubimpactcrater:AmericanGeophysicalUnionFall

Aeromagnetic anomaly modeling, Chicxulub crater 195

Meeting,SanFrancisco,California:EOS,Transactions,80(46),F595.

Rebolledo,M.,UrrutiaFucugauchi,J.,2004,Magnetostratigraphyoftheimpactbrecciasandpost-impactcarbonatesfrombore-holeYaxcopoil-1,Chicxulubimpactcrater,Yucatan,Mexico:MeteoriticsandPlanetarySciences,39,821-830.

Rebolledo-Vieyra,M.,Urrutia-Fucugauchi,J.,2006,MagnetostratigraphyoftheCretaceous/TertiaryboundaryandearlyPaleocenesedimen-tarysequencefromtheChicxulubimpactcrater:EarthPlanetsandSpace,58,1309-1314

Rebolledo-Vieyra,M.,Urrutia-Fucugauchi,J.,Trejo-Garcia,A.,Marin,L., Sharpton, V., Soler, A., 2000, UNAM’s Scientific Shallow DrillingProgramintotheChicxulubimpactcrater:InternationalGeologyReview,42,928-940.

Sharpton,V.L.,Dalrymple,G.B.,Marín,L.,Ryder,G.,Shuraytz,B.,Urrutia-Fucugauchi,J.,1992,NewlinksbetweentheChicxulubimpactstructureandtheCretaceous/Tertiaryboundary:Nature,359,819-821.

Sharpton,V.L.,Burke,K.,Camargo-Zanoguera,A.,Hall,S.,Lee,S.A.,Marín,L.E.,Suárez-Reynoso,G.,Quezada-Muñeton,J.M.,Spudis,P.D.,Urrutia-Fucugauchi,J.,1993,Chicxulubmultiringimpactbasin:Sizeandothercharacteristicsderivedfromgravityanalysis:Science,261:1564-1567.

Urrutia-Fucugauchi,J.,Pérez-Cruz,L.,2008,Post-impactcarbonatedepo-sitionintheChicxulubimpactcraterregion,Yucatanplatform,Mexico:CurrentScience,95,241-252.

Urrutia-Fucugauchi,J.,Perez-Cruz,L.,2009,Multiring-forminglargebolideimpactsandevolutionofplanetarysurfaces:InternationalGeologyReview,51,1079-1102.

Urrutia-Fucugauchi,J.,Marín,L.,Sharpton,V.,1994,ReversepolaritymagnetizedmeltrocksfromtheKTChicxulubstructure,Yucatanpeninsula,Mexico:Tectonophysics,237,105-112.

Urrutia-Fucugauchi J., Marin, L., Trejo-Garcia, A., 1996, UNAM Scientific drillingprogramofChicxulubimpactstructure.Evidencefora300kilometercraterdiameter:GeophysicalResearchLetters,23,1565-1568.

Urrutia-Fucugauchi, J., Morgan, J., Stoeffler, D., Claeys, P., 2004a, The Chicxulub Scientific Drilling Project: Meteoritics and Planetary Science,39,787-790.

Urrutia-Fucugauchi,J.,Soler,A.M.,Rebolledo-Vieyra,M.,Vera-Sánchez,P.,2004b,Paleomagneticand rockmagnetic studyof theYaxcopoil-1impactbrecciasequence,Chicxulubimpactcrater,Mexico:MeteoriticsandPlanetaryScience,39,843-856.

Urrutia-Fucugauchi,J.,Chavez-Aguirre,J.M.,Pérez-Cruz,L.,delaRosa,J.L.,2008,ImpactejectaandcarbonatesequenceintheeasternsectorofChicxulubcrater:ComptesRendusGeosciences,341,801-810.

Veermesch,P.M.,Morgan,J.,2008,StructuralupliftbeneaththeChicxulubimpactstructure:JournalofGeophysicalResearch,113(B7),B07103.

WardW.C.,Keller,G.,Stinnesbeck,W.,Adatte,T.,1995,Yucatansubsur-facestratigraphy:ImplicationsandconstraintsfortheChicxulubimpact:Geology,23,873-876.

Weidie,A.E.,1985,GeologyoftheYucatanPlatform,PartI,inWard,W.C.,Weidie,A.E.,Back,W.(eds.),GeologyandhydrologyoftheYucatanandQuaternarygeologyofnortheasternYucatanPeninsula:NewOrleans,Louisiana,NewOrleansGeologicalSociety,1-19.

Manuscripreceived:February12,2008Correctedmanuscriptreceived:August12,2009Manuscriptaccepted:November23,2009