Seismic Evidence for Blind Thrusting of the Northwestern Flank of the Venezuelan Andes Bruno De Toni

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12-1993 Seismic evidence for blind thrusting of the northwestern flank of the Venezuelan Andes Bruno De Toni

James N. Kellogg University of South Carolina - Columbia, [email protected]

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Publication Info Published in Tectonics, Volume 12, Issue 6, 1993, pages 1393-1409. De Toni, B. & Kellogg, J. N. (1993). Seismic evidence for blind thrusting of the northwestern flank of the Venezuelan Andes. Tectonics, 12 (6), 1393-1409. ©Tectonics 1993, American Geophysical Union

This Article is brought to you by the Earth, Ocean and Environment, School of the at Scholar Commons. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. TECTONICS, VOL. 12, NO. 6, PAGES 1393-1409, DECEMBER 1993

SEISMIC EVIDENCE FOR BLIND La Fria andE1 Vigia (Figures1 and 2). In thispaper we THRUSTING OF THE NORTHWESTERN presentgeological and geophysical evidence for basement- FLANK OF THE VENEZUELAN ANDES involvedthrusting of the VenezuelanAndes toward the Maracaibo Basin. Bruno De Toni INTEVEP, S.A., Caracas, Venezuela GEOLOGIC SETTING

JamesKellogg The regionalgeologic setting of thearea can be describedas Departmentof GeologicalSciences, University of South a mountainfront adjacentto a foredeepbasin. The mountain Carolina, Columbia frontrepresents a Precambrian-Paleozoic igneous-metamorphic complexpartially covered with sedimentsof Mesozoicand Abstract. Surfacegeology and seismicand well datafrom Cenozoicages. The foredeepis an asymmetricstructural basin the northwestern flank of the Venezuelan Andes indicate thatcontains up to 12 km of sedimentarysection. The early overthrustingof Andeanbasement rocks toward the adjacent Miocene to Pleistocenemolasse deposits up to 6 km in MaracaiboBasin along a blind thrustfault. The frontal thicknessdominate the sedimentarysection. The oldest monoclineis interpretedas the forelimbof a northwestward sedimentaryrocks that crop out in the mountainfront are vergingfault-related fold deformedover a crustal-scaleramp. Jurassicred bedswhich are unconformablyoverlain by a The Andean block has been thrust 20 km to the northwest and Cretaceoussequence up to 2 km in thickness.From early uplifted 10 km on a rampthat dipsabout 20o-30 ø Cretaceousto middleEocene, sediments were deposited in an southeastward.The thrustfault rampsup throughcrystalline extensivepassive continental margin setting (Figure 3). basementrocks to a decollementhorizon within the shaly Coarseclastic sediments of the Rio Negro Formation, units of the Cretaceous Colon-Mito Juan formations. bioclasticcarbonates of the Apon Formation,and siliciclastic Backthrustsin themonocline produce a wedgegeometry and sedimentsof the AguardienteFormation were depositedfirst in reducethe amountof blind sliprequired on the decollement the initial stageof the Cretaceoustransgressive event. northwestof the Andes. The rigid Andeanuplift wascaused Limestones,calcareous shales, and black shalesof the Capacho by northwest-southeastcompressive tectonic forces related to andLa Luna formationsrepresent anoxic facies deposited theconvergence of theCaribbean plate, the Panama volcanic duringthe maximummarine transgressiol•of late Cretaceous arc,and northwestern South America. The thick(up to 6 km) (Cenomanian-Campanian)age. Regressionbegan in the latest molassedeposits accumulated in the foredeepbasin indicate Cretaceous(Maestrichtian) with marinedeposition of the shaly thatthe Venezuelan Andes started to riseas earlyas the early Colon Formation and the sandier Mito-Juan Formation. This Miocene.However, a lateMiocene intramolasse unconformity regressionevent continued through the Paleocene and middle marksthe beginningof the formationof the monoclineand the Eocenewith depositionof sandstones,siltstones, shales, and greatestuplift. Thecrustal-scale fault-related fold modelmay coalsof the OrocueGroup in a deltaicenvironment followed explain structuralfeatures seen in otherareas of basement- by fluvial sandstonesof the Mirador Formation. involved foreland deformation. Unconformablyoverlying this is the upperEocene paralic facies of the Carbonera Formation which is in turn INTRODUCTION transitionallycovered by lacustrineshales of the Oligocene Leon Formation. Mioceneto Recentaged sediments represent Over thepast several years, diverse tectonic models have continentaldeposits (Guayabo Group) derived mainly from beenproposed to explainthe Venezuelan Andes uplift. Bucher erosionof the VenezuelanAndes uplift. [ 1952],Gonzalez de Juana [1952], Ilargraves and Shagam The sedimentsin the mountainfront are foldedinto a steep [1969],and Shagam[1972] described the VenezuelanAndes as northwestwarddipping monocline. The dip of strataranges a mega-anticlinecontaining a horst-grabenblock complex and from 25 ø to overturned, and the strike is northeast-southwest. limitedat bothflanks by high-anglereverse faults. Rod Numerousfaults, mainly orientednorth northwest-south [ 1960], Deratmiroff [ 1971], Schubert[ 1985], White [ 1985], southeastand northeast-southwest,cut and offset the Boesiet al. [1988],and Monsalve [ 1988]proposed that the outcroppingstratigraphy. The Rio Mocotiesfault is VenezuelanAndes is a "mushroomlike"transpressional uplift considered to be the southwestern continuation of the Bocono withshear deformation in theBocono fault system and right-lateralstrike-slip fault. imbricatethrusting toward both Andean fronts. Hospers and The gravity field of the area showsa Bougueranomaly in Van Wijnen [ 1959],Kellogg and Bonini [1982, 1985], the northAndean trough of nearly- 150 mGal [Bonini et al., Giegengack[1984], and Kohn et al. [1984] notedthe 1977]. This large negativeanomaly in the southernedge of asymmetryof the gravityfield [Belliziaet al., 1976] and the Maracaibo Basin indicates that the basin and the Andes are suggestedthat the Venezuelan Andes uplift was caused by not in local isostaticequilibrium. crustalshortening along a southeastdipping "Wind River type" thrustfault. TectonicSetting This paperfocuses on the structuralinterpretation of seismic The intra-Mioceneunconformities indicate a majorAndean sectionstraversing the northwesternflank of the Venezuelan orogenywhich has been relatedto thecollision of thePanama Andesand the southernedge of theMaracaibo Basin between island arc with the South American continent [Mann and Burke, 1984; Eva et al., 1989]. The resultingcollision and Copyright 1993 by the American GeophysicalUnion. the obliquesubduction of the Carnegieridge beneath the Andes in Ecuadormay havecaused the upliftof theEastern Cordillera Paper number 93TC01893 of Colombia and the detachment of the North Andes block or 0278-7407/93/93TC-01893510.00 microplatealong the Boconoand the East Andean frontal fault 1394 DeToni and Kellogg: Venezuelan Andes

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Fig. 1. Locationmap of the studyarea. system[Pennington, 1981' Kellogg et al., 1985]. The limited (1988-1991) showof the orderof 1 to 2 cm/yr convergence cumulativedisplacement on the Boconofault [Stephan,1977; betweenthe Caribbean,Panama, and North Andeanplates Gonzalezde Juanaet al., 1980; Schubert,1982; Giegengack, [Vegaet al., 1991]. "Thick-skinned"basement-involved 1984], despitethe relativelyrapid present-day slip rate of 1 deformationhas been correlated with magmaticgaps and low- cm/yr [Schubert,1980; Aggarwal, 1983], suggeststhat most angleor flat subductionin thePampeanas Range of Argentina of the northeastward movement of the North Andes block [Jordanet al., 1983;Jordan and Allmendinger, 1986] andthe occurredin the last 5 or 10 m.y. [Kellogg, 1984; Macellari, North AmericanCordillera [Lipman et al., 1971; Burchfieland 1984]. Davis, 1975; Coney, 1976; Dickinsonand Snyder, 1978]. Globalplate motionmodels predict west northwest-east southeastconvergence for the Caribbeanand South American Comparisonsto OtherBasement-lnvolved Foreland plates[Jordan, 1975; Minster and Jordan,1978; DeMets et al., Deformations 1990]. The north-southvector component is even greaterfor Caribbean-NorthAndean convergence when the NorthAndean Structural features in the Venezuelan Andes are similar to movement northeastward relative to South America on the thoseassociated with theLaramide orogeny (40-70 m.y.) in Boconofault systemis added. The directionof maximum the centraland southernRocky Mountains of the United shorteningin the VenezuelanAndes is northwest-southeast. States. The basementblock tectonicstyle of the Laramide Kellogg and Bonini [1982, 1985] and Kellogg [1984] orogenyis characterizedby the movementof basementblocks suggestedthat this vectorof deformationis relatedto slow alongreverse faults. The overlyingsediments in manyareas convergence(1.7 + 0.7 cm/yr) betweenthe Caribbeanplate and fold overreverse faulted basement blocks forming the the North Andeanmicroplate at the SouthCaribbean marginal monoclines[Steams, 1971; Steamsand Weinberg, 1975; fault (Figure4). This compressivedeformation was also Matthews,1978; Narr, 1993;Narr and Suppe,submitted responsiblefor the upliftsof the Sierrade Perijaand the Sierra manuscript]analogous to thebruchfalten (fault folds)of Stille Nevadade SantaMarta. Present-dayamagmatic subduction [1925]. Local secondarynormal faults occur [Wise, 1963; alongthe SouthCaribbean marginal fault is indicatedby active Howard, 1966; Steams,1971], but gravity dataand foldingand thrustingin the SouthCaribbean deformed belt Consortiumfor ContinentalReflection Profiling (COCORP) [Lehneret al., 1983;Ladd et al., 1984] andby a southeast deepseismic reflection profiling show that regional shortening dippingWadati-Benioff zone [Dewey, 1972;Pennington, far exceedsextension [Berg, 1962; Berg and Romberg, 1966; 1981' Kelloggand Bonini, 1982;Toto andKellogg, 1992]. Smithson et al., 1978]. The structuralrelief on the basement Pennington[1981] andDaniels [1991] estimatedthe dip of the surfaces exceeds 12 km for the Wind River and Sweetwater Wadati-Benioffzone as 20 ø. Preliminaryresults from Central uplifts [Pmchaet al., 1965]. and SouthAmerica (CASA) GlobalPositioning System Other activeAndean mountain belts that are structumlly (GPS) satellitegeodetic measurements over a 3-yearperiod analogousto the basement-involvedLaramide orogeny in the De Toni andKellogg: Venezuelan Andes 1395

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GEOLOGICALAiGNE STRATIGRAPHIC DEPOSITIONAL andAllmendinger, 1986]. A similarstyle has been recognized TIME MA. UNITS ENVIROMENT in middleand northern Germany [Stille, 1910; Lotze, 1938], the Arbuckle-WichitaMountains [Fichter, 1954], the Permian PLEISTOCENE 2- m FLUVIAL basinof westTexas [Elam, 1969],and the Laramide fault • BETIJOQUE PLIOCENE O ALLUVIAL blockuplifts and related monoclines of the Colorado Plateau 5_L9 [Kelly, 1955]. O 10-•6-,• /• DATA AND METHOD MIOCENE • r5 FLUVIO-DELTAIC ISNOTU Thedata used in thisstudy include seismic sections, surface EL PALMAR COASTAL PLAIN geology,and the Friata-IX well log [DeToni, 1990]. Five 25 seismiclines traversing the Andean mountain front between La OLIGOCENE 36 LEON PARALICTO U CARBONERA LAGOONAL Friaand E1 Vigia (Figure2) wereselected for thisstudy. To interpretthe seismic profiles, the time sections were convertedto depthsections: the travel times of prominent EOCENEM 39-••49- L MIRADOR FLUVIAL seismichorizons were converted to approximatedepths in 54 meters,using stacking velocities. Wherever possible, these PALEOCENE OROCUEGROUP PARALIC-DELTAIC seismichorizons were correlated with theexposed surface 66 M ITO JUAN stratigraphy.The geologic solutions presented in thispaper OPEN COLON MARINE arequalitative descriptions anddo not presume todescribe the z structurequantitatively (i.e., the cross sections are not 74- LALUNA •: OPENMARINE balanced/retrodeformable). CRETACEOUS CAPACHO < ANOXlC :•- AGUARDIENTE CARBONATE PLATFORM SEISMIC INTERPRETATION APON DELTAIC

RIO NEGRO FLUVIAL In seismicprofiles of thearea, two seismic provinces are recognized:the Andean frontal monocline and the foredeep basin.The southern portion which corresponds to the frontal • BASEMENT monoclineis characterizedby discontinuousseismic horizons Fig. 3. Stratigraphyof Ihenorth Andean flank. dippingto thenorthwest. The major seismic reflectors can be correlatedwith geologic units exposed at thesurface. The middleand southern Rocky Mountains include the Sierra de northernportions of theseismic sections correspond to the Perijaon the Colombian-Venezuelan border and the Sierra foredeepbasin. This zone shows well-defined, continuous Nevadade SantaMarta of Colombia[Kellogg and Bonini, seismicreflectors dipping gently to thesoutheast. The deeper 1982],the Peruvian Andes [Audebaud et al., 1973],and the reflectors are discontinuousat the mountain front and cannot SierrasPampeanas of Argentina[Jordan et al., 1983;Jordan be correlatedwith thereflectors present in thefrontal

80 ø 60 ø F•UL T N

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Fig.4. Present-dayplate motions (solid arrows) relative to the north Andean block showing average slip rates(in centimeters per year) during the last 5-10 m.y. [after Kellogg and Bonini, 1985]. De Toniand Kellogg: Venezuelan Andes 1397

monocline.As a resultof thislack of lateralcorrelation, some hangingwall cutoffs. The thrust fault slips basinward into a Cretaceousunits in theforedeep basin were identified by beddingplane decollement in theUpper Cretaceous Colon- correlationof acousticcharacter with an interpreted seismic Mito Juanshales (Figures 8, 10, 12, 14,and 16). This line anda synthetictrace for a well in theLos Manuelesoil interpretationrequires the presence of a regionalsoutheast field (Figure5). dippingramp south of the sections.The structural configurationof the frontal monocline iscomplicated by Frontal Monocline northwestdipping backthrusts and southwest dipping high- anglereverse faults. Mesozoicand Cenozoic rocks are exposed in theAndean In thesection A-A' (Figures6, 7, and8), UpperCretaceous frontalmonocline in a narrowbelt that follows the regional andTertiary units are folded in theLa Friataanticline by a northeast-southwesttrend. Here the Cretaceousrocks were seriesof backthrusts.One backthrust fault is predicted in a upliftedup to 10km above the same units in theflanking beddingplane decollement in theColon-Mito Juan formations foredeepbasin. The exposed Cretaceous and Tertiary beds steppingup into a higherdetachment at thebase of the showmoderate tosteep northwestward dips. These dips are Paleocene.Although prediction of thisfault explains at least consistentover much of thestudy area. To thesoutheast the 2 kmof shorteningin the Lower Tertiary units, the major dipof themonocline steepens, and in someplaces the beds are shorteningin the anticline occurs within the Oligocene units. overturned. The seismic data constrainthe structural TheFriata-lX well log shows an unusually thick sequence interpretationat depth. (1658m) of OligoceneLeon Formation shales (Figures 7 and Basedon the seismic and surface geology information, the 8). Thethick shale sequence isinterpreted asa duplex frontalmonocline has been interpreted asthe hanging wall consistingof five overlappingramp anticlines. The solution forelimbof a majorfault bend fold structure [Suppe, 1983]. A shownin Figure8 requiresapproximately 17km of slipat the blindthrust fault is located in a beddingplane decollement Oligocene/Eocenecontact. If, however,the original thickness beneaththe mountainfront monocline. Reflectors correlated of the OligoceneLeon Formationin the line of the section to theLa Luna Formationand olderrocks are involvedin the was500-800 m, onlyone or twooverlapping ramp anticlines

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II• I1 Fig.6. Seismic section A-A' showing thelocation ofthe Friata-lX well. and 4-8 km of slip wouldbe required. Structuralsolutions of this duplex structure.Shallow onlapping reflections in the with one or two rampsare compatiblewith the dip log data upperpart of the prof'des(Figures 13 and 15) suggesta period from the Friata-lX well (Figure7), which suggeststwo or of Plio-Pleistoceneerosion and monoclinegrowth. threedistinct dip domains. The fault bendfold interpretationfor the Andeanfront would In seismicsection B-B' (Figure9), subhorizontalreflectors explain the 12 km of verticaluplift of the pre-Cretaceousunits at 4.1-4.4 s underliesteeply dipping reflectors in the traversedby the sections.A minimumlateral shortening of 20 monocline.The subhorizontalreflectors are interpretedas km is estimatedfor the interpretedstructures. undeformedCretaceous units in the footwall (Figure 10) beneatha beddingplane decollement in the Upper Cretaceous ForedeepBasin Colon-Mito Juan shales. Structuraldiscontinuity of seismicreflections in the upper The foredeepbasin is the structuraldepression of the part of the profile C-C' (Figure 11) wasrelated to a backthrust southernMaracaibo Basin that is adjacentand parallel to the fault within the Miocene units(Figure 12). The Rio Escalante VenezuelanAndes. The basinis an asymmetrictrough that fault in sectionD-D' (Figures13 and 14) and E-E' (Figures15 containsup to 12 km of Cretaceousand Tertiary sediments. and 16) is alsoa backthrustwith 1-3 km of displacement.The This sedimentarysection is dominatedby more than6 km of Rio Escalantefault traceis obliqueto the mountainfront Mioceneto Recentmolasse deposits derived from the erosion (Figure2). In sectionD-D' thisbackthrust appears to stepup of the risingAndes. A majorregional unconformity separates from a decollementat the top of the Paleocene,whereas in this thick clasticwedge into two differentmolasse units sectionE-E' the fault is steeperand probably rooted in a (Figures5, 6, and 17). Structuraldeformation of the foredeep beddingplane in Cretaceousunits. Two more minor basinis mainly restrictedto the mountainfront; however, backthrustsare shownin Figure 16: onein the uppermolasse some minor normal faults offset seismic horizons correlated sequenceat the front of the monoclineand anotherin the with the Cretaceousunits (Figures6, 8, and 15). The Cretaceous south of the Rio Escalante fault. displacementof the faultsis apparentlyabsorbed or truncated Two imbricatedfault-related folds steepen the frontal in the reflectionfree zoneproduced by the CretaceousColon- monoclinein sectionD-D' (Figures13 and 14). Steepeningof Mito Juan formation shales. middleMiocene strata indicates post middle Miocene formation We attributethe structuralconfiguration of the foredeepand De Toni andKellogg: Venezuelan Andes 1399 1400 De Toniand Kellogg: Venezuelan Andes

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a ! B deformingover a crustal-scaleramp (Figures 18a and 18b). Estimatesof the fault dip rangefrom 20o-25ø basedon gravity models[Kellogg and Bonini, 1982, 1985] to about27 ø from N45W S45E fault bendfold theoryfor a 38ø frontallimb dip [Suppe,1983]. 600 500 400 300 200 100 The simplified solutionshown in Figure 18b showsa low- angle(20 ø) ramp, (1) from fault bendfold theoryfor the 20ø- 23 ø frontal limb dips in profiles A, C, and D (Figures8, 12, and 14) and (2) so that the monoclineon the southeasternflank of the VenezuelanAndes could be explainedas the backlimbof the same fault bend fold that formed the northwestern flank. The low-angle ramp lies within the rangeof solutions compatiblewith the gravity data [Kelloggand Bonini, 1982, 1985]. The high dip anglesshown in profilesB, C, D, and E (Figures 10, 12, 14, and 16) are interpretedas frontal limb dips producedby imbricatethrust faults not shownin Figure 18b. The Andes are thrust 20-30 km northwestward toward the MaracaiboBasin. Vertical relief on the top of basementis more than 12 km. The fault rampsup to a decollement horizon within the Cretaceous Colon-Mito Juan formation shales. The northwestward movement of the Andean basement foldedthe hangingwall sedimentarysequence and produced the steepmonocline of the northAndean flank. Deep subhorizontalreflectors beneath the monoclineare interpreted as undeformedCretaceous units in the footwall (Figures9 and 10). Backthrustsin the monoclineproduce a wedgegeometry [Tearpockand Bischke, 1991] andreduce the amountof blind slip requiredon the decollementnorthwest of the Venezuelan Andes. Our interpretationis supportedby new seismiclines [Audemard,1991] that showa major decollementin the Upper CretaceousColon shalesand backthrustingwithin Colon, Lower Tertiary,and Molasseunits. Backthrustsconsist of fault bendfolds, fault propagationfolds, and duplexes. Northwestverging duplexes involve the basement. Nonscaledrock modelsof low-anglereverse faults deformed underconfining pressure show late stagebasement structures developedthrough fracture, imbricate faulting, and rotation of Fig. 9. Seismicsection B-B'. Beneaththe steeplydipping basement blocks between zones of cataclasis [Chester et al., reflectors in the monocline note the subhorizontal reflectors at 1988]. Overlyingthe upliftedbasement block in the modelsis 4.1-4.4 s. Dashed lines indicate the location of reflectors a continuous,flat-topped, asymmetric kink fold in the veneer interpretedas beingassociated with the La Luna Formation. havingone near-verticallimb. The steeplydipping limb is analogousto the monoclineon the northwesternflank of the Venezuelan Andes. theregional southeastward tilting of theseismic reflectors to flexureof the lithosphereas a responseto tectonicloading by Timing of Deformation the Venezuelan Andes. The normal faults at the base of the sedimentarysection in theforedeep basin may be secondary The thick molassedeposits accumulated in the foredeep featuresproduced by thebending stresses. basinprovide the mostdirect evidence for timingthe Andean thrustingand uplift. Thesesediments, represented by the E1 DISCUSSION GuayaboGroup (El Palmar,Isnotu, and Betijoqueformations), suggestthat the VenezuelanAndes started to rise as early as CrustalScale Fault-Related Folding and WedgeStructure the early Miocene,with the formationof the monoclineand the greatestuplift in late Miocene to Pleistocenetimes. The seismicsections presented in thispaper show The E1 GuayaboGroup has always been described as one considerableshortening by imbricatebackthrust and uninterruptedcycle of continentalsedimentation [Gonzalez de breakthroughfaults within the monoclineon thenorthwestern Juana et al., 1980]. However, the seismic data show an front of the VenezuelanAndes. Sedimentsmonoclinally importantintramolasse unconformity, indicating that two drapedover a verticalor normalbasement fault would be differentsedimentation cycles are separated by a periodof extendedby normalfaulting or ductilethinning [Brown, 1988] tectonicinterruption. This unconformityis mostpronounced which is not observed.The reversefaulting observed in the in the westernportion of the studyarea (Figure 17). In •edimentstherefore indicates reverse faulting and shorteningin outcropsof the north Andeanmountain front, the intramolasse '.hebasement. We havetherefore interpreted the Venezuelan discontinuityis not readilyobserved; however, in the Andesas a northwestwardverging fault bend fold structure southwesternflank (TachiraDepression) the unconformitycan 1402 DeToni and Kellogg: Venezuelan Andes

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9 LOWERCRETACEOus LA I0 PRE-CRETACEOus 11

12 SEISMIC= REFLECTIONS0,21 13,

Fig. 12. Structuralinterpretation of sectionC-C' showingseismic reflectors and apparentbedding dips from surface measurements.

be distinguishedand occursbeneath the fluvial latestMiocene with our interpretationof therange as a fault-relatedfold sedimentsof the La CopeFormation [Macellari, 1984]. This structuredeformed over a crustal-scaleramp. Theramp dips intra-Miocene event has also been found in seismic sections of about 20o-30¸ southeast,and the Andean basementwas thrust the Barinas-ApureBasin and in mostof the intermontane 20-30 km northwestward toward the Maracaibo Basin. The basinsof northwesternColombia [Meier et al., 1987;Chigne, blindthrust fault stepsinto a beddingplane decollement 1985; Eva et al., 1989]. horizon within the Cretaceous Colon-Mito Juan formation SUMMARY shales. The northwestward movement of the Andean basement The seismicreflection profiles of the northwesternflank of foldedthe hanging wall sedimentarysequence and produced a the VenezuelanAndes presented in thispaper are consistent steepmonocline. Backthrusts in the monoclineproduce a

N30W S15E

700 600 500 400 300 200 lOO I :::1,..:•!•::•:•!:r-: ..:-.-•.::?•.::,•_•12.•,•,•...... • •:• .... -• ..... • ...... - _ •: ...... -

z o

Fig. 13. Seismic sectionD-D'. 1404 DeToni and Kellogg: Venezuelan Andes

RIO ESCALANTE S15E FAULT 700ß 600ß .500ß 400ß 300ß 200ß 1'100ß LALUNA

LATEST MIOCENE - PLIO-PLEISTOCENE ß

KIT, Km

LOWER MIOCENE

OLIGOCENE • EOCENE . •

UPPER CRETACEOUS

ß LOWERCRETACEOUS LALUNA Fm

PRE-CRETACEOuS

REFLECTIONS Km.

Fig. 14. Structuralinterpretation of seismicsection D-D' showingseismic reflectors and apparent bedding dips. Imbricatesoutheast dipping thrusts within the basement and northwest dipping backthrusts within thesedimentary cover steepen the monocline. Discordant reflectors between SP350 and SP450 suggest a late Mioceneunconformity associated with the formationof the monocline.

N45W S05E

8OO 7OO 6OO 500 400 300 2,oo 1O0

, 0 IBlilll.,,.,•,, IIIlfilll•', ',' o '•..'.T':..•..•' .'•.: •/.-•:.?'•-:• .•.• ,•.

...... •. .- ...... •

3_•. 3

Fig. 15. Seismicsection E-E'. DeToni and Kellogg: Venezuelan Andes 1405

E' E I

8OO 7o0 600 5oo 4oo 30o 200 ioo ß ß , , , , , ß S05E

RIO ESCALANTE LA LUNA Fm FAULT N45W

LATEST MIOCENE PLIO-

LOWEROL I GOCENEMIOCENE / EOCENE / U

LOWERCRETACEOUS LA LUNAFm PRE- CRETACEOUs

I SEISMIC REFLECTIONS- 0Kl•mm 2

Fig. 16. Structuralinterpretation of sectionE-E'. The Rio Escalantefault is a backthrustwith a trace obliqueto the mountainfront (Figure2). The discordanceof shallowreflectors at SP500 is interpretedas beingproduced by growthof the monocline.

S80W N80E

O0 200 300 400 500 600 700 800 900 I I I

_L.... •l__- ...... '"'•'---•'•••:,s•-•L•,-•.•---•,_••,•,....'.::.t....'•'-"•••'"'--•--'••.•- .....•.• '• ...... "' ,•g..' ,•,_.--,...... - -. -.____-.:-•--:•.'•.•'.••:•-. o

z o

5 - •.• ...... •• •.. 4-

Fig. 17. Seismicline F-F' showingtruncation of reflections(solid arrows) beneath the intramolasse unconformity.This unconformityis mostpronounced in the westernportion of the studyarea. 1406 DeToni and Kellogg: Venezuelan Andes

CARIBBEAN SEA

NTA MARTA MASSIF

MARACAIBO CARIBBEAN MOUNTAINS

APURE- BARINA$ BASIN

CO L OM B / x•'•'x

o,o Km

Fig. 18a. Tectoniclocation map for schematiccross section of the VenezuelanAndes (Figure 18b).

BOUGUER ANOMALY b

-50-

<1::-]oo -

-150-

RIO MOCOTIES -- 200 FAULT APURE - BARINAS NW MARACAIBOBASIN VENEZUELANANDES BASIN sE --o

o lO 2o i i i

Fig. 18b. Schematiccross section of the VenezuelanAndes located in Figure18a. We haveinterpreted the VenezuelanAndes as a fault bendfold structuredeforming over a crustal-scalesoutheast dipping ramp. Estimatesof the fault dip rangefrom 20o-25ø basedon gravitymodels [Kellogg and Bonini, 1982, 1985] to about27 ø from fault bendfold theoryfor a 38ø frontal limb dip [Suppe,1983]. In the solutionshown (20ø ramp) the monoclineon the southeasternflank of the VenezuelanAndes is the backlimbof the fault bend fold. The Andes are thrust 20-30 km northwestward toward the Maracaibo Basin. Vertical relief on the top of basementis more than 12 km. Backthrustsin the monoclineproduce a wedgegeometry and reducethe amountof blind slip requiredon the decollementnorthwest of the VenezuelanAndes. DeToni and Kellogg: Venezuelan Andes 1407

wedgegeometry and reduce the amount of blindslip required althoughthe formation of themonocline and the greatest uplift on the decollement northwest of the Venezuelan Andes. Rock occurred in late Miocene to Pleistocene time. The intramolasse modelsof low-anglereverse faults deformed under confining unconformitymay be relatedto theinitial lateMiocene pressureshow late stagebasement structures similar in shape collision of the Panama volcanic arc with northwestem South to thepredicted fault bend fold. America. The monocline on the northwestern flank of the Venezuelan Andesis the resultof crustalshortening in a northwest- Acknowledgments.Christopher Kendall advised and directed southeastdirection. This shorteninghas been attributed to the all thestratigraphic aspects of thestudy. We thankReginald convergenceof theCaribbean plate and the North Andean Shagamand Wayne Narr for criticalreviews. Maraven, S.A., microplateat the Caribbeanmarginal fault. Venezuela,generously provided all theseismic information. Two molassecycles separated by an intm-Miocene AlbertBally kindly provided a copyof FelipeAudemard's discontinuityare present in the southwestempart of the Ph.D. thesis. The secondauthor was supportedin part by a foredeepbasin. The firstcycle corresponds to theMiocene E1 grantfrom the Petroleum Research Fund of theAmerican Palmarand Isnotu formations. The secondcycle corresponds ChemicalSociety. We wantto thankINTEVEP, S.A., to the late Miocene-Plio-PleistoceneBetijoque Formation. Venezuela,for supportingresearch by thefirst author at the Thesethick (up to 8 km) continentalmolasse deposits indicate Universityof SouthCarolina and for authorizingthe thatthe Andean uplift hadbegun as early as early Miocene, publicationof thispaper.

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