University of South Carolina Scholar Commons Faculty Publications Earth, Ocean and Environment, School of the 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] Follow this and additional works at: https://scholarcommons.sc.edu/geol_facpub Part of the Earth Sciences Commons 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 C•RIBBE•N SEA C•RIBBœ•N Sœ• 12 ø I VENEZUELA COLOMBIA 11 ø MARACAIBO BASIN CARIBBEAN 10 o MOUNTAINS STUDY AREA •- APURE-BARINASBASIN __ _..,.,. _• r.,'"' •--X.,,, j_. o o, ,o,o ß I 75 ø W 72 ø 71 ø 70 ø 69 ø 68 ø 67 ø 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 many areas 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.