acosta et al 2007 strike-slip deformation within colombian andes c

From: RIES, A. C., BUTLER, R. W. H. & GRAHAM, R. H. (eds) 2007. Deformation of the Continental Crust: TheLegacy of Mike Coward. Geological Society, London, Special Publications, 272, 303–319.0305-8719/07/$15 © The Geological Society of London 2007.

Strike-slip deformation within the Colombian Andes

JORGE ACOSTA1,2, FRANCISCO VELANDIA3, JAIRO OSORIO3,LIDIA LONERGAN2 & HÉCTOR MORA3

1EXGEO-CGG, Maracaibo, Venezuela (e-mail: [email protected])2Department of Earth Science and Engineering, Imperial College, Royal School of Mines,

Prince Consort Road, London SW7 2BP, UK3Ingeominas, Diagonal 53# 34–53, Bogotá, Colombia

Abstract: The Colombian Andes are characterized by a dominant NE structural trend,which is offset by ENE-trending right-lateral and NW-trending left-lateral structures.NE-trending faults are either dip-slip or oblique thrusts, generated as a result of a trans-pressive regime active since at least Palaeogene times. NW-trending faults are considered to bereactivated pre-Cretaceous extensional structures. Right-lateral shear on ENE-trending faultshas resulted from oblique convergence between the Nazca Plate and the Northern Andes.Major changes in the geometry of the oblique-plate convergence between the Nazca andSouth American plates have generated the northward ‘escape’ of the Northern Andes andstress–strain partitioning within the mountain belt. These strike-slip structures have exertedimportant controls on sedimentation, source-rock distribution, fluid flow and ore mineraliza-tion during Cenozoic times. The interpretation of the Northern Andes as a mountain beltaffected by strike-slip deformation provides a structural context in which to reassess theexploration plays.

Each of these cordilleras has a different composi-tion and evolution as a result of different tectonicprocesses that have affected the region sinceMesozoic times. The Central Cordillera origi-nated in response to Triassic subduction andconsequent volcanic and igneous activity(Barrero et al. 1969; Barrero & Vesga 1976;Bartok et al. 1981). Triassic and Jurassic back-arc rifting generated a predominantly NE–SWstructural trend that influenced the sedimen-tation in the adjacent Magdalena Valley andEastern Cordillera. The Western Cordillera iscomposed of oceanic crust and deformed deepmarine sediments, representing an accretionarycomplex established in the Late Cretaceous(de Freitas et al. 1997). The Eastern Cordillerarepresents the inversion of thick Mesozoic andTertiary sedimentary basins (see Taboada et al.2000).

Eastward subduction of the Nazca Platebeneath the Northern Andes occurs at c.60 mm a−1 (Trenkamp et al. 2002) along the west-ern margin of Ecuador and Colombia (Fig. 1).Additionally, north of latitude 8°N, the Carib-bean Plate is moving ESE at an average velocityof 20 mm a−1 (Trenkamp et al. 2002). As a resultof the oblique convergence of both the Nazcaand the Caribbean Plates with the NorthernAndes, the Andean block (Fig. 1) is movingnortheastwards relative to the South AmericanPlate (Pennington 1981; Kellogg et al. 1985;

The Colombian Andes have been interpretedas an assemblage of terraines that have beenaccreted to South America with a dominantNNE structural trend (Etayo-Serna et al. 1983;López & Barrero 2003). This interpretation hasled to the assumption of plane strain deforma-tion, orthogonal to the strike of the main struc-tures, which, in Colombia, is almost parallel tothe continental margin (Fig. 1). For this reason,the Colombian Andes have been described as aclassical fold and thrust belt (Mojica & Franco1990; Schamel 1991; Cooper et al. 1995). How-ever, oblique ENE and NW strike-slip deforma-tion has been recognised in the region (Feininger1970; Boinet et al. 1986; Diederix et al. 1987;Cuervo 1995; Velandia & Komuro 1998; Montes2001; Ujueta 2001; Branquet et al. 2002; Acostaet al. 2004) and the importance of these struc-tures in the evolution of the belt has, for the mostpart, been neglected. In this paper, a variety ofgeological, geophysical, geodetic and geodyna-mic information has been integrated to examinethe origin and evolution and of some of thesestrike-slip fault systems within the ColombianAndes.

Regional setting

The Andean Belt from northern Colombia toEcuador is divided into three mountain ranges(cordilleras), which merge near 1°N latitude.

304 J. ACOSTA ET AL.

Freymueller et al. 1993; Kellogg & Vega 1995;Mann 1995; Trenkamp et al. 2002).

Within the Andean Block, NE- and NW-trending thrust faults are offset by ENE-trendingright-lateral and NW trending left-lateral strike-slip faults. Whereas some researchers considerthese structures as local accommodation struc-tures around folds and thrusts (Camargo 1995;Cooper et al. 1995; Branquet et al. 1999a,b;Corredor 2003), in this paper it is argued thatthey are regionally significant and representa transpressive regime affecting the entireNorthern Andes.

The Algeciras Fault system

The Algeciras Fault system occurs in the centralpart of the Eastern Cordillera in SW Colombiaand continues southward for more than 800 kminto Ecuador and the Gulf of Guayaquil(Velandia et al. 2005) (Fig. 1). Features of

neotectonic activity have been identified alongstraight segments of the Algeciras Fault, whichare associated with right-lateral slip (Chorowiczet al. 1996; Vergara 1996; Velandia & Komuro1998). These features include synthetic and anti-thetic Riedel faults, principal displacement zones(PDZ), pull-apart basins (such as lazy-S shapedreleasing bends, extensive and rhomboidal-shaped and releasing sidestep basins) and minorfolds, located oblique to the main trace of thefault system (Fig. 2). The generation and bound-aries of the pull-apart basins are dominatedby transverse NW basement faults (e.g. theSibundoy Basin). Strike-slip indicators mainlyconcentrate along ENE segments, whereas asso-ciated thrust faults occur along NE-trendingsegments. Changes in the orientation of these

Fig. 2. The Algeciras Fault system (after Velandiaet al. 2005). Inset images are Landsat TM 5 scenes.

Fig. 1. Tectonic setting of the Colombian Andes.

305STRIKE-SLIP WITHIN THE COLOMBIAN ANDES


segments are also controlled by NW-trendingstructures.

The Algeciras Fault system is classified asa right-lateral strike-slip complex structure, withan important vertical component in which sedi-mentary cover and basement rocks are involved.Velandia et al. (2005) describe this fault as a zoneof simple shear, caused by the oblique conver-gence between the Nazca Plate and the NorthernAndes. It marks the boundary of the neotectonictranspressive regime in the Northern Andeswhich begins in Ecuador and continues intoColombia and Venezuela.

The Quetame Massif, which lies at thenorthern end of the Algeciras Fault system, is

interpreted as a major transpressional structure,whereas active volcanism along the Colombian–Ecuadorian border at the southern end of theAlgeciras Fault System indicates transtension.

Upper Magdalena Valley

The Upper Magdalena Valley is an intermontanebasin that lies between the Eastern and CentralCordilleras. It has been divided into the Girardotand Neiva sub-basins, which are separated bythe Natagaima Uplift (Beltrán & Gallo 1979;Corrigan 1979). As shown in Figures 3 and 4, theNeiva sub-basin exhibits three structural styles,as follows.

Fig. 3. Digital topographic model (DEM) of the Upper Magdalena Valley with main structural elements.


(1) NNE-trending thrusts carry pre-Cretaceous crystalline and Lower Cretaceousrocks over Cenozoic strata (Chusma, El Agrado–Dina and Rivera faults, Fig. 4). According toButler & Schamel (1987), the Chusma Fault is apre-existing basement structure that was reacti-vated during the Late Eocene–Early Oligocene.The El Agrado–Dina Fault is a thrust fault asso-ciated with the Chusma Fault, which, along withother minor thrust faults, forms an east-vergingimbricate fan system. The west-verging RiveraFault constitutes the boundary of the EasternCordillera with the Magdalena Valley.

(2) ENE-trending, right-lateral strike-slipsteep faults form a left-stepping array (e.g. LaPlata, Santa Helena, Platanillal and Hobo faults,Fig. 4) that affects pre-Cretaceous basement toNeogene rocks (Fig. 4). Drag folds, observednext to some of these faults, demonstrate thestrike-slip nature of the structures (e.g the SantaHelena Fault). Some of these faults are lateralramps for the north and NE reactivated faults ofthe Central Cordillera foothills (e.g. Chusma–LaPlata system, Butler & Schamel 1987). However,the ENE-trending system continues along theNeiva sub-basin under Quaternary deposits andeven affects the Eastern Cordillera (Figs 3 and 4).A large number of active neotectonic featuresoccur along these faults.

(3) NW-trending, left-lateral strike-slip steepfaults involve pre-Cretaceous basement to Neo-gene rocks (Neiva and Paso de Bobo faults, Fig.4). Some workers (e.g. Renzoni 1994; Velandia

2001) have shown that these faults have beenactive since at least Early Cretaceous times andcontrolled Cretaceous and Cenozoic sedimenta-tion along the Upper Magdalena Basin. In addi-tion, Neogene to Quaternary basic volcanicrocks, at the junction of these NW-trendingfaults and NE-trending structures (Velandia2001), indicate that the structures are still active.

Central Cordillera and Middle MagdalenaValley

The central part of the Central Cordillera com-prises igneous and metamorphic rocks affectedby a NE-trending system (Palestina Fault), anENE system (Ibagué Fault), a NW system (ArmaFault) and an arcuate fault system that boundsthe cordillera to the west (Romeral Fault system)(Fig. 5). This last system is a suture zone alongwhich oceanic crust collided obliquely with acontinental margin, 65–49 Ma ago (Barrero et al.1969).

The Palestina Fault system is a N30ºE-trending right-lateral zone that cuts through theCentral Cordillera (Fig. 5) and is assumed tohave developed as a result of the oblique collisionof the oceanic crust during the Late Cretaceous(Feininger 1970). Strike-slip deformation alongthis system, (1) generated the San Lucas Serrania,a transpressive duplex located at the northernend, (2) caused an over-step where draggingand right-lateral displacement of basement faults

Fig. 4. DEM of Neiva sub-basin with major faults. 1, Chusma Fault; 2, El Agrado-Dina Fault; 3, Rivera Fault; 4,La Plata Fault; 5, Santa Helena Fault; 6, Platanillal Fault; 7, El Hobo Fault; 8, Neiva Fault; 9, Paso de BoboFault.


Fig. 5. DEM and map of main faults and volcanos of the Central Cordillera (CC) and Middle Magdalena Valley(MV). SLC, San Lucas Serranía; RmV Romerales Volcano; CBV, Cerro Bravo Volcano; RV, Ruiz Volcano;QV, Quindio Volcano; TV, Tolima Volcano; MV, Machin Volcano; WC, Western Cordillera; CV, Cauca Valley;EC, Eastern Cordillera.

occurred on the central part, and (3) createdoblique right-lateral and normal faults that areactive and control the Quaternary magmatismat the southern end of the system (Fig 5). Inaddition, an analysis of the magmatic rocks inthis region during the present study showed thatit has migrated from north to south since theEocene. Similarly, reactivation of NW-trendingfaults during this time has affected the horsetailstructure of the Palestina Fault system and there-fore migration of magmatism and reactivation ofNW-trending faults is closely related. Hence the

emplacement of the volcanic bodies in this partof the Central Cordillera contrasts with thatobserved at the Colombia–Ecuador border.

The Ibagué Fault system right-laterallyoffsets the Central Cordillera by 25 km (Figs 5& 6) (Montes et al. 2005). This N70ºE-trendingsystem is a left-stepping array that can be tracedthrough the Central Cordillera from the CaucaValley to the Magdalena Valley; it may continue

Fig. 6. Neotectonic map of the Ibagué Fault.


into the Eastern Cordillera as the Viani Fault.The main strand of the fault is a succession ofshutter ridges, pull-apart basins and syntheticfaults within the Ibagué Fan (Fig. 6).

Palaeoseismological studies along the mainstrand of the Ibagué Fan indicate an averagestrike-slip rate of 0.77 mm a−1 for the last 15 Ka(Montes et al. 2006). Assuming a constant sliprate of 0.77 mm a−1, would imply that the IbaguéFault has been active from Late Eocene to EarlyOligocene times to account for the 25 km oflateral displacement along the Central Cor-dillera. However, Montes et al. (2006) also deter-mined strike-slip rates as high as 3.8 mm a−1 forthe Ibagué Fault, which, if constant over geologi-cal time, would indicate that activity on the faultwas much more recent (Middle to Late Miocene).In this study, it is assumed that the average of thestrike-slip rates reported by Montes et al. (2006)is probably the valid number to use and that theIbagué Fault became active in the Middle to LateOligocene.

NW-trending faults cut basement rocks ofthe Central Cordillera and Upper MagdalenaValley (Fig. 7). These faults are continuous fromthe western foothills through the Central Cordil-lera to the eastern foothills. The main structure ofthis system is the Arma Fault, which is an obliquenormal, left-lateral fault. Gold-bearing igneousdykes are found only along the NW-trendingfaults (Lozano & Murillo 1983).

A set of NE-trending, steeply dipping pre-dominantly normal left-lateral faults have alsobeen identified in the western foothills of theEastern Cordillera and Middle MagdalenaValley, which affect Neogene to Quaternarysediments (Acosta et al. 2004). The NW-trendingstructures are clearly related to focal mechanismsat depths of 24–40 km (Fig. 7), and confirm thereactivation of the NW–SE-trending faults, aspreviously proposed by several workers inregional studies of the Northern Andes (Acosta1983; Gómez 1991; Ujueta 2001; Velandia &De Bermoudes 2002).

The Bucaramanga–Santa Marta Fault has arelatively straight trace to the north of the MiddleMagdalena Valley (Campbell 1968; Boinet et al.1986). The deformation zone of this N30ºW-trending, left-lateral strike-slip fault can betraced for c. 500 km through northern Colombia(Fig. 8) The fault zone is defined by a set of

approximately parallel faults that splay from,and rejoin, the main fault strand within a 10 kmwide strip. A set of NW-trending low to moder-ately dipping thrust faults, striking subparallelto the main strand, also occur within the faultzone.

Seismic profiles, perpendicular to the mainstructures on the western block of the Bucara-manga Fault (Fig. 8), show typical elements ofmany thrust belts around the world, includingbasin inversion, thin-skinned thrusting anddecoupling of the post-rift cover, out-of-sequence thrusting and thick-skinned basementinvolved thrusting. However, in this studyfeatures typical of strike-slip movement, associ-ated with the Bucaramanga Fault, have also beenobserved: (1) complex unconformities; (2) largestructural changes along strike such as non-cylindrical folding of the detachment level; (3)change from open to isoclinal folding alongthe axis of the detachment, showing non-planardeformation.

Eastern Cordillera

The core of the Eastern Cordillera is formedby metamorphic and sedimentary rocks ofPalaeozoic age and igneous extrusive and intru-sive rocks of Jurassic age. Longitudinal andtransverse tectonic controls have exposed a thicksequence of Cretaceous sediments of differingfacies and thickness in the axial zone of thecordillera. Cenozoic and unconsolidated Quater-nary deposits partially cover the region, makingit difficult to follow some of the structures.

Strike-slip faulting in the axial zone of theEastern Cordillera has been identified by severalworkers, who proposed that these structures areregionally related to subvertical or high-anglefaults at depth (Kammer & Mojica 1996;Taboada et al. 2000; Sarmiento 2002). De Freitaset al. (1997) supported this interpretation, butadditionally proposed that the NW-trendingstructures represent rift cross-faults.

The Bucaramanga Fault system ends to thesouth in the axial zone of the Eastern Cordillera,in a compressional duplex structure that showsthe left-lateral movement of the system (Fig. 9)(Velandia 2005). NE-trending right-lateral faultsmerge with the duplex to the south generatingan arcuate complex structural pattern in theregion.

In the Paipa geothermal area, NW- and ENE-trending strike-slip structures have also beenidentified (Fig. 9). Right-lateral displacementalong the ENE-trending faults, which affects thesedimentary sequence, was determined by striae

Fig. 7. (a) Digital Elevation Model of centralColombia, (b) stereonet plot of Neogene andQuaternary faults, (c) shallow focal mechanismsolutions in the western foothills of the EasternCordillera and Middle Magdalena Valley (b and cmodified from A costa et al. 2004).


along the fault surfaces. These ENE-trendingfaults offset NE thrusts and are in concordancewith the ENE strike-slip faults observed in theCentral Cordillera (e.g. Ibagué Fault), and there-fore are interpreted as a younger fault systemrather than lateral ramps associated with thethrusts.

NW-trending left-lateral shear zones, whichoccur to the SE, are associated with a Neogene

volcanic body that is the thermal source of thePaipa geothermal field. Regionally, the trace ofthe shear zone continues to the SE, whereanother volcanic body and the Iza geothermalsystem occur (Velandia 2005). These faults areinterpreted as pre-existing extensional basementstructures that were reactivated during theAndean tectonic phase and facilitated the magmaemplacement.

Fig. 8. Interpreted seismic profiles across the Bucaramanga Fault, Seismic data courtesy of Ecopetrol. Verticalscale in two-way travel time, to 5 seconds for sections A, B, and C; and 6 seconds for section D.


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Partitioning within the Colombian Andes

NW-trending strike-slip deformation within theColombian Andes is interpreted to be associatedwith pre-existing basement structures. However,some other questions arise, such as: what con-trols the ENE-trending strike-slip deformation,the timing of the deformation and the relation-ship between the NW-, NE- and ENE-trendingstrike-slip structures? To answer these questions,the changes in the dynamics and geometry of thesubducting Nazca Plate under South Americatook place are considered.

The subduction zone of the Nazca Plate,under the Northern Andes, is affected by

erosional and accretionary processes within thecoupling zone. Submarine topographic anoma-lies, such as the Carnegie Ridge, the YaquinaGraben and other fossil ridges, have led to a seg-mentation of the subduction zone. Additionally,the geometrical relationship between the conver-gence vector and the shape of the trench has ledto strain partitioning resulting from the fact thatthe convergence vector can be divided into twovectors, as proposed by Toro & Osorio (2002):(1) a vector orthogonal to the trench, indicatedby the deformation within the coupling zone(Fig. 10), which favours shortening and inversionof NE-trending structures within the continent;(2) a vector parallel to the trench, which is

Fig. 10. Tectonic map of the Nazca plate and Northern Andes. CGR, Carnegie Ridge; NP, Nazca Plate; YG,Yaquina Graben; MR, Malpelo Ridge; HF, Hey Fault; CR, Coiba Ridge; CP, Coiba Microplate; JF JordanFault, SP, South American Plate; RF, Romeral Fault; PF, Palestina Fault; ArF, Arma Fault; IF, Ibagué Fault;GF, Garrapatas Fault; AF, Algeciras Fault; DGM, Dolores-Guayaquil Megashear. The seismicity plotted inprofiles A, B, C and D illustrates the dip of the subducting slab. Vertical scale of A and C is 150 km; B and D is250 km. Seismic data from Ingeominas earthquake catalogue.


transferred into the continent, generating newENE-trending shear structures and facilitatingthe reactivation of NW-trending pre-existingfaults.

As a result, four distinct segments can beidentified along the Nazca subduction zonein Northern South America (Orozco 2004), asfollows.

(1) In the Ecuador segment, between the Gulfof Guayaquil and Esmeraldas in northernEcuador (Fig. 10), the convergence vector isalmost orthogonal to the trench, leading to aminimum of strain partitioning, as shown by thegeneration and reactivation of shortening struc-tures that dominate the Andean Cordillera inEcuador. ENE-trending strike-slip structures arepresent in the southern part of the segment alongthe Dolores–Guayaquil Megashear (Dumont &Benítez 1996). Volcanic bodies to the north of theDolores–Guayaquil Megashear and absence ofdeep seismicity are the main tectonic processeswithin this segment. The lack of deep seismicitywas used by Gutscher et al. (1999) to propose aflat slab under Ecuador. However, the volcanismseems contradictory to this hypothesis, but itcould occur as a result of the volume compen-sation in a transtensional zone, related to majorENE-trending systems (e.g Algeciras Faultsystem).

(2) In the Tumaco Segment, between Manta(Ecuador) and the mouth of the Patia River (Fig.10), the convergence vector is 60º oblique to thetrench, favouring the transfer of displacementand deformation to the continent. ENE-trendingright-lateral strike-slip faults developed withinthe continent, such as those in the UpperMagdalena Valley and along the Algeciras Faultsystem. This segment represents a huge rupturezone along the trench where subduction earth-quakes of magnitude Mw 8.8 (1906), Mw 7.9(1942) and Mw 7.8 (1958) have been recorded.However, the scarcity of deep seismic activitymakes it difficult to trace the subduction slabunder the continent. Parallelism and changes indirection of the volcanic belt at the Colombia–Ecuador border and the trench at the same lati-tude suggest that the geometry of the subductionzone changed.

(3) The Buenaventura Segment, between theGarrapatas and Hey faults to the south andnorth, respectively (Fig. 10), is characterized byaccretion in the coupling zone (Cediel et al.2004), shallow and deep seismicity, and the pres-ence of a volcanic arc. The convergence vectoris semi-orthogonal to the trench (80º), leadingto the generation of ENE-trending right-lateralstrike-slip faults within the continent, such as theGarrapatas and Ibague faults systems.

(4) The Coiba Segment lies between the Heyand Jordan faults forming the Coiba Microplate(Pennington 1981) (Fig. 10). This microplate wasa part of the Nazca Plate, from which it splitabout 8 Ma ago to release the displacement ofthe Cocos and Nazca plates (Sayares & Charvis2003). As a result, the Panama Fracture Zoneand the Hey and Jordan faults were formed.

Currently the neighbouring Nazca Plate(to the south) and Panama Block (to the north)are moving faster (60 mm a−1 and 30 mm a−1,respectively) eastward than the Coiba Microplatewhich is moving at 25 mm a−1 (Mora 1995), hencemaking the subduction process less probable inthe region. The absence of deep seismicity andvolcanism in the Coiba Segment is consistentwith these observations. Stress is directly trans-ferred to the continent because of the orthogonalconvergence of the block, generating shorteningin the Baudo Serrania.

The effect of the orthogonal convergenceof the Coiba Microplate and the convergencevector of the Panama Block generates a SE-trending main stress direction within the conti-nent, allowing tensional and left-lateral shearalong pre-existing NW-, north- and NE-trendingstructures.

Global Positioning System data

GPS data collected from 1994 to 2003 within theNorth Andean Block and plotted with respect toSouth America confirm that part of the conver-gence vector is transferred to the continent, asexpressed by Trenkamp et al. (2002), with valuesof displacement that reach 21 mm a−1 with anazimuth ranging from 58º to 89º. An elastic lock-ing and aseismic slip imply a slip interface that islocked by enough friction to permit part of theNazca Plate velocity to be transferred to the over-riding South America Plate with the plate stillsliding into the mantle with part of the originalplate velocity (Trenkamp et al. 2002). The defor-mation caused by these vectors can be interpretedas: (1) continental elastic–plastic deformation,part of which will be recovered in an elastic slipevent, such as an earthquake, on the subductionzone and part of which permanently deforms theAndean crust; (2) active tectonic faulting obliqueto the trench, where the deformation is homo-geneously distributed in simple shear zones, asproposed by Folgera et al. (2002).

Results from GPS data obtained before,and after, the 1999 earthquake, plus the ENE-trending strike-slip faulting indicate that bothdeformation mechanisms are simultaneouslyacting in the Colombian Andes. Therefore, either


transpressional or transtensional systems will bedeveloped, and wide continental areas will beaffected by rotational shear.

In addition, there is a possible visco-elasticoverprinting deformation on a subset of thosevectors between 2º and 4ºN (White et al. 2003).However, the visco-elastic part is not the mainsignal that is being observed. Visco-elastic pro-cesses are longer wavelength and elastic pro-cesses are shorter wavelength phenomena. Inother words, when an earthquake occurs, theelastic effect is generally over in a year or two butvisco-elastic processes continue for decades orcenturies.

Timing and relationship between structures

Tectono-stratigraphic studies of Colombia havedemonstrated the existence of pre-Cretaceousrifting which gave rise to NE-trending normalfaults and NW-trending cross-fault systems(Cooper et al. 1995; Acosta 2002). At the end ofthe Mesozoic (66 Ma), the Caribbean Plate rela-tive motion changed in the west from northeast-ward to eastward and began to underthrustnorthern South America (Mattson 1984). Thisdrove the oblique accretion of the Western Cor-dillera along the Romeral Fault from 68 to 49 Ma(Barrero et al. 1969), causing uplift and erosionin the Central Cordillera (Cooper et al. 1995).

As a result of the oblique collision of theoceanic crust during the Late Cretaceous, strike-slip systems, such as the Palestina Fault, wereactive (Feininger 1970). It is therefore assumedthat simple shear deformation occurred withinthe Northern Andes during this time, as pro-posed by Tikoff & Teyssier (1994) and Teyssieret al. (1995).

The Farallon–Phoenix Plate separated intothe Nazca and Cocos plates at about 27 Ma,increasing the convergence rate between theNazca and South American plates (Mattson1984). This event coincided with the initiationof the Ibague Fault system and the other ENE-trending right-lateral strike-slip fault systemswithin the Northern Andes. Additionally, NW-trending left-lateral strike-slip fault systemsstarted to be reactivated almost at the same time,suggesting that the convergence angle, althoughoblique, became more orthogonal than it hadbeen previously.

The change in the convergence angle, therheological heterogeneities within the region andthe action of the newly formed ENE-trendingright-lateral strike-slip faults have facilitatedthe continuous differential reactivation of pre-existing basement faults since the Oligocene.Some NE-trending basement structures, such asthe Chusma Fault, inverted as oblique thrusts

by taking advantage of the partition generatedby ENE-trending structures that acted as lateralramps. NW-trending rift cross-faults, reactivatedas oblique left-lateral normal faults togetherwith the ENE-trending right-lateral faults, actedas barriers for the Cenozoic sedimentation in theinter-montane valleys.

The accretion of the arcuate Panama Blockin the northwestern corner of Colombia (Duque-Caro 1990), combined with the generation of theCoiba Microplate as a result of the formation ofthe Panama fracture zone and Hey and Jordanfaults, was responsible for the major episode ofMiocene deformation in the Colombian Andes.This last major event in the region led to apresent-day maximum horizontal stress direc-tions of 112° and 138° (Castillo & Mojica 1990).In addition, Acosta et al. (2004), based onmeasurement of kinematic data, earthquakefocal mechanisms and borehole breakout data,inferred a NW-trending principal incrementalstrain axis (shortening axis) for the region atthe present, which may have extended back tothe Late Neogene, as no significant changes toplate movement directions are thought to haveoccurred since 11 Ma (e.g. Daly 1989).

Therefore, the NW-trending incrementalstrain is parallel or subparallel to the NW-trending pre-existing faults. This favours theirreactivation and opening as tension faults, alongwhich hydrothermal fluids and Neogene magmashave risen.

Implications for mineral and hydrocarbonexploration

The different rheological properties of the rockson either side of these strike-slip structures andtheir different thicknesses exert a pronouncedcontrol on the structural style. On one side tightcomplex structures are developed, whereas onthe other wide and simple folds are formed asis observed in the Upper Magdalena Valley.This pattern could affect the petroleum systemif strike-slip fault systems act as migrationpathways.

The ENE- and NW-trending strike-slipfaults are associated with two different processesthat might be either favourable or damagingfor the structural trapping of hydrocarbons.Continuous reactivation of pre-existing base-ment faults plus the generation of new strike-slipstructures enhances fracturing that is accom-panied by infilling of fluids, some of whichgenerate hydrothermal–sedimentary deposits;for example, the occurrence of emeralds as pro-posed by Branquet et al. (1999a), and ore mineraldeposits, as suggested by Lozano & Murillo


(1983). Major concentrations of minerals, hydro-thermal fluids and volcanism (especially cindercones) are usually associated with the junction ofthe NE-, NW- and ENE-trending systems.

The continuous deformation process has alsoled to damage of pre-existing ore mineral depos-its. The enhanced heat flow arising from hydro-thermal fluid circulation along the strike-slipfault systems may have made organic hydrocar-bons overmature. Emplacement of magma canoccur, as in the Paipa and Iza geothermal fieldsin the Eastern Cordillera, affecting sedimentarysequences and emplacing porphyritic bodiesalong shear zones in the Central Cordillera,which can destroy any previously ore-enrichedsedimentary sequences.

Conclusions

Stress and strain partitioning within the North-ern Andes is due to the oblique convergencevector of the Nazca Plate and to the changes inthe geometry of the coupling zone, which inducea transpressive regime in the region. This parti-tioning was expressed during Early Cenozoictimes along transcurrent fault systems, sub-parallel to the margin (e.g. Palestina Faultsystem), and then along ENE-trending right-lateral structures, such as the Ibagué Fault, sincethe Late Palaeogene.

This partitioning also favoured the inversionof pre-existing NE-trending faults as obliquestructures and the reactivation of pre-existingNW-trending faults as left-lateral structures.A conjugate movement with a right-lateral senseof motion and partitioning along left-lateralstrike-slip faults led to a counterclockwiserotation of the Andean Block and northwardexpulsion of the whole block.

The existence of strike-slip deformation alongthe Colombian mountain belt is favourablefor the occurrence of ore-mineral deposits andprovides a new structural style that should beconsidered in the of assessment hydrocarbonexploration plays.

We thank P. Cobbold and an anonymous reviewer forhelpful comments, Douglas Hamilton for help with theEnglish, A. Rees and R. Graham for editorial revisions,and Ingeominas for permission to publish DEM data.This research was inspired by discussions with MikeCoward while J. Acosta conducted his PhD underMike’s supervision.

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