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Relationship of Mesozoic graben development, stress, shortening magnitude, and structural style in the Eastern Cordillera of the Colombian

E. TESO´ N1*, A. MORA1, A. SILVA1, J. NAMSON2, A. TEIXELL3, J. CASTELLANOS1, W. CASALLAS1, M. JULIVERT3, M. TAYLOR4, M. IBA´ N˜ EZ-MEJI´A5 & V. A. VALENCIA6 1Instituto Colombiano de Petro´leo, Ecopetrol, Bucaramanga, 2Namson Consulting Inc., San Clemente, CA 92672, USA 3Departament de Geologia, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain 4Department of Geology, University of Kansas, Lawrence, KS 66045, USA 5Department of Geosciences, The University of Arizona, Tucson, Arizona, USA 6School of Earth & Environmental Sciences, Washington State University, Pullman, Washington, USA *Corresponding author (e-mail: [email protected])

Abstract: We use the Eastern Cordillera of Colombia as an example in early stages of inversion orogen showing still modest values of shortening. The style of deformation recorded in this oro- genic chain seems to be strongly influenced by two main factors. The first is the pre-compression geometry of the basin, conditioning the strong heterogeneity imparted by a trough filled with to Neocomian sediments limited by and Palaeozoic high-angle walls. The second factor is the orientation of the stress regime with respect to the main normal faults during the inversion. If the stress field is of pure compression, the normal faults are not extensively inverted and the deformation is accommodated mainly in terms of footwall shortcuts. On the other hand, in transpressive regimes the inversion of the former normal faults is more common and the footwall shortcuts are not dominant structures. No significant lateral variations in tectonic shorten- ing are found in the Eastern Cordillera. Finally we emphasize the role of buckle folds in the internal parts of the inversion orogens and give a cautionary note when interpreting these structures in terms of fault-related folding using the well-documented example of the Soapaga fault area.

Inverted orogens remain not completely understood older and stronger rocks, often crystalline . in their origin, evolution and geometry of related This results in a large rheological difference faults and folds. Typically, the development of between the rift basin fill and its margins. inverted orogens begins by continental rifting that A change in plate kinematics may result in a leads to the initial development of an upper crustal shift from a tensional to compressional stress state extensional basin, commonly filled with conglo- in the former rift basin. In these cases, contractional merates and in terrestrial or shallow deformation (inversion) will be strongly influen- marine settings. As extension progresses, the zone ced by the extensional structures formed previ- of accommodation deepens, evolving into a deep ously, which can act as structural barriers between marine basin with fine- to medium-grained sedi- rocks of different rheologies as described above. ments deposited over the previous succession. Fin- Most models of orogenic wedges developed over ally, shallow marine to transitional sediments are the past decades are applied to accretionary prisms deposited over larger areas while subsidence rates or fold-and-thrust belts in convergent margin set- decrease as the system becomes dominated by ther- tings (e.g. Davis et al. 1983; Lallemand et al. 1992; mal processes (e.g. Allen & Allen 2005 and refer- DeCelles & Mitra 1995; Constenius 1996; Meigs ences therein). Owing to their protracted history, et al. 1996; Meigs & Burbank 1997; Mitra & Sus- rift systems are mechanically anisotropic, with rift sman 1997), without the heterogeneities and strong basin fills typically composed of relatively weak rheological contrasts characteristic of inverted basin sedimentary rocks that in some cases are poorly settings. consolidated (e.g. Sarmiento-Rojas 2001). Outside The Eastern Cordillera of Colombia is an intra- the rift margins common lithologies consist of cratonic chain derived from the inversion of a

From:Nemcˇok, M., Mora, A. & Cosgrove, J. W. (eds) 2013. Thick-Skin-Dominated Orogens: From Initial Inversion to Full Accretion. Geological Society, London, Special Publications, 377, 257–283. First published online June 11, 2013, http://dx.doi.org/10.1144/SP377.10 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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Neocomian Rift system (Colletta et al. 1990; Casero years by members of Ecopetrol S.A. Additionally, et al. 1997). Some tectonic models describing the a large dataset of thermochronology and vitrinite evolution of the Eastern Cordillera of Colombia reflectance also aided in identifying faults and assume a simple convergent margin accretion- folds (see Mora et al. 2013; Jimenez et al. this ary wedge-like structure dominated by flat detach- volume, in press; Caballero et al. this volume, in ments and low-angle thrust ramps resulting in press, 2013). Four cross sections were constructed over-estimates of total shortening with values up (Fig. 2) to include the areas where remarkable geo- to 50% (Dengo & Covey 1993; Roeder & Chamber- logical features are present and where the geological lain 1995). In contrast, shortening estimates that data collected are good enough to produce confi- assume the inversion of a rift basin lead to differ- dence in the determination of the geometry of the ent values (Cooper & Williams 1989 and references structures. In addition, the four sections were therein) with specific values for the Eastern cordil- restored to determine the geometry pervious to the lera of 20–25% (Cooper et al. 1995; Mora et al. deformation and the amount of shortening. 2008). Intermediates values of shortening of about The structural data presented are used to for- 30% are obtained by Colletta et al. (1990) and mulate a discussion on the influence of the past Toro et al. (2004). and present stress regime in the northern Colom- In this paper we describe the tectonic configur- bian Andes. Statistical analysis on the distribution ation of the Central Segment of the Eastern Cordil- of folds and faults along the Eastern Cordillera lera of the Colombian Andes based on new detailed was incorporated to describe mechanisms of fold- observations and a series of synthetic transects ing during the deformation. Finally, the different (four), with special emphasis on the initial rifting mechanisms of folding and faulting observed in stage and its role during subsequent compression. the Eastern Cordillera are discussed based on new Together with companion papers in this volume, data and previous studies and we also intend to we intend to shed more light on the processes deter- correlate these different folding mechanisms with mining the geometry of faults and folds associ- different mechanical conditions. ated with the structural inversion of rift basins into orogenic belts. Stresses, different rheologies and relative histories of deformation interplay to pro- Basement of the Northern Colombian duce a picture that definitely departs from conven- Andes tional wedge models and suggests that shortening assessments are lower than previous models pro- Establishing a link between different basement posed. We argue here that our geometric descrip- blocks and provinces underlying the Meso-Ceno- tion supported on robust data is not only the basic zoic sedimentary infill of the Eastern Cordillera deformation framework for other chapters in this is crucial for our discussion, in addition to under- volume, but also constitutes an exceptionally docu- standing how this pre-existing framework influ- mented template pilot area for the unique defor- enced the structural development of the thrust-belt mation and folding mechanisms that characterize upon inversion. As documented below, the main moderately deformed inversion orogens. extensional limits of the Mesozoic rift basin, which initially controlled the thickness of syn-rift clas- tic sedimentation, also appear to have played a Methods major role controlling the geometry of later com- pressional structures, which seems to be recurrent To recognize the pre-inversion basin geometry in over time. the Eastern Cordillera, recent field work and map- The Proterozoic and Palaeozoic basement of ping included new maps from up to 40% of the the Eastern Cordillera appears as isolated massifs area of the Eastern Cordillera carried out by Eco- and culminations exposed along the cordilleran petrol in the framework of this study and focused strike, namely the Garzo´n, Quetame, Floresta and on the centre and eastern side of the range. We also Santander massifs (Fig. 3). The contrastingly differ- used the detailed maps of Mora (2007) and Parra ent age patterns that each exhibits nicely exempli- (2008) for the southern part of the eastern foot- fies the diversity of the crust that underlies the hills, the Farallones anticline and the Medina area northern Andes. The Garzo´n massif consists of gran- (Fig. 1). Normal faults were identified in the field ulites and gneisses of Grenvillean affinity (Alvarez based on fault geometry and kinematics, bed cut- 1981; Kroonenberg 1982; Restrepo-Pace 1995), offs along mapped faults, changes in thickness of and is characterized by two high-grade meta- sedimentary units and sedimentary facies. Fully morphic events dated by zircon U–Pb at c. 1020 and partially inverted faults, footwall shortcuts and and c. 990 Ma that rework late thin-skinned flat detachments were identified in magmatic and sedimentary protoliths (Cordani map view based on data collected over the last 10 et al. 2005; Ibanez-Mejia et al. 2011). Crust of Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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Fig. 1. Geological map of the Eastern Cordillera of Colombia and Middle Magdalena Basin (modified from Mora et al. 2008; Parra et al. 2009a), showing the localization of the cross-sections presented in Figures 2 and 6 and the detailed map of Figure 5. WC, Western Cordillera; CC, Central Cordillera; EC, Eastern Cordillera. similar metamorphic age is also known to occur in those found in the Garzo´n affect the late Palaeo- and the proximal foreland underlying sediments of the early Mesoproterozoic crust of the Amazon Craton Putumayo basin, just east of the Garzo´n massif, (Ibanez-Mejı´a et al. 2011). The geochronological where metamorphic events of the same age as data available suggest that the Tesalia–Guaicaramo Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

260 E. TESO´ N ET AL.

Fig. 2. Geological cross sections through the Eastern Cordillera of Colombia showing the bivergence and the thick-skinned style. See location in Figure 1. fault system now marks the boundary between The Santander massif displays a protracted Pro- Grenville-age, accreted, para-autochthonous terranes terozoic to mid-Cenozoic tectonometamorphic to the west, and autochthonous Amazonian crust history that still awaits to be fully unravelled; the to the east. The Garzo´n crystalline rocks are overlain availability of published geochronological data by a clastic sequence of the La Jagua obtained by robust methods such as zircon U–Pb is formation, and no evidence of lower-Palaeozoic very limited (Restrepo-Pace 1995; Do¨rr et al. tectonothermal events has been reported in this area. 1995; Cordani et al. 2005). The complexity of the The Quetame massif consists of presumably results obtained for the Santander Massif makes lower Palaeozoic low- and medium-grade meta- age determinations still ambiguous for many of the sedimentary rocks (quartzites and phylites), whose dated units. The general consensus is that the Bucar- minimum age is constrained by their stratigra- amanga gneiss, a sillimanite–cordierite-bearing phic position below the Devonian unconformity metasedimentary unit in the lower part of the succes- (Renzoni 1968), and by the crystallization age of sion, corresponds to the Grenville age basement in a lower granite (483 + 10 Ma) intrud- the area (Cordani et al. 2005). Deformed and unde- ing them (Horton et al. 2010). These metasedi- formed granitoids that appear in spatial association mentary rocks appear tightly folded and display with the metasedimentary Silgara´ formation, com- a foliation, although precise age constraints for the posed of slates to meta-sandstones in the core of metamorphism associated with the deformational the massif, have shown crystallization ages c. event are still unavailable. The Floresta massif is 480 Ma (Restrepo-Pace 1995), and they have been comparable to the Quetame as it is composed of assumed to constrain a minimum depositional age metapelitic units underlying non-metamorphosed for the protoliths of the metasedimentary sequence. Devonian strata of the Tibet formation. Like the However, more recent but yet unpublished detrital Quetame massif, more constraints on the age of zircon U–Pb data is revealing that some successions protolith sediments and deformation of the metase- of metasedimentary units previously assigned to dimentary sequences are currently lacking. the Silgara´ formation were deposited during or Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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Fig. 3. Shaded relief of the northern corner of South America showing the main basement massifs and their nature.

after the , and have likely experienced magmatic or metamorphic events outside the – metamorphism adding further present limits of the Eastern Cordillera. However, complexity to the deformational history of the recent U–Pb zircon geochronological studies per- massif. This Proterozoic to Palaeozoic basement formed in magmatic and high-grade metamorphic was later affected by widespread to units of the Central Cordillera are starting to early Jurassic magmatism (Do¨rr et al. 1995) and reveal an important role of Permian–Triassic tec- migmatization. tonic events in shaping the basement of the Northern Out of the Eastern Cordillera domain, recently Andes. After the pilot study of Vinasco et al. (2006), studied boreholes that pierced crystalline rocks which first showed the widespread occurrence of under the show that its Permian–Triassic magmatism in the Central Cordil- basement is composed of biotite- to sillimanite- lera, other studies have reported granites of similar grade pelitic schists that yield late Permian maxi- age also in the Central Cordillera (Restrepo-Pace mum ages for protolith sedimentation (youngest & Cediel 2010), in the basement underlying the detrital zircons), later affected by lower Triassic Plato-San Jorge basin (Montes et al. 2010), in the mid- to high-grade metamorphism. Other Precam- Santa Marta massif (Cardona et al. 2011) and in brian basement outcrops are known from the Santa the Guajira peninsula (Weber et al. 2010). The Marta and Guajira massifs in the Caribbean coast extent of the Permian–Triassic orogen in Colombia (Cordani et al. 2005; Cardona et al. 2010), as well can also be potentially traced as far east as the as from the Central Cordillera (Serrania de las Santander massif and with a limit roughly coinci- Minas; Ibanez-Mejia et al. 2011). As for Palaeo- dent with the Boyaca´ fault to the south of the San- zoic basement, no conclusive evidence exists in tander massif, given some of the new observations support of the occurrence of lower Palaeozoic mentioned above. Evidence from studies conducted Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

262 E. TESO´ N ET AL. in Venezuelan territory suggests that the early and inverted normal fault at this latitude (4–58N; see late Palaeozoic orogens continue towards the NE, details in Jimenez et al., this volume, in press). extending along the Merida Andes (Burkley 1976) and under the proximal Llanos foreland (Feo- Extensional faults in the Pajarito–Nunchı´a areas. Codecido et al. 1984). Also, plutonic units from To the north of the Farallones massif, the exten- the El Bau´l uplift in the central Venezuelan fore- sional basin configuration in the Nunchı´a–Pajarito land, with crystallization ages dated at c. 290 and area (Fig. 1) was poorly understood prior to this 490 Ma by U–Pb SHRIMP, document the extent study, so a detailed mapping campaign was carried of the Palaeozoic orogens that seem to be spatially out to identify and characterize the main faults superposed along this part of the margin. and their kinematics. A series of extensional faults of early age were found towards the interior of the Eastern Pre-compressional basin configuration Cordillera. These faults, herein referred to as the of the Eastern Cordillera San Ignacio normal fault system, are located west of the Pisba village in the Pa´ramo de San Ignacio Experimental models of inversion tectonics wid- region (Figs 1 & 5). Further east, the west-dipping ely demonstrate that graben configuration strongly Paya Lower Cretaceous normal fault (Figs 5 & 6), influences the geometry of contractional structures together with the San Ignacio normal fault system, (e.g. Krantz 1991; Nalpas & Brun 1993; Nalpas defines the Pisba graben, which could be the 1994; McClay 1995; Yamada & McClay 2004; southern continuation of the cretaceous Cocuy Amilibia et al. 2005; Marques & Nogueira 2008). Basin described by Fabre (1985). The tectonic sig- Defining the geometry and timing of rift-bounding nificance of the Pisba Graben is similar to the Gua- normal faults is a fundamental challenge in under- tiquı´a Graben in the Farallones–Medina area. standing the evolution of natural inverted orogens. The Paya fault preserves normal offset for the Berrasian to units (Macanal, Alto de Caqueza and Fomeque formations), and was active Pre-compressional configuration of the up to the late because the top of the eastern foothills area formation records no evidence of normal faulting (Figs 2 & 6). The Fomeque formation is ,500 m Previous accounts of the Cretaceous rift geometry, thick in the fault footwall, but in the hanging wall deformation style and chronology of inversion refer our minimal thickness estimates exceed 1000 m to the southern part of the eastern foothills, and (since the base of the formation is not observed). include Mora et al. (2006), Mora et al. (2010b) The Paya fault is linked to the Pajarito fault (Fig. and Parra et al. (2009a, b). Here we add new 4), which transfers slip into the Servita´ fault. The results of field mapping in the northern part of the Pajarito fault is interpreted as a Cenozoic contrac- eastern foothills. These new field observations, tional oblique structure nucleated in the transfer combined with the analysis of seismic reflection zone between both normal faults during Cenozoic profiles and oil wells, allow us to understand the tec- contraction. tonic evolution of the eastern foothills of the north- Based on the field observations above, the con- ern Colombian Andes. figuration of the eastern margin of the foothills is similar in the northern and southern sectors. Com- Extensional faults in the Medina–Quetame areas. prehensive reconstruction during the Neocomian The Medina–Quetame region is described by requires understanding of the Cusiana fault, which Mora et al. (2008, 2010a) in terms of a main inver- is the easternmost contractional fault along the sion anticline (Farallones anticline) bounded by eastern foothills, and has been interpreted to be an continuous, west-dipping master faults that were inverted normal fault by Cooper et al. (1995) and originally normal faults of Neocomian age and are by Jimenez et al. (this volume, in press). The Guai- now inverted as thrusts (e.g. Servita´ fault; Fig. 4). caramo thrust fault also appears to be a continuous This is in stark contrast with the more segmented, feature along most of the eastern foothills. less continuous east-dipping faults bounding the anticline to the west (e.g. Naranjal and San Juanito faults). Both fault systems define the Neocomian Jurassic–Lower Cretaceous extensional Guatiquı´a graben (Mora et al. 2008), which faults in the internal parts of the Eastern coincides with the Cenozoic Farallones anticline Cordillera basement culmination. The faults to the west do not appear to have been significantly inverted The Jurassic to Early Cretaceous extensional struc- during Cenozoic shortening. East of the Servita´ tures within the inner parts of the Eastern Cordillera and Tesalia the Guaicaramo fault is the easternmost remain poorly characterized. The moderate amount Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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Fig. 4. Geological map of the Eastern Cordillera and Middle Magdalena Basin (modified from the Ingeominas map (available at http://www.ingeominas.gov.co/). Over this map we have superposed the main faults of the Eastern Cordillera classified by their nature (see text for discussion). Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

264 E. TESO´ N ET AL.

Fig. 5. Geological map covering the area from the eastern foothills to the Floresta antiform at the Pisba latitude. Note the position of the Pisba graben, bounded by the Pajarito and Paya fault to the East and by the San Ignacio normal fault system to the West. Note location of the seismic lines and cross-sections shown in Figures 12 and 13. of structural inversion and exhumation precludes the level of erosion is lower and the Mesozoic the exposure of the lowermost part of the Mesozoic normal faults are well exposed. extensional faults and inferences about the fault The Boyaca´ fault is one of the most signifi- geometry are usually based on facies analysis of cant faults exposed in the axial part of the East- the outcropping formations (Cooper et al. 1995; ern Cordillera (Fig. 1). The thickness variations of Kammer & Sanchez 2006) or on the reconstruction Jurassic and lower Cretaceous sedimentary rocks of balanced cross sections (e.g. Colletta et al. 1990; in the hanging wall and footwall are significantly Toro et al. 2004). The same approach was used in large: on the basis of field mapping, a total thick- this work, but it was based on field observations ness of c. 5000 m has been estimated, while the foot- from the area around the Floresta massif, where wall contains approximately 1000 m of the same Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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Fig. 6. Two end member modes of deformation of the eastern foothills. (a) The deformation style of the northern part of the eastern foothills, where the tectonic regime is of pure compression. (b) The style of the southern part, where transpression dominates (see location in Fig. 1). Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

266 E. TESO´ N ET AL. time interval, which is consistent with an inverted but along-strike change in rift geometry has been normal fault interpretation (Cooper et al. 1995; proposed by Sarmiento-Rojas et al. (2006). Toro et al. 2004). Based on detailed work using bedding thicknesses and facies analysis, Kammer & Sanchez (2006) interpreted the Soapaga fault Jurassic–Lower Cretaceous extensional (Fig. 1) as another inverted normal fault that is faults in the western foothills of the Eastern structurally associated with the Boyaca´ fault. Coo- Cordillera per et al. (1995) interpret this region as a single tectonic block tilted to the west that is bounded on As discussed by Moreno et al. (this volume, in the western side by the Boyaca´ fault and on the press) and Caballero et al. (this volume, in press, eastern side by the Guaicaramo fault. This forms a 2013) inverted rift structures in the Magdalena tectonic domain referred to as Cocuy sub-basin foothills are much rarer than along the eastern foot- (Fabre 1985, 1987). Our field observations east of hills. One notable exception is the Suarez fault the Floresta massif revealed the set of normal faults located east of the Los Cobardes anticline and the referred as the San Ignacio Normal Fault System Bituima and Minipi faults in the southern part of (Fig. 5), which deforms the whole Lower Cretac- the Magdalena foothills at about 58N latitude (Figs eous succession and bounds the western margin 1 & 4). Significant stratigraphic thickness changes or the Cocuy Basin. This configuration occurs at on each side of the main faults have been reported least up to the , when carbonate rocks since the oldest works. of the upper part of the Tibasosa formation may Similarly, basement-involved uplifts of dimen- represent the first post-faulting rocks that depo- sions and style similar to the Farallones or sitionally overlie the San Ignacio Normal Fault anticlines, demonstrably related to fault inversion System and the Soapaga and Boyaca´ faults. in the eastern and central part of the Cordillera, do In the northernmost part of the study area exist in the Magdalena foothills, and might be (Section 1 in Fig. 2), the distribution of Jurassic bounded by Neocomian syn-rift structures. They and lower Cretaceous sedimentary rocks led Cor- particularly include the Los Cobardes anticline redor (2003) to the identification of the Labateca (Fig. 1), but other folds like the El Pen˜on Anticline and Servita´ faults as inverted structures (Fig. 4). and the Villeta anticlinorium to the south may be These structures were originally west-dipping nor- associated with former grabens like the Paya and mal faults of Jurassic age. The Bucaramanga fault Guatiquı´a grabens in the eastern foothills as was can be interpreted in the same way: the Jurassic proposed by Corte´s et al. (2006), after comparing sedimentary thickness in the hanging wall (west- the stratigraphic thickness of the lower cretaceous ern side of the Bucaramanga fault) is higher than rocks in the Magdalena basin and in the western in the footwall (eastern side), where it is locally foothills. absent. Following the earlier reasoning, the maps available and the distribution of the sedimentary Jurassic rocks suggest that the Bucaramanga fault Contractional reactivation of the Eastern can be kinematically linked to the Soapaga and Cordillera rift structures Boyaca´ normal faults, defining a Jurassic exten- sional horsetail structure (Fig. 4) at the southern The post-Cretaceous inversion that gave rise to end of the Bucaramanga fault. the Eastern Cordillera (see Mora et al. 2013 for a South of the Floresta massif the contractional detailed discussion on the timing of deformation) displacement of both the Soapaga and the Boyaca´ occurred in a strongly compartmentalized basin. faults decreases along-strike to the south, result- Jurassic–Cretaceous normal fault systems repre- ing in decreased structural relief. The extensional sent a border between the stronger rocks of the rift non-reactivated fault interpreted in front of the margins and the Santander–Floresta Massif high, Soapaga fault (Section 2, Fig. 2) is equivalent to and the weaker and unconsolidated sediments of the San Ignacio Fault System bounding different the basin fill. This configuration is unfavourable to syn–rift sequences on each side of the inferred generating a simple tectonic wedge initiating at the fault (see Section 2, Fig. 2). The equivalent of the western boundary that migrates to the east. More Boyaca´ fault in Section 3 (Fig. 2) is interpreted as distributed deformation, strongly influenced by a partially inverted normal fault bounding thicker inherited structures, would be expected, as shown syn-rift units to the west. Further south, in addi- in experimental analogue models (i.e. Yamada & tion to the Neocomian normal faults documented McClay 2004; Marques & Nogueira 2008) and by Mora et al. (2006) and Mora et al. (2009) in the other inversion orogens like the Pyrene´es (Mun˜oz eastern foothills, there are few documented ances- et al. 1986; Mun˜oz & Santanach 1987; Teixell 1996; tral normal faults in the internal part of the East- 1998; Verge´s & Garcı´a-Senz 2001), the High and ern Cordillera towards the Bogota´ Plateau region, Middle Atlas (Teixell et al. 2003; Arboleya et al. Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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2004) and the eastern Argentinean Andes (Carrera Kammer (1996) and Kammer & Sanchez (2006) et al. 2006; Carrera & Mun˜oz 2008). for the eastern margin of the Floresta massif, but contractional reactivation has not been studied in detail. To the south, in the Sabana de Bogota´ area, Inversion tectonics in the eastern foothills the main research about the pre-compressional The geological map of the eastern flank of the configuration was conducted by Corte´s (2004) and Eastern Cordillera (Fig. 1) shows two areas with Corte´s et al. (2006). contrasting tectonic styles: (1) a southern segment, The Soapaga fault has a well developed east- at the latitude of the Farallones anticline, which is vergent hanging wall antiform with an overturned characterized by strong structural relief; the topo- frontal limb (Fig. 2). This fault has been interpre- graphic difference between the Palaeozoic rocks ted as an inversion structure by Ayala-Calvo et al. of the Quetame massif and the same unit in the (2005) and Kammer & Sanchez (2006) based on exceeds 10 km. Despite the large ver- differences in the stratigraphic thickness in the tical separation between the two, shortening is not hanging wall of the fault, but field observations of efficiently transferred outside the original rift (c. a prior extensional phase are not conclusive since 8–10 km away from the main inversion fault; the footwall does not expose Jurassic–Lower Creta- see e.g. Section 4, Figs 2 & 6). In contrast, (2) the ceous rocks for comparison. In contrast, the Boyaca´ northern segment around the Yopal area is char- fault can be clearly viewed as an inversion fault acterized by less structural relief (c. 6 km), and on the basis of exposed Jurassic and lower Cretac- deformation is more effectively transmitted to the eous sequences in the footwall juxtaposed against rift margin over a distance of more than 30 km from sequences that are different in facies and thickness the main inversion fault (Section 2, Figs 2 & 6). in the hanging wall. The difference in sedimentary As described earlier, the configuration of both thickness between the Jurassic and Hauterivian basin segments prior to shortening is similar (e.g. rocks of the two blocks is about 4800 m. The pre- Guatiquı´a and Pisba grabens), but both grabens are sent day geometry is consistent with reverse slip probably not directly linked because of the pres- at the surface level, juxtaposing the Jurassic Rusia ence of the transfer fault described by Sarmiento- formation over the Cretaceous Chipaque forma- Rojas et al. (2006). Furthermore, the shortening tion. Both the Soapaga and Boyaca´ faults are for both is not significantly different (Fig. 6), east-verging thrusts linked to the north with the with 25 km of shortening in the northern part of Bucaramanga strike-slip fault. The Soapaga fault, the eastern foothills and 22 km in the southern part located in the more frontal position, can be inter- of the eastern foothills. In the southern section, preted as (1) another inverted normal fault or (2) a shortening is accommodated by the main inversion footwall shortcut of the Boyaca´ fault. The second fault, with little deformation transmitted out of the hypothesis implies that the Soapaga fault is a more rift margin. In the frontal part of the Tesalia fault, recent fault developed during the contraction that only two discrete structures are observed and are merges at depth with the Boyaca´ fault. In any the Mirador fault, which is interpreted as a basement case, if the Soapaga fault is interpreted as an inver- shortcut (Mora et al. 2006), and the Guaicaramo sion fault, the precursor normal offset was limited: fault as the frontalmost structure also localized by the thickness of the Giro´n formation in the hang- a pre-existing normal fault. ing wall of the Soapaga fault is c. 500–600 m, In contrast, structures in the northern part of potentially equivalent to the maximum extensional the eastern foothills are characterized by more dis- offset. tributed deformation. The main normal faults are The absence of exposure of the lower part of not completely inverted, and the deformation front the Mesozoic sequence to the south and SW of the dominantly comprises footwall shortcuts expressed Floresta massif and the Sabana de Bogota´ area as imbricate fans of folds and thrusts. The amount make it impossible to identify inversion structures of exhumation is also different in both areas, with in the internal parts of the eastern Cordillera to exhumation in the southern area exceeding 10 km the south of the Boyaca´ fault termination. The (Mora et al. 2008), while in the northern section, scarce seismic profiles available are not conclusive although it is not small, exhumation is less than either. 7 km as indicated by the erosion level (Fig. 6). North of the Floresta massif, exposure of Juras- sic and Lower Cretaceous rocks is widespread, and inversion structures crop out. The Suarez fault Structural inversion in the central Eastern represents one example of an inversion structure Cordillera (Fig. 4; see also cross sections in Caballero et al. this volume, in press, 2013). The hanging wall of The extensional features within the internal parts this fault shows a sequence of the Jurassic Giro´n of the eastern Cordillera have been inferred by formation more than 2.5 km thick in the hanging Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

268 E. TESO´ N ET AL. wall (Julivert 1958; Navas 1963; or even more than over the basement. Jurassic thicknesses and facies 4.5 km according to Cediel 1968), contrasting with information is limited so the nature of this fault 0–1100 m in the footwall (Julivert 1958; Ward remains unclear. However, fault orientation and et al. 1974). position are similar to the Paya–Pajarito fault, so On the other hand, the Bucaramanga fault is Corredor’s (2003) interpretation appears viable. not consistent with the classic inversion interpret- ation since slip magnitude shows along-strike vari- Structural inversion on the western flank ations. For example in the northern portions of the fault, north of Bucaramanga, rocks are older and of the Eastern Cordillera topography is higher on the eastern side of the fault, The four main faults of the western margin of while in the south, close to the Floresta massif, the the Eastern Cordillera (Lebrija, La Salina, Bituima relationships are the opposite. and Cambao faults) are thought to be inversion Near the Bucaramanga area, the eastern block of structures (Moreno et al., this volume, in press; the fault exposes Cretaceous sediments directly Caballero et al. this volume, in press, 2013). Inter- unconformable overlying the basement (in some estingly, the deformation front trends c. N30E and areas with a decametric conglomeratic layer inter- the main inversion faults are obliquely oriented en preted as Giro´n formation in between). On the echelon between N5E and N10E, with the La western side of the fault, the Jurassic Jordan and Salina fault relaying to the Lebrija fault, the Giro´n formations are always well developed with Bituima fault relaying the La Salina fault, and local thickness exceeding 1100 m (Ward et al finally the Bituima fault relaying into the Cambao 1974). Accordingly, this relationship can be inter- thrust fault (Fig. 4). preted as a Jurassic normal fault that has not been A much more detailed picture of the deforma- inverted in the Bucaramanga area (see cross sec- tion nature of this flank of the Eastern Cordillera tion 1 in Fig. 2 and cross sections in Caballero is presented by Corte´s et al. (2006) and Moreno et al. (this volume, in press, 2013) and instead et al. (this volume, in press), and the chronology of structural inversion, the vertical normal separ- of deformation is discussed in detail by Mora ation has possibly increased during the Cenozoic et al. (2013). compression. To the east of the Bucaramanga fault are the Servita´ and the Labateca faults (Fig. 4), which Current state of stress and GPS data are thought to be inversion structures following Corredor (2003). The Servita´ fault is west-dipping The stress regime for the northern Andes is rev- and the Jurassic Jorda´n and Giro´n formations exist ealed by shallow earthquake focal mechanisms only in the hanging wall of the fault, while on the together with neotectonic analysis (Ego et al. 1996; eastern flank the Lower Cretaceous Tibu´ Mercedes Colmenares & Zoback 2003; Corte´s et al. 2005; formation directly overlies the Palaeozoic base- Fig. 7). Earthquakes define two main tectonic ment. Furthermore, the Giro´n formation observed domains – one south of the latitude of Bogota, in the footwall of the Servita´ fault exhibits wedge related to eastward subduction of the Nazca plate, geometry with increasing thickness towards the and a northern one associated with southward sub- fault (Corredor 2003). The current configuration duction of the Caribbean plate. GPS data collected shows the Giro´n formation of the western flank during the 1990s in the northern Andes (Trenkamp thrust over Cenozoic formations to the east. This et al. 2002; White et al. 2003; Egbue & Kellogg observation, in addition to wedge geometry and 2010) indicate eastward movement of the main the absence of the Jurassic beds in the footwall, is tectonic blocks for the northern domain relative to consistent with an inversion structure. stable South America. Unfortunately, only three Located 30–40 km east of the Servita´ fault is GPS data points were obtained for the study area the Labateca fault, which was interpreted as an in those papers (Bogota´, Bucaramanga, and Villavi- inversion structure by Corredor (2003). In contrast cencio). Shallow (,60 km) earthquake focal mech- with the Servita´ fault, the interpretation as an inver- anisms indicate a transpressive regime, while the sion structure of this fault is not straightforward. data from the northern domain is consistent with The fault is west-dipping with east-directed slip nearly pure compression oriented orthogonal to the placing Precambrian to Jurassic rocks over Ceno- main faults (Fig. 7). zoic sediments. The hanging wall of the Labateca Another indication of the present-day stress fault shows the Jurassic Giro´n or the Triassic– regime comes from borehole breakouts, which are Jurassic over the Precam- zones of failure at the well borehole wall formed brian or Palaeozoic basement, while the footwall of by the applied horizontal stress field (e.g. Gough the fault only exposes the basement 30 km to the east & Bell 1982; Zoback et al. 1985). This method of the fault and there again the Jurassic is observed provides an estimate of the present-day stress field Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

EASTERN CORDILLERA STRUCTURAL STYLE 269

Fig. 7. Tectonic map of northern South America including stress regimes after Colmenares & Zoback (2003) and Corte´s et al. (2005), and GPS data after Trenkamp et al. (2002). for zones with a simple structure, and caution should domains for the Eastern Cordillera with the limit be exercised in structurally complex areas (Camac between them located between 48N and 58N. et al. 2006). For this study the breakouts of 16 wells (Fig. 8) located along the eastern foothills of the Eastern Cordillera have been analysed. Again, Palaeostress assessments two clearly separated domains appear in the east- ern foothills: the first is south of the Upia River The extrapolation of the current stress state back (Fig. 8), where the breakouts trend east–west to in geological time is not a straightforward exercise. WNW–ESE, oblique to the main inversion faults The work by Corte´s et al. (2005) for the Guad- but nearly perpendicular to most of the fold axis uas and Zipaquira´ areas using stress inversion of orientations; the second, a northern domain, shows fault slip datasets suggests that the orientation of more disperse breakouts but still coherent with a the maximum principle stress was west–east to compression direction oriented NW–SE. When WSW–ESE from the late Cretaceous to late Paleo- comparing the breakout directions with the folds cene time. The maximum principal stress direction axis orientations for the northern domain, the per- changed to WNW–ESE during the main defor- pendicularity between both is notorious; the mean mational stage (Neogene) forming the present East- fold axis is approximately N035E while the break- ern Cordillera. No more significant changes in the outs are oriented at N310E. maximum principle stress direction were detected Our borehole breakout data are in agreement by Corte´s et al. (2005) for the last 55 Ma. Assum- with results obtained by Mora et al. (2010a), and ing that this correlation is correct, you can extra- their combination with the analysis of fold direc- polate the present day stress regime to the past tions supports the existence of two different stress with a moderate degree of confidence, in spite Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

270 E. TESO´ N ET AL.

Fig. 8. Two main tectonic domains of the Eastern Cordillera. The northern domain is characterized by pure compression while the southern one is characterized by transpression. The orientations of faults and folds in both domains are indicated by rose diagrams. of the uncertainties characteristic of palaeostress analysis was conducted for two areas, in the Med- analysis. ina Basin of the eastern foothills and at the Middle A complementary approach to palaeostresses Magdalena Valley to the south of the Rio Horta consists of analysing fold axis orientations. Grow- Syncline (Fig. 8). ing folds developed under the buckling mechan- The area located to the south of the previous ism acquire an axial plane perpendicular to s1 and line shows a unimodal but disperse distribution of parallel to s2 in isotropic rocks. Deviations from fold axes with a modal orientation of about N008E, this behaviour may occur when folds develop in and a mean of N015E (Fig. 8). Faults in this south- anisotropic rocks, but those deviations tend to ern domain show polymodal distribution, where involve the dip of the axial plane rather than the at least three populations of faults can be distin- strike direction (i.e. Biot 1957; 1961 Biot et al. guished (Fig. 8). The most frequent population is 1961; Ghosh 1966; Hudleston 1973; Smith 1977). north–south and roughly coincides with the orien- In fault-related folding, variations in fold geome- tation of some segments of the main inversion try can occur in both strike and dip of the axial faults in the Middle Magdalena Valley margin. plane, especially if the folds overprint older faults The second population is oriented approximately (e.g. in inversion tectonics) or in lateral thrust N030E and coincides with the orientation of the ramps. Following these assumptions, analysis of main inversion faults in the eastern foothills. Fin- folds developing far from the main inversion faults ally, a small population with N340E orientation is or transfer zones may provide a rough estimate of not related to known inversion faults. The con- the stress state under them where produced. This clusion from this analysis is that fold orientation Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

EASTERN CORDILLERA STRUCTURAL STYLE 271 does not coincide with the orientation of the main Shortening estimates along the Eastern inversion faults of the eastern foothills, but it is Cordillera close to the orientation of the main inversion faults in the Middle Magdalena Valley (Fig. 8). Four restored cross sections (Figs 1, 2, & 9) were On the other hand, in the area located to the constructed to estimate shortening across and to north, the distribution of fold axes is unimodal evaluate along-strike variations along the Eastern with little dispersion. The mean and modal orien- Cordillera. The sections were constructed using the tations cluster at approximately N035E. The fault classical methodology and assumptions (Dahlstrom orientation for the northern sector also show a unim- 1969) in addition to incorporating the 2DMove odal distribution with mean and mode at N035E software. The cross sections are mainly based on (same as folds), coinciding with the orientation of surface geology, but we used wells and seismic the main inversion faults (Fig. 8). lines when available, especially for the Middle All datasets analysed, folds, breakouts, focal Magdalena valley and eastern foothills. mechanisms and palaeostress indicators are consist- The northernmost section (Section 1; Figs 1, 2 & ent with an almost steady state of stress from the 9) is located to the north of 78N between the Sirirı´ beginning of the main Andean compression in the and Nuevo Mundo areas. The main deformation Miocene if we follow the work of Corte´s et al. is concentrated in the eastern margin where a con- (2005) to recent times. In conclusion, two stress siderable amount of shortening is taken up by domains can be separated: a southern one charac- thrust stacking in the frontal part, while the short- terized by transpression in the eastern foothills ening accommodated in the western and central and pure compression in the Middle Magdalena parts is much more modest. The total shortening Valley margin, and a northern one characterized value for this section is 69 km. The Bucaramanga by almost pure compression from the eastern foot- fault is located in the central part of this section so hills to the Middle Magdalena Valley. The area the interpretation of total shortening value obtained north of latitude 6.58N was not analysed in detail is not straightforward. but seems to have a transpressive character at least Section 2 is located 100–140 km to the south of in the eastern margin of the Bucaramanga fault, the first and was constructed between the city of where the Eastern Cordillera bends toward the Yopal and the Opo´n oilfield area (Figs 1, 2 & 9). north and the eastern deformation front changes in This section is comparable to Section 1, with most direction from NNE to NNW (Figs 1 & 8). of the total shortening concentrated along the

Fig. 9. Restoration of the four cross sections presented in Figure 2. The value of shortening is indicated for each one in kilometres. No shortening gradients can be deduced. From these restorations the pre-compression configuration of the basin can be deduced. Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

272 E. TESO´ N ET AL. eastern margin, but a considerable amount of short- ening is also accommodated by structural inversion of normal faults in the central and western parts of the section along the Boyaca´ and La Salina faults. The total amount of shortening that resulted from section restoration is 69 km, exactly the same as in the northern section. The Section 3 is located about 50 km south of Section 2, between the Nunchı´a Syncline in the eastern foothills and the Rio Horta area in the Middle Magdalena valley. Like the other transects, thick-skinned tectonics dominates much of the section (especially the central and eastern parts), although there is a thin-skinned belt in the western- most part of the section (Fig. 2). The total amount of shortening from section restoration is about 80 km (Fig. 9), being mainly accommodated by the Fig. 10. Three cross-sections from the Santander massif thick-skinned structures. near Bucaramanga (after Julivert 1970). Note that The southernmost cross section differs from the basement rocks are folded by buckling and the main others in total width and structural style, but has faults dissect previous buckling structures. See location similar amounts of shortening (62 km) according in Figure 8. to our restoration. Most of the section displays thick-skinned deformation, as do the previous ones, with very little deformation in the central descriptions by Julivert (1958, 1959, 1970) of the part (Bogota´ plateau). In this section, most of the Santander Massif and surrounding areas, the pres- deformation focuses on the building of a large base- ence of Cenozoic-age buckle folds is ubiquitous ment culmination (Quetame Massif). On the other and well documented, especially for the internal hand, in the western margin a thin-skinned belt parts of the Eastern Cordillera (Fig. 10). The later ( area), equivalent to that of Section 3, interpretation of Kammer (1993) for the Santander takes up a considerable amount of shortening. Massif is very similar to Julivert’s interpretation Based on these restorations, there are no signi- and also based on extensive fieldwork campaigns. ficant along-strike variations of total shortening Also dealing with buckling in the Eastern Cordillera along the central Eastern Cordillera of Colombia, are the works of Kammer (1997) and Mora & the main differences lying in the precise locus of Kammer (1999) and Kammer & Mora (1999). shortening. Thus, while in the Section 1 deformation Kammer (1997) produced a detailed analysis of is dominantly absorbed on deep rooted basement- the folds located to the north of the Bogota´ involved thrust faults in the east, and buckle folds Plateau and to the SE of the Arcabuco anticline in the interior, Section 2 shows some deforma- that support a buckling model for their origin. The tion absorbed in the west and central parts by struc- upper layers are folded at the expense of homo- tural inversion of pre-existing normal faults. In geneous shortening in the deeper beds, manifested Section 3 the deformation is distributed along a by the occurrence of foliation in the lower Cretac- larger number of structures of both types, thin- eous units (Mora & Kammer 1999). Again these and thick-skinned, while in Section 4, which also folds cannot be explained by the presence of fault shows distributed deformation, shortening is con- ramps or fault-bends but by a buckling process centrated in a thick-skinned culmination in the (Kammer & Mora 1999). east and a thin-skinned thrust belt in the west. During the 1990s and early 2000s the models of Dengo & Covey (1993) and Roeder & Chamberlain (1995) were widely accepted. The Roeder and Styles of folding in the Eastern Cordillera Chambelain model displays the Eastern Cordillera as an east-verging fold and thrust belt (Fig. 11c). The role of the buckling in the Eastern Cordillera In the model, all folds are interpreted in terms of has been debated since the earliest studies. The FBF or FPF, and the tectonic shortening proposed first works about the structures of the eastern Cor- for the Eastern Cordillera is 190 km (on a section dillera (Julivert 1958, 1959, 1970) carefully des- 313 km long). The Dengo & Covey (1993) model cribe a large number of basement and cover folds (Fig. 11b) is even more controversial in that the not directly related to faults (i.e. that cannot interpretation of each fold of the cross section is be ascribed to the models of fault-bend folding, in terms of FBF or FPF. This interpretation leads FBF, or fault-propagation folding, FPF). In the to the development of a model where the original Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

EASTERN CORDILLERA STRUCTURAL STYLE 273

Fig. 11. Five tectonic models proposed for the Eastern Cordillera of Colombia. Models are chronologically ordered. The value of tectonic shortening for each section is indicated in kilometres. Models A, D and E propose the inversion of the main former normal faults while models B and C propose a model based on newly formed faults (see discussion in text).

Neocomian rift is decapitated by structures propa- models take into account the existence of base- gating from the western plate boundary, that is, ment folds in the internal parts of the Eastern the Central and Western Cordilleras. The tectonic Cordillera. The total tectonic shortening in the shortening in this model is 150 km (on a section of Colletta et al. (1990) model is 105 km (313 km), about 300 km long). Both models are opposed to and 69 km (in a section of 313 km) for the Cooper Colletta et al.’s (1990) and Cooper et al.’s (1995) et al. (1995) model. As evident from the sections models (Fig. 11a, d), which consider the main reproduced in Figure 11, models considering buckle thrust structures as derived from the inversion of folds in the internal pats of the Eastern Cordillera pre-existing normal faults. In addition, these later imply smaller amounts of shortening than models Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

274 E. TESO´ N ET AL. in which all the folds are interpreted as fault-bend to ramp transition to the east of the Soapaga fault or fault-propagation folds. These differences in to produce the west-dipping structural panel that shortening values are greater than 50%, so proper characterize the area (Fig. 12a). The minimum interpretation of the folding mechanism is crucial fault displacement has to be at least 12 km, and in terms of understanding not only the geometry part of the footwall flat of at least this length must and kinematics of the Eastern Cordillera and other be located below the Floresta antiform for internal inversion orogens, but also in terms of oil prospects consistency (Fig. 12b). A second model proposed (oil generation, migration and trapping). by Kammer (1996) infers the Soapaga fault as a In addition to evidence presented by Julivert steeply dipping structure, and all hanging-wall (1958, 1959, 1970), Kammer (1993, 1997) about folds are interpreted as detachment folds formed Cenozoic-age buckle folds in the internal parts of by the buckling mechanism. This work is based on the Eastern Cordillera, we discuss the structural detailed field mapping, and the detailed geometry style of the frontal part of the Soapaga fault as a of fold and fault planes. The regional dip of about example about the evolution of the deformation 158 of the whole footwall block of the Soapaga style for the Eastern Cordillera. The area selected, fault is still unexplained (Fig. 12c). from the immediate footwall of the Boyaca´ fault Rodrı´guez (2009) presents a model for the Soa- to the Pisba Syncline (Figs 1 & 5), was interpreted paga fault based on gravimetric and magnetic sus- by Dengo & Covey (1993) as a simple thrust belt ceptibility anomalies and interpretation of seismic with displacement transferred from the Boyaca´ reflection profiles. This analysis suggests that the and Soapaga faults in which all the folds were Soapaga fault is a high-angle planar basement- viewed as fault-bend or fault-propagation folds. If rooted structure, without a flat-ramp transition as correct, this interpretation requires a footwall flat required by Dengo & Covey’s (1993) model.

Fig. 12. Two tectonic models for the internal parts of the Eastern Cordillera. (a) Dengo & Covey’s (1993) model proposing the Soapaga fault as a low-angle structure. (b) The restoration of Dengo & Covey’s (1993) section needs a 20 km flat of the Soapaga fault below the Floresta anticline and a flat ramp transition as shown in the figure. (c) Kammer’s (1996) model proposing a high-angle fault and buckle folds in the footwall of the Soapaga fault (see location in Fig. 5). Legend is as follows: B, basement; Gi, Giro´n formation; C, Concentracio´n formation; Ch, Chipaque formation; F, Fomeque formation; G, Guadalupe formation; P, Picacho formation; S, Socha formation; U, Une formation. Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

EASTERN CORDILLERA STRUCTURAL STYLE 275

Additionally, the geometry proposed by Rodrı´guez sense. If shortening is oriented orthogonal to the pre- (2009) is consistent with the model proposed by exisiting normal fault, homogeneous strain and Kammer (1996) and the model presented in this folding will result instead of a reversal in slip sense work. if the rheological and frictional conditions are In the region of the Pisba syncline and the favourable. San Ignacio anticline, the first-order folds have Figure 14 is a schematic model illustrating the a l of 30–50 km and amplitudes of about 8 km mechanical behaviour of an inversion fault in with second-order parasitic folds with wavelengths Mohr–Coulomb space. Plane A represents the frac- ,5 km with amplitude of about 2 km (Fig. 2). Our ture plane of an intact rock generated when the Mohr field observations agree with Kammer (1996) that circle touches the Mohr–Coulomb failure envelope. the second-order folds are detachment folds for- This fracture plane nucleates at an angle close to med by shortening transferred from the Soapaga 308 with s1 according to the failure criterion (228 fault to the footwall block. The faults affecting the for the case specific case of the Blair dolomite second-order folds are not continuous features as (Handin 1969) taken as the basis for Fig. 14). The is commonly observed in thrust belts dominated frictional criterion for a Byerlee material (Byerlee by ramp-related folds, but they are only locally 1978) describes the behaviour of fractured rock observed and discontinuous in detachment- with negligible cohesion. The sliding envelope for dominated fold belts (e.g. Sherkati et al. 2005; a frictional coefficient (m) of 0.85 (an average for Burberry et al. 2008). In addition, locally faults most rocks) is also represented. At any given are observed to truncate at high angles, both fold stress state, the sliding envelope for fractured rock limbs suggesting that folds and faults are not would yield before the Coulomb fracture envelope directly linked. for intact rock. The shaded area in Figure 14 (I) In our view the Dengo & Covey (1993) model represents the orientation of pre-existing planes fails to explain the main geological features of the (from B to C) that could potentially slip before the area. This model requires a thrust fault to the east generation of a new fracture (A). Reactivation of of the Mongua syncline (Fig. 12a) in a flat over a pre-existing high-angle normal fault (plane D in ramp relationship to generate a west-dipping panel Fig. 14) will occur not only when the strength of in the hanging wall. This thrust fault is not obser- fault is lowered by increasing the pore-fluid pres- ved; rather a system of non-reactivated exten- sure (see, e.g. Sibson (1985)), but also when the sional faults of lower Cretaceous age comprises the stress regime is not purely compressional, but trans- dominant faults (San Ignacio Normal Fault Sys- pressive. In this second case, the shear stress across tem; Fig. 13a). The large west-dipping structural the fault plane can be high enough to allow the panel observed in the area, together with the Pisba fault plane to exceed the frictional strength of the Syncline, probably formed by a buckling mech- fracture. anism involving basement. This folding cannot For the specific case of the Eastern Cordillera, be explained by a fault-bend folding mechanism the southern transpressive domain of the eastern because there are no faults in the area capable of gen- foothills (Fig. 8) shows numerous examples of erating such structures. Structures similar to the normal fault reactivation (Fig. 6b) and the basement Pisba anticline and the South Ignacio Anticline are in the internal parts does not show extensive buckle observed all along the internal parts of the Eastern folds (Section 4, Fig. 2). The latter can be explained Cordillera, including secondary folds observed with the total shortening accommodated by the along the Santander Massif and El Pen˜on, Arcabuco inversion of the normal faults along the eastern and Cobardes Anticlines (see Sections 1–3, Fig. 2). margin of the Cordillera. On the other hand, defor- mation in the northern sector (Fig. 6b), where the stress regime is mainly of pure compression, is Discussion and conclusions dominated by buckling and homogeneous strain in the internal parts of the rift domain (Sections 1–3, Structural data and cross-sections presented in this Fig. 2), and probably with a contribution of reacti- work support the view that the first-order control vated weak fault zones in the internal parts of the on the contractional geometry of the Eastern Cor- system. When deformation migrated towards the dillera is the pre-existing structural fabric related foreland, low-angle basement shortcuts formed to the development of Mesozoic rift basins. A in the footwall of the pre-existing normal faults common example is the juxtaposition of Jurassic– (Fig. 6a) because the normal fault geometry was Neocomian sedimentary rocks in buttress unconfor- non-optimally oriented (i.e. steeply dipping) for mity with adjacent rift flanks composed of Pre- thrust sliding. and Palaeozoic crystalline basement. We An interesting aspect is related to the presence or suggest that shortening applied obliquely to the pre- absence of overturned forelimbs in the frontalmost existing normal fault enhances a reversal in slip structures. It is conspicuous how, in the southern Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

276 E. TESO´ N ET AL.

Fig. 13. Tectonic model for the internal part of the Eastern Cordillera presented in this work. (a) shows a cross-section equivalent of the two presented in Figure 12 based on extensive mapping and interpretation of the seismic lines of the ANHSP program. (b) is the ANHSP-2005-02 line interpreted and (c) is the same line uninterpreted. Note as the Soapaga fault is a moderate dipping fault without flat-ramp transitions. See legend in Figure 12.

cross sections (especially on the eastern flank of hand, in the areas where orthogonal compression the Eastern Cordillera), the frontalmost structures is dominant, the stresses are not focused over the are associated with tight to overturned frontal previous structures and new fault planes as foot- anticlines coinciding with those areas where trans- wall shortcuts are preferred to accommodate the pression appears to be dominant. We hypothesize deformation. that the role of transpression focusing stresses Some faults, such as the Boyaca´ and the Suarez along either inherited structures or shortcuts origi- faults, for example, have been inverted following nated under transpressional regimes also prompts a the same plane used during normal faulting. This high strain area close to the inverted planes (over- fact, as explained above, is unusual if it is con- turned or steeply dipping forelimbs). On the other sidered only pure compression, so the reactivation Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

EASTERN CORDILLERA STRUCTURAL STYLE 277

Fig. 14. Mohr diagram based on experiments with Blair dolomite (Handin 1969) that illustrates that only the discontinuities placed in a small range of orientations with respect to s1 (between planes B and C) are able to slide before the generation of new fractures (see text for explanation). of these faults could have been influenced by trans- basement folds at the culmination of the Santander pression, which seems likely given the kinematic Massif in Section 1). In the end, the second-order linkage with the left-slip Bucaramanga fault in the ‘basement’ involved buckling folds are the prod- central parts of the Eastern Cordillera. uct of similar deformation mechanisms to the first- We have also differentiated various mecha- order inversion-related anticlines with the difference nisms to deform areas of prior extension (Fig. 15). that they are not directly associated with inversion The first mechanism is related to the largest first- structures except in cases where the normal fault order anticlines in the Eastern Cordillera (e.g. Faral- acts like a buttress (see folds in the southern sec- lones, Cobardes and Arcabuco). These basement tion at the core of the Quetame Massif west of non- folds, which are similar in size, amplitude and reactivated normal faults). It is worth noting that length, are controlled by pre-existing rift-bounding both types of basement folds are associated with normal faults. In some places the deformation is the lowest exposed structural levels (Kammer 1997; extreme, and even a trishear (Allmendinger 1998; Mora & Kammer 1999) of the sedimentary cover Erslev 1991) basement involved deformation mech- under amounts of overburden exceeding 10 km. anism could model the geometric features. In con- (e.g. notice that, in all the areas, the lowermost trast, second order type 1 folds follow the Mitra sedimentary rocks are reset for ZFT, or for ZHe (2002, 2003) mechanical principles of folding: for at least, showing that they reached maximum palaeo- low-cohesion materials and a low-friction basal temperatures higher than 180 8C; Parra et al. 2009; detachment with overlying shaley horizons and a Mora et al. 2013; Moreno et al., this volume, in relatively thick overlying cover. In these folds the press). The pure detachment folds are restricted to deformation is accommodated by symmetric upper structural levels. detachment folds independent of prior tectonic We hypothesize that, given the difficulties in inheritance (see folds E of the Soapaga fault in Sec- reactivating ancestral normal faults, this type of tions 2 & 3). fold corresponds to earlier structures during the A basal detachment promotes flexural slip and strain (work) hardening phases before actual reacti- the development of symmetric box folds in regions vation and brittle faulting occurs (Wojtal & Mitra with high strain accumulation in front of a princi- 1986). Penetrative strains in folds passively trans- pal basement fault. On the other hand, if a duc- ported on top of reactivated listric normal faults tile basal horizon is absent because there are no document that the rocks that were folded and significant rheological contrasts between differ- deformed were still under high temperatures and ent horizons or between basement and cover, homo- later exhumed by brittle faulting. geneous flattening and basement folding produces New field observations document the absence of second-order type 2 folds. Under high strains, the fault bend folds in the internal parts of the Eastern second-order type 2 folds are asymmetric (e.g. Cordillera, but fault bend folds are observed in the compare the asymmetric basement folds in the frontal eastern foothills as documented by Linares Quetame Massif in Section 4 v. the symmetric (1996), Rowan & Linares (2000), Mora (2007) Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

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Fig. 15. Classification of the Eastern Cordillera folds according to their nature and origin. The legend for the cross sections is the same as in Figure 2. and Jimenez et al. (this volume, in press), along the can be observed. In addition they are younger and Yopal and Guaicaramo faults to the south (second certainly Late Miocene or (see Jimenez order type 3 folds). Interestingly, they are well docu- et al., this volume, in press), and therefore formed mented to affect only the sedimentary cover and are under larger amounts of sedimentary cover. It there- present in areas where a ductile basal detachment fore makes sense that more than 2 km of overburden Downloaded from http://sp.lyellcollection.org/ by guest on November 8, 2018

EASTERN CORDILLERA STRUCTURAL STYLE 279 above the basal detachment facilitated their growth Allmendinger, R. W. 1998. Inverse and forward numeri- by faulting and slip along the basal detachment, cal modeling of trishear fault-propagation folds. Tec- instead of growing in amplitude as detachment tonics, 17, 640–656. folds. An interesting transition to this folding style Alvarez, J. 1981. Granulitas Charnoquiticas Y Rocas is the Provincia detachment fold in the western Relacionados Del Macizo De Garzon, Cordillera Oriental (Colombia). Ingeominas, Bogota. extreme of Section 1. Here, Miocene deposits are Amilibia, A., McClay, K. R., Sabat, F., Mun˜ oz, J. A. & thinner than in the eastern foothills and it appears Roca, E. 2005. Analogue modelling of inverted that there is not yet enough overburden on top of oblique rift systems. Geologica Acta, 3, 251–271. the growing structure to produce a fault rather Arboleya, M. L., Teixell, A., Charroud, M. & Juli- than increase the amplitude of folding. vert, M. 2004. A structural transect through the The Eastern Cordillera, as with many inver- High and Middle Atlas of Morocco. Journal of sion orogens around the world (e.g. the Argentinean African Earth Sciences, 39, 319–327. Cordillera Oriental, the High and Middle Atlas of Ayala-Calvo, R. C., Veloza, G. E. et al. 2005. Paleo- ´ ´ ´ Morocco, the Iberian chain of Spain) is clearly a magnetısmo y Mineralogıa Magnetica en las unidades del Mesozoico de Bucaramanga y el Macizo de Flor- thick-skinned dominated orogen and the role of esta. Geologı´a Colombiana, 30, 49–66. the basement involved deformation is non-negli- Biot, M. A. 1957. Folding instability of layered visco- gible. Thin-skinned tectonics is restricted to the elastic medium under compression. Proceedings of external parts of the orogen in the foothills adja- the Royal Society London, A242, 444–454. cent to the Middle Magdalena Valley or the Biot, M. A. 1961. Theory of folding of stratified visco- Llanos basin (Fig. 2). For example, comparing the elastic media and its implications in tectonics and oro- Eastern Cordillera with the High Atlas of Morocco genesis. Geological Society of America Bulletin, 72, (Teixell et al. 2003) reveals a very similar struc- 1595–1620. tural style: in both orogens, the internal parts of Biot, M. A., Ode, H. & Roever, W. L. 1961. Experimen- tal verification of the theory of folding of stratified the chains are characterized by basement-involved viscoelastic media. Geological Society of America Bul- buckle folds that are not fault related, and in the letin, 72, 1621–1631. external parts of the orogens, thin-skinned defor- Burberry, C. M., Cosgrove, J. W. & Liu, J. G. 2008. mation dominates. In both the Eastern Cordillera Spatial arrangement of fold types in the zagros and the High Atlas, total tectonic shortening is simply folded belt, Iran, indicated by landform mor- fairly modest at about 25% or less, and appears to phology and drainage pattern characteristics. Journal be accommodated along reactivated normal faults, of Maps, 2008, 417–430. with some basement-involved buckling and the Burkley, L. A. 1976. Geochronology of the Central development of footwall shortcuts. Venezuela Andes. Thesis, Case Western Reserve Uni- versity, 150. Collectively, strain localization over protracted Byerlee, J. 1978. Friction of rocks. Pure and Applied periods; along the eastern margin of the early Geophysics, 116, 615–626. Palaeozoic deformed belt, the eastern boundary of Caballero, V., Mora, A. et al. In press. Tectonic con- Jurassic–Lower Cretaceous rifting and the east- trols on sedimentation in an intermontane hinterland ern extent of Andean deformation, highlights the basin adjacent to inversion structures: the Nuevo importance of pre-existing structural fabrics as pro- Mundo Syncline, Middle Magdalena Valley, Colom- posed by Ring (1994) on the reactivation of bia. In: Nemcˇok, M., Mora, A. & Cosgrove, J. W. orogenic belts. (eds) Thick-Skin-Dominated Orogens: From Initial Inversion to Full Accretion. Geological Society, London, Special Publications, 377, http://dx.doi.org/ The authors are indebted to Mauricio Parra, Michal 10.1144/SP377.12 Nemcok, Victor Caballero, Juan Carlos Ramirez, Isaid Caballero, V., Parra, M., Mora, A., Lopez-Arias, C., Quintero, Joel Saylor, Carlos Javier Sanchez and Camilo Rojas, L. E., Quintero, I. & Horton, B. K. 2013. Higuera for fruitful suggestions and comments. 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