Influence of salt in the tectonic development of the frontal thrust belt of the eastern Cordillera (Guatiquía area, Colombian Andes)

Geologic maps, seismic lines, and data from a dry exploration well were used to develop a new structural model for a segment of the eastern foothills of the Eastern Cordillera of Colombia, emphasizing the role of salt tectonics. Milestones in the deformation history of the Guatiquía foothills were studied by sequential section restoration to selected steps. Uncommon structural geometries and sparse salt occurrences were interpreted in terms of a kinematic evolution in which Cretaceous salt migration in extension produced a diapiric salt wall, which was subsequently welded during the main episodes of the Andean compression, when the salt wall was squeezed generating a large overturned flap. Salt-weld strain hardening resulted in breakthrough thrust-ing across the overturned flap in late deformation stages. We have evaluated a pattern of salt tectonics previously unrecognized in the foothills thrust belt, which may be significant in other parts of the external Colombian Andes.


Introduction
The prolific thrust belt of the eastern foothills of the Eastern Cordillera (EC) of Colombia has been intensively investigated since the discovery of the Cusiana giant oil field in 1992.Diverse authors have illustrated via structural cross sections, and kinematic restoration models the foothills structure as consisting of an eastverging thrust system with imbricate fans and duplexes, conforming to ramp-flat geometries (Dengo and Covey, 1993;Cazier et al., 1995;Cooper et al., 1995;Rowan and Linares, 2000;Toro et al., 2004;Martinez, 2006;Mora et al., 2006Mora et al., , 2010Mora et al., , 2013;;Tesón et al., 2013;Teixell et al., 2015).These works emphasize tectonic inversion of former extensional faults, giving rise to either basementinvolved or thin-skinned thrusts.
Within the south-central part of the eastern foothills, the Guatiquía area (Figure 1) shows features that differ from the more standard ramp-flat, fault-related folding reported for other segments of the thrust belt.A large overturned panel of Cretaceous rocks is exposed in the frontal thrust sheet and was crossed by the Anaconda-1 exploration well.The well targeted a subthrust play with fault-bend antiformal culminations comparable to Cusiana, which was not encountered.Rather, the footwall of the emergent Mirador thrust comprised the inverted limb of a complex syncline (Kammer et al., 2005;Mora et al., 2008).
We aim to provide an explanation for differences in structural style in the EC external thrust belt.Based on seismic lines, maps, the occurrences of salt in old mines, and a detailed analysis of the postdrilling information from the Anaconda-1 well, we propose a new kinematic model for the Guatiquía segment, which emphasizes the influence of long-lived salt tectonics, especially during early deformation stages (Cretaceous extension and early Cenozoic contraction), which has been masked by the later stages of the Andean compression.The model accounts for early layer tilting, diapir squeezing, and the formation of an overturned flap, with similarities to other remarkable recumbent folds in thrust settings recently interpreted as salt related (e.g., Graham et al., 2012;Rowan et al., 2014), and it bears implications for other compressional areas in which the role of salt can be overlooked due to diapir welding and/ or dissolution.

Geologic setting
The EC is the easternmost branch of the northern Andes of Colombia (Figure 1).It is a 110-200-km-wide intracontinental mountain belt related to transmission of stresses to the South American plate by the accretion of arcs in the northwestern Andes and appears as a doubly verging thrust system that formed during the Ceno-zoic by the inversion of a Mesozoic back-arc rift (Colletta et al., 1990;Cooper et al., 1995).
The rifting history of the EC had a main phase during the early Cretaceous when a graben system approximately coincident with the present Cordillera formed (Etayo et al., 1969;Cooper et al., 1995;Sarmiento-Rojas, 2001).Cretaceous sediments (up to 8-10 km thick) are dominantly marine.Terrestrial deposits indicative of an overfilled basin appear in the late Maastrichtian-Paleocene. Uplift of the EC was associated with a progressive emersion of the former rift due to tectonic inversion and with subsidence in the adjacent foreland basins such as the Llanos basin in the eastern side (Figure 1).
The study area is located in the eastern foothills of the EC between the Guatiquía and Guamal rivers (Figure 1), in which the thrust belt overrides the Llanos basin and is associated with a topographic increase from 400 to 1700 m above sea level (asl).

Tectonostratigraphic evolution
The stratigraphy of the study area is sketched in Figure 2. We refer the reader to Renzoni (1968), Etayo et al. (1969), Mora et al. (2006), andMora andParra (2008) for detailed references.Basement, which crops out in the Quetame massif and in the Mirador thrust hanging wall (Figure 1), consists of lower Paleozoic metamorphic rocks (Quetame Group), and the overlying Devonian-Carboniferous Farallones Group, that accumulated in extensional grabens (Mora et al., 2006).
The early Cretaceous depositional settings were highly variable and disparate over short distances, due to extensional tectonism (Mora et al., 2006(Mora et al., , 2009)).In paleogeographic reconstructions (e.g., Sarmiento-Rojas et al., 2006), the early Cretaceous graben does not reach the southernmost part of the EC foothills, being limited by the Naranjal transfer paleofault, south of which the Albian overlies directly the Paleozoic (Figure 1).During the Albian-Late Cretaceous, the basin became dominated by thermal subsidence, with laterally expansive deltaic sandstones and deeper water shales (Une, Chipaque, and Gualdalupe Formations; Figure 2).
Toward the end of the Cretaceous, the EC became part of a large basin in front of the Central Cordillera, disrupted by growing folds that controlled the deposition of fluvial to transitional Paleogene units (Gomez et al., 2005) (Figure 2).The foreland basin megasequence in the eastern foothills and Llanos basin started in the mid-late Oligocene, as indicated by subsidence, exhumation, and provenance analysis in the fluvial Carbonera Formation (Parra et al., 2009a(Parra et al., , 2009b;;Horton et al., 2010).Coarse clastic influx by mid-Miocene times of the Guayabo conglomerate into the Llanos basin records the "Andean" main growth of the EC (Cooper et al., 1995;Hoorn et al., 1995;Branquet et al., 2002;Toro et al., 2004), continuing until recent times as recorded by synsedimentary deformation and very young apatite fission track ages (Mora et al., 2008).

Data and methods
Geologic maps available included the national quadrangle at 1:100,000 scale (Pulido et al., 1998) and those in Mora et al. (2006Mora et al. ( , 2013) ) and Mora and Parra (2008).Based on these maps and our own structural data, we generate a new map synthesis presented in Figure 1.We analyzed 1271 km of seismic profiles across the area provided by ICP-Ecopetrol.Synthetic seismograms of the Anaconda-1 well were generated for seismic-well calibration.In addition, the formational tops and data from the Anaconda well (Figure 3) were used to constrain the cross sections.
The seismic quality in the foothills is typically poor; stratigraphic horizons are irregularly imaged, but fault planes are often well imaged, suggesting distinctive rocks formed or injected along the fault zones.In the Llanos basin, reflectors are continuous and gently dipping (Figure 4).Structural transects were constructed with the goal to illustrate the lateral structural variation and using the maximum available surface and subsurface data (Figure 5).Time-depth conversions used replacement velocity of the seismic surveys and were calibrated with the Anaconda-1 well.Cross-section BB′ (Figure 1) is the most representative and was modeled by kinematic restoration in sequential steps, honoring available thermochronological data (Mora et al., 2008;Jimenez et al., 2013).

Structural elements of the foothills in the Guatiquia and Guamal areas
The innermost element of the area is the Quetame massif, a prominent basement uplift that is bounded by the Servitá fault (Figure 1 and 5).In front of it, the foothills can be divided into two segments by the Naranjal transfer fault (Figure 1).North of this fault (Guatiquía area), the Cretaceous displays an antiformal structure (Buenavista anticline) cut by the northwestdipping Mirador thrust along its core, which separates a normal limb region with gentle folds and a basement exposure from a large overturned limb that forms the   (Kammer et al., 2005;Mora et al., 2006Mora et al., , 2008)).The Anaconda-1 well indicated the existence of several splays of the Mirador thrust segmenting the footwall overturned panel (Figures 3 and 5).South of the Naranjal fault (Guamal area), the structure consists of a system of open folds related to imbricate thrusts (Toro et al., 2004;Mora and Parra, 2008;Mora et al., 2010).The Llanos basin is less deformed except in the vicinity of the mountain front, and a tabular late Cretaceous to Neogene succession is cut by blind thrusts and relict normal faults visible on seismic data (Figure 4b).The thrusts that are basement involved and steeply dipping, and in the case of the Chichimene oil field (Figure 1), demonstrably derive from the inversion of previous extensional faults (Kluth et al., 1997).

Evidence for salt tectonics
The long overturned limb of the Guatiquía foothills suggests folding mechanisms that differ from simple fault-bend folding.The northern end of the Buenavista anticline is the locus of the Upín and La Campana salt mines, which have produced since the sixteenth century.The salt deposits of these mines are found within the earliest Cretaceous shales (Wokittel, 1960).In the Upín   , 1972).Salt layers are disturbed, with variable attitude from moderately to steeply dipping, in response to regional tectonism and possibly diapirism (McLaughlin, 1972).The fact that neither overturned Buenavista breccia nor basement is found in the surface or the subsurface is consistent with a detachment level for the overturned panel coinciding in stratigraphic position with the evaporites of the salt mines.On the other hand, numerous salt springs are found in the region (e.g., Salinas de Cumaral, 20 km north of Villavicencio).
In the Guateque-Medina area, north of the study area, a brecciated evaporitic layer below the Macanal Formation hosts emeralds and gypsum deposits (Figure 6) (Cheilletz and Giuliani, 1996;Branquet et al., 2002), in which fluid-inclusion studies revealed Na-Ca-K-bearing hypersaline chlorine brines responsible for emerald and pyrite crystallization by deep-seated formation waters heated by burial, thereafter dissolving evaporites by interaction with salt diapirs (Giuliani et al., 1995).
The Anaconda-1 well reported high concentrations of chlorides in the mud system at a fault zone above the Villavicencio-Mirador thrust (see Figure 3), attributed to salty water of crystalline salt (Chevron Petroleum  , 1996, Anaconda-1 -Final Geological Report).In the light of this, high reflectivity of fault zones in seismic profiles may be due to their content of salt.It is noteworthy that the southern end of the overturned panel coincides with the limit of the Lower Cretaceous basin at the Naranjal transfer fault, which reinforces its stratigraphic control, likely the occurrence of the early Cretaceous evaporitic formation.
We thus interpret that the association of an overturned panel and salt in the Buenavista anticline area is not accidental but indicative of a causal relationship.Strongly overturned fold limbs associated to salt diapir squeezing have been reported elsewhere (Graham et al., 2012;Rowan et al., 2014).Salt tectonic influences have never been reported in the foothills of the EC, but doubly verging folds and systematic limb overturning in the Sabana de Bogotá have been recently associated by Teixell et al. (2015) with salt-related detachment folding preceded by early diapirism of Cretaceous salt.Explanations for the association of thrust ramps and asymmetric  1998), but on the basis of the indicators described above, we favor and will explore a model combining thrust faulting and diapirism.

Kinematic modeling of the Villavicencio section
Section B-B' (Villavicencio transect) was selected for kinematic modeling to illustrate our proposed model for the evolution of the Guatiquía foothills (Figure 7).The modeling includes extensional faulting, salt diapirism, and tectonic inversion.In the proposed model, the Cretaceous salt is assumed to start moving very early during the extensional episode (Figure 7f), as is common in salt-bearing basins (Jackson and Vendeville, 1994).Comparison of outcrop data and the Anaconda-1 well reveals marked thickness variations in the Lower Cretaceous across the Mirador and Servitá faults, which indicates former extensional faulting.In the hanging wall of the Mirador thrust, zircon (U-Th)/He ages are reset but zircon fission-track ages are not (Parra et al., 2009b;Jimenez et al., 2013), indicating that the basement of this unit was buried to reach temperatures between 180°C and 250°C from Cretaceous to recent times.In contrast, zircon fission tracks are reset in the hanging wall (HW) of the Servitá fault, which indicates a greater burial in this unit.According to our resto-ration, the maximum burial depths were probably attained during Paleogene times (Figure 7e).
The Buenavista anticline may have originated as a salt wall associated with an extensional fault (Figure 7), later squeezed during the compression, so the salt formation has been largely removed.The early activity of salt, together with the extension, may have contributed to the change in thickness observed in the synrift sequences as described above.Salt continued to rise into structurally thinned zones during early Paleocene-Eocene times.The Anaconda-1 well found upper Eocene sediments unconformable over the Maastrichtian, potentially attesting for uplift in the salt wall (Figure 7e), and the Mirador Formation drilled by the well showed a very high formation of water salinity.Because this unit is attributed to fluvialshallow marine environments, the anomalous salinity may be ascribed to a diapir growing at the surface and being partially dissolved during early Eocene times.Alternatively, the structure may have been originated during the early Paleogene as an eroded, salt-cored detachment fold, although compressional deformation as old as that has never been recognized in the Guatiquía-Guamal foothills (Toro et al., 2004;Jimenez et al., 2013).
During the main Andean orogeny episode, extensional faults were reactivated and the Paleozoic basement uplifted (Figure 7), as occurs all along the cordillera (Mora et al., 2008).The squeezing of the diapir and the inversion of the Servitá fault are interpreted to be contemporaneous and attributed to the late Oligocene to early Miocene (Figure 7d), in agreement with the zircon fission tracks (ZFT) and zircon Helium (ZHE) ages that document an exhumation from approximately 180°C to 120°C from the mid Oligocene to the Pliocene (Jimenez et al., 2013).Mid-Oligocene cooling ages (29 AE 2.3 Ma; Jimenez et al., 2013) indicate that the Andean exhumation commenced immediately after the maximum burial attained during the late Eocene-early Oligocene (Figure 7d and 7e).Subsequently, with the complete closure of the diapir stem by squeezing, it could no longer accommodate shortening, and this caused the initiation of new thrusts cross cutting the antiformal structure (Figure 7c).The basement-involved reactivation of the salt weld gave rise to the present Mirador thrust and a series of small imbrications in the inverted flank of the former diapir.
For the restoration of the imbricate thrusts that segment the steep flank of the old diapir in front of Mirador thrust, we assumed a break-back sequence of propagation (Figure 7a-7c) on the basis of (1) the leading imbricate thrusts are fossilized under the molassic sediments and (2) in the restoration, the Mirador thrust, interpreted as the reactivation of the salt weld, was almost vertical prior to thrust imbrication of the steep flank of the diapir.Displacement on this imbricate sequence is attributed to late Miocene to recent times, in accordance with approximately 3-Ma apatite fission track ages (Mora et al., 2008).The Mirador thrust is the last to be formed within the system; late blind thrusting within the foreland basin, in which seismic data show involved  the entire molassic sequence, is represented as contemporaneous (Figure 7a).

Conclusions
We suggest that the fault boundaries of the Mesozoic extensional basins as well as the mechanical behavior of the infill (in particular weak evaporites) played a major influence in the tectonic configuration of the foothills of the EC.Cross sections across the eastern foothills in the Guatiquía-Guamal segment highlight a lateral variability in which from the north to south, the structural style changes from thickskinned tectonic inversion and large-scale folding producing an anomalous overturned forelimb (Guatiquía area) to a simple thrust imbricate fan (Guamal area), via a transfer fault inherited from the Mesozoic extension.
The restoration of a regional transect through the Villavicencio area illustrates a complex kinematic evolution characterized by extensional, contractional, and salt tectonics.This area constituted the western and southern edge of the Cundinamarca extensional basin during the early Cretaceous.The area accumulated thick marine sediments including evaporitic layers that markedly influenced the entire history of deformation.The present-day Buenavista faulted anticline is interpreted as a former salt wall associated with the extensional Mirador fault, later squeezed during continuous shortening.The large overturned limb of the anticline is compatible with this process.In late shortening stages, welding of the diapir resulted in break-through thrusting across the overturned flap.
We proposed a previously unrecognized pattern of diapirism for the EC foothills.The new interpretation for the Guatiquía area leads to envisage that the known salt occurrences in the EC may be signs of a larger evaporitic depositional system, underestimated in previous interpretations, whose influence in terms of salt tectonics may cover wide parts of the EC of Colombia.
Sandstone with interbedded shale Shale with interbedded sandstone, and coal at the base Fine sandstone interbedded with shale at the top Interbedded claystone, and shale, locally silt, and sand at the base Siltstone with interbedded sandstone Shale, and claystone

Figure 4 .
Figure 4. Selected seismic profiles for cross section B-B′ (Figures 1 and 5).(a) Interpreted seismic line CHVRB-1993-105, with continuous reflections characterizing the basin, chaotic reflections in the foothills, and high-amplitude reflections for the deep Villavicencio-Mirador fault plane and minor faults above (b) Interpreted seismic line V-1988-1065 from the Llanos basin, showing very continuous and subhorizontal reflectors.In general, the sedimentary sequence thins to the east, although a more detailed interpretation shows that some late Cretaceous-early Paleogene reflectors onlap underlying ones and are restricted to the proximal foredeep.In the distal foredeep, the molasse sediments are folded over blind thrusts that affect the Cretaceous-Paleogene succession.

Figure 5 .
Figure 5. Cross sections A-A′, B-B′, and C-C′ across the Guatiquía and Guamal segments of the EC foothills (see Figure 1 for location).Note the structural variation and the significant thickness change of the Cretaceous sequence from the north to south.Cross section B-B′ (the Villavicencio section) is kinematically restored in Figure 7.
u y s u b b a s in F lo re s ta -S a n ta n d e r m a s s if

Figure 6 .Figure 7 .
Figure 6.Paleogeographic reconstruction to early Cretaceous times, showing the extension of the Berriasian-Valanginian sea, the extent of the Berriasian evaporitic depositional system, and the main active tectonic structures (not restored for orogenic shortening).Also shown are the present-day location of emerald mines (on Berriasian-Valanginian layers), the Berriasian salt and gypsum mines, and saline springs (constructed on the basis of data from Etayo et al. (1969) and Sarmiento-Rojas et al. (2006).