Oblique transpression in the western thrust front of the Colombian Eastern Cordillera

Oblique transpression in the western thrust front of the Colombian

Eastern Cordillera

J. Acostaa,b,*, L. Lonergana, M.P. Cowarda

aDepartment of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UKbIngeominas, Diagonal 53 No 34-53 Bogota, Colombia

Received 1 November 1999; accepted 1 June 2004

Abstract

New kinematic data reveal that the main faults of the western foothills of the Colombian Eastern Cordillera are a series of left-lateral to

oblique thrusts that are offset by steeply dipping, northwest-trending, left-lateral, strike-slip faults. Kinematic data were collected from the

main structures that dominate the 350-km length of the western foothills of the Eastern Cordillera, north of Bogota (La Salina–Bituima,

Cambras, Dos Hermanos–Alto del Trigo, and Bucaramanga faults). These data indicate that transpression is responsible for the Cenozoic

formation of the folds and thrusts that deform the Mesozoic and Cenozoic sedimentary sequences of the Middle Magdalena Valley.

Kinematic and structural data suggest that the La Salina–Bituima fault changed from a reverse fault to a fault with a left-lateral, strike-slip

sense of displacement during middle to late Miocene times. As it propagated, a transpressive zone was generated in the western foothills of

the Eastern Cordillera, with the development of arcuate, oblique-reverse secondary faults on the leading edge of the Magdalena basin

(e.g. Cambras, Dos Hermanos). This development implies that during the Neogene (?), the tectonics of the western foothills were dominated

by nonplanar deformation with shortening accompanied by significant left-lateral, strike-slip displacements. These new data indicate that the

oblique convergence vector imposed by convergence among the Nazca, Caribbean, and South American plates is not fully partitioned in

space but instead must be distributed in a diffuse zone of transpressional deformation along the western margin of the Eastern Cordillera and

its associated foreland basin.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Indentation; Kinematics; Northern Andes; Oblique faulting; Transpression

1. Introduction

The oblique convergence of the Nazca and Caribbean

plates with the South American plate is considered

responsible for the transpressive tectonic regime that

resulted in the formation of the northernmost Andes of

Colombia, Ecuador, and Peru during the Cenozoic (e.g.

Burke et al., 1984; Ego et al., 1996). In addition, the marked

trend change that occurs between the Central (Peru and

Bolivia) and Northern (Ecuador and Colombia) Andes has

also been attributed to the transition between orthogonal

plate convergence in the south and oblique plate

0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jsames.2004.06.002

* Corresponding author. Fax: C44-571-2223764.

E-mail address: [email protected] (J. Acosta).

convergence in the north (Stephan et al., 1986; Delouis et

al., 1996). In the Central Andes, the main structural grain

trends NW, broadly orthogonal to the plate convergence

vector between the Nazca and South American plates. In

Ecuador and Colombia, the structural trend changes

significantly to the NE, which reflects the complicated

oblique, three-plate convergence in the Northern Andes

(Pennington, 1981; Ego et al., 1996). Although tectonic

models for the Colombian Andes have predicted that the

oblique interaction of the three plates should generate

dextral transpressional deformation in the Andean Cordil-

leras south of 58N and sinistral transpressional deformation

north of 58N (Ego et al., 1996), little structural evidence

from field studies in the Colombian Andes has been

presented to support these models. Most of the two-

dimensional structural reconstructions of the deformation

in the western foothills of the Eastern Cordillera assume

Journal of South American Earth Sciences 17 (2004) 181–194

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J. Acosta et al. / Journal of South American Earth Sciences 17 (2004) 181–194182

plain strain, orthogonal to the strike of the main thrusts, and

ignore any strike-slip contribution to the deformation field

(Colletta et al., 1990; Dengo and Covey, 1993; Cooper et al.,

1995; Roeder and Chamberlain, 1995). South of 58N,

focal mechanism solutions for recent earthquakes suggest

that the dextral transpression predicted by plate conver-

gence models exists currently (Ego et al., 1996), and a

recent structural study (Montes, 2001) concludes that the

Piedras-Giradot foldbelt, located just south of 58N in the

Eastern Cordillera, was formed by dextral transpression

during the Cenozoic.

We present the results of a detailed structural investi-

gation carried out along the western margins of the Eastern

Cordillera and its associated foreland basin, the Middle

Magdalena Valley, which extends over an area of

30,000 km2 between 4 and 78N. We present new kinematic

data collected along the major faults in the fold-and-thrust

belt and use them to explain the structural configuration and

kinematic evolution of the western foothills of the

Colombian Eastern Cordillera during the Cenozoic.

2. Geological setting: Eastern Cordillera and Magdalena

basin

In Colombia, the Andes are less than 300 km wide in the

south at the border with Ecuador, widen northward to more

than 600 km at a latitude of w78N, and trend generally NNE

(Fig. 1). A belt of late Neogene deformation extends from

the Sinu basin in the north (at the junction of the Magdalena

and north Panama accretionary wedge complexes) in a

southeasterly direction to the foothills of the Eastern

Cordillera, which border the Llanos foreland basin. In

Colombia, the Andean belt is divided into four principal

mountain ranges (Cordilleras) separated by long intermon-

tane valleys (Fig. 2). The Magadalena Valley, located

between the Central and Eastern Cordilleras, formed a

foreland basin in the westward-verging Eastern Cordillera

during the Andean orogeny (late Paleogene–early Neogene)

(Schamel, 1991; Gomez et al., 2003). We focus on the

Middle Magdalena basin (which covers an area of

approximately 30,000 km2) and the adjacent western foot-

hills of the Eastern Cordillera.

The few studies of the tectonics and stratigraphy of the

eastern Middle Magdalena foreland basin and western

foothills of the Eastern Cordillera tend to be regional in

scope (Julivert, 1959; Julivert, 1961; De Porta, 1966; Mojica

and Franco, 1990; Schamel, 1991; Cooper et al., 1995;

Gomez et al., 2003) The structure of the region is generally

described as a fold-and-thrust belt (Schamel, 1991; Cooper

et al., 1995; Mojica and Franco, 1990), but published studies

of the structural evolution that address the kinematics of the

deformation are notably lacking.

The Middle Magdalena basin is bound to the north by the

NW-trending, left-lateral Bucaramanga strike-slip fault

and to the south by the Piedras–Giradot foldbelt (Fig. 2).

Three NE–SW-trending, left-stepping reverse faults define

the eastern edge of the Middle Magdalena Valley. From

north to south, they are the La Salina–Bituima, Dos

Hermanos–Alto del Trigo, and Cambras faults. Displace-

ment is transferred in a SSW direction from the La Salina to

the Dos Hermanos and then onto the Cambras fault in an en

echelon piggyback fashion (Schamel, 1991). The La Salina

structure is a reactivated, inverted, normal fault (Colletta

et al., 1990; Dengo and Covey, 1993; Cooper et al., 1995),

and its southerly extension within the basin is known as the

Bituima fault. North of La Salina, the Chucurı fault forms

the main thrust fault along which the Eastern Cordillera

Jurassic rocks are displaced over the Cenozoic sedimentary

rocks of the Middle Magdalena Valley. The N–S-trending

Suarez fault, east of the Chucurı fault, is an important

sinistral strike-slip fault that appears to splay off the

Bucaramanga fault (Fig. 2).

Along most of their 600 km length, the western foothills

of the Eastern Cordillera and Middle Magdalena basin

exhibit Cenozoic thrust structures. The structural style

varies markedly along-strike. In most cases, the boundaries

between regions of different structural styles are defined by

0458–0708-trending, basement-involved faults. Across these

faults, there are marked facies and thickness variations of

Cretaceous sediments (Acosta, 2002). These basement-

controlled structures give rise to clear segmentation of the

Andean foreland. In a comprehensive recent study based on

the interpretation of a regional grid of two-dimensional

seismic reflection profiles and surface mapping, Acosta

(2002) distinguishes the following principal styles of

deformation in the region:

1.
Thin-skinned deformation where the thrusts have
detached along a decollement level in Late Cretaceous

sediments. Usually the decollement ramps up in front of

preexisting basement structures and crops out at the

surface as a thrust parallel or subparallel to the

preexisting basement fault.

2.
Thick-skinned inversion, developed in two phases: Late
Eocene–Early Oligocene and Miocene. The formation of

broad, hanging-wall anticlines in Cretaceous rocks,

short-cut thrusts, and folding of structures developed

prior to the inversion of east-dipping normal faults

characterize this style of deformation.

3.
Thick-skinned basement thrusting along the western
foothills of the Eastern Cordillera and Middle Magdalena

basin, characterized by fault-bend fold structures and

folding of preexisting thin-skinned structures.

4.
Thin-skinned, out-of-sequence thrusts of Miocene to
Pliocene age along the internal part of the foothills.

3. Fault kinematics

Previous regional scale kinematic reconstructions and

palinspastic models of the Eastern Cordillera thrust belt

Fig. 1. (a) Major structural features of the Colombia Andes with observed velocity vectors (arrows) relative to stable South America at 95% confidence.

GPS vectors from Trenkamp et al. (2002). (b) Regional tectonics of northern South America and the Caribbean.

J. Acosta et al. / Journal of South American Earth Sciences 17 (2004) 181–194 183

Fig. 2. Generalized geological map of part of the Eastern Cordillera and Middle Magdalena Valley (modified from INGEOMINAS, 1995).


have calculated restored geometries and amounts of short-

ening along transects perpendicular to the trend of the

mountain belt, assuming plain strain deformation (e.g.

Colletta et al., 1990; Dengo and Covey, 1993; Cooper et al.,

1995). These models were constructed for transects north of

Bogota, the western end of which fall within our study area.

The presence of the large regional Bucaramanga strike-slip

fault that clearly offsets the whole Eastern Cordillera and

predictions of sinistral transpressional deformation north of

58N (Ego et al., 1996) prompted us to investigate the

kinematics of the region in more detail, particularly to

assess the potential importance of oblique shortening and

transpression in the development of the structures that form

the Eastern Cordillera and Middle Magdalena basin.

The study area is covered by a tropical rainforest, and as

a result, the major faults tend to be poorly exposed, and

geological traverses are difficult to make. In total, 24 road-

or riverside sites along the La Salina–Bituima, Cambras,

Dos Hermanos–Alto del Trigo, Chucuri, Suarez, and

Bucaramanga faults were found where good fault and

fault-related kinematic data could be collected. Kinematic

indicators typically include mineral lineations, crystal fibers

(slickensides), grooves or striae on the fault surface, and

extension veins and stylolites (Doblas, 1998). In the study

area, most indicators measured were calcite crystal fiber

lineations and striae on fault.

The data were sorted into parallel or subparallel plane

subgroups, which we assumed at any one site were related to


the main structure. Other fault planes were then grouped

into populations with similar dip, strike, and displacement

sense; four groups of faults with different kinematics were

identified.

3.1. La Salina–Bituima and Chucurı faults

The La Salina–Bituima fault has been interpreted as a

west-verging, steeply-dipping thrust fault that displaced

early Cretaceous sedimentary rocks of the Eastern Cordil-

lera over late Cretaceous and Cenozoic rocks of the Middle

Magdalena basin (Fig. 2) (Colletta et al., 1990; Dengo and

Covey, 1993; Cooper et al., 1995). Most previous work also

Fig. 3. Lower-hemisphere stereoplots of structural data from the La Salina-Bituima

circles represent fault plains and related slickenside lineations.

infers that this fault is an inverted late Jurassic–early

Cretaceous normal fault. Julivert (1961) originally mapped

the Chucurı fault as a monocline and interpreted it to be the

surface manifestation of a basement fault. However,

because of its clear expression in the field, Mojica and

Franco (1990) identify it as a thrust that juxtaposed

Jurassic and early Cretaceous rocks over late Cretaceous

and Cenozoic sediments of the Middle Magdalena Valley

(Fig. 3a).

Two sets of slickenside lineations were measured on

the La Salina–Bituima fault system (Fig. 3a). The first,

from sites 2a, 3a, and 4a, shows that the fault system is

formed by steeply dipping faults with a reverse sense of

, Cambras, Dos Hermanos, Chucuri, Suarez, and Bucaramanga faults. Great


shear. The second set, from sites 1, 2b, 3b, 4b, and 6,

reveals a series of steeply dipping, left-lateral, and oblique

faults with a reverse sense of shear toward the NNE. This

second set of lineations also appears on the Chucuri fault.

If considered in isolation, at any one site, the oblique

kinematics might be explained as occurring in response to

local fault linkage/lateral ramp formation (e.g. site 4).

However, the consistency and persistence of the oblique

kinematics along the entire 250-km length of the

combined fault systems suggests that it reflects the main

displacement sense of these faults and cannot be attributed

to local effects.

The presence of the two sets of slickensides at sites 2, 3,

and 4 suggest that they were formed during two different

stages of displacement of the La Salina–Bituima fault

system. The relative timing of these two stages is not clear

in the field. However, several authors have proposed that the

reverse sense of displacement for the La Salina–Bituima

fault occurred during the Eocene, associated with the

inversion of a pre-existing normal fault (e.g. Colletta et al.,

1990; Dengo and Covey, 1993; Cooper et al., 1995).

Transcurrent movement is thought to have occurred during

the Late Miocene in response to the accretion of the Panama

block. At site 5, a NW-dipping thrust with orthogonal

displacement links the Chucurı and La Salina-Bituima fault

systems (site 5, Fig. 3a). This linkage fault is assumed to be

a minor backthrust of the La Salina–Bituima fault system,

which was later overthrust by the Chucurı fault.

3.2. Bucaramanga and Suarez faults

Since at least the middle Eocene (Boinet et al., 1986;

Acosta, 2002), the Bucaramanga fault appears to have acted

as a strike-slip fault, and though the most recent GPS data do

not directly measure present-day strike-slip displacements

across it, regional data are consistent with sinistral slip

(Trenkamp et al., 2002). Using well data, Kellogg (1984)

has suggested that the Santa Marta massif and Sierra de

Perija were displaced northwestward along the Bucara-

manga-Santa Marta fault during the Oligocene (Fig. 1). In

the Late Miocene and Pliocene, sinistral strike-slip

deformation continued in the north with a horizontal offset

of w100 km, but reverse movement occurred along the

southern strands of the fault (Boniet et al., 1986). The

transition from strike-slip faulting in the north to the reverse

movement in the south implies that a significant component

of strike-slip displacement must have been accommodated

on Eastern Cordillera thrusts (Vargas et al., 1981; Boinet

et al., 1986) (Fig. 3b).

The N–S-trending Suarez fault joins the Bucaramanga

fault at its northern end. On the basis of the morphology of

neotectonic features, Vargas and Nino (1992) propose that it

is a reverse fault with a significant component of sinistral

strike-slip motion along it, which has transported Jurassic

and early Cretaceous rocks over Cretaceous sedimentary

rocks.

Lineation data confirm that the Bucaramanga and Suarez

faults are steeply dipping, left-lateral, strike-slip faults (sites

7–10, Fig. 3b). Two more sets of planes also were measured

along the Bucaramanga fault, one of which trends

subparallel to the main structure. These planes are

interpreted as riedel shears. The second set is formed by

steeply dipping, right-lateral faults, which are particularly

well exposed on Jurassic granites of the Santander massif at

sites 9 and 10. These faults are interpreted as antithetic

riedel shears (Fig. 3b).

3.3. Cambras and Dos Hermanos faults

The next set of faults along which kinematic data were

collected are the Cambras (also known as Rıo Seco, Belta,

or Cambao fault) and Dos Hermanos faults (Fig. 3c). These

faults form the frontal structures of the Eastern Cordillera

on the margins of the Magdalena basin in the southern

foothills (Fig. 2).

The Cambras thrust forms the most westerly structure of

the southern foothill imbricate thrust stack. Mapping and

interpretation of seismic reflection profiles show that the

Guaduas syncline is carried piggyback in the hanging wall

of the Cambras thrust (Fig. 4). On the eastern side of the

Guaduas syncline, the Cambras and Bituima faults link with

the same detachment level in the basement (Fig. 4).

Lineation data collected along both the Cambras thrust

(sites 12, and 13, Fig. 3c) and its northward continuation, the

Dos Hermanos thrust (site 14, Fig. 3c), show predominantly

SE to NW-directed thrust motion with only a minor

component of oblique slip. Thus, moving to the west and

south in the foothills belt, the amount of oblique motion on

the thrust structures appears to diminish. The data collected

at site 11 to the west of Bogota show strong dextral oblique

motion (Fig. 3c), which is related to dextral motion on the

ENE-trending Ibague fault.

3.4. ESE strike-slip faults

A set of steeply dipping, southeast-trending (1208),

predominantly left-lateral, strike-slip faults that offset the

Cambras and Dos Hermanos thrust faults are observed south

of latitude 68N (Fig. 5). These faults exhibit similar

kinematics to some earthquake focal plane solutions for

recent events of basement faults in the area (Fig. 6, sites 1

and 2, 24 and 40 km deep, respectively). The kinematic

indicators observed on the previously discussed thrust faults

suggests that these E-ESE-striking, left-lateral faults might

be lateral ramps to the thrust system. However, the

predominant NW–SE direction, left-lateral shear sense of

motion, and direct relationship with basement faults

enable us to confirm that they are reactivated NW–SE

faults, as proposed by several authors in regional studies

of the Northern Andes (Acosta, 1983; Gomez, 1991; Ujueta,

2001; Velandia and De Bermoudes, 2002).

Fig. 4. E-W cross-section across the Guaduas syncline. (a) Seismic profile H-78-10, (b) structural interpretation of the seismic profile, (c) interpreted cross-

section from the seismic profile.


3.5. Incremental strain

Borehole breakout data from wells in the Magdalena

basin and the Llanos Plains show that the present-day

maximum horizontal stress direction is 112 and 1388,

respectively (Castillo and Mojica, 1990). Over an extensive

area, including the foreland basins on either side of the

Eastern Cordillera, the present-day maximum horizontal

Fig. 5. Lower-hemisphere stereoplots of structural data from NW-striking, left-lateral, strike-slip faults. Great circles represent fault plains and related

slickenside lineations.


compressive stress trends NW–SE. Using measured kine-

matic data, earthquake focal plane solutions, and borehole

break-out data, we infer a NW-trending principal incre-

mental strain axis (shortening axis) for the region, which

may have extended to the late Neogene, in that no

significant changes to plate movement directions are

believed to have occurred since 11 million years BP.

Stratigraphic offset of the top of the Cretaceous,

interpreted from seismic sections (e.g. Fig. 4) and the

results of the kinematics, enable us to calculate the left-

lateral fault displacement. For the Cambras and Dos

Hermanos faults, 4–5 km of left-lateral offset is estimated,

according to the results of the oblique sense of shear

observed. Values of 5–42 km of left-lateral offset are

estimated for the La Salina–Bituima fault. The greatest

values are obtained when pure strike-slip motion is applied.

However, the presence of the two sets of slickensides in this

fault indicates an intermediate value of offset for this

structure.

4. Discussion

At a regional scale, the formation of the principal

structures observed in the western foothills of the Eastern

Cordillera and Middle Magdalena basin are controlled by

oblique convergence due to the interaction of the Nazca,

South American, and Caribbean plates and the Panama

Fig. 6. Lower-hemisphere stereoplots of focal mechanism solutions from the Eastern Cordillera. Note that the active deformation is largely related to strike-slip

motion.


block. When integrated with GPS studies conducted by

others (e.g. Trenkamp et al., 2002) and previous plate

tectonic reconstructions (e.g. Burke et al., 1984; De Mets

et al., 1990), the kinematic results indicate that a differential

distributed shear has been applied to the north Andean

crustal lithosphere since the Late Neogene. In addition,

differential distributed shear could have been applied since

the Late Paleogene (Fig. 7), as supported by (1) the

accretion of the Sinu-San Jacinto sedimentary wedge in the

north part of Colombia during the Eocene as a result of a

major change in Caribbean plate motion (Duque-Caro,

1979); (2) left-lateral faulting along northern Colombia

since the late Oligocene, which resulted in the uplift of the

Santa Marta massif and the Sierra de Perija along the

Bucaramanga fault (Kellogg, 1984); and (3) sinistral motion

along the La Salina-Bituima fault since the Middle–Late

Miocene, when the Panama block accreted onto north-

western Colombia (Duque-Caro, 1990).

The accretion of the arcuate Panama block in the

northwestern corner of Colombia was responsible for the

major episode of Miocene deformation in the Colombian

Andes (Duque-Caro, 1990). The Panama block is bound

to the north and east by the arcuate Uramita fault zone

and to the south by the Isthmina fault zone (Fig. 1).

Fig. 7. (a–c) Schematic reconstruction of the northern Andean margin, showing the direction of influence (arrows) of the different tectonic plates over the

northern Andean block and particularly over the Middle Magdalena Valley and Eastern Cordillera during the Cenozoic. No attempt has been made to remove

crustal shortening in the northern Andes. Modern coastlines are used. (d) Schematic reconstruction of the current deformation in the Andean block. Arrows

indicate the approximate direction and velocity of the plates.


Sandbox models of deformation associated with an

obliquely converging rigid indenter generate structures

with very similar geometries and kinematics to those

observed in northwestern Colombia, where the Uramita,

Rio Atrato, and Baudo faults (Fig. 1) are equivalent to the

curved oblique faults that form a component of dextral and

sinistral slip ahead of the indenter (Fig. 8). The experiments

also show that a right-lateral shear zone develops at the

blunt southern end of the indenter (Fig. 8), is comparable to

the Isthmina fault zone that trends 0608 (Fig. 1), and is

characterized by 0608-trending steeply plunging tight folds

that are cut by strike-parallel faults (Duque-Caro, 1990). It

seems likely, therefore, that the Panama block acted as a

more-or-less rigid indenter during oblique convergence

Fig. 8. Experimental evolution of sandbox models of oblique convergence, collision, and indentation of a rigid block.


between the South American and Caribbean plates. As a

result, contraction occurred across the northwestern margin

of Colombia, which led to the formation of the Baudo

Serrania, Uramita, Rio Atrato, Isthmina, and Baudo faults

(Fig. 1). In addition, the Andean block was shortened, and it

internally deformed on oblique thrust faults.

Previous theoretical studies of oblique subduction

predict that the relationship between the plate convergence

vector and the orientation of the collisional margin control

the amount of strain partitioning that occurs across the

trench and deformed continental margin (e.g. McCaffrey,

1992), which seems evident in the Northern Andes. The

recent synthesis of a 10-yr CASA GPS experiment in

northwestern South America by Trenkamp et al. (2002)

clearly documents that the Andean margin is undergoing

shortening perpendicular to the margin, with the oblique

component of the Nazca plate subduction partitioned onto

faults subparallel to the Andean margin. The velocity of the

convergence between the Nazca plate and northern South

America varies along the continent margin (Fig. 1). At

Ecuador, the rate is 70 mm/yr, but it decreases to 35 mm/yr

in the northern part of the Colombian Pacific region.

Therefore, thrusting and dextral transpressional deformation

occurs in the south part of the Colombian Andean

Cordilleras. Furthermore, the Caribbean plate is moving

at a rate of 10–15 mm/yr toward the Colombian Andes,

and the Panama block is indenting northwestern Colombia

at 8–21 mm/yr, which results in a sinistral component of

shear on N–NW-oriented structures. It therefore, is to be

expected that the Northern Andes are ‘escaping’ northward,

in agreement with the results of Trenkamp et al. (2002), who

indicate such an ‘escape’ at velocities of 6G2 mm/yr.

This finding also agrees with the prediction of Ego et al.

(1996), who propose dextral transpressional deformation in

the Andean Cordilleras south of 58N (e.g. Ibague fault) and

sinistral transpressional deformation north of 58N, where the

western foothills of the Eastern Cordillera and Middle

Magdalena are located (e.g. Bucaramanga and La Salina–

Bituima fault systems).

When the GPS data are plotted relative to the different

plates and blocks located in northern South America, the

same sense of motion can be deduced (Fig. 9). Furthermore,

a conjugate movement of counterclockwise rotation and

expulsion of the Andean block is suggested. The counter-

clockwise rotation of the Andean block would induce a

dextral sense of motion along the NE–SW-trending major

faults in the region, including the Ibague, Palestina, and

Guaicaramo (Figs. 1 and 7). Because of the indentation of

the Panama block and the Caribbean plate in the northern

part of the Colombian Andes, the strain is partitioned

in the western foothills of the Eastern Cordillera along

NNW–SSE, left-lateral, strike-slip faults (e.g. partitioning

has driven the northward displacement of the Santa

Marta massif along the Bucaramanga and Bocono faults

Fig. 9. Present-day relative motions of plates and ‘blocks’ in the Northern Andes, GPS data (modified from Mora, 1995).


and changed the displacement of the La Salina–Bituima

fault from thrust to strike-slip at the edge of the Eastern

Cordillera). This model requires further testing with, for

example, paleomagnetic data.

At a latitude of approximately 48N, a set of reactivated

northwest-striking, left-lateral, strike-slip faults forms a

conjugate set with the dextral Ibague fault, which is a

neotectonic structure with a clear surface expression (Fig. 1).

This conjugate set is almost orthogonal to the present motion

of the Nazca plate with respect to the northern Andes at that

latitude. Therefore, we infer that the current boundary

between dextral and sinistral transpressional deformation in

the study area is located in the southernmost part of the

Middle Magdalena basin at 48N, as opposed to the 58N first

proposed by Ego et al. (1996).

Arcuate oblique thrusts (Cambras and Dos Hermanos)

were also observed in the western foothills of the Eastern

Cordillera and Middle Magdalena basin. Very similar

structures form in sandbox experiments of transpressive

settings (e.g. Richard, 1990; Cobbold et al., 1991; Fig. 10),

in support of the interpretation that these oblique thrusts

formed in transpressive conditions during the change in

deformation from thrusting to left-lateral, strike-slip move-

ment along the La Salina–Bituima fault.

Fig. 10. Faulting in a sandpack above an oblique-slip (left-lateral) basement fault. (a) Surface view of grid of surface markers, originally square in undeformed

state, now deformed and offset across faults. Faults are classified as strike-slip (plain, with half arrows to show sense), normal (ticks on downthrown side in

direction of throw), or oblique reverse (triangle pointing down-dip in direction of throw). (b) Surface view of vectors of total horizontal displacement (arrows)

for selected nodes of surface grid. Vectors are to scale; they span distances between the original position of nodes (dots at bases of arrows) and the final

positions (points of arrows). Faults separate blocks with nearly rigid behavior (mainly translation, some rotation). Arcuate reverse faults terminate at poles of

relative block rotation. Modified from Richard (1990); Cobbold et al. (1991).


5. Conclusions

When integrated with regional studies of Nazca,

Caribbean, and Panama plate convergence vectors, our

new data show that the tectonic history of the western

margin of the Eastern Cordillera in Colombia is complex.

Faults that have been interpreted to have accommodated

only dip-slip movements have been shown to have oblique

motion. Left-lateral fault motion plays an important role on

both NNE- and NW-trending structures. The kinematics of

these faults is the result of a conjugate motion of counter-

clockwise rotation and the expulsion of the Andean block.

The counterclockwise rotation of the Andean block induces

a dextral sense of motion along the NE–SW-trending major

faults in the region, including the Ibague, Palestina, and

Guaicaramo faults. Because of the indentation of the

Panama block and the Caribbean plate in the northern part

of the Colombian Andes, the strain is partitioned in the

western foothills of the Eastern Cordillera along NNW–

SSE, left-lateral, strike-slip faults, such as the Bucaramanga

and La Salina–Bituima faults.

The La Salina–Bituima is an inverted normal fault,

which changed through time from a thrust to an oblique,

left-lateral, strike-slip fault as a result of the strain

partitioning generated by the indentation of the Panama

block. The Cambras, Dos Hermanos, and Honda faults are

thrusts and oblique faults with a transport direction from SE

to NW. These faults branched out from the La Salina–

Bituima fault as a result of transpression during the change

of displacement kinematics along the La Salina–Bituima

fault system.

Acknowledgements

We acknowledge Colciencias for doctoral funding to J.

Acosta and support from the Royal Society to L. Lonergan.

Ingeominas is thanked for funding the fieldwork through

the ‘Cartografia geologica y tematica del territorio

Colombiano’ program. Special thanks to J. Cosgrove,

M. Belayneh, M. Sepehr, and M. Brown for comments

and suggestions on previous versions of this manuscript. We

also thank F. Corredor for a thorough review. Stereographic

plotting software for the Apple Mac written by Hugo Ortner

(1991–1998) was used for Figs. 3, 5 and 6.


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Oblique transpression in the western thrust front of the Colombian Eastern Cordillera

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