930

ABSTRACT

New apatite fi ssion-track data, paleoele-vation estimates from paleobotany, and recently acquired geological data from the Eastern Cordillera of Colombia document the onset of increased exhumation rates in the northeastern Andes at ca. 3 Ma. The Eastern Cordillera forms an effi cient oro-graphic barrier that intercepts moisture-laden winds sourced in the Amazon low-lands, leading to high rainfall and erosion gradients across the eastern fl ank of the range. In contrast, the drier leeward west-ern fl ank is characterized by lower rates of deformation and exhumation. In light of the geological evolution of the Eastern Cordil-lera, the combination of these data sets sug-gests that the orographic barrier reached a critical elevation between ca. 6 and ca. 3 Ma, which ultimately led to protracted, yet more focused erosion along the eastern fl ank. Sequentially restored structural cross sec-tions across the eastern fl ank of the Eastern Cordillera indicate that shortening rates also have increased during the past 3 Ma. From fi ssion-track and structural cross-section balancing, we infer that accelerated exhumation led to increasing tectonic rates on the eastern fl ank, creating a pronounced topographic and structural asymmetry in the Eastern Cordillera. The tectonic and climatic evolution of this orogen thus makes

it a prime example of the importance of cli-matic forcing on tectonic processes.

Keywords: Apatite fi ssion tracks, deformation, balanced cross sections, erosion, orographic barriers, paleoelevation, paleoclimate.

INTRODUCTION

Important advances have been made in the understanding of climatic forcing of orogenic evolution over the past several decades (Koons, 1989; Molnar and England, 1990; Beaumont et al., 1992; Masek et al., 1994; Willett, 1999; Montgomery et al., 2001; Molnar, 2004; Strecker et al., 2007). Probably one of the most important conclusions has been that active shortening and tectonic uplift may be localized in areas where protracted erosion impacts tectonically active landscapes. Here, focused high precipitation and exhumation result from orographic barriers that intercept moisture and generate powerful erosional regimes on the windward fl anks of an orogen (Horton, 1999; Montgomery et al., 2001; Reiners et al., 2003; Thiede et al., 2005; Barnes and Pelletier, 2006). Beaumont et al. (1992) and Willett (1999) argued that the degree of asym-metry of an orogen may be highly dependent on climatic (e.g., precipitation) gradients. Their conclusions rely on numerical modeling of an idealized orogen with long-term precipitation focused on one side and can also be reproduced in numerical and sandbox models. These models show that long-term erosion and sedimentation patterns are fundamental in determining struc-

tural styles and evolution (Dahlen and Suppe, 1988; Mugnier et al., 1997; Koyi et al., 2000; Whipple and Meade, 2004; Hoth et al., 2006). If such conditions are associated with wedge-shaped thrust belts, focused precipitation and erosional removal of rock may ultimately lead to out-of-sequence thrusting (Mugnier et al., 1997; Koyi et al., 2000; Hilley and Strecker, 2004). In contrast, more arid, interior sectors of an orogen may experience less tectonic activity due to the increase of lithostatic stresses resulting from fail-ure to evacuate erosional materials (e.g., Sobel and Strecker, 2003; Hilley and Strecker, 2005).

The Himalayas, the southern and central Andes, and the New Zealand Alps are premier examples of orogens where interactions between tectonics and climate have been documented and where the interplay between them may have fundamentally infl uenced the evolution of indi-vidual mountain ranges and intraorogenic pla-teaus (Montgomery et al., 2001; Koons et al., 2002; Sobel et al., 2003; Sobel and Strecker, 2003; Thiede et al., 2005; Strecker et al., 2007). In the northern Andes, where strong precipita-tion gradients also exist, such relations have not been explored. The NNE-oriented northern Andes have created important precipitation gra-dients across the Western and Eastern cordille-ras (Fig. 1). In fact, with ~7 m of annual precipi-tation (Fig. 1), the humid fl anks of these ranges receive one of the highest amounts of precipi-tation on Earth. This sector of the Andes thus lends itself to a study of the interplay between tectonic activity, the spatiotemporal evolution of climatic gradients, and exhumation patterns.

For permission to copy, contact editing@geosociety.org© 2008 Geological Society of America

GSA Bulletin; July/August 2008; v. 120; no. 7/8; p. 930–949; doi: 10.1130/B26186.1; 13 fi gures; 3 tables.

Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia

Andrés Mora†

Mauricio ParraManfred R. StreckerEdward R. SobelInstitut für Geowissenschaften, Universität Potsdam, Karl Liebnecht-Str 24, D14476 Potsdam-Golm, Germany

Henry HooghiemstraVladimir TorresInstitute for Biodiversity and Ecosystem Dynamics (IBED), Paleoecology and Landscape Ecology, Faculty of Science, University of Amsterdam, Kruislaan 318, 1098 SN Amsterdam, Netherlands

Jaime Vallejo JaramilloPetrobras-Colombia, Carrera 7, No 71-21, Torre B. Edifi cio Bancafé, Bogotá, Colombia

†E-mail: andresmora@googlemail.com

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 931

The eastern boundary faults of the Eastern Cordillera, here referred to as the Guaicaramo fault system (Fig. 1), constitutes a major plate boundary separating the structural domain of the Northern Andes microplate from the South American plate (Aggarwal, 1983). Between 4° and 5° N, the eastern fl ank of the Eastern Cordillera has accommodated signifi cantly more shortening, and basement rocks crop out at elevations ~2 km higher compared to the western fl ank (Toro et al., 2004; Mora et al., 2006; Fig. 2). In addition to the structural asymmetry, the distribution of rainfall is also asymmetric, and the more humid eastern fl ank strongly contrasts with the drier western parts (Fig. 1). Nevertheless, interactions between climate and tectonics remain poorly known. If signifi cant feedback between tectonics and cli-mate indeed existed in the Eastern Cordillera, it would likely be detectable through an erosional gradient. Therefore, we evaluated the role of erosional denudation in the context of the styles

and timing of the structures that constitute the mountain range. We collected fi eld structural data and constructed geologic cross sections to assess the degree of overall shortening and removal of cover units. We then carried out apa-tite fi ssion-track thermochronology to quantify the long-term role and timing of erosional denu-dation. In a third step, we examined published data on the paleofl oristic evolution of the area, the late Cenozoic sedimentary record, and ther-mochronology data to reconstruct range-wide late Cenozoic surface uplift, denudation, and tectonism. In our attempt to unravel tectonics and climate interactions we compare different domains in the Eastern Cordillera with remark-ably different Plio-Pleistocene denudation and uplift histories and explain their patterns in light of interactions between tectonically and climatically controlled migration of deforma-tion. Our investigation reveals that the uplift of the Eastern Cordillera generated an effec-tive orographic barrier for easterly moisture-

bearing winds, resulting in strong precipitation gradients across the range. Focused precipita-tion and removal of rocks conspire in sustaining high exhumation rates on the windward fl anks, whereas the drier leeward side is characterized by low rates, emphasizing the pivotal role of climate in the evolution of mountain ranges.

REGIONAL GEOLOGY

The Eastern Cordillera is the easternmost range in the northern Andes (Fig. 1). It is an inversion orogen that coincides with a Lower Cretaceous rift (Colleta et al., 1990; Sarmiento-Rojas, 2001; Mora et al., 2006). Structurally, the Eastern Cordillera can be divided into three major areas perpendicular to strike, including the western and eastern marginal thrust belts with associated basement uplifts, and the Bogotá Basin (Fig. 1). Situated between two mountain ranges, the Bogotá Basin forms a central, low-relief highland with outcrops of Cretaceous and

B

0 80 KmN

Precipitation (mm)

5700

650

6°

5°

4°

-72°-73°-74°

PKPK

A

B

B'

A'

PK

SbBA

B

B'B'

A'A'

PKPK

SbBSbBA

B

B'

A'

125

4

40

159

89 52129

441

1548

PK

PK

SbB

5000 m

0 m

0 80 KmN

6°

5°

4°

-72°-73°

Swath profilein Fig 3A Fig 2A

PLATE

PLATEECNAZCA SOUTH

AMERICANPLATE

CARIBBEAN

-74° 0°

6°

-73°

Fig 2B

Yopa

l Thr

ust

Yopa

l Thr

ust

Gua

icar

amo

Thrus

t

Gua

icar

amo

Thrus

t

Leng

upa T

hust

Leng

upa T

hust

La S

alin

a Th

rust

Ho n

da T

hrus

t

Ho n

da T

hrus

tB

ituim

a T

hust

Bitu

ima

Thu

st

Cam

bao

Cam

bao

Mag

dalena T

hrus

t

Mag

dalena T

hrus

t

Servi

tá Fault

Servi

tá Fault

RNRNRN

RBRBRB

RSRSRS

RURURU

RGtRGtRGt

RGyRGyRGy

RHRHRH

RMRMRMLa

Sal

ina

Thru

st

A

Figure 1. (A) General features of the Colombian Eastern Cordillera. Regional structure with the main thrusts bounding the major topo-graphic breaks in the Eastern Cordillera. River discharges (values in red in m3/sec) of the main rivers in the eastern and western fl anks clearly showing higher values in the eastern side rivers fl owing toward the Orinoco (e.g., Guayuriba—RGy; Guatiquía—RGt; Humea—RH; Upía—RU. Western rivers fl owing toward the Magdalena—RM (e.g., Negro—RN, Bogotá—RB, and Sumapaz—RS rivers) do not manage to cut across the structures. SbB—Sabana de Bogotá basin. The box in the smaller map shows the location of the bigger area. EC—Eastern Cordillera. (B) Precipitation map compiled from data of 424 meteorological stations for the past 20 yr from Instituto de Hidrología, Meteorología y Estudios Ambientales de Colombia (IDEAM). The eastern slopes of the Eastern Cordillera clearly constitute a present-day orographic barrier for winds, therefore concentrating precipitations to the E.

Mora et al.

932 Geological Society of America Bulletin, July/August 2008

Guyana Shield Basement

Paleogene

Neogene

LKUK

LKLK

T

3.51(7)3.51(7)

4.05(8)4.05(8)

0.49(5)0.49(5)

0.77(2)0.77(2)

0.27(1)0.27(1) 1.38(3)1.38(3)4.45(4)4.45(4)

3.51(7)

4.05(8)

0.49(5)

74°W

4°N

4°30'N0.77(2)

0.27(1) 1.38(3)4.45(4) FAFAFASJ

FSJ

FSJ

F

STFSTFSTFM

SCM

SCM

SC

GTGTGT

LKUK

PK

LK

NFNFNF

LK

PK

T

D

C

E

E'

D'

C'

GuatiquiaGuatiquiaGuatiquiaGuatiquia

HumeaHumeaHumeaHumea

GuayuribaGuayuribaGuayuribaGuayuriba

Anaconda WellAnaconda WellAnaconda Well

101010

99

8877

6655 44

33

2211

17171711-1411-1411-14 15-1615-1615-16

10 kmFig. 5

0

N

30

km

Upper Cretaceous

Lower Cretacous

Gomez (2001) AFT Profiles

AFT sample location (this study)

5°

2.6 ± 0.3 Ma2.7 ± 0.3 Ma

3.0 ± 0.4 Ma2.9 ± 2.1 Ma

2.3 ± 0.6 Ma2.6 ± 0.6 Ma2.6 ± 1.1 Ma

2.8 ± 0.4 Ma

2.6 ± 0.7 Ma2.4 ± 0.7 Ma

2.8 ± 0.4 Ma2.3 ± 0.4 Ma

3.8 ± 0.7 Ma0.9 ± 0.2 Ma

1.2 ± 0.4 Ma0.8 ± 0.3 Ma

1.6 ± 0.4 Ma

30 km

Triassic-Jurassic

Paleozoic-Carboniferous

Eastern flank

Western flank

SbBSbBSbB

GS VA

B

Paleozoic-Devonian

Andean Basement

SbB

A

Figure 2. (A) Digital elevation model of the Eastern Cordillera including the deeply dissected Eastern fl ank, the central fl at-lying Sabana de Bogotá basin and the topographically lower western fl ank. GS—Guaduas syncline; VA—Villleta anticlinorium. (B) Topographic and gen-eralized geologic map of the area studied in detail (see location in Fig. 1). Apatite fi ssion-track (AFT) sample locations are shown in red, and vitrinite refl ectance values are shown in black. Abbreviations: FA—Farallones anticline; GT—Guaicaramo thrust; LK—Lower Cretaceous units; MSC—Mirador short-cut fault; NF—Naranjal fault; PK—Pre-Cretaceous units; SbB—Sabana de Bogotá; SJF—San Juanito fault; STF—Servita fault; T—Tertiary units.

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 933

Tertiary sedimentary units. Cenozoic shorten-ing in the Bogotá Basin is minor and primar-ily related to folding and to a lesser degree to thrusting (Julivert, 1970). Nonetheless, the basin is topographically high, with elevations of more than 2500 m above sea level (Figs. 1 and 2). The western fl ank of the Eastern Cordillera constitutes west-vergent basement thrusts with offsets on the order of more than 20 km (Fig. 1). The eastern fl ank is similar to the western fl ank because it forms an east-vergent thrust belt where shortening is concentrated. In the east, thrusting has uplifted basement rocks to higher elevations than on the western fl ank, resulting in basement exposure at elevations of ca. 4 km (Figs. 1 and 2). In contrast, the low relief and lack of major internal structures in the Bogotá Basin illustrate that this region was not uplifted by tectonic stacking within the basin, but rather passively uplifted by the principal thrusts on both fl anks of the ranges (see cross section in Fig. 1). At the latitude of the study area, these are the Guaicaramo fault system on the eastern fl ank and the Bituima, Cambao, and Honda faults in the west (Fig. 1).

A late Paleogene to Neogene foreland basin sequence to the east refl ects the uplift history of the adjacent basement highs in the Eastern Cor-dillera. This sequence overlies late Cretaceous to early Paleogene transitional, to shallow marine facies and is characterized by pronounced facies changes in its lower part (Parra et al., 2005a). The upper section of the sequence, ranging from early Miocene to late Miocene (Parra et al., 2006), is composed of continental strata whose depositional environment records the transition from marine to terrestrial conditions that were characterized by meandering rivers and proxi-mal alluvial fan systems (Parra et al., 2006).

TOPOGRAPHY AND FLUVIAL NETWORK

Between 3.5° and 6° N, the eastern fl ank of the range is remarkable for its topographic and climatic contrasts compared to the Sabana de Bogotá and the western branch of the East-ern Cordillera. For instance, the eastern fl ank has the highest mean elevations and a jagged topography with deep river canyons that have dissected the basement rocks (Figs. 1–3). In addition, the eastern fl ank constitutes an effec-tive orographic barrier that enhances precipita-tion on this side of the mountain belt and leaves leeward sectors dry (Fig. 1). For example, while the eastern fl ank receives ~5000 mm/yr of rainfall, the Sabana de Bogotá and the west-ern fl ank receive ~1000 mm/yr and 2000 mm/yr, respectively (Fig. 1). Accordingly, the most important fl uvial discharge in the Eastern Cor-

dillera is concentrated on the east and feeds tributaries of the Orinoco River (Fig. 1). Rivers on the eastern fl ank are shorter than rivers in the Sabana de Bogotá and the western fl ank, chan-nel slopes are steeper and are either perpendicu-lar or moderately oblique to the structural grain (Figs. 1–3). Along the western fl ank, the princi-pal rivers have only succeeded to crosscut major structures in a few locations and after having fl owed a long distance parallel to subparallel to them (Fig. 1).

METHODS

Structural and Stratigraphic Data

We have compiled a comprehensive surface stratigraphic and structural database for the study area (e.g., Julivert, 1963; Guerrero and Sarmiento, 1996; Mora and Kammer, 1999; Mora and Parra, 2004; Parra et al., 2005b; Parra et al., 2006), including all available oil indus-try seismic and well-data sets provided by the Agencia Nacional de Hidrocarburos (ANH) of Colombia. We use these data to retrodeform and estimate amounts of shortening in two cross sections across the eastern fl ank of the Eastern Cordillera, as well as two regional cross sec-tions across the entire orogen. Shortening esti-mates were obtained using conventional line-length balancing techniques (e.g., Dahlstrom, 1969). Depths to detachment were estimated using seismic refl ection profi les and geometri-cal approaches, such as area-balancing methods (e.g., Mitra and Namson, 1989) and basement-uplift balancing methods, similar to techniques applied in the Rocky Mountains (e.g., Erslev, 1986; see Fig. 4 and Appendix for details).

Vitrinite Refl ectance Analysis

Macerals (like vitrinite) are major compo-nents of coal or organic matter in sedimentary rocks. Macerals increase their refl ectance (i.e., refl ectivity as measured under the microscope) with increasing temperatures (e.g., Bustin et al., 1990). Vitrinite refl ectance (R

o) is the most

widespread measurement of the maturity of organic matter as related to heating, commonly associated with burial. Thus, vitrinite-refl ec-tance analysis can provide the maximum paleo-temperatures reached by sedimentary rocks dur-ing burial. Normal vitrinite refl ectance values in organic matter (R

o) typically range between 0.2

at maximum paleotemperatures of <30 to 6 °C for rocks that experienced maximum paleotem-peratures >250 °C. Intermediate values can be converted to paleotemperature estimates using an adequate kinetic model (e.g., Barker and Pawlewicz, 1994). In our study, vitrinite mea-

surements were made at 100 points in polished whole-rock blocks with a Leica MPV3 photo-microscope. Polarized refl ected light was used to measure the mean random percentage refl ec-tance in oil. Most of the samples were from coals. Our vitrinite data from the lowermost Cretaceous rocks (Berriasian) were compared with vitrinite data from equivalent units avail-able in the national oil industry (Chevron, 1997, personal commun.; Ecopetrol, 2005, personal commun.). The two data sets are consistent. In a following step, the vitrinite data were used to estimate the maximum paleotemperatures and to derive amounts of removed overburden. Samples from units, ranging from Barremian to Oligocene age, were also used to assess the regional paleothermal history and amounts of removed overburden. Geothermal gradients were also available from ~678 wells, mostly from the foreland areas.

Apatite Fission-Track Analysis

Fission-track dating relies on crystal dam-age produced by the spontaneous fi ssion of 238U in apatite or zircon (e.g., Wagner and Van den Haute, 1992). The number of spontaneous tracks is proportional to the age and the ura-nium content of the crystal (e.g., Green et al., 1989a). A typical total annealing temperature for apatite is ~120°; above this temperature on geological timescales, tracks are removed com-pletely (annealed). This temperature is variable, depending principally on the F/Cl ratio of the apatites and the cooling rate (Gallagher et al., 1998; Carlson et al., 1999; Ketcham et al., 1999). Apatite fi ssion tracks (AFT) remain stable only below 60 °C (Gleadow et al., 1986). At tem-peratures in the partial annealing zone (PAZ), between 60 °C and the total annealing tem-perature, tracks are partially or totally erased. If the apatites cooled suffi ciently fast through the PAZ, the calculated age would coincide with the time when the apatite-bearing rocks cooled through the closure temperature. However, if a sample spends a signifi cant time in the PAZ, the age and confi ned track-length distribution will be modifi ed. In regions undergoing contraction, erosional denudation following thrusting is one of the primary mechanisms of cooling (e.g., Coutand et al., 2006).

In this study, we assess cooling driven by erosional denudation using AFT (e.g., Sobel and Strecker, 2003) in samples that cooled from temperatures higher than the total anneal-ing temperatures, as determined by vitrinite refl ectance data. Therefore, we only obtain information about the last cooling event (e.g., Green et al., 1989a; Green et al., 1989b). Samples were collected along two elevation

Mora et al.

934 Geological Society of America Bulletin, July/August 2008

Distance (km) Distance (km)300 0 0 300

Elevation (m)

W E

Guayuriba

Guatiquía

HumeaNegro

Sumapaz

Bogotá

B

W E

Minimum

Mean

MaximumSabana de Bogotá

Eastern FlankWestern Flank

40 80 120 160

Distance (km)

0

0

2000

4000

Ele

vatio

n (m

)

A

Figure 3. (A) Swath topographic profi le across the central segment of the Eastern Cordillera showing higher minimum, maximum, and mean elevations in the eastern fl ank compared to the western fl ank. (B) Topographic profi les along main rivers of the western fl ank (Bogotá, Negro and Sumapaz Rivers) and eastern fl ank (Guatiquía, Guayuriba, and Humea rivers) of the Eastern Cordillera. The Bogotá River can be identi-fi ed in Figure 1A as the river having a discharge of 40 m3/sec; conversely, the Negro River has been marked with a discharge of 125 m3/sec. The Sumapaz River is located south of the Bogotá Basin. At the eastern fl ank, the Guayuriba in Figure 1A has a discharge of 159 m3/sec, the Guatiquía 89 m3/sec, and the Humea 129 m3/sec. Rivers along the eastern side of the chain are shorter and have higher slopes.

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 935

profi les to decipher age and elevation trends and derive apparent exhumation rates (e.g., Sobel and Strecker, 2003). Thermochrono-logic data alone do not provide rock uplift or paleoelevation. We therefore compare surface uplift history deduced from published paleofl o-ristic data with the denudation history obtained from apatite fi ssion-track analysis.

In the following sections we present 17 new AFT ages from the eastern fl ank of the East-ern Cordillera and compare them to published AFT data from the western fl ank to examine the Late Cenozoic exhumation histories of the two regions. Data and methodological details are presented in Tables 1 and 2.

STRUCTURAL STYLES AND TOTAL SHORTENING ESTIMATES FROM THE EASTERN CORDILLERA

A correct assessment of the structural styles of the Eastern Cordillera is required to support shortening estimates. In our cross sections we assume that the principal marginal thrusts in the Eastern Cordillera fl atten at depth (Fig. 4) to transfer deformation throughout the middle crust from the west (e.g., the Central Cordil-lera), as has been observed in other inverted intra-plate basins (e.g., Chapman, 1989; Rob-erts, 1989; Deeks and Thomas, 1995; Sinclair, 1995) and other inversion orogens (Beauchamp

et al., 1996). Based on this consideration, we deduced the depth to a mid-crustal detachment using a similar method as the applied by Col-leta et al. (1990) and Cortés et al. (2006) (see Appendix and Fig. 4). In our cross sections, we modeled the Eastern Cordillera as a biver-gent orogen with two main detachments. The assumption that the eastern detachment is the principal structure and the western, east-dip-ping detachment is a backthrust (Figs. 1 and 4) is supported by a major change in the character of the basement in the eastern Llanos foreland. The eastern Guaicaramo fault system (Fig. 1) separates a phyllitic basement domain reported ubiquitously in the Eastern Cordillera from the

0

12

24

36 km

0

12

24

36

12

24

36 km

0

12

24

36 km

0

12

24

36

0

12

24

36

Retrodeformed state

Deformed state

Exess area: 1340 km

Total shortening = 58 km

0

Cam

bao

Th

rust

Bit

uim

a Th

rust

Len

gu

pá

Fau

lt

Gu

aica

ram

o T

hru

st

A

B

Figure 4. (A) Cross section A–A′ (location in Fig. 1) and digital elevation model of the Eastern Cordillera at the latitude of the Bogotá Basin (Sabana de Bogotá). The inferred subsurface geometry implies that the Bogotá basin is a piggy-back basin uplifted on top of the western marginal thrust faults like the Cambao and Bituima faults. (B) Retrodeformation of cross section in Figure 4A, and deformed state cross section. The comparison of both allows calculation of a total shortening of 58 km in the Eastern Cordillera. The area marked on the deformed state cross section is the excess area used (location in Fig. 1) to estimate depths to detachment (see Appendix for details).

Mora et al.

936 Geological Society of America Bulletin, July/August 2008

migmatite and igneous basement rocks of the Guyana shield.

In a subsequent step, additional geometric considerations allow us to model the Sabana de Bogotá as an area passively transported on top of the gently dipping back limb of the basement thrusts of the western marginal thrust belt (Fig. 4). Accordingly, we modeled the boundary of the westward-dipping panel east of the Sabana de Bogotá as an active hinge that coincides at depth with a bend in the main west-dipping detachment (Fig. 4). Therefore, in our cross section A–A′, surface uplift in the Sabana de Bogotá is not necessarily depen-dent on movement along the eastern Guaicar-amo fault system (see Appendix and Fig. 4). The validity of this assumption is underscored by observations from the Magdalena thrust system. Here, where the western marginal thrust system (detaching at ca. 20 km depth) is superseded by the Magdalena thrust system with a fl at detachment at <10 km toward the south (compare cross sections in Figs. 2B and 4), the Eastern Cordillera becomes an asym-metric block, tilted to the west by the eastern marginal thrusts and still detaching at more than 20 km.

Inversion tectonics plays a fundamental role in the present-day structure of the eastern fl ank of the Eastern Cordillera (Mora et al., 2006). We summarize the structural style of this area from west to east. East of the Sabana de Bogotá, several basement highs are bounded by Lower Cretaceous normal faults, such as the San Juanito and Naranjal faults (Figs. 5–7). To the east, the Lower Cretaceous Servitá normal fault was reactivated during the Andean orogeny as a major basement thrust, in contrast to the San

Juanito and Naranjal faults, which have under-gone only minor contractional reactivation. Therefore, the latter two faults are passively uplifted by the Servitá fault. The Farallones anticline (Fig. 7) is a typical inversion structure (Mora et al., 2006) in the hanging wall of the Servitá fault and east of the San Juanito fault. East of the Servitá fault lies the Guaicaramo thrust (Figs. 1 and 2), inferred to have formed in response to the Servitá fault hanging-wall block pushing from behind (Mora et al., 2006). The Guaicaramo and Servitá faults are an integral part of the Guaicaramo fault system, which converges into the basal west-dipping main detachment (Fig. 4).

The Anaconda exploratory well and seismic lines in its vicinity provide good evidence that the main basement thrusts on the eastern fl ank (e.g., Mirador shortcut) dip between 20° and 30° (e.g., Narr and Perez, 1993; Mora et al., 2006; and Fig. 5). Using these fault geometries, we obtained consistent shortening estimates of ~30 km in three cross sections along the east-ern fl ank of the Eastern Cordillera (Figs. 4, 6B, and 7; see Appendix for details). Similar low-angle geometries are supported by seismic lines over the frontal faults along the western marginal thrust belt (e.g., Cambao and Honda thrusts). Relatively gently dipping fault angles demonstrate that vertical uplift estimates can only represent a minor component of much larger amounts of slip along the fault planes. These, and all other previously mentioned considerations, allow us to calculate a total of ~8 km shortening for the northern cross section (A–A′) along the Eastern Cordillera (Fig. 4; see Appendix for details). Overall, ~30 km of shortening along the eastern fl ank and ~23 km

along the western fl ank contrast with ~5 km of shortening in the Bogotá basin (Fig. 4).

NEW APATITE FISSION-TRACK DATA FROM THE EASTERN FLANK OF THE EASTERN CORDILLERA

The Guayuriba and Guatiquía river profi les (Figs. 2, 6, and 7) were collected in deeply incised valleys where the shallowly dipping lower Cre-taceous sedimentary sequence unconformably overlies the basement near the drainage divide. Therefore, samples from lower elevations in the river valleys should correspond to deeper crustal levels. Samples were collected in lower Creta-ceous units. Additional material was collected from basement rocks below the basal Cretaceous unconformity and at lower elevations. The set of samples analyzed represents the youngest ages reported to date in the Eastern Cordillera.

TABLE 1. APATITE FISSION-TRACK DATA Rho-S Rho-I P(x²) Rho-D Sample Latitude

(°N) Longitude

(°W) Lithology Elevation

(m) XI

(104 tracks/cm2)

NS (106

tracks/cm2)

NI

(%) ( 105 tracks/cm2)

ND Age (Ma)

U (ppm)

1 4.1614 73.6702 Meta-tuff 846 30 4.303 118 3.403 9334 76.1 1.202 4926 2.7 ± 0.3 35.4 2 4.1624 73.6690 Meta-tuff 831 40 3.378 75 2.806 6228 15.7 1.198 4926 2.6 ± 0.3 29.3 3 4.1986 73.6447 Conglomerate 567 30 2.716 56 2.277 4695 41.2 1.380 5748 3.0 ± 0.4 20.6 4 4.1962 73.7124 Sandstone 921 10 1.628 2 1.205 148 86.8 1.195 4850 2.9 ± 2.1 12.6 5 4.2029 73.7951 Meta-tuff 1200 30 3.342 13 4.530 1762 97.8 1.193 4926 1.6 ± 0.4 47.5 6 4.2674 73.8003 Meta-tuff 2215 30 3.120 16 3.598 1845 57.9 1.460 5089 2.3 ± 0.6 30.8 7 4.2933 73.7828 Meta-tuff 2963 30 4.158 24 3.640 2101 76.6 1.239 5089 2.6 ± 0.5 36.7 8 4.2925 73.7834 Meta-tuff 2954 20 3.047 6 2.605 513 86.0 1.214 5089 2.6 ± 1.1 26.8 9 4.2245 73.8941 Meta-tuff 1556 40 4.518 54 3.603 4307 74.2 1.217 4926 2.8 ± 0.4 37.0 10 4.3705 73.9008 Sandstone 1458 29 4.953 31 2.850 1784 49.7 1.213 4926 3.8 ± 0.7 29.4 11 4.4885 73.7312 Meta-tuff 3662 20 4.215 12 3.534 1006 6.2 1.193 4850 2.7 ± 0.7 37.0 12 4.4599 73.7083 Meta-tuff 3041 30 4.918 14 4.593 1307 79.0 1.263 5089 2.4 ± 0.7 45.7 13 4.4938 73.6809 Meta-tuff 2699 21 4.743 48 4.417 4470 35.6 1.432 5748 2.8 ± 0.4 38.6 14 4.4716 73.6931 Meta-tuff 2137 20 2.164 15 2.917 2022 0.0 1.388 5748 2.3 ± 0.8 26.3 15 4.4304 73.6802 Sandstone 1644 27 1.704 10 3.564 2092 54.5 1.419 5748 1.2 ± 0.4 31.4 16 4.4949 73.6570 Sandstone 2018 30 9.478 9 3.154 2995 97.7 1.406 5748 0.8 ± 0.3 28.0 17 4.4813 73.5575 Granite 2078 30 9.681 22 2.509 5701 86.6 1.347 5557 0.9 ± 0.2 23.3 Note: Apatites used for apatite fission-track (AFT) analysis were separated from whole rocks following conventional heavy liquid and magnetic methods. The separates were mounted in thin sections and polished to expose the internal surfaces of the grains and then etched for 20 s in 5.5 N nitric acid at 21 °C, to reveal fossil tracks. Samples were irradiated at Oregon State University TRIGA (Training, Research, Isotopes, General Atomics) reactor, and neutron fluxes were monitored withthe CN5 standard glass. Induced tracks in muscovite external detectors were revealed by etching at 21 °C in 40% HF acid. Fission tracks were counted with a Leica DMRM microscope with drawing tube located above a digitizing tablet and a Kinetec computer-controlled stage driven by the FTstage program (Dumitru, 1993). Fission- track ages were determined by measuring the mean track density and U concentration of the sample using the external detector method (Gleadow, 1981). Ages were calculated using the zeta calibration method (Hurford and Green, 1983). Samples were counted by Andrés Mora using a zeta value of 362 ± 7. Xl—number of crystalls counted; NS—spontaneous tracks; NI—induced tracks; ND—total number of tracks counted for determining track density.

TABLE 2. DPAR DATA Sample Sample

(field code)Dpar n Standard

deviationCorrected

Dpar 1 BV-277 3.222 82 0.244 2.861 2 BV-279 2.789 36 0.222 2.477 3 BV-427 2.940 35 0.402 2.610 5 BV-423 2.371 7 0.201 2.105 6 BV-192 2.459 9 0.322 2.184 7 BV-195 2.300 14 0.303 2.042 8 BV-196 2.129 5 0.168 1.890 9 BV-90 2.933 55 0.259 2.604 10 BV-151 2.567 12 0.213 2.279 11 FT-1 2.621 6 0.369 2.327 12 FT-6 2.269 9 0.293 2.015 13 SJ-1 2.621 31 0.133 2.327 14 SJ-5 2.769 7 0.120 2.459 15 SJ-2a 2.691 5 0.193 2.389 16 SJ-3 2.710 2 0.279 2.406 17 HUMEA 2.388 19 0.164 2.120 Note: Dpar values were corrected according to RayDonelick values (Sobel and Seaward, 2006).

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 937

Along the Guatiquía River profi le (Figs. 6 and 7 and Table 1), we analyzed six samples collected at elevations ranging from 1900 to 3600 m. The samples have no clear age and elevation trend. However, ages are younger in the hanging-wall block of the San Juanito fault compared to the footwall. East of this structure, all ages are younger than 1.6 Ma (maximum 1.2 ± 0.4 and minimum of 0.8 ± 0.2 Ma) and in the footwall to the west, all of them are older than 1.5 Ma (maximum age 2.8 ± 0.4 and minimum of 2.3 ± 0.8 Ma). Dpar has been defi ned by Donelick et al. (2005) as the arithmetic mean, fi ssion-track etch fi gure diameter parallel to the crystallographic c-axis, and is positively corre-

lated with chlorine content in apatites. Our Dpar measurements show similar kinetic character-istics for all of the samples along this profi le; these samples are close to the Fish Canyon apa-tite composition (Ketcham et al., 1999). There-fore, we assume that age differences are not due to compositional differences (Table 2).

In the Guayuriba River profi le (Fig. 6), located south of the Guatiquía River profi le and along strike, four basement samples were collected between 1200 and 2963 m (sam-ples 5–8, Figs. 2 and 6). Ages increase with elevation from 1.6 ± 0.4 Ma at 1200 m to 2.6 ± 0.5 Ma at 2954 m (Fig. 6). The age and eleva-tion plot yields an apparent denudation rate of

1.7 ± 1 mm/year (Fig. 6). In addition to these samples, we collected material from the low-ermost Cretaceous units (samples 3 and 10) or basement units located very close to the basal Cretaceous unconformity in different structural positions (samples 1, 2, 4, and 9). The ages from those samples have similar values between 2.6 ± 0.5 and 3.0 ± 0.4 Ma. These cooling ages are independent of the tectonic block sampled. Fur-thermore, ages from samples of nearly the same stratigraphic horizon are very similar in the dif-ferent blocks of the major structures. Basement samples from lower elevations in the Guayuriba River valley (e.g., samples 5 and 6, Figs. 2 and 6) have younger cooling ages than samples

0

1

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3

Two

-way

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ime

(s)

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Two

-way

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velt

ime

(s)

4 km

Top Cretaceous

Paleozoic Basement

Servitá

Fault

Top Basement

Mirador Thrust

Top Basament

Tertiary

Anaconda-1 well

Anaconda-1 well

Figure 5. Interpreted migrated seismic line near the Ana-conda-1 well. It shows clearly the low-angle basement sheet of the frontal Mirador thrust. See location in Figure 2.

Mora et al.

938 Geological Society of America Bulletin, July/August 2008

10

9123

48

765

11

1615141312

Mir

ado

rth

rust

S an

Juan

ito

Fau

lt

Guayuriba River Profile

Guatiquíia River ProfileA

Lower Cretaceous units

Upper Paleozoic units

Andean metamorphic basement

Neogene molasse deposits

Tertiary (Paleogene-non-molassic Neogene) units

Upper Cretaceous units

2. 2.7 ± 0.3 Ma

3. 3.0 ± 0.4 Ma4. 2.9 ± 2.1 Ma

8. 2.6 ± 1.1 Ma

7. 2.6 ± 0.6 Ma

6. 2.3 ± 0.6 Ma

5. 1.6 ± 0.4 Ma

9. 2.8 ± 0.4 Ma

10. 3.8 ± 0.7 Ma

11. 2.6 ± 0.7 Ma15. 1.2 ± 0.4 Ma

14. 2.3 ± 0.8 Ma

13. 2.8 ± 0.4 Ma

12. 2.4 ± 0.7 Ma16. 0.8 ± 0.3 Ma

D D´W E

C C´W E

2

0

4 km

2

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4 km

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-6 km

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-2

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2

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-4

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4

1. 2.6 ± 0.3 Ma

1. 2.6 ± 0.3 Ma Apatite fission-track sample and age1 AFT sample location (projected)

Serv

ita

fau

lt

Nar

anja

lfa

ult

B

ER = 1.74 ± 1.0 mm/yrr2 = 0.94

Figure 6. (A) Cross section D–D′ showing the projected location of apatite fi ssion-track (AFT) samples along the Guatiquía River profi le. (B) Cross section C–C′ along the Guayuriba River profi le showing the projected samples and an age elevation plot. Samples 5–8 constitute the vertical profi le sampled whose ages were used in the age elevation plot above. Calculated exhumation rates in this profi le are 1.7 ± 1.0 mm/yr, based on error weighted linear regression using Isoplot (Ludwig, 2003) as described by Blythe et al. (2007).

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 939

collected in Cretaceous or basement rocks at higher elevations in this transect. The youngest sample stratigraphically comes from a horizon ~1 km above the base of the Cretaceous strata and has the oldest cooling age (3.8 ± 0.7 Ma). It appears that cooling-age trends are conditioned by elevation and stratigraphic position, sug-gesting that younger rock units were exhumed prior to older units, as expected in the simplest structural setting. Nonetheless, structures do not appear to have conditioned cooling-age patterns in this profi le. However, Dpar data from sam-ples east of the Servitá fault have larger values, closer to the Fish Canyon apatite composition, whereas Dpar values to the west are closer to the Durango composition (Table 2). Therefore, the eastern samples likely have a closure tempera-ture ~20 °C higher than the western samples (Ketcham et al., 1999).

INTERPRETATION OF THE NEW AFT DATA

To constrain the amount of overburden removed during cooling, estimates of apatite-closure temperature and past geothermal gradi-ents are critical variables. Ketcham et al. (1999)

estimated closure temperatures for Durango and Fish Canyon apatites. Our samples in the Guayuriba River profi le west of the Servitá fault (Figs. 2 and 6B) have Dpar values close to that for Durango apatite (Table 2); therefore, we assume closure temperatures between 107 and 126 °C for cooling rates between 10 and 100 °C/Ma, respectively. In contrast, Dpar values close to those of Fish Canyon apatite in the Guatiquía River transect and in the Guayuriba River pro-fi le east of the Servitá Fault suggest closure tem-peratures between 128 and 148 °C for cooling rates between 10 and 100 °C/Ma.

Based on maximum paleotemperatures esti-mated from vitrinite refl ectance data, compared with the thickness of the sedimentary units (Figs. 8 and 9; Table 3), we calculate a Cam-panian to Oligocene geothermal gradient of ~20 °C/km. This value is similar to the pres-ent average geothermal gradient obtained from wells in the eastern foreland regions (Bachu et al., 1995). However, Mancktelow and Grase-mann (1997) demonstrated that in an active tec-tonic regime with high exhumation rates, normal geothermal gradients may increase by 60% due to advection. Therefore, we use an approximate value of 30 ± 2 °C/km for the geothermal gradi-

ent of areas undergoing denudation at high rates along the eastern fl ank of the orogen.

A compilation of unpublished data from Eco-petrol and new vitrinite refl ectance data shows R

o (vitrinite refl ectance; see our new data in

Table 3) values greater than 4% for the Lower Cretaceous rocks that hosted our AFT samples, implying maximum paleotemperatures above 250 °C (Barker and Pawlewicz, 1994; Table 3 and Figs. 2, 6, and 7). Clearly, AFT data from the same units represent cooling of completely reset samples, and therefore only the record of the last episode of the cooling event.

Given the assumptions concerning closure temperatures and geothermal gradient, our data suggest ~3–5 km of overburden exhumation during the past 3 Ma (Figs. 10 and 11), imply-ing cooling rates of 1.0–1.6 mm/yr. These rates are toward the lower range of that obtained from the elevation profi le (1.7 ± 1.0 mm/yr; Fig. 6). Mancktelow and Grasemann (1997) argue that apparent cooling rates on the order of 2 ± 1 mm/yr derived from elevation profi les can be overestimated and even doubled in set-tings with high relief expressed over 10–20 km wavelengths due to the infl uence of nonplanar isotherms. Therefore, we rely more on the rates

15. 1.2 ± 0.4 Ma

16. 0.8 ± 0.3 Ma

17. 0.9 ± 0.2 Ma

San

Juan

ito

Fau

lt

Serv

itá

Fau

lt

Farallones Anticline

Gu

aica

ram

oTh

rust

Mir

ado

rTh

rust

E E´NW SE

0.27% 0.60% 0.77% 1.38%4.45%

0

-8 km

-4

44.16%15 16 17

Tertiary (Paleogene-non-molassic Neogene) units

1. 2.6 ± 0.3 Ma Apatite fission track sample and age1 AFT sample location (projected)

0.60% Vitrinite relflectance value

Lower Cretaceous units

Upper Paleozoic units

Andean metamorphic basement

Neogene molasse deposits

Upper Cretaceous units

Figure 7. (A) Regional cross section E–E′ across the eastern fl ank of the Eastern Cordillera with projected average values of vitrinite refl ectance measurements and projected additional apatite fi ssion-track (AFT) ages along the Guatiquía River profi le. See Figure 2B for cross-section location.

Mora et al.

940 Geological Society of America Bulletin, July/August 2008

Figure 8. Measured strati-graphic thicknesses in two different structural domains across the study area. To the left, the sequence west of the Servitá fault and in the Bogotá basin shows that the youngest widespread (pre-deformation) formation is of late Eocene age, whereas Oligocene and Mio-cene rocks are absent or have a reduced thickness. To the right, the sequence east of the Servitá fault with a condensed Paleo-gene thickness but an increased Neogene thickness.

kms

11USME Late Maastrichtian

to Eocene thickness(after Julivert, 1963;Mora and Kammer, 1999)

Late Eocene toMiocene thickness(Mora and Parra, 2004;Parra and Mora, 2005)

Cretaceous EarlyEocene thickness(Mora et. al, 2006)

Late Cretaceous(Mora and Kammer, 1999)

Lower Cretaceousthickness(Mora and Kammer, 1999;Mora et. al, 2006)

REGADERA

GUAYABO

LEON

CARBONERA

GUADALUPE

MACANAL

BUENAVISTA

CHIPAQUE

UNE

CUERVOSMIRADOR

BOGOTA

GUADUAS

GUADALUPE

CHIPAQUE

UNE

FÓMEQUE

LAS JUNTAS

MA

CA

NA

L-B

UE

NA

VIS

TA

CACHO

10

9

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1

0

BOGOTA BASIN MIRADORSHORTCUTHANGING WALL

SERVITA FAULTHANGING WALL

0

50

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300

0.0 2.0 4.0 6.0 8.0 10.0

Amount of overburden (km)

Max

imu

m P

aleo

tem

per

atu

re (°

C)

Campanian-Oligocene (18.3 °C/km)Berri

asian-C

ampanian (44.0 °C

/km)

1

2

3

4

5

6

Figure 9. Inferred maximum paleotemperatures from vitrinite samples in Table 3 (using Barker and Pawlewicz [1994] kinetic model) versus theoretical amounts of overburden on top of them, assuming that all the measured amounts of burial cov-ering the different units (Fig. 8 and Table 3) in areas where those rocks are preserved equals the eroded thickness in areas where they have been eroded and where we performed vitrinite analysis. The thick lines in the plot show inferred Campanian to Oligocene geothermal gradients (using samples from units of those ages). The same procedure was followed to calculate Ber-riasian to Campanian geothermal gradients. The two differ-ent trends roughly coincide fi rst with the Berriasian to Aptian synrift phase, thus with high geothermal gradients (Sarmiento-Rojas, 2001) and second with the post-Albian post-rift phase (Sarmiento-Rojas, 2001).

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 941

of 1.0–1.6 mm/yr derived from estimates of overburden removal.

The total thickness of the Meso-Cenozoic sedimentary sequence in areas adjacent to our sampling locations is between 7.5 and 10 km (Julivert, 1963; Mora and Kammer, 1999; Mora et al., 2006; Fig. 8). Our vitrinite-refl ectance data suggest that a comparable thickness of overburden once covered the lower Cretaceous units in the AFT sample locations prior to the onset of denudation (Table 3 and Figs. 8 and 9). Sedimentological and basin analysis show that denudation of these ranges has been active since

middle Oligocene time (Parra et al., 2005a; Parra et al., 2007). In addition, Parra et al. (2007) cal-culated a total of 3 km of unroofi ng between that time and the late Miocene. Our thermochrono-logical data documents that 3–5 km of rock were exhumed from the studied section over the past 3 Ma. Comparing these two data sets reveals that exhumation rates must have accelerated at some point during the Pliocene (Fig. 10).

The exhumation history documented in our study involves rocks eroded from above the Servitá fault and the frontal Mirador shortcut (Figs. 5, 6, 7, and 11). The 4–5 km of eroded

material on top of the Mirador shortcut during the past 3 Ma roughly equal its total vertical throw, calculated by comparing the position of the Lower Cretaceous hanging-wall and foot-wall cutoffs (Figs. 5, 6, and 11). Assuming that the Pleistocene surface elevation at the posi-tion of the most distal thrust sheets, such as the Mirador shortcut, was roughly the same as the present-day elevation of ~1000 m, the Mira-dor shortcut was probably initiated during the past 4 Ma (Figs. 6 and 11). Data from seismic refl ection profi les and the Anaconda-1 explor-atory well reveal that the Mirador shortcut dips ~20–30° W (Fig. 5). Using this information, we estimate that ~13–15 km of shortening has occurred during the past 3 Ma. This is ~50% of the total shortening across the eastern foothills. These values are based in incrementally bal-anced retrodeformations of the cross sections in Figures 6 and 7 (see incremental restorations in Fig. 11). Compared with total shortening estimates in the Eastern Cordillera (Fig. 4), the calculated Plio-Pleistocene shortening along the eastern fl ank represents ~25% of the total accumulated Cenozoic shortening of the entire orogen. The similar AFT ages from the hanging wall of the Mirador shortcut and the hanging-wall block of the Servitá fault demonstrate that both structures formed in a forward-breaking or synchronous sequence of thrusts, with the Ser-vitá hanging-wall block pushing from behind (e.g., Mora et al., 2006). The ongoing tectonic activity of these structures is underscored by numerous fault scarps and tilted river terraces (Robertson, 1989).

In contrast, in the Guatiquía River vertical profi le, the only marked jump in AFT ages coin-cides with a fault (see Figs. 6 and 7, samples 11–16). Our Dpar data show that apatite com-position is very similar on both sides of the structure. Therefore, either the western foot-wall block of the San Juanito fault apparently commenced exhumation ca. 1 Ma prior to the hanging wall or the amount of overburden was greater to the east, so that the ages now found at the surface are younger. The latter case could be expected, given the character of the San Juanito fault as an inherited extensional structure.

Interestingly, east of the San Juanito fault the Lower Cretaceous unconformity in the hinge zone of the Farallones anticline lies at the same elevation as west of the San Juanito fault (Figs. 2 and 7). Younger ages east of this fault may indi-cate that the Lower Cretaceous unconformity in the Farallones anticline may have reached similar elevations later compared to the equiva-lent horizon in the footwall of the San Juanito fault. Thus, at least part of the fold amplifi ca-tion process in the Farallones anticline may be subsequent to movement along the San Juanito

1

2

3

4

5

6

7

8

51015202530

Oligocene MioceneEarly Late Early Middle Late

0.1 mm/yr

0.2 mm

/yr

0.5

mm

/yr

1mm

/yr

Calculated amount of removed materialin the Miocene (Parra et al. 2006)

Calculated amount of removedmaterial in the past 3 Ma (this work)

Timing of removal from source areas (m.y.)

Tota

l am

ou

nt

of o

verb

urd

en (k

m)

TABLE 3. VITRINITE REFLECTANCE DATA Sample Age Overburden

(m) Ro mean

(%) n Maximum

paleotemperature (°C)

Ro range (%)

1 Late Eocene ~300 0.27 100 30 ± 15 0.18–0.37 2 Turonian-Santonian 4000 0.77 100 115 ± 17 0.62–0.94 3 Cenomanian 4800 1.38 100 162 ± 25 0.95–1.69 4 Berriasian 7500–10,000 4.45 69 256 ± 25 3.20–5.68 5 Turonian-Santonian 3500 0.49 63 78 ± 9 0.43–0.54 6 Barremian? 7000 3.51 100 237 ± 20 3.00–4.43 7 Barremian? 7000 2.46 100 208 ± 20 2.00–3.23 8 Berriasian ~8500 4.05 83 249 ± 8 3.7–4.5

Figure 10. Compilation of inferred amount of eroded material versus time in the study area as deduced from our data and data from Parra et al. (2007).

Mora et al.

942 Geological Society of America Bulletin, July/August 2008

0

-8

-16

-24

-32

- 8

-16

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-32

Shortening: 2.3 km

Middle Miocene

1000 m

0

-8

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A

Maastrichtian-Paleocene retrodeformed state

0 km

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Total Shortening: 30.0 km

Present

0

-8

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-8

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Shortening: 14.1 km

ca. 3 Ma

4000 m

0 km

10 km H=V

GTSJF STF

MSCFA

E E

Figure 11. (A) Incremental retrodeformation of cross section E–E′ (location in Fig. 2) using the constraints from paleoaltimetry (from palynofl ora), exhumation (apatite fi ssion-track [AFT]) and total burial from Ro data. On the other hand, many published data (Julivert, 1963; Helmens, 1990; Kammer, 2003; Gómez et al., 2005) constitute pieces of evidence showing that most of the folding in the western part of the section (Bogotá area) is older than Pliocene at least. Shortening at each stage is calculated with line-length balancing. The methods to deduce fault geometry at depth where there is no seismic control are explained in the Appendix. (B) Incremental retrodeformation of cross section C–C′ (location in Fig. 2), using similar constraints as in the previous one. The frontal shortcut geometry is defi ned with seismic profi les and the Anaconda-1 well. Abbreviations as in Figure 2B. (Continued on following page.)

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 943

fault. Drainage patterns in this area may sup-port this assessment. For example, the Gua-tiquía River fl ows east from the footwall block of the east-dipping San Juanito normal fault, cutting through basement lithologies (Fig. 2). Farther east, the river reaches the hanging-wall block and is defl ected almost 90° to the south near the trace of the San Juanito fault, cutting weak black shales. If an imaginary straight line parallel to the trace of the Guatiquía River in the footwall block of the San Juanito fault were extended ESE, the headwaters of the Humea River would be reached farther east (Fig. 2). We hypothesize that the Guatiquía River was formerly linked to the Humea fl owing directly to the east. In such a scenario incision through basement lithologies during growth of the Far-allones anticline may have caused the Humea to be beheaded and the Guatiquia River to be

defl ected or even captured by the present-day lower Guatiquía River (Fig. 2). After such a capture, the defl ected Guatiquía River may have begun to erode the less resistant litholo-gies of the synrift shales adjacent to the San Juanito fault in the downdip sectors of the tilted hanging wall. This hypothetical geomorphic scenario requires earlier exhumation of the San Juanito footwall block and later folding of the Farallones Anticline, an assessment which is supported by our AFT data.

A remarkable feature of the AFT data is the similarity of young ages of two samples east of the San Juanito fault. The fi rst sample is from a Lower Cretaceous sandstone with an AFT age of 0.8 ± 0.2 (sample 16). The second is from a pre-Silurian granite located to the east (Fig. 7), with an AFT age of 0.9 ± 0.2 Ma (sample 17). The granite underlies the Paleozoic sequence

(Segovia, 1965). Our cross sections show that today the granite sample lies stratigraphically 3 km below the Lower Cretaceous sample (Figs. 7 and 11). Similar ages in these samples that underwent very different amounts of exhu-mation can be explained, if the frontal Servita fault is interpreted as a listric fault that fi rst uplifted and exhumed the frontal, easternmost parts of the hanging-wall block rather than the back limb (Figs. 7 and 11A). In addition, the reconstructed amount of exhumed material in the cross sections (Fig. 11A), calculated from vitrinite-refl ectance data and measured thick-nesses, depicts progressively increasing amounts of total exhumation from the back limb to the frontal areas of the eastern fl ank. In support of our interpretation, structural models show that pronounced and progressively increasing differ-ential exhumation between the back limb of a structure and the deformation front (Fig. 11A) is expected and can be considered to be typical of reactivated listric faults (e.g., Mitra, 1993; McClay, 1995). A listric geometry for the east-ern fl ank frontal thrusts has been previously proposed based on cross-section balancing (Restrepo-Pace et al., 2004; Cortés et al., 2005; Cortés et al., 2006; Mora et al., 2006).

Samples with ages younger than 1 Ma imply removal of 3–4 km of rock in ~1 m.y. One possibility explaining this phenomenon is that such an increase in denudation may have been caused by accelerated incision. However, some additional effect due to isotherm advec-tion cannot be discarded (e.g., Mancktelow and Grasemann, 1997).

APATITE FISSION-TRACK DATA FROM THE WESTERN FLANK

Gómez (2001) and Gómez et al. (2003) ana-lyzed 15 apatite samples from different Meso-Cenozoic stratigraphic units from the Guaduas syncline and the Villeta anticlinorium at 4.5° N (Fig. 2A) to unravel the timing of shortening, exhumation, and related synorogenic sedimen-tation on the western fl ank of the Eastern Cor-dillera. These authors used apatite fi ssion-track dating, track-length distributions, and vitrinite-refl ectance analysis to assess the thermal events associated with tectonism. Their conclusions are mostly based on modeling rather than individual ages. The fi rst event they identify is cooling from maximum temperatures of around 130–100 °C between 65 and 30 Ma, associated with erosion of more than 3–4 km of sedimentary cover. A second event, bracketed between 15 and 5 Ma, indicates cooling from temperatures of ~110 °C and erosion of ~3 km of rock in the hanging wall of the main thrusts of the western fl ank. Only four of the samples (Gómez, 2001) have totally

0 km

-24

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-8

-16

-8

0 km

-24

-16

-8

0 km

-16

-8

0

-24

-16

-8

0 km

-16

-8

10 km H=V

Maastrichtian-Paleocene retrodeformed state

ca. 3 Ma

Present

0

-24

B

Shortening: 12.7 km

Total Shortening: 28.1 km

C C

STFMSC

NF FA

Figure 11 (continued). (B) Incremental retrodeformation of cross section C–C′ (location in Fig. 2), using similar constraints as in the previous one. The frontal shortcut geometry is defi ned with seismic profi les and the Anaconda-1 well. Abbreviations as in Figure 2B.

Mora et al.

944 Geological Society of America Bulletin, July/August 2008

reset ages. None of them provided ages younger than 5 Ma.

It is worth emphasizing again that our sam-ples from the eastern fl ank have younger cooling ages and refl ect a larger magnitude of late Ceno-zoic exhumation than samples from the western fl ank. Most of our samples are from structurally deeper levels and were obtained either from basal Cretaceous units or from basement rocks. The only comparable samples from the western fl ank are the two Lower Cretaceous samples of Gómez (2001). In these samples, apatites were totally reset, but the ages are substantially older than the average age of our samples. Combining both data sets reveals a pronounced asymmetry in the degree of exhumation, indicating a signifi -cantly younger onset and higher rate of exhuma-tion along the eastern, compared to the western fl ank of the orogen (Figs. 1 and 2, Table 1).

Gómez et al. (2003) also present data for a shortening event along the western fl ank that occurred between late Miocene and prior to 5 Ma. They argue that this was associated with exhumation documented in the AFT data. Gómez et al. (2003) date the deposition of the Honda Group, in the hanging wall of the more frontal Honda thrust (Fig. 1), between 12.7 and 6 Ma. This unit is tilted ~30° to the west. Over-lying undeformed volcanic-ash–bearing terraces of late Pliocene age provide a minimum age for motion along this fault (Gómez et al., 2003). Furthermore, based on detailed mapping and detailed stratigraphic work in a region 200 km along strike from Gomez study area, Restrepo-Pace et al. (2004) suggest that ~75% of the total shortening along the western fl ank of the East-ern Cordillera must have occurred before the late Miocene. Although this proportion may change southward, important pre-late Miocene shorten-ing has been documented along the entire west-ern fl ank (e.g., Cortés et al., 2005)

PALEOALTIMETRY AND PALEOFLORA

Paleoaltimetry data in the Andes are sparse and often subject to signifi cant errors and uncer-tainties (e.g., Gregory-Wodzicki, 2000; Blisniuk and Stern, 2005; Garzione et al., 2006). Paleo-fl ora data have been used in the assessment of paleotopography in Bolivia (e.g., Graham et al., 2001) and Colombia. In the northern Andes, Van der Hammen et al. (1973), Wijninga (1996), and Hooghiemstra et al. (2006) analyzed the fossil fl oras of the late Miocene to late Pliocene fl u-viatile Tilatá Formation in outcrops exposed in the Bogotá Basin above 2400 m. Using the near-est living relatives method, they correlated fossil pollen spectra from a 6–3 m.y. sedimentary sec-tion (Andriessen et al., 1993) with similar pollen spectra from modern vegetation and found a clear

trend of environmental evolution (Fig. 12). The oldest pollen assemblages (late Miocene, Salto de Tequendama sections) including Alchornea, Bombacaceae, Humiriaceae, Ilex Iriartea, Mau-ritia Malpighiaceae, Melastomataceae, and Pro-tium, indicate a tropical lowland environment. In contrast, the youngest pollen assemblages from the ca. 3 Ma Guasca section mainly com-prise taxa such as Eugenia, Hedyosmum, Myrica Weinmannia Oreopanax, characteristic of high-elevation (>2000 m) Andean forest.

Wijninga (1996) argued that the present topography of the Eastern Cordillera does not prevent pollen from the tropical lowland for-est in the Magdalena Valley from reaching the high plain of the Sabana de Bogotá. Probably a similar situation has prevailed since at least late Miocene time because pollen of all adjacent vegetation zones is represented in younger sedi-ments of the Bogotá Basin (Wijninga, 1996). However, the proportion of Andean pollen in the Pliocene record of the Bogotá Basin steadily increases in the younger sections, until reaching amounts similar to those in the present-day high elevation Andean forests. Van der Hammen et al. (1973), Wijninga (1996), and Hooghiemstra et al. (2006) proposed that the observed change in pollen spectra implies that a major temperature change must have affected the Bogotá Basin. In their interpretation, this change cannot be solely explained by the effects of late Cenozoic global cooling (Hooghiemstra et al., 2006). Rather, the pollen data are interpreted to refl ect a major change in paleoelevation between 6 and 3 Ma, from less than 1000 m to the present-day eleva-tion of ~2600 m (Fig. 12; Wijninga, 1996).

After the main phase of topographic growth, simultaneous with folding in the Bogotá Basin, the deposition of ~600 m of undeformed fl uvio-lacustrine sediments (Helmens and Van der Hammen, 1994; Hooghiemstra and Cleef, 1995; Hooghiemstra et al., 2006) from ca. 3.2 Ma illustrates that the Bogotá Basin evolved into a subsiding, isolated intermontane basin. The most recent study of the sedimentary and palynological record of these sediments was published by Torres et al. (2005) based on the Funza-2 borehole, located in the depocenter of the Bogotá Basin. The chronology was initially established through tephrochronology (Andries-sen et al., 1993), but more recently astronomical tuning to the pollen record was applied (Torres, 2006). The Funza-2 site provides one of the best terrestrial records of environmental change for the past 3.2 Ma. This data set documents climate change during the past 3 Ma, but no further sur-face uplift in the Bogotá Basin or adjacent areas can be deduced (Hooghiemstra, 1984; Hooghi-emstra and Cleef, 1995; Van’t Veer and Hooghi-emstra, 2000; Hooghiemstra et al., 2006; Tor-

res, 2006). We therefore conclude that the main phase of topographic growth in the Eastern Cor-dillera must have occurred between 6 and 3 Ma. This means that even the eastern third of the Eastern Cordillera probably remained at similar elevations as today, because there are no indica-tors in the pollen data of further surface uplift in areas adjacent to the Bogotá basin during the past 3 Ma (Torres, 2006).

DISCUSSION

Our new thermochronological data reveal a complex interaction between tectonics, cli-mate, and exhumation in the Eastern Cordillera of Colombia. We document an acceleration of exhumation rates on the eastern fl anks of this range by ca. 3 Ma, compared to the previous 30 Ma. However, compared with similar data sets from the western fl ank of the orogen, it appears that focused and accelerated denuda-tion solely characterizes the eastern deforma-tion front. The spatial pattern of exhumation has thus been asymmetric in the Eastern Cordillera from late Pliocene to present. Asymmetry is also visible at different levels in the geological evo-lution of both areas. We document that greater total amounts of shortening have occurred on the eastern versus the western fl ank. In addition, basement exposures occur in the east but are absent in the west (Fig. 1). Accelerated shorten-ing rates of ~5mm/yr (see Appendix and Figs. 4 and 11 for shortening calculations) are restricted to the east during the Pliocene. Finally, topogra-phy, precipitation patterns, and the characteris-tics of the fl uvial network follow this disparate spatiotemporal evolution. An important ques-tion, however, is whether climate and exhuma-tion have been active or passive with respect to the spatial and temporal trends in tectonic evo-lution. In the following discussion we propose a scenario involving the interaction between climate and structures to explain the geological evolution observed in this part of the Andes.

Areas with high mean elevation typically coincide with thickened crust; therefore the tim-ing of crustal thickening presumably coincides with the timing of growth of topography and the establishment of high-relief conditions (e.g., Isacks, 1988). As in many other areas of the Andes (e.g., Allmendinger et al., 1997), crustal thickening in the Eastern Cordillera was mainly accomplished through shortening, because the contribution of magmatic addition can be con-sidered to be minor and uplift related to litho-spheric delamination appears unlikely based on the overall tectono-magmatic evolution of this part of the Andean orogen (e.g., Sarmiento-Rojas, 2001). The specifi c structural style of the Eastern Cordillera (Figs. 1, 2, and 4) shows

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 945

that crustal shortening is mostly concentrated on both margins of the mountain belt. Activ-ity along the marginal thrusts raised the central plain of Bogotá, and the timing of topographic growth in the central plain can be related to active shortening along both marginal thrust systems, which set the stage for the subsequent accelerated climate-driven processes.

Any hypothesis about the infl uence of cli-mate on the temporal patterns of shortening and topographic growth, however, must take into account the paleoclimatic history prior to and during uplift. Molnar and Cane (2002) suggested that the global climate system dur-ing Mio-Pliocene time was in a permanent paleo-El Niño state. According to their predic-tions the northwestern corner of South Amer-

ica would have been much drier than today during such conditions. If this were true for the Neogene of northern South America, present-day precipitation patterns must be used with extreme caution in characterizing interactions between orography and precipitation dur-ing the Plio-Pleistocene. However, palyno-logical, macroscopic paleofl ora evidence, and stable isotope data from fossil and present-day growth bands in mollusks suggest that foreland areas east of the Eastern Cordillera were humid and characterized by similar precipitation pat-terns, at least since middle Miocene time (e.g., Lorente, 1986; Hoorn, 1994; Van der Ham-men and Hooghiemstra, 2000; Kaandorp et al., 2005; Hoorn, 2006). Therefore, we posit that the impact of global climate change on erosion

processes in the Pliocene (e.g., Molnar, 2004) was negligible in this region.

In keeping with the regional paleoclimate data, we suggest that the Eastern Cordillera at the latitude of Bogotá was at low elevation and was characterized by a tropical lowland climate during late Miocene time, as proposed by Wijninga (1996) and Hooghiemstra et al. (2006). Topography increased between 6 and 3 Ma in the Eastern Cordillera (Wijninga, 1996) as a consequence of movement along the main boundary thrusts in the eastern and western foothills. Presumably when topography reached a critical elevation, an orographic barrier was created that forced easterly moisture-bearing winds to focus precipitation along the eastern side of the orogen, as happens in the present.

Inferredpaleo

altitude(m)

2500

2000

1500

1000

500

0

1 2 3 4 5 6 16 Age (Ma)

L. Pliocene

E. Pliocene

Middle? Miocene

3.7 ± 0.5 Ma

5.3 ± 1 Ma

Funza-2

Guasca 103

Subachoque 39

Salto de Tequendama I and IIRió Frió 17

Facatativá 13

0-3 Ma (present elevation 2550 m)

Biozones

IV-VII

III

IIB

IIA

I

QUATERNARY PLIOCENE MIOCENE

Figure 12. Inferred paleoelevation from reconstructed altitudinal vegetation belts based on the characteristic pollen and paleobotanical associations in sections Salto del Tequendama-I and -II, Rio Frio-17, Subachoque-39, Facatativa-13, and Guasca-103, and sediment core Funza-2. Sections are located in the outer parts of the Bogotá Basin. Uncertainties in age control and inferred paleoaltitude are shown as arrows. Biozones I to VII refer to stages in uplift history and paleobiogeography of main (arboreal) taxa of the Eastern Cordillera (after Van der Hammen et al. [1973] and Wijninga [1996]).

Mora et al.

946 Geological Society of America Bulletin, July/August 2008

The regional precipitation map (Fig. 1) shows that fault-bounded ranges 0.7–1 km higher than the adjacent undeformed foreland, such as the Serranía de las Palomas, apparently have no pronounced effect on amount and distribution of rainfall in this environment. In other tectonically active regions, such as the Sierras Pampeanas or the Patagonian Andes (Sobel and Strecker, 2003; Blisniuk et al., 2005), and the Eastern Cordillera of Bolivia (Masek et al., 1994), higher threshold elevations between 2000 and 2500 m are needed to generate pronounced precipitation gradients. We thus infer that such conditions were proba-bly attained toward the end of the phase of relief growth between 6 and 3 Ma.

If true, focused precipitation on the eastern fl ank would have generated an effective, east-ward fl owing fl uvial system with higher dis-charge that cut deeper canyons compared to the modern rivers of the western fl ank. Accord-ingly, we suggest that some of the differences in AFT ages associated with the different fault blocks can also be linked to differential river incision, such as in the Guatiquía River pro-fi le. Thus, some of the documented cooling occurred through river incision. If such relation-ships indeed had been in place during the past 3 Ma, then climate-driven focused denudation and accelerated rates would be the expected result. Accelerated denudation rates during the past 3 Ma along the eastern side of the East-ern Cordillera would have caused unloading of the fault-bounded ranges, prompting faster movement along the main thrusts, as proposed by Hilley et al. (2005) for orogens strongly conditioned by inherited structures like the Eastern Cordillera. Support for the viability of this model comes from the comparison of tim-ing and amount of shortening along the west-ern (Gómez et al., 2003; Restrepo-Pace et al., 2004; Cortés et al., 2005; Montes et al., 2005) and eastern fl anks (Fig. 11). Crustal shortening has been concentrated on the eastern fl ank dur-ing the past 10 million years, with peak values of ~5 mm/yr during the past 3 Ma (Figs. 1, 2, 4, and 11). Therefore, peak shortening rates would have occurred subsequent to uplift of the Bogotá Basin, as supported by the pollen data (Wijninga, 1996). Thus, enhanced mass removal may have favored, and probably accel-erated motion along the shortcut faults on the eastern fl ank since mid-Pliocene time.

Such accelerated shortening, involving ~25% of a total 60 km orogenic shortening and 50% of the total shortening along the eastern fl ank, is expected to generate a signifi cant amount of lithospheric fl exure in adjacent areas that are not being uplifted. The exact thickness of cor-responding units in the adjacent Llanos foreland basin to the east has never been established, due

to poor chronological constraints. However, the Funza-2 drill hole from the Bogotá Basin records ~0.6 km of lacustrine sediments depos-ited in an area undergoing no deformation dur-ing that time (Helmens and Van der Hammen, 1994; Torres, 2006). We speculate that the onset of continuous deposition and subsidence in the Bogotá Basin at 3.2 Ma (Helmens and Van der Hammen, 1994; Torres, 2006) refl ects enhanced tectonic thickening in adjacent, actively deform-ing areas, such as the eastern foothills. The lacus-trine conditions in the Bogotá Basin prevailed until ~17,000 yr B.P. (Hooghiemstra, 1984). We infer that this was the result of internal drainage, aided by the drier conditions in the highlands, which prevented fl uvial systems from effi cient downcutting and sediment evacuation. We sug-gest that reduced rainfall caused ineffi cient drainages in the leeward highlands to the west, while active tectonism associated with high rain-fall along the eastern fl ank caused deep incision of high-gradient rivers. Probably because of the ongoing tectonic activity along the east, these rivers have not yet been able to reach the Bogotá Basin by headward erosion. Intense, localized precipitation may thus have resulted in higher rock uplift rates and a profound infl uence on the drainage system compared to areas to the west.

CONCLUSION

New structural and thermochronologic data combined with published paleofl ora and ther-mochronologic information allow the contrast-ing deformation, exhumation, and surface-uplift histories in different areas of the Eastern Cordil-lera of Colombia to be evaluated from the late Pliocene to the present. The fundamental trigger causing asymmetry in the orogenic processes is the initial topographic growth between 6 and 3 Ma that built an orographic barrier that sub-sequently intercepted easterly moisture-bearing winds leading to focused erosion. Our data sug-gest that this focused erosion increased mass removal along the eastern fl ank of the orogen to values comparable to tectonically controlled advection of material, therefore enhancing tec-tonic mass fl ux. In contrast, due to the eastern orographic barrier, precipitation was reduced in the western sectors of the orogen during the same period, and the basin became internally drained. Consequently, mass removal and tec-tonic fl ux were reduced. We propose that such disparate behavior ultimately enhanced or even-tually produced the regional tectonic asymme-try. Focused, climate-driven denudation in the Eastern Cordillera is therefore a fundamental process that has profoundly modifi ed the rates and the location of tectonic deformation in this part of the Andean orogen.

APPENDIX

Fault Geometries and Depth to Detachment Calculations:

A correct calculation of shortening depends on a well-constrained fault geometry. Our interpretation of a listric fault shape is favored with the exhuma-tion pattern documented in this work (Fig. 8), remi-niscent of that simulated in inverted listric normal faults (Mitra, 1993; McClay, 1995). Otherwise the forward-breaking sequence of deformation from W to E along the eastern deformation front, documented with apatite fi ssion-track (AFT) analysis in the Guay-uriba profi le, does not favor the geometrical assump-tion of steeply dipping fault planes at depth for the basement faults while the frontal thrusts are low angle. Second it has been observed by means of deep seismic refl exion profi les, reaching more than 7 sec two-way traveltime (TWT) (e.g., Deeks and Thomas, 1995; Sinclair, 1995), that many inverted master faults in intra-plate inverted grabens have a listric shape. We further assume inversion reactivating mostly the same extensional fault planes, following observations by Mora et al. (2006).

With these assumptions an approximate depth to detachment can be found based on the fact that, assuming plain strain deformation, the area of cross-sectional structural relief equals total shortening mul-tiplied by depth to detachment (e.g., Hossack, 1979; Mitra and Namson, 1989; Fig. 4). This relationship has been used already in the Eastern Cordillera by Colleta et al. (1990) and Cortés et al. (2006). With a total shortening of 58 km calculated from line-length balancing in our cross section (Fig. 4) and an excess area of 1340 km2, a depth to detachment of about 23 km in the undeformed state was obtained assuming the west-dipping fault as a main detachment.

Through an iterative process using 2D Move©, we tried to extrapolate the dip data from the area adjacent to the Anaconda well to the west and at depths bigger than 5 km. We searched for a fault plane that cutting-sequence-up fi ts better to the observed spatial and tem-poral evolution of rock uplift, exhumation, and surface uplift during the late Cenozoic and the predicted depth to detachment as well. We tried different fault-related deformation mechanisms but found that a rigid block rotation along a circular fault, such as proposed by Erslev (1986) in the Rocky Mountains, reproduced better the observations at different moments in time (Fig. 4). This geometry was used also by Jordan and Allmendinger (1986) in the Sierras Pampeanas and Kley and Monaldi (2002) in the Santa Barbara Sys-tem. As proposed by Erslev (1986), the back-limb dip angle of a given basement uplift (Δ tilt) can be related to fault curvature (1/R, where R is the radius of curva-ture) and slip along the circular fault segment by the following relationship (Fig. 13):

Δ tilt = 180 SRA

/πR.

Using as an example cross section E–E′ (Fig. 7), we only know the back-limb dip angle (7°) from draw-ing a tangent line to the base of the best constrained horizon, that is the base of the Aptian Une Forma-tion in section E (Figs. 7 and 13). Then we assumed a radius of curvature of a circle that fi ts approximately the surface trace of the fault and the depth to detach-ment, where the rotation axis is located above the place where the back limb is horizontal, in our case the axis of the Bogotá Basin (Figs. 4, 7, 11, and 13). Given the uncertainties, we obtained a range of val-ues, but we fi nally chose those variables that had the

Climatic forcing of orogenic evolution

Geological Society of America Bulletin, July/August 2008 947

best fi t with depth to detachment and rock uplift, sur-face uplift, and exhumation through time. With such radius of curvature, we calculated the slip along the main listric inverted fault assuming an additional constraint—a retrodeformed state where lowermost Cretaceous hanging-wall and footwall cutoffs fi t to each other (which equals back rotating 7° the back limb to zero dip) plus some differential subsidence in the downthrown block of the Servitá fault during the Lower Cretaceous, assuming the Farallones Anti-cline as an amplifi ed rollover of the listric detachment as interpreted by Mora et al. (2006). This means an additional 1° rotation along the fault (Figs. 11 and 13). Finally we found the best fi t with a total rotation of 8°, ~21 km of slip along the main fault and a radius of curvature of about 158.5 km (Figs. 11 and 13). We used those geometries to calculate the shortening val-ues obtained in our cross sections.

Another subsequent step was to incrementally ret-rodeform the deformed state cross sections (Fig. 11). In this step we used the mentioned considerations plus the data of total amount of overburden from vitrinite refl ectance and removed overburden at each stage cal-culated from our thermochronological data.

ACKNOWLEDGMENTS

The authors are indebted to Peter Molnar, Richard W. Allmendinger, John Suppe, Jose María Jaramillo, Thomas van der Hammen, and Brian Horton for fruit-ful suggestions, and Birgit Fabian for graphic work. Gerold Zeilinger helped generate the digital elevation model (DEM) and the swath profi les. Careful reviews by Douglas Burbank, David Montgomery, and Associ-ate Editor Brendan Murphy greatly helped our inter-pretation. Seismic and well data were kindly provided

by Javier Cardona and Mauricio Blanco (Agencia Nacional de Hidrocarburos) and German Rodriguez (Sipetrol). Sandra Passos (Ecopetrol) allowed us to use vitrinite refl ectance data from Ecopetrol. Carlos Costa Posada from IDEAM Colombia provided data of precipitation and river discharge for the past 20 years from the Eastern Cordillera. Vitrinite refl ectance analy-sis was carried out at the Departamento de Geocien-cias (Universidad Nacional) and Ingeominas, with the cooperation of Gladys Valderrama, Luis Jorge Mejía, Andreas Kammer, and Luis Ignacio Quiroz. Oscar Fernandez, from Midland Valley, provided a 2D Move license. Elias Gomez kindly provided some of his orig-inal data from the western fl ank. Financial support for fi eld work and analysis came from Petrobras Colombia and the German Science Foundation to M. Strecker (Str 373/19-1). A. Mora and M. Parra thank the German Academic Exchange Service (Der Deutsche Akademis-che Austauschdienst [DAAD]) and the Leibniz Center for Earth Surface and Climate Studies at Potsdam Uni-versity for funding their studies at Potsdam University. M.R. Strecker also thanks the A. Cox fund of Stanford University for additional support.

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0

-12 km

-8

-4

0

-12 km

-8

-4

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Slip = 21.5 km

Figure 13. Back-limb dip (7°), rotation angle (8°), and total slip along the Servitá fault, taken to constrain fault geometry in section E–E′.

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