Climatic forcing of asymmetric orogenic evolution ... - GeoScienceWorld

Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia Andrés Mora† Mauricio Parra Manfred R. Strecker Edward R. Sobel Institut für Geowissenschaften, Universität Potsdam, Karl Liebnecht-Str 24, D14476 Potsdam-Golm, Germany

Henry Hooghiemstra Vladimir Torres Institute for Biodiversity and Ecosystem Dynamics (IBED), Paleoecology and Landscape Ecology, Faculty of Science, University of Amsterdam, Kruislaan 318, 1098 SN Amsterdam, Netherlands

Jaime Vallejo Jaramillo Petrobras-Colombia, Carrera 7, No 71-21, Torre B. Edificio Bancafé, Bogotá, Colombia

ABSTRACT New apatite fission-track data, paleoelevation 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 efficient orographic barrier that intercepts moistureladen winds sourced in the Amazon lowlands, leading to high rainfall and erosion gradients across the eastern flank of the range. In contrast, the drier leeward western flank is characterized by lower rates of deformation and exhumation. In light of the geological evolution of the Eastern Cordillera, the combination of these data sets suggests 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 flank. Sequentially restored structural cross sections across the eastern flank of the Eastern Cordillera indicate that shortening rates also have increased during the past 3 Ma. From fission-track and structural crosssection balancing, we infer that accelerated exhumation led to increasing tectonic rates on the eastern flank, creating a pronounced topographic and structural asymmetry in the Eastern Cordillera. The tectonic and climatic evolution of this orogen thus makes †

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it a prime example of the importance of climatic forcing on tectonic processes. Keywords: Apatite fission 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 flanks 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 asymmetry 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-

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

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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 wedgeshaped 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 failure 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 influenced the evolution of individual mountain ranges and intraorogenic plateaus (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 precipitation gradients also exist, such relations have not been explored. The NNE-oriented northern Andes have created important precipitation gradients across the Western and Eastern cordilleras (Fig. 1). In fact, with ~7 m of annual precipitation (Fig. 1), the humid flanks of these ranges receive one of the highest amounts of precipitation 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.

Climatic forcing of orogenic evolution

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Figure 1. (A) General features of the Colombian Eastern Cordillera. Regional structure with the main thrusts bounding the major topographic breaks in the Eastern Cordillera. River discharges (values in red in m3/sec) of the main rivers in the eastern and western flanks clearly showing higher values in the eastern side rivers flowing toward the Orinoco (e.g., Guayuriba—RGy; Guatiquía—RGt; Humea— RH; Upía—RU. Western rivers flowing 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.

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 flank of the Eastern Cordillera has accommodated significantly more shortening, and basement rocks crop out at elevations ~2 km higher compared to the western flank (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 flank strongly contrasts with the drier western parts (Fig. 1). Nevertheless, interactions between climate and tectonics remain poorly known. If significant feedback between tectonics and climate 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 field structural data and constructed geologic cross sections to assess the degree of overall shortening and removal of cover units. We then carried out apatite fission-track thermochronology to quantify the long-term role and timing of erosional denudation. In a third step, we examined published data on the paleofloristic evolution of the area, the late Cenozoic sedimentary record, and thermochronology 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 remarkably different Plio-Pleistocene denudation and uplift histories and explain their patterns in light of interactions between tectonically and climatically controlled migration of deformation. Our investigation reveals that the uplift of the Eastern Cordillera generated an effective orographic barrier for easterly moisture-

bearing winds, resulting in strong precipitation gradients across the range. Focused precipitation and removal of rocks conspire in sustaining high exhumation rates on the windward flanks, 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; SarmientoRojas, 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, lowrelief highland with outcrops of Cretaceous and

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Figure 2. (A) Digital elevation model of the Eastern Cordillera including the deeply dissected Eastern flank, the central flat-lying Sabana de Bogotá basin and the topographically lower western flank. GS—Guaduas syncline; VA—Villleta anticlinorium. (B) Topographic and generalized geologic map of the area studied in detail (see location in Fig. 1). Apatite fission-track (AFT) sample locations are shown in red, and vitrinite reflectance 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.

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Climatic forcing of orogenic evolution Tertiary sedimentary units. Cenozoic shortening in the Bogotá Basin is minor and primarily 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 flank of the Eastern Cordillera constitutes west-vergent basement thrusts with offsets on the order of more than 20 km (Fig. 1). The eastern flank is similar to the western flank 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 flank, 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 flanks 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 flank and the Bituima, Cambao, and Honda faults in the west (Fig. 1). A late Paleogene to Neogene foreland basin sequence to the east reflects the uplift history of the adjacent basement highs in the Eastern Cordillera. 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 proximal alluvial fan systems (Parra et al., 2006).

dillera is concentrated on the east and feeds tributaries of the Orinoco River (Fig. 1). Rivers on the eastern flank are shorter than rivers in the Sabana de Bogotá and the western flank, channel slopes are steeper and are either perpendicular or moderately oblique to the structural grain (Figs. 1–3). Along the western flank, the principal rivers have only succeeded to crosscut major structures in a few locations and after having flowed 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 industry 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 flank of the Eastern Cordillera, as well as two regional cross sections across the entire orogen. Shortening estimates were obtained using conventional linelength balancing techniques (e.g., Dahlstrom, 1969). Depths to detachment were estimated using seismic reflection profiles and geometrical approaches, such as area-balancing methods (e.g., Mitra and Namson, 1989) and basementuplift balancing methods, similar to techniques applied in the Rocky Mountains (e.g., Erslev, 1986; see Fig. 4 and Appendix for details). Vitrinite Reflectance Analysis

TOPOGRAPHY AND FLUVIAL NETWORK Between 3.5° and 6° N, the eastern flank of the range is remarkable for its topographic and climatic contrasts compared to the Sabana de Bogotá and the western branch of the Eastern Cordillera. For instance, the eastern flank 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 flank constitutes an effective orographic barrier that enhances precipitation on this side of the mountain belt and leaves leeward sectors dry (Fig. 1). For example, while the eastern flank receives ~5000 mm/yr of rainfall, the Sabana de Bogotá and the western flank receive ~1000 mm/yr and 2000 mm/ yr, respectively (Fig. 1). Accordingly, the most important fluvial discharge in the Eastern Cor-

Macerals (like vitrinite) are major components of coal or organic matter in sedimentary rocks. Macerals increase their reflectance (i.e., reflectivity as measured under the microscope) with increasing temperatures (e.g., Bustin et al., 1990). Vitrinite reflectance (Ro) is the most widespread measurement of the maturity of organic matter as related to heating, commonly associated with burial. Thus, vitrinite-reflectance analysis can provide the maximum paleotemperatures reached by sedimentary rocks during burial. Normal vitrinite reflectance values in organic matter (Ro) typically range between 0.2 at maximum paleotemperatures of 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 photomicroscope. Polarized reflected light was used to measure the mean random percentage reflectance 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 available 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 damage produced by the spontaneous fission of 238 U in apatite or zircon (e.g., Wagner and Van den Haute, 1992). The number of spontaneous tracks is proportional to the age and the uranium 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 completely (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 fission tracks (AFT) remain stable only below 60 °C (Gleadow et al., 1986). At temperatures in the partial annealing zone (PAZ), between 60 °C and the total annealing temperature, tracks are partially or totally erased. If the apatites cooled sufficiently 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 significant time in the PAZ, the age and confined track-length distribution will be modified. 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 annealing temperatures, as determined by vitrinite reflectance 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

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Figure 3. (A) Swath topographic profile across the central segment of the Eastern Cordillera showing higher minimum, maximum, and mean elevations in the eastern flank compared to the western flank. (B) Topographic profiles along main rivers of the western flank (Bogotá, Negro and Sumapaz Rivers) and eastern flank (Guatiquía, Guayuriba, and Humea rivers) of the Eastern Cordillera. The Bogotá River can be identified 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 flank, 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.

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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).

profiles to decipher age and elevation trends and derive apparent exhumation rates (e.g., Sobel and Strecker, 2003). Thermochronologic data alone do not provide rock uplift or paleoelevation. We therefore compare surface uplift history deduced from published paleofloristic data with the denudation history obtained from apatite fission-track analysis. In the following sections we present 17 new AFT ages from the eastern flank of the Eastern Cordillera and compare them to published AFT data from the western flank 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 flatten at depth (Fig. 4) to transfer deformation throughout the middle crust from the west (e.g., the Central Cordillera), as has been observed in other inverted intra-plate basins (e.g., Chapman, 1989; Roberts, 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 Colleta et al. (1990) and Cortés et al. (2006) (see Appendix and Fig. 4). In our cross sections, we modeled the Eastern Cordillera as a bivergent orogen with two main detachments. The assumption that the eastern detachment is the principal structure and the western, east-dipping 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

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Mora et al. TABLE 1. APATITE FISSION-TRACK DATA U NS NI ND Age Rho-S Rho-I P(x²) Rho-D XI Elevation (Ma) (ppm) (m) (104 (106 ( 105 (%) 2 2 2 tracks/cm ) tracks/cm ) tracks/cm ) 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 with the 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). Fissiontrack 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. Sample

Latitude (°N)

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Lithology

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 dependent on movement along the eastern Guaicaramo 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 flat detachment at 2000 m) Andean forest. Wijninga (1996) argued that the present topography of the Eastern Cordillera does not prevent pollen from the tropical lowland forest 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 sediments 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 reflect a major change in paleoelevation between 6 and 3 Ma, from less than 1000 m to the present-day elevation 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 fluviolacustrine 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 (Andriessen 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 surface uplift in the Bogotá Basin or adjacent areas can be deduced (Hooghiemstra, 1984; Hooghiemstra and Cleef, 1995; Van’t Veer and Hooghiemstra, 2000; Hooghiemstra et al., 2006; Tor-

res, 2006). We therefore conclude that the main phase of topographic growth in the Eastern Cordillera 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 indicators 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, climate, and exhumation in the Eastern Cordillera of Colombia. We document an acceleration of exhumation rates on the eastern flanks of this range by ca. 3 Ma, compared to the previous 30 Ma. However, compared with similar data sets from the western flank of the orogen, it appears that focused and accelerated denudation solely characterizes the eastern deformation 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 evolution of both areas. We document that greater total amounts of shortening have occurred on the eastern versus the western flank. In addition, basement exposures occur in the east but are absent in the west (Fig. 1). Accelerated shortening rates of ~5mm/yr (see Appendix and Figs. 4 and 11 for shortening calculations) are restricted to the east during the Pliocene. Finally, topography, precipitation patterns, and the characteristics of the fluvial network follow this disparate spatiotemporal evolution. An important question, however, is whether climate and exhumation have been active or passive with respect to the spatial and temporal trends in tectonic evolution. 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 timing 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 considered to be minor and uplift related to lithospheric delamination appears unlikely based on the overall tectono-magmatic evolution of this part of the Andean orogen (e.g., SarmientoRojas, 2001). The specific structural style of the Eastern Cordillera (Figs. 1, 2, and 4) shows

Geological Society of America Bulletin, July/August 2008

Climatic forcing of orogenic evolution

Biozones

IV-VII

Inferred paleo altitude (m) Funza-2 2500

0-3 Ma (present elevation 2550 m)

Guasca 103

III IIB

L. Pliocene 3.7 ± 0.5 Ma

2000

Facatativá 13

1500

Subachoque 39

IIA

E. Pliocene 5.3 ± 1 Ma

1000

Rió Frió 17

Salto de Tequendama I and II

I

Middle? Miocene

500

0 1

2

QUATERNARY

3

4

5

6

PLIOCENE

16 Age (Ma)

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]).

that crustal shortening is mostly concentrated on both margins of the mountain belt. Activity 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 influence of climate 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 during Mio-Pliocene time was in a permanent paleo-El Niño state. According to their predictions 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, presentday precipitation patterns must be used with extreme caution in characterizing interactions between orography and precipitation during the Plio-Pleistocene. However, palynological, macroscopic paleoflora 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 patterns, at least since middle Miocene time (e.g., Lorente, 1986; Hoorn, 1994; Van der Hammen 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.

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Mora et al. 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 probably attained toward the end of the phase of relief growth between 6 and 3 Ma. If true, focused precipitation on the eastern flank would have generated an effective, eastward flowing fluvial system with higher discharge that cut deeper canyons compared to the modern rivers of the western flank. Accordingly, 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 profile. Thus, some of the documented cooling occurred through river incision. If such relationships 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 Eastern 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 timing and amount of shortening along the western (Gómez et al., 2003; Restrepo-Pace et al., 2004; Cortés et al., 2005; Montes et al., 2005) and eastern flanks (Fig. 11). Crustal shortening has been concentrated on the eastern flank during 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 accelerated motion along the shortcut faults on the eastern flank 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 flank, is expected to generate a significant amount of lithospheric flexure in adjacent areas that are not being uplifted. The exact thickness of corresponding units in the adjacent Llanos foreland basin to the east has never been established, due

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to poor chronological constraints. However, the Funza-2 drill hole from the Bogotá Basin records ~0.6 km of lacustrine sediments deposited in an area undergoing no deformation during 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) reflects enhanced tectonic thickening in adjacent, actively deforming areas, such as the eastern foothills. The lacustrine 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 fluvial systems from efficient downcutting and sediment evacuation. We suggest that reduced rainfall caused inefficient drainages in the leeward highlands to the west, while active tectonism associated with high rainfall along the eastern flank 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 influence on the drainage system compared to areas to the west. CONCLUSION New structural and thermochronologic data combined with published paleoflora and thermochronologic information allow the contrasting deformation, exhumation, and surface-uplift histories in different areas of the Eastern Cordillera 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 subsequently intercepted easterly moisture-bearing winds leading to focused erosion. Our data suggest that this focused erosion increased mass removal along the eastern flank of the orogen to values comparable to tectonically controlled advection of material, therefore enhancing tectonic mass flux. 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 tectonic flux were reduced. We propose that such disparate behavior ultimately enhanced or eventually produced the regional tectonic asymmetry. Focused, climate-driven denudation in the Eastern Cordillera is therefore a fundamental process that has profoundly modified 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 exhumation pattern documented in this work (Fig. 8), reminiscent 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 fission-track (AFT) analysis in the Guayuriba profile, does not favor the geometrical assumption 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 reflexion profiles, 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 crosssectional structural relief equals total shortening multiplied 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 cuttingsequence-up fits better to the observed spatial and temporal 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 System. 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 curvature) 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 drawing a tangent line to the base of the best constrained horizon, that is the base of the Aptian Une Formation in section E (Figs. 7 and 13). Then we assumed a radius of curvature of a circle that fits approximately the surface trace of the fault and the depth to detachment, 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 values, but we finally chose those variables that had the

Geological Society of America Bulletin, July/August 2008

Climatic forcing of orogenic evolution by Javier Cardona and Mauricio Blanco (Agencia Nacional de Hidrocarburos) and German Rodriguez (Sipetrol). Sandra Passos (Ecopetrol) allowed us to use vitrinite reflectance 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 reflectance analysis was carried out at the Departamento de Geociencias (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 original data from the western flank. Financial support for field 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 Akademische Austauschdienst [DAAD]) and the Leibniz Center for Earth Surface and Climate Studies at Potsdam University for funding their studies at Potsdam University. M.R. Strecker also thanks the A. Cox fund of Stanford University for additional support.

8°

REFERENCES CITED

Slip = 21.5 km

Dip = 7° 0

0

-4

-4

-8

-8 -12 km

-12 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′.

best fit with depth to detachment and rock uplift, surface 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 fit 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 Anticline as an amplified 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 fit 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 values obtained in our cross sections.

Another subsequent step was to incrementally retrodeform 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 reflectance and removed overburden at each stage calculated 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 fruitful suggestions, and Birgit Fabian for graphic work. Gerold Zeilinger helped generate the digital elevation model (DEM) and the swath profiles. Careful reviews by Douglas Burbank, David Montgomery, and Associate Editor Brendan Murphy greatly helped our interpretation. Seismic and well data were kindly provided

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