Restrepo et al_2009_Long-term erosion and exhumation of the “Altiplano Antioqueño_EPSL

Earth and Planetary Science Letters 278 (2009) 1–12

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Earth and Planetary Science Letters

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Long-term erosion and exhumation of the “Altiplano Antioqueño”, Northern Andes(Colombia) from apatite (U–Th)/He thermochronology

Sergio A. Restrepo-Moreno a,⁎, David A. Foster a, Daniel F. Stockli b, Luis N. Parra-Sánchez c

a Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USAb Department of Geology, University of Kansas, Lawrence, Kansas 66045, USAc ICNE, Universidad Nacional de Colombia, Medellín, Colombia

⁎ Corresponding author. 241 Williamson Hall PO Box2120, USA. Tel.: +1 352 3922231; fax: +1 352 3929294.

E-mail address: [email protected] (S.A. Restrepo-Mo

0012-821X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.epsl.2008.09.037

a b s t r a c t
a r t i c l e i n f o
Article history:
The Antioqueño Plateau (A Received 6 October 2007Received in revised form 9 September 2008Accepted 24 September 2008
Editor: R.W. Carlson

Keywords:(U–Th)/He datingrelict landscapemorphotectonicserosion/exhumation ratesNorthern AndesAntioqueño Plateau

P) in the northern Cordillera Central, Colombia, is the largest high elevationerosional surface in the Northern Andes. Apatite (U–Th)/He thermochronometry (AHe) of samples collectedfrom two elevation profiles spanning ∼2 km of exhumed crustal sections reveal the long-term erosionalexhumation of the AP. Sample profiles exhibit AHe ages that increase with elevation from ca. 22 Ma (∼760 m)at the bottom of regional scarps to ca. 49 Ma (∼2350 m) on top of the AP. A marked inflection point in ageversus elevation data at ca. 25 Ma defines the bottom of the exhumed post-Oligocene He partial retentionzone (He-PRZ). Elevation-invariant ages below ca. 25 Ma record the onset of rapid exhumation and surfaceuplift of the AP that led to river incision. A subtle change in slope within the He-PRZ, ca. 41 Ma, is interpretedas a less intense, exhumation-related cooling episode. These two exhumation pulses coincide with the Proto-Andina and Pre-Andina orogenic phases previously proposed for the Colombian Andes, and are synchronouswith tectonically driven exhumation events reported for the Peruvian, Bolivian and Argentinean Andes, andfor some orogenic systems in the Caribbean. The pulses are correlated with variations in the rates ofconvergence between Nazca (Farallon) and South America documented for the Middle Eocene and the LateOligocene suggesting continental-scale controls on uplift and denudation throughout the Andean range. AHedata provide an average erosion rate of ∼0.04 mm/yr for the last 25 million years. Erosion rates during theexhumation pulses were in the order of ∼0.2–0.4 mm/yr. Similarity between AHe profiles indicates the wholeAP was uplifted and exhumed as a coherent structural block, corroborating previous structural evidence forthe rigidity and coherence of this crustal block in the Northern Andes. Our results are in agreement withtectonostratigraphic data in the Magdalena and Cauca basins and with proposed scenarios for paleogeo-graphic evolution in the Northern Andes.

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1. Introduction

Defining the morphotectonic evolution of the Andean Cordillera iscritical for understanding the tectonic processes driving uplift,climatic influences of the topography, the source of sediment tooceans and continental sedimentary basins, and changes in paleogeo-graphy (Hoorn et al., 1995; Gregory-Wodzicki, 2000; Harris and Mix,2002; Gómez et al., 2005; Sobolev and Babeyko, 2005). Despite itsimportance in the geodynamic evolution of a geologically complexregion, the Northern Andes in Colombia remain poorly studiedcompared to many segments of the Andes to the South. Little isknown about the morphotectonic evolution of some major provinces,including the Antioqueño Eastern Massif (Botero, 1963), which is apolymetamorphic complex (Restrepo and Toussaint, 1982), intrudedby large Mesozoic calc-alkaline plutons of the Antioqueño and Ovejas

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Batholiths (González, 2001). Geomorphologically, the AntioqueñoEastern Massif encompasses the Altiplano Antioqueño (Arias, 1995;Page and James,1981), hereafter the Antioqueño Plateau (AP), which isthe largest high elevation erosional surface in the Northern Andes. TheAP is preserved as a wide, slightly dissected surface extending for tensof kilometers, locally and deeply incised by the Medellín–Porce fluvialsystem (Figs. 1 and 3). A better understanding of the long-termlandscape development and erosional exhumation history of the AP isneeded for defining the Cenozoic morphotectonic and uplift history ofthe Northern Andes, the paleogeographic evolution of the region, andthe relationships between erosion and tectonics (Van der Hammen,1960; Van der Hammen et al., 1973; Duque-Caro, 1990; Hoorn et al.,1995; Gregory-Wodzicki, 2000; Gómez et al., 2005; Hooghiemstraet al., 2006). The findings of this study also have implications forrefining morphotectonic response to variations in convergence ratesbetween Nazca (Farallon) and South American plates during theCenozoic (Pardo-Casas and Molnar, 1987).

Elevated erosional surfaces affected by deep and localized fluvialincision, such as the AP (Figs. 1 and 3), constitute an important source

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Fig. 1. Shaded relief map of the Northern Andes, Colombia. The Andean mountain chain in Colombia is divided into three separate ranges: Cordillera Occidental (Western Range),Cordillera Central (Central Range), and Cordillera Oriental (Eastern Range). The Antioqueño Plateau in the northern Cordillera Central is part of the Antioqueño EasternMassif (Botero,1963), which is bounded by major fault zones Cauca-Romeral fault system (R) to the west and Palestina fault system (P) to the east. The Guaicáramo (G) and Boconó (B) faults definethe limit between the Andean Block and the autochthonous South American Plate (craton).

2 S.A. Restrepo-Moreno et al. / Earth and Planetary Science Letters 278 (2009) 1–12

of morphotectonic information, retrievable from low-temperaturethermochronologic data. Such surfaces, usually referred to as relictsurfaces, have played an important role in the reconstruction of themorphotectonic evolution of active (Kennan et al., 1997; Coltorti andOllier, 1999; Ollier and Pain, 2000) and passive continental margins(Peulvast and Sales, 2005). Elevated, relict erosional surfaces, indicatethat portions of an orogen have not achieved equilibrium betweenerosion and tectonics (Coltorti and Ollier, 1999; Ollier and Pain, 2000).It is generally thought that these surfaces develop by planation closeto base level (Davis, 1899) and are subsequently uplifted via tectonicprocesses to their present position (Arias, 1995; Coltorti and Ollier,1999). The non-equilibrium character of these denudational featuresimplies lagged response to tectonic forcing, which allows their use forreconstructing regional, long-term erosional exhumation as well aspatterns of surface/rock uplift and topographic evolution (Clark et al.,2005; Schildgen et al., 2007).

Cenozoic uplift and erosion in the northern Cordillera Central isrecorded by sedimentary units (e.g., associated litho facies and fluvialpaleoflows) in the Middle Magdalena Valley to the east (Van derHammen, 1960; Gómez et al., 2005), the Lower Magdalena Valley tothe north (Van der Hammen, 1960; Reyes et al., 2002), and the Caucavalley to the west (Grosse, 1926). The traditional interpretationsuggests that since the latest Cretaceous the eastern flank of theCordillera Central has operated as the source area for synorogenicdetritus to the Middle and Lower Magdalena Valleys (Reyes et al.,2002; Gómez et al., 2005). The region was also considered as a sourceof terruginous material for the Terciario Carbonífero de Antioquiaduring the Oligo–Miocene (Grosse, 1926), a coal-bearing sedimentarysequence later renamed Amagá Formation (González, 2001), situatedin the Cauca valley. Cenozoic uplift in the Northern Andes of Colombiahas been also invoked as the cause of substantial paleogeographicchanges that took place in the region, particularly, the reorientation–reorganization of the Orinoco and Amazon fluvial systems (Hoornet al., 1995).

We present thermochronological evidence from apatite (U–Th)/Heanalysis from two vertical profiles in the AP (Fig. 3) that reveal theerosional exhumation history of the plateau with implications for thetiming of tectonic pulses of the Andes and the topographic evolutionof the range during and after uplift.

2. Study site: the Antioqueño Plateau

2.1. Physiographic and Geologic overview

From west to east, the Colombian Andes are composed of threemain mountain belts known as Cordillera Occidental, CordilleraCentral, and Cordillera Oriental. These Cordillera are separated bytwo inter-Andean depressions occupied by the Cauca and Magdalenarivers (Figs. 1 and 2). The Magdalena River yields ∼1000 t/km2 yr ofsediment to the delta which is the highest sediment yield of any largeriver in South America along the Caribbean and Atlantic coasts(Restrepo and Syvitski, 2006). Such sediment yield is roughlyequivalent to denudation rates on the order of ∼1 mm/yr. Similarfigures have been reported for the AP based on a temporal series ofbathymetric surveys in five reservoirs in the AP (Giraldo, 2005).Augmented erosion in the Colombian Andes has been attributed toanthropogenic activities, e.g., deforestation, agriculture, and develop-ment of infrastructure (Restrepo and Syvitski, 2006). However,quantitative data regarding long-term, background erosion rates forthe area are inexistent.

The three major cordilleras belong to two distinct geologicdomains that are separated by the Romeral Fault system (Figs. 1and 2). The western province consists of accreted oceanic crust and isexposed in the Cordillera Occidental and the lower western flank ofthe Cordillera Central. The eastern province is underlain by con-tinental basement and is exposed throughout most of CordilleraCentral and Cordillera Oriental (Taboada et al., 2000; Tapias et al.,2006). The Cordillera Oriental is a fold-and-thrust belt that consists of

3S.A. Restrepo-Moreno et al. / Earth and Planetary Science Letters 278 (2009) 1–12

polydeformed, continental Precambrian and Paleozoic metamorphicand igneous rocks overlain by thick Paleozoic to Mesozoic sedimen-tary sequences (González, 2001; Taboada et al., 2000).

The Cordillera Central is characterized by modern volcanic activityrelated to the subduction of the Nazca plate and a basement consistingof a Mesozoic–Cenozoic plutonic arc. This study focuses on acordilleran segment located from 5°45′ to 7° latitude North withoutmodern volcanism. Within the study area (Fig. 2), the CordilleraCentral encompasses a pre-Mesozoic to Mesozoic polymetamorphiccomplex (Restrepo and Toussaint,1982) composed of low- tomedium-grade metamorphic rocks of the Cajamarca Complex (Maya andGonzález, 1995), and high-grade metamorphic rocks of the El RetiroGroup and Las Palmas Gneiss (González, 2001). Lower Paleozoic orPrecambrian age has also been proposed for this metamorphic core(Hall et al., 1972). The metamorphic basement is covered by theunmetamorphosed shallow marine to epicontinental sedimentarysequences San Luis (Feininger et al., 1972), San Pablo, La Soledad (Hallet al., 1972), and Abejorral (Etayo-Serna, 1985), that are EarlyCretaceous in age (132–127 Ma). Metamorphic and sedimentaryunits were intruded at a shallow crustal level (∼4 km) by Meso–Cenozoic batholiths, including the Batolito Antioqueño and Batolito deOvejas plutons, and overlain by Late Eocene continental-arc volcanicrocks (González, 2001). Finally, Late Neogene volcaniclastic sequences

Fig. 2. Geologic units of Colombia and geology of the study site, northern Cordillera Central. Tmarine sedimentary sequences Early Cretaceous in age (Kisp, Ksls, Kissl) and the Late Cretadotted grey). Other units include: Q=Quaternary fluvial and alluvial deposits; Ngc+gas=Formation (fluvial); Ngro=Mesa Formation (detrital, continental). Ksvb=Volcanic Barrosobelonging to the western oceanic province. Jsde=Segovia Batholith, Jts=Sonsón Batholith,Pzone which represents the boundary between the Eastern Continental Province (approx(approximately coinciding with the Cordillera Occidental). (Geologic information was adapt

associated with modern volcanic activity in the Cordillera Centralmantle the region (Arias, 1995). The northern portion of the CordilleraCentral is considered to be part of an exotic terrane,whichwas affectedby Devonian–Carboniferous, Permo–Triassic (Hercynian), and Cretac-eous tectono-metamorphic events (Restrepo and Toussaint, 1982,1988; Toussaint, 1993).

The Antioqueño and Ovejas batholiths are texturally and composi-tionally homogenous. Approximately 95% of the samples collected fromthis units have been classified as quartzdiorite–granodiorite (Feiningerand Botero, 1982; Saenz, 2003). Previously published thermochronolo-gic data from the Antioqueño Batholith and other associated plutonsinclude biotite K/Ar ages, which range from 68±3 Ma (Feininger andBotero,1982) to 90±6Ma (Restrepo,1975), three biotite Rb/Sr ages from56±7.4 to 66±9.2Ma (Fujiyoshi et al., 1976); twowhole-rock Rb/Sr agesof 82±8 and 98±27 Ma (Ordóñez and Pimentel, 2001); zircon fission-track dates from 49.1±2.5 to 67.1±2.1 Ma and fourteen apatite fission-track ages varying between 49.1±1.2 and 28.7±1.5 Ma (Saenz, 2003).The large spectrumof ages can be explainedby thewide range of closuretemperatures of the thermochronometric systemsemployed and shouldbe interpreted as cooling rather than formation ages. Although the Rb–Sr age of 98±27 Ma has been proposed as the “best estimate ofcrystallizationage for theAntioqueñobatholith” (OrdóñezandPimentel,2001), recent zircon U–Pb spot analyses (LA-ICP-MS) give ages between

he area is dominated by the polymetamorphic Cajamarca Complex (Pz), small patches ofceous calk-alkaline intrusion of the Antioqueño Batholith and associated stocks (Ksta,(Neogene sediments from Combia Formation (volcaniclastic continental) and AmagáFormation (volcanic, marine) and Ksu=Urrao Formation (clastic-chemical, marine)

=metamorphic rocks of Neoproterozoic age. The Cauca Almaguer Fault is a paleosutureimately coinciding with the Cordillera Central) and the Western Oceanic Provinceed from González, 2001 and Tapias et al., 2006.)

Fig. 3. Three dimensional representation (top) and simplified topographic/geologiccross section A–A′ (bottom) across the Antioqueño Plateau. The flat portion of theplateau has an average elevation of ∼2,500 m. MP=Medellín–Porce fluvial valley,CG=Cauca Gorge (CG). The CG coincides with the Cauca–Romeral (Cauca–Almaguer)fault system (shear zone). Sampled profiles indicated with black and open small circles.Páramo de Belmira within Las Baldías Range is at, ∼3600 m of elevation. (Shaded reliefDEMwas generated fromGTOPO-30 elevation data. (Geologic informationwas obtainedfrom González, 2001; and Tapias et al., 2006).


71±1.2 and 77±1.7 Ma suggesting that formation occurred close to thetransition Campanian–Maastrichtian (Restrepo et al., 2007).

2.2. The Antioqueño Plateau

The AP is an extensive (N5000 km2) low-relief geomorphicdomain, which appears to be a relict surface located in the northern-most portion of the Cordillera Central (Arias, 1995). It is flanked in theeast and west by the Magdalena and Cauca river valleys, respectively(Fig. 1). Mean elevation in the AP is ∼2500m and it possesses subduedinternal topography characterized by rolling hills with local reliefb40m and slopes b10°. Its overall geometry resembles an asymmetric,truncated pyramid with its steepest margin corresponding to theCauca River canyon (Figs. 1 and 3).

The AP is dominated by a dendritic drainage network and developedover gently rolling terrain where most hill slopes are mantled bysaprolitic horizons up to 100 m thick. Inselbergs (e.g., bornhardts andtors) are conspicuous, sporadic geomorphic features throughout the APand in some cases create local relief N300m. To the north and northeastthe AP descends in a series of step-like, less extensive erosional surfacesinto the lowlands of the Lower Cauca and Magdalena Rivers. Internallow-relief of the AP is interrupted by steep regional escarpments anddeeply incised gorges (Figs. 1 and 3). In some localities, steep

topographic margins expose up to 3200m of crustal sectionwith slopesin excess of 50°. Most of the planar topography of the AP coincides withthe Antioqueño Batholith, one of the largest (7600 km2) and mostcontinuous lithological units in the Cordillera Central. The prominenttopographic features within the AP, like the Las Baldías Range andPáramo de Belmira in thewestern margin of the plateau, are held up bythemore resistantmetamorphic units (Figs.1–3). About 150kmsouth ofthe study area, the plateau‘s low-relief rapidly transitions into a morerugged portion of the Cordillera Central dominated by active volcanoes.This part of the orogen, in clear contrast to the AP, has a geometry thatbears resemblance to a symmetric pyramid terminated in highersummits reaching up to ∼5000 m.

The AP has been interpreted to have developed as an erosionsurface near sea level in Paleocene time and that was uplifted indiscrete episodes during the Cenozoic, and modified by fluvial activitycombinedwith the unique weathering properties of granitic rocks in atropical setting (Scheibe, 1919; Arias, 1995). While this is theprevailing view, the AP's morphotectonic history is still poorlyconstrained, and the age of the surface is strongly debated. Despitethe fact that exhumation and paleoelevational history of the APremain virtually undocumented, remnants of shallow, marine sedi-mentary sequences formed during an Early Cretaceous marinetransgression point to the origin of the region as a low-elevation,low-relief domain.

No other elevated low-relief erosional surfaces as extensive as theAP have been reported in other parts of the Northern Andes.Nevertheless, similar geomorphic features of lesser extent, referredto in the literature as “elevated planation surfaces” (Coltorti and Ollier,1999), “high altitude paleosurfaces” (Kennan et al., 1997), and “erosionplateaus” (Ollier and Pain, 2000), are found in Ecuador, Peru, andBolivia. Quantitative data that constrain erosion rates, landscapeevolution, and uplift history of these important geomorphic features isstill scarce.

2.3. Tectonic setting

Orogeny in the Northern Andes is primarily driven by the collisionof allochthonous terrains and interactions between several tectonicplates and micro plates (Taboada et al., 2000; Cortes and Angelier,2005). This has resulted in heterogeneous deformation compared tothe Central Andes (Taboada et al., 2000; Trenkamp et al., 2002; Coateset al., 2004; Cortes and Angelier, 2005). Most authors agree that theregional tectonic history has been dominated by the subduction of twooceanic plates, Nazca and Caribbean, beneath continental SouthAmerica and by interactions of two lithospheric provinces, namely,the North Andean and the Panama-Chocó blocks (Duque-Caro, 1990;Taboada et al., 2000; Trenkamp et al., 2002; Coates et al., 2004; Cortesand Angelier, 2005) (Fig. 4). The Nazca plate is undergoing rapid steepsubduction along the length of the Colombia–Ecuador trench at a rateof ∼54 mm/a, and is responsible for the volcanism in the CordilleraCentral (Taboada et al., 2000; Trenkamp et al., 2002). The Caribbeanplate is being subducted slowly from the northwest at ∼20 mm/a, at ashallowangle, andwithout an associated volcanic arc (Trenkamp et al.,2002; Cortes and Angelier, 2005). During Paleocene until Miocene, thecollision of the Caribbeanplatewith South America resulted in uplift inthe Cordillera Central (Toussaint, 1999). Starting in the Neogene,subduction was blocked in the collision area between the Panama–Chocó block and the Cordillera Occidental of Colombia and thisongoing collision is producing deformation in the three cordilleras(Taboada et al., 2000; Cortes and Angelier, 2005).

Modern stress regimes are accommodated by major structureswithin the North Andean Block, which is part of the South Americanplate (Taboada et al., 2000), and within the Panama–Chocó block(Duque-Caro,1990; Coates et al., 2004; Cortes and Angelier, 2005). TheBoconό and Guaicáramo faults separate the North Andean Blockproper from the South American Plate (Figs. 1 and 4).The North

Fig. 4. Modern tectonic setting. Generalized map of tectonic plates and crustal blocks around the Northern Andes of Colombia. Arrows and numbers indicate, respectively, thedirection and velocity of plate's motion. (Adapted from Cortes and Angelier, 2005; Coates et al., 2004; Taboada et al., 2000; Duque-Caro, 1990; Trenkamp et al., 2002.).


Andean Block is escaping rigidly at 6±2 mm/a to the northeast withthe South American craton acting as a rigid buttress while thePanama–Chocó block is in active collision at a rate of ∼25 mm/yr(Trenkamp et al., 2002). Finally, three lithospheric blocks withdifferent relative strengths are invoked to describe the kinematics ofthe Northern Andes of Colombia (Montes et al., 2005). The area thatencompasses the AP is characterized as a lithospheric unit of highrigidity (Montes et al., 2005).

Twomajor variations in the rate of convergence between the Nazca(Farallon) and South American plates have been reported for theEocene and Oligocene (Hey, 1977; Wortel, 1984; Pardo-Casas andMolnar, 1987; Somoza, 1998). These changes may have determinedmorphotectonic, deformational and magmatic trends in the wholeAndes (Mégard, 1984; Pilger, 1984). Collision of the Panama–Chocóblock is considered as the trigger of the modern topographicconfiguration of the Northern Andes and has been invoked as thecause of the most recent phase of uplift and exhumation in the region(Eu–Andina Phase of Van der Hammen, 1960), as well as the driver ofLate Miocene–Pliocene uplift of the Cordilleras Occidental andOriental that has produced most of the present day structural relief(Duque-Caro, 1990; Kroonenberg et al., 1990; Taboada et al., 2000;Gómez et al., 2005; Hooghiemstra et al., 2006).

The Cordillera Central may represent a crustal-scale, positiveflower structure (Taboada et al., 2000; Gómez et al., 2005). The age ofmajor deformation of the Cordillera Central at the latitude of thisstudy is pre-middle Eocene, as evidenced by the widespread MiddleMagdalena Valley unconformity (Gómez et al., 2005). Morphotectonicevolution of the Cordillera Central has exerted a major control on thetectonostratigraphic development of the Magdalena and proto-Cordillera Oriental basins (Gómez et al., 2005). Elevated areas in theCordillera Central were a source of detrital material for basins on both

sides of this range. Good examples are the Late Cretaceous to presentsedimentary sequences in the Magdalena Basin to the east (Reyeset al., 2002; Gómez et al., 2005) and the continental, coal-rich AmagáFormation with N1500 m of sediments deposited on the west side ofthe Cordillera Central (Grosse, 1926; González, 2001).

3. Apatite (U–Th)/He thermochronology

AHe dating is awell-established thermochronometer with tempera-ture sensitivity between ~40 °C and 80 °C (House et al., 1998; Farley,2002; Ehlers and Farley, 2003; Stockli, 2005), a range that is lower thanthat of any other routinely used thermochronometer. Assuminggeothermal gradients of 20–30 °C/km and mean annual surfacetemperature of ∼15 °C, this temperature range is equivalent to depthsof ∼1.2–3 km. AHe analysis has been used in diverse tectonic settings toinvestigate several geologic processes in the upper crust, includinglandscape and topographic development (House et al.,1998; Clark et al.,2005), orogenic exhumation in collisional orogens (Reiners et al., 2003),uplift/erosion in transpressive environments (Spotila, 2005), develop-ment of rifted margins and escarpments (Persano et al., 2002),and footwall exhumation in extensional settings (Stockli et al., 2000;Stockli, 2005).

AHe thermochronometry of samples collected from near-verticalelevation profiles enables study of the timing, magnitude, and rate ofcooling of rocks as they traverse the upper 1–4 km of the earth's crust,the realm strongly influenced by upper-crustal tectonic and erosionalprocesses (Ring et al., 1999; Ehlers and Farley, 2003; Spotila, 2005;Stockli, 2005; Reiners and Brandon, 2006). Improved reconstructionsof erosional exhumation by AHe are achieved where (1) geothermalgradients have remained relatively stable through time (Mooreand England, 2001); (2) denudation rates are not excessively high

Table 1Summary of (U–Th)/He results for Matasanos and La García vertical transects

Samplenumber

Age ± Std. dev. A/G Mass U Th He Ft Elev. Lat N/LonW–transect(Ma) (Ma) (mg) (ppm) (ppm) (ncc/mg) (m)

SR-9 43.6 2.2 1.6 3/3-2 2.8 17.5 9.5 76.5 0.70 2370 6.46/75.37 – MSR-11 43.4 2.2 2.5 3/2-1 2.8 13.5 7.7 54.0 0.70 2230 6.46/75.37 – MSR-15 48.9 2.4 3.9 5/2 4.6 24.8 3.4 112.3 0.73 2100 6.45/75.37 – MSR-19 40.7 2.0 2.8 5/3-2 4.0 17.7 8.0 54.0 0.73 1990 6.45/75.36 – MSR-6 33.7 1.7 2.5 3/3 9.4 20.3 29.5 82.3 0.72 1710 6.43/75.37 – MSR-2 36.6 1.8 0.8 3/2 4.0 8.4 10.8 34.5 0.70 1520 6.47/75.38 – MSR-40 31.4 1.6 2.5 4/2 3.6 44.4 55.4 156.7 0.71 1500 6.41/74.44 – MSR-41 25.1 1.3 3.5 3/2 3.7 42.5 56.7 115.6 0.70 1380 6.41/75.41 – MSR-CC3 24.2 1.2 2.6 3/2 8.0 18.7 39.7 63.4 0.80 1070 6.76/75.12 – MSR-CC2 23.9 1.2 2.8 3/2 3.1 29.0 18.2 67.0 0.70 1000 6.80/75.14 – MSR-CC1 22.8 1.1 1.5 3/2 3.6 19.0 15.0 39.7 0.70 760 6.86/75.18 – MSR-26 46.7 2.3 3.4 3/3-2 5.7 14.1 15.3 76.1 0.75 2350 6.38/75.59 – GSR-31 42.9 2.1 1.1 3/3 5.6 14.9 15.1 68.2 0.71 2170 6.37/75.59 – GSR-32 41.3 2.1 0.5 3/2 4.3 13.1 13.9 57.6 0.70 2110 6.38/75.60 – GSR-45 45.7 2.3 1.5 4/2 4.1 8.3 9.7 42.0 0.71 2015 6.36/75.59 – GSR-46 40.8 2.0 2.7 3/3 9.2 10.7 11.3 49.2 0.74 1900 6.36/75.59 – GSR-48 32.2 1.6 1.8 4/3-1 4.6 14.7 15.3 49.1 0.70 1710 6.35/75.58 – GSR-44 26.6 1.3 3.8 3/2 3.5 16.7 15.9 45.3 0.69 1640 6.34/75.58 – G

The column marked as “A/G” indicates the number of aliquots (A) and the number of grains (G) per aliquot analyzed for each sample, for instance, in sample SR-9 a total of 3 aliquotswere analyzed and each aliquot was prepared using two to three grains. Averagemass of aliquots run for each sample is indicated in column “Mass (mg)”. 147Sm concentrations (ppm)in all samples analyzed were virtually 0 or below the detection limits.

Fig. 5. Relation between apparent apatite (U–Th)/He ages and elevation integrated forLa García (black squares) and Matasanos (gray triangles) vertical profiles. Error bars are±2σ. Bottom of the exhumed He-PRZ marked by inflection point at ∼1500 m. Upperboundary of the He-PRZ appears to be beyond the maximum elevation sampled. Slopesand R2 values of the regression lines by segments appear in front of the segment'ssymbol for the two exhumation pulses discussed in the text. Block arrows in grayindicate the duration of the Pre-Andina and Proto-Andina orogenic phases of Van derHammen (1960).


(N5mm/yr) as to promote significant heat advection in the upper crust(Brown and Summerfield, 1997; Mancktelow and Grasemann, 1997);(3) pre-existing topography was not rough, i.e., topographic amplitudeN3 km, topographic wavelength b20 km at denudation rates N0.5 mm/yr(Stüwe et al., 1994); and 4) surface relief amplitude has not changedsignificantly since the rocks cooled through the closure temperature,as this has a strong effect on the slope of the age-elevationrelationship and can lead to overestimation/underestimation ofexhumation rates (Braun, 2002). Although AHe can be used toreconstruct exhumation histories even when these criteria are notmet, uncertainties associated with these four factors are minimized ifa vertical profile for sample collection is available and if samples arecollected over a large range of elevations. Extended vertical profilesallow a more thorough analysis of features displayed in age-elevationprofiles such as exhumation events, helium partial retention zone(He-PRZ), and paleogeothermal gradients, which permit betterconstraints on denudation (Stockli et al., 2000; Braun, 2002; Farley,2002). The AP possesses geologic, morphologic and tectonic char-acteristics that make it a perfect scenario to undertake AHe lowtemperature thermochronology studies along vertical profiles.

4. Methods

The AHe technique is based on the accumulation of 4He (α particles)resulting from the radioactive decay of the parent isotopes 235U, 238U,232Th, and 147Sm. Thermokinetics of helium diffusion and α-particlestopping distances in apatite (∼20 μm) are well constrained so thatmeasurements of 4He and its parent isotopes by mass spectrometryallows calculation of the system's closing time (House et al., 1999;Farley, 2000, 2002). Measurements were made by degassingmultigrainaliquots through laser heating and evaluating 4He by isotope-dilutiongas source mass spectrometry, followed by determination of U, Th, andSm on the same crystals by isotope-dilution ICP-MS. Mean (U–Th)/Heages were calculated on the basis of 3–5 apatite replicate analyses andreported age uncertainties (2σ) reflect the reproducibility of replicateapatite analyses (∼6%) of laboratory standard samples (Stockli et al.,2000).

AHe analyses were conducted on samples from two separateelevation profiles located ∼40 km apart in the central portion of theAP. The La García and Matasanos–Porce profiles are situated on aregional scarp developed on the northern flank of the Medellín/PorceRiver canyon. Samples were collected at elevation intervals of ∼70 m,

spanning the largest possible range of paleocrustal depths (∼2 km) inthe exposed structural relief along this margin of the AP (Fig. 3).Apatite concentrates were obtained through the conventional methodof heavy liquid and magnetic susceptibility separation. Nineteensamples yielded good quality inclusion-free, euhedral, non-fracturedapatite with N65 µm in the prism width, suitable for AHe analysis(Farley, 2002; Ehlers and Farley, 2003). AHe apparent ages weremeasured at the (U–Th)/He Laboratory of the University of Kansasfollowing themethods described in Farley (2002), Reiners et al. (2003)and Stockli et al. (2000), and available as a Supplemental file.

5. Results

U, Th and He concentrations, Ft correction for grain dimensionsand alpha ejection corrected He ages are reported in Table 1 for the LaGarcía and Matasanos–Porce profiles. All individual AHe sample

Fig. 6. Relation betweenapparent apatite (U–Th)/He ages (this study; black squares)+AFTages (Saenz, 2003; grey circles) and elevation. Error bars are ±2σ. Gray dotted lines andblock arrows indicate the extent of the Pre-Andina Phase (Van der Hammen, 1960).


aliquot analyses displayed excellent reproducibility. Similarity in theage-elevation relationships for both profiles indicates consistency ofthe data sets and allowed us to combine AHe data into one compositevertical transect (Fig. 5). Most aliquots yielded ages with standarddeviations b2.0 Ma. Only one sample, SR-11, yielded an aliquot withirreproducible 4He concentrations (see Data supplement) and AHe ageof 1088.8 Ma. This was likely caused by parentless 4He hostedby mineral or fluid U- and/or Th-bearing micro-inclusions, which isthe most common reason for irreproducible ages (Stockli et al., 2000;Farley, 2002; Ehlers and Farley, 2003). Small, acicular mineralinclusions were found in some resin-mounted transparent crystalsfrom sample SR-11 when analyzed under high magnification.Spontaneous fission-track distributions in other apatite grains fromthe same separates show negligible zoning, therefore, α-emission

Fig. 7. Simplified sketch illustrating the extent of the crustal section removed since ca. 25 Ma25 million years ago. Right panel is the actual condition with remnants of a Miocene surfacAndean phase (Eu-Andina II–IV, 12 MA to present). Deep and localized incision of the Meddisplacement of point A to A′. Bottom of He-PRZ represents the paleodepth at which A–HAP=Antioqueño Plateau.

corrections, which assume homogenous U and Th distributions(Farley, 2002), can be considered adequate for the population ofgrains analyzed.

AHe ages in the La García vertical transect increase systematicallywith elevation from 26.6±1.3 Ma at the bottom of the scarp to 46.7±2.4Ma at the level of the plateau. Virtually identical resultswere obtainedin theMatasanos transect, with samples ranging in age from22.8±1.1Mato 48.9±2.4Ma (Fig. 5 and Table 1). Variations in AHe ageswith elevationin this study are positively correlated and resemble theoretical Heretention vs. depth curves (Wolf et al., 1998) and age-elevation relation-ships obtained for other He vertical-profile studies (Stockli et al., 2000;Reiners et al., 2003). Positive correlation in our age-elevation profiles,similarity in slope between previous apatite fission-track data (Saenz,2003), andmodern topography of the AP suggest that relief changes havebeen negligible since Eocene time, therefore, topographic corrections bythe admittance ratio (Braun, 2002; Reiners et al., 2003) to ourdata arenotrequired. Further, the combination of U and Th concentrations in ourapatite grains (average 19 ppm) and previous apatite fission-track agesreported for the same geologic province (26–45 Ma, Saenz, 2003) fallwithin the figures recently proposed as the range over which excessiveHe retention due to radiation damage, and concomitant increases in AHeages should be negligible (Green et al., 2006). Even though these aregeneral guidelines, they enhance our confidence that the He kinetics weused in interpreting ourdata are a reasonable assumption, so that noneofour AHe apparent ages are potential overestimates of the time of coolingthrough the He partial retention zone.

The age-elevation relationship for all data is characterized by threesegments (Fig. 5). Segment 1 between 760 and 1600 m displays AHeages from∼22 to ∼26Ma, exhibits very low data dispersion (R2=0.98),and has a slope of 0.24 mm/yr. Segment 2 at sample elevationsbetween 1600 and 2000 m shows a broader range of AHe ages from26–41 Ma (R2b0.2) and a lower slope than the other two segments(0.02 mm/yr). Segment 3 at elevations N2000 m is characterized byapparent ages from ∼41 to 49 Ma, and data scatter higher than insegment 1 but lower than in segment 2 (R2b0.5) and a slope of0.08 mm/yr.

(dotted area). Left panel represents crustal conditions at the onset of major exhumatione that has begun to be obliterated by uplift related denudation during the most recentellín and Porce rivers is shown. Rock uplift discussed in the text is represented by thee ages were zero before onset of rapid exhumation at 25 Ma. PB=Páramo de Belmira,


6. Discussion and interpretation

Our AHe results complement apatite and zircon fission trackthermochronology from the Antioqueño Batholith and associatedplutons providing additional detail to constrain the exhumationhistory of the region during Cenozoic time. The AHe data reveal a

marked cooling event at ca. 23 Ma (Fig. 5). We interpret the almostinvariant He ages of segment 1 as representing a rapid exhumationpulse starting at ∼25 Ma. Segment 2 of the age-elevation profile isinterpreted to be part of the lower segment of the Oligo–Miocene He-PRZ and may represent a period of tectonic quiescence lasting∼17 million years. The change in slope at ∼41 Ma (∼2000 m) is not


Fig. 8. Simplified cartoon of cross-sections depicting the paleogeographic and geologic evolthe ancestral Cordillera Central was covered by a shallow sea and marine sedimentary unplate separated from the Cordillera Central by the ancestral Romeral Fault System. (b) SAntioqueño Batholith (Ksta), a shallow (epizonal) intrusion emplaced into Paleozoic–Pmetamorphism). Initial phase of uplift in the region. Cordillera Central emerges leadingmarginal basins (Proto Magdalena and Proto Cauca basins respectively). Progradation tsedimentation are prevalent over the region of the Proto-Cordillera Oriental. (c) Paleoceneastern margin of the Cordillera Oriental basin is now pronounced. Cordillera Central contiReactivation of vertical movement along the Cauca–Romeral and Palestina fault systems. (Orogeny in Colombia. No deposition taking place in most of Colombia. Exposure of the futurPhase, 24–21 Ma: Major phase of uplift across the Colombian Andes, uplift on the threeOccidental and Oriental still low standing. Localized subsidence in the Cauca and MagdaleMiocene to Present, Eu-Andina Phases I–IV, 18–0Ma: Major uplift and initial inversion of throoted normal faults. Amagá formation is folded, Colombian Andes attain the actual topogbeen removed to expose the bulk of this massive igneous unit. (Major events in this figure ar

as well defined as the one at 25 Ma (1500 m). It is probably indicativeof an exhumation event that was less significant than the Oligoceneepisode.

Apatite fission-track data (Saenz, 2003), although not derivedfrom vertical profiles, provide additional insight to evaluate the AHechange in slope between segments 2 and 3 at ca. 41 Ma. All of theapatite fission-track apparent ages are equal or older than AHe ageswith a mean age of ca. 46 Ma (Fig. 6). The samples show unimodalfission-track length populations with a mean value of 14.1 μmindicating relatively rapid cooling in Eocene time (Saenz, 2003). Thisfinding corroborates our interpretation that the segment 3 of theage-elevation plot records an Eocene exhumation event. The average

and Molnar, 1987; Restrepo and Toussaint, 1988; Restrepo et al., 2007; Tapias et al., 2006;

ution of the northern Andes at the latitude of the study site. (a) Hauterivian–Coniacian:its were being deposited. The Proto-Cordillera Occidental was part of the subductingantonian–Maastrichtian: Increased convergence rates lead to the magmatism of therecambrian crystalline basement and lower Cretaceous marine sequences (contactto a regional unconformity and becomes source of sediment to eastern and westerno the east begins in the eastern margin of the Cordillera Central. Subsidence ande, Laramic Phase, 60 Ma: Generalized coastal deposition took place, inversion of thenued to be gradually uplifted. Initial phases of uplift of the Cordillera Occidental begin.d) Lower to Middle Eocene, Pre-Andina Phases I–II, 54–45 Ma: climax of Pre-Andeane Middle Magdalena Valley as depocenter. (e) Oligo-Miocene transition, Proto-Andinacordilleras. Amagá formation begins to develop in the Cauca depression. Cordillerasna depocenters. Nazca plate subduction begins after splitting of the Farallon plate. (f)e Cordillera Oriental by reactivation of inherited Jurassic–Cretaceous steep and deeplyraphic configuration. The majority of the overburden of the Antioqueño Batholith hase compiled from: this study; Gómez, 2005; González, 2001; Grosse,1926; Pardo-Casas

AHe age for segment 3 (∼41 Ma) is a minimum age for exhumationas these samples resided in the lower temperature part of the HePRZ when exhumation slowed. Nearly concordant AHe and apatitefission-track ages over a crustal section of ∼1500 m (Fig. 6) indicatethat rocks were exhumed from temperatures above the apatitefission-track partial annealing zone (N110 °C) to below most of theAHe PRZ. This suggests that rocks within this zone cooled ca. 50 °Cin 5 million years, which, at a gradient of 25 °C/km, impliesunroofing on the order of 2 km, i.e., ∼0.4 mm/yr. Basal conglom-erates in the Magdalena and Cauca basins deposited at this timealso suggest significant erosion of the AP during the Eocene (e.g.,Gómez et al., 2005; Van der Hammen, 1960).

Van der Hammen, 1961).


The AHe results are consistent with interpretations that kilometer-scale uplift and exhumation in the Northern Andes, as well as in otherportions of the Andes, took place in discrete pulses in the Cenozoic(Steinman, 1929; Van der Hammen, 1960; Mégard et al., 1984). Theseexhumation events occurred during or just after rapid rates ofconvergence between the Farallon (Nazca) and South America(∼150 mm/yr) documented for the Middle Eocene and the LateOligocene (Pardo-Casas and Molnar, 1987), suggesting a relationshipbetween orogeny and subduction dynamics.

Such pulses of exhumation are separated by interludes of tectonicquiescence of durations in excess of 15 million years, spans duringwhich low-standing erosional surfaces (peneplain in the Davisiansense) can develop. The inflection point between segments 1 and 2 ofthe age-elevation plot (Fig. 5) defines the bottom of He-PRZestablished towards the end of the quiescence period between ca. 45and 25 Ma when erosional processes lead to the development of theproto-AP. This pre-early Miocene He-PRZ was exhumed at ca. 25–23 Ma. The base of this fossil He-PRZ (∼80 °C paleo-isotherm) is at amodern depth relative to the present AP surface of ∼1000 m. Beforeexhumation rates increased at ca. 25 Ma, the break in slope was at adepth of about 2.6 km, assuming a geothermal gradient of 25 °C/km anda mean surface temperature of 15 °C. This implies rock uplift ofapproximately 1.5 km (Fig. 7). Based on the assumed paleogeothermalgradient and the temperature difference across the He-PRZ (∼40 °C),the top of the pre-Miocene He-PRZ is probably at∼3100m,which is theelevation average in the Las Baldías Range on top of the AP (Fig. 3).

If our assumptions for the geothermal gradient and pre-existingtopography are correct, only about 900 m of plateau have been removedby erosion in the last 25 Ma, yielding a low erosion rate for an activeorogen (∼0.04 mm/yr). Erosion in the summits of the Las Baldías Range(e.g., PáramodeBelmira)musthavebeenvery limited, i.e., less than200min the last 25 million years (∼0.008 mm/yr), and such surfaces probablyrepresent a position closer to a “true” relict of the original plateau. Ourinterpretation implies very contrasting efficiencies in erosion on thegranitic units of the Antioqueño Batholith (∼0.04 mm/yr) relative to themetamorphic units in Las Baldías Range (0.008 mm/yr) where less than200mof crustalmaterial (and in some cases b100m) have been removedsince LateOligocene. The spatially restricted remnants of Early Cretaceousmarine sedimentary strata present in the area are portions of theoverburden removed by erosion during the development of the presentAP since 25 Ma, as well as during previous exhumation pulses in theEocene and Paleocene.

The low values for background erosion rates derived from ourstudy are in agreement with denudation rates for subdued relief andsoil mantled regions suggested by other authors (Ahnert, 1970;Summerfield and Hulton, 1994) supporting the idea that the AP is arelict surface that has not adjusted to modern erosional conditions ofthe orogen. Such rates are in clear contrast relative to proposed figuresof modern erosion in the region, i.e., ~1 mm/yr (Giraldo, 2005;Restrepo and Syvitski, 2006). The discrepancy may be explained bythe intensification of erosional process due to anthropogenic influencein the Magdalena basin, a problem not addressed in detail in thisinvestigation, but that certainly warrants further consideration.

Chronology and intensity of exhumation pulses of the APdocumented in this study coincide remarkably well with the timingand relativemagnitude of the orogenic phases Proto-Andina (∼24Ma)and Pre-Andina II (∼43 Ma) proposed for the Northern Andes ofColombia by Van der Hammen (1960) based on a stratigraphic andpaleobotanical study. Further, these phases of erosion occurred at thesame time as the discrete orogenic events in Peru known as the Incaicin the Eocene, and The Quechua 1 in the Early Miocene (Mégard,1984). The Late Oligocene–Early Miocene exhumation event recordedin our study is consistent with documented erosional phases for thePeruvian Andes and the Bolivian Eastern Cordillera, where Gregory-Wodzicki (2000) reported an erosive phase at ca. 22 Ma that removed∼2 km of overburden. For the Eastern Cordillera of the Andes of

central Peru apatite fission track data record two exhumation pulsesduring the Neogene at ca. 21 Ma and between 12 Ma and the Present,which removed about ∼4 to 6 km of overburden in the past 30 millionyears (Laubacher and Naeser, 1994). Similarly, an exhumation pulse at∼40 Mawas reported by Benjamin et al. (1987) in his study of fission-track thermochronology for the Bolivian Andes, whereas analogoustiming for Eocene cooling events associated with erosion have beenindicated for the Sierras Pampeanas in Argentina (Coughlin et al.,1998)and northern Bolivia (Barnes et al., 2006). Rapid exhumation events inmountain ranges in the Caribbean region (Rojas-Agramonte et al.,2006) have also been recognized for the Late Eocene and Mid to LateOligocene. This evidence suggests that Cenozoic exhumation pulses inthe Northern Andes and the Caribbean are controlled by continental-scale tectonics. The trigger of these accelerated pulses of erosion maybe related to well documented changes in convergence rates betweenthe Nazca (Farallon) and the South American plates (Hey, 1977; Pardo-Casas and Molnar, 1987; Somoza, 1998), which are in turn related tomajor reorganization of the plates such as the one caused by the breakup of the Farallon Plate into Nazca and Cocos plates.

The period of relative tectonic quiescence between the twoexhumation pulses provides enough time for the development of anerosional surface on which the Terciario Carbonifero de Antioquia ofOligocene–Miocene age was deposited (Grosse, 1926). Our results arealso in agreement with the tectonostratigraphic development ofsedimentary sequences in the Magdalena Inter Andean depressions(Reyes et al., 2002; Gómez et al., 2005) corroborating that the CordilleraCentralwas anelevatedmassif that functioned as an important sourceofdetrital material to major sedimentary basins during the Paleogene andNeogene. Late Paleogene–Neogene sedimentary sequences found in theAP (Parra, personal communication) imply that fluvial activity andconcomitant obliteration of the original erosional surface in the AP havebeen in operation throughout the Neogene.

AHe data in this study do not record post–Middle Mioceneexhumation events, i.e., the Eu–Andina orogenic phase of Van derHammen (1960), which has been invoked as the phase responsible forthe modern configuration of the Andean range and which has alsobeen recorded in many massifs throughout the Andes. The fact thatthe Eu-Andina phase is not verified by AHe data, in conjunction withlow spatially-averaged, long-term erosion rates derived in this study,constitutes compelling evidence that the AP is in geomorphicdisequilibrium.

Themodern expression of the AP developed through a long history(Fig. 8) of multiple phases of erosional surface formation at lowelevation (peneplanation) beginning with its emergence in theSantonian–Early Masstrichtian (Van der Hammen, 1960). The Eoceneand Oligocene–Miocene exhumation events here reported wereinitiated after kilometer-scale orogenic uplift accompanied by fluvialincision and erosion that removed part of the cover of the AntioqueñoBatholith including the majority of the Cretaceous marine formations.

Although there seems to be an agreement in regards to the timingof the major phases of surface uplift-denudation for the Andes,paleoelevation of the region remains for the most part undocumen-ted. Gregory-Wodzicki (2000) suggested that the Colombian Andeswas at ∼40% of its modern elevation by 4 Ma. Similar results, e.g.,uplift of the Colombian Andes concentrated in the last 10 Ma andintensified in the Pliocene, were obtained by Van der Hammen et al.(1973) and by Hooghiemstra et al. (2006) from paleobotany studies inthe Cordillera Oriental. We suggest that the age of the most recentpeneplain in the AP is Miocene. During the Pre-Andina (Eocene) andProto-Andina (Oligo–Miocene) phases the area experienced kilo-meter-scale uplift followed by erosion surface development (reliefobliteration) during intervals of tectonic quiescence. The actualelevation of the AP was attained with the onset of the Eu-Andinaphase, particularly the sub-phases II through IV (12 Ma to present)when the erosion surface raised from close to sea level (e.g., 500m) toca. 3600 m. However, reliable paleoelevation data for the Cordillera


Central is scant so the question about the progression of elevationwithin the context of orogenic pulses and uplift phases in the regionhas only been addressed indirectly, particularly in relation to theevolution of depocenters in the Magdalena Basin and its develop-ment from a foreland basin to an Interandean basin during theCenozoic (Gómez et al., 2005; Reyes et al., 2002).

Deep incision in the AP along the Medellín–Porce river is probably arecent feature developed during surface uplift in the Pliocene. Moderngeomorphic expression of the AP is the result of recent fluvial processescontrolled by nick point propagation. The closest point to a relictMesozoic surface may be found at the summits of Páramo de Belmira,althoughglacial erosionduringPaleocene–Holoceneglacial periodsmayhave removed some of it (∼100 m). The saprolitic cover of the AP isabsent above 3300 m elevation where the landscape is dominated byremnants of glacial landforms and truncated summits.

Similarity in the age versus elevation plots for both profiles (Fig. 5) isinterpreted as indicating exhumation of the entire AP as a discrete unit.Our results support the hypothesis that the Northern Cordillera Centralis part of a structurally coherent crustal blockwithin the orogen (Monteset al., 2005). The rigidity of the Northern Cordillera Central block, wheretheAP is located,mayhave controlled thenearly circularoutcroppatternof the Antioqueño batholith. Cross-sections presented by Cortes andAngelier (2005) show a major structural zone at ∼7°N/75°–77°W withhigh-angle, reverse faults flanking the AP (Palestina and Romeral faultsystems in the east and west respectively). These structures areinherited from the basement and bound the AP block. Rigidity of theblock and bounding faultsmay have controlled the structural continuityof the area aswell as themode of emplacement and overall shape of theAntioqueño batholith, thus imposing litho-structural control in thegeomorphic evolution of the AP. Reactivation of these major crustalstructures during periods of enhanced convergencemay have facilitateduplift of the AP as a coherent block.

The Cenozoic uplift and exhumation history of the northernCordillera Central of Colombia resulted from plate-tectonic reorganiza-tion and directly affected themorphotectonic evolution of the AP aswellas the development of the sub-Andean fold-and-thrust belt and the filland tectonostratigraphic evolution of the intramontane and forelandbasins to the east (Magdalena, Llanos Basin) and west (Cauca). This isparticularly clear at the end of the Oligocene when the Farallon platebrokeup into theNazca andCocos plates (Wortel,1984; Pardo-Casas andMolnar, 1987). Augmented convergence rates induced synchronoustectonic and geomorphic activity on the overriding plate (i.e., uplift,exhumation, deformation, shortening, etc.) fromArgentina to Colombia.A similar effect was also observed during an earlier phase of rapidconvergence in themiddle Eocene. Additional control to themore recentmorphotectonic history in the area is related to thewest-to-eastmotionof the Caribbean plate and the collision of the Panama–Chocó block at∼10 Ma, which closed the Panama isthmus between 3.7 and 3.4 Ma(Duque-Caro, 1990) triggering the most recent orogenic phase in theregion and giving the Colombian Andes its present topographicconfiguration (Kroonenberg et al., 1990). These main phases of upliftof the Northern Andes led to important paleogeographic events (Hoornet al., 1995) and to erosion of a large volume of sediments resulting inthick basal sandstones and molasse successions in the sub-Andeanbasins (Van der Hammen,1960; Gómez et al., 2005). Age control for twoof thesemajor periods is accurately providedbyourAHedatafittingwellthe spatiotemporal framework for the evolution of the ColombianAndesin the context of continental tectonics.

Acknowledgements

This work is part of Sergio Restrepo's Ph.D. investigation. Sergiowants to thank his family for continued support. The study was fundedby the Compton Foundation, American Geological Institute, NationalScience Foundation (SEAGEP Program), Corporación para la Investiga-ción y el Ecodesarrollo Regional (NGO Colombia), University of Florida

Department of Geological Sciences, Geological Society of America(to Restrepo), and NSF EAR-0414817 (to Stockli). We thank CarlosSuescún and Camilo Polanco for their help in the field, and StephanieBrichau and others at the University of Kansas (U–Th)/He Laboratory forlaboratory assistance. The Geomorphology Group at ICNE-UniversidadNacional de Colombia provided unpublished information and valuablediscussions. Juan D.Moreno and Patricia Monsalve were of great help ingenerating the graphics that accompany the text. Thanks are also due tothe reviewers of this manuscript, B. Kohn and J. Hourigan, as their inputand criticism helped improve the scientific quality of the article.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.epsl.2008.09.037.

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