The integration of restored basin geometry and internal features of syntectonic units (e.g., stratal architecture, thickness, sandstone composition) with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth (spatial/temporal loading confi guration). Using this approach, we determined the Maastrichtian-Cenozoic polyphase growth of the Eastern Cordillera of Colombia, an inverted Mesozoic extensional basin. The record of this growth occurs in an Andean (post-middle Miocene) thrust belt (the Eastern Cordillera) and in adjacent foreland basins, such as the Llanos Basin to the east. This approach permitted the identification of fi ve tectono-stratigraphic sequences in the foreland basin and fi ve phases of shortening for the Eastern Cordillera. Thermochronological and geochronological data support the spatial and temporal evolution of the orogen-foreland basin pair. The geometry of tectonic loads, constrained by fl exural models, reveals shortening events of greater magnitude for the uppermost two sequences than for pre-middle Eocene sequences. Tectonic loads for the late Maastrichtian-middle Eocene phases of shortening were less than 3 km high and 100 km wide. For the late Eocene-middle Miocene phase, tectonic loads changed southward from 6 km to less than 4 km, and loads were wider to the north. The strong Andean inversion formed today's Eastern Cordillera structural confi guration and had equivalent tectonic loads of 10-11 km.Integrated analysis is necessary in polyphase orogenic belts to defi ne the spatial and temporal variation of tectonic load and foreland basin confi gurations and to serve studies that seek to quantify exhumation and threedimensional analyses of thrust belts. For the Bayona et al. 1172Geological Society of America Bulletin, September/October 2008 Eastern Cordillera, thermochronological sampling must span the width of the Eastern Cordillera rather than be concentrated in a single range.

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The timing of orogeny in the northern Andes and the mechanism driving it are still debated. We have studied the age, composition and provenance of granitoids and sandstones of the Santa Marta Massif and Rancheria Basin, northern Colombia, to relate deep-seated and surface tectonic processes attending the Late Cretaceous-Palaeogene history of the northern Andes. Our results indicate the development of five tectonic episodes: (1) collision of northwestern South America with a 92-80 Ma Caribbean arc (70 Ma); (2) late-collisional to early-subduction metamorphism and magmatism (65 Ma); (3) distal accumulation of a thick siliciclastic sequence (60-58 Ma); (4) renewed arc magmatism (58-50 Ma); and (5) magmatic quiescence and block uplift (post-50 Ma). The first episodes are related to the onset of subduction, and the last episode is related to shallow subduction and oblique convergence. Similar events in Colombia and Ecuador reveal that the Late Cretaceous-Eocene orogeny of the northern Andes was influenced by the collision and subduction of the Caribbean oceanic plate.

The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (∼5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (≥0.2 Km/My) during 65–58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (≥0.32 Km/My) during 50–40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40–25 Ma (≥0.15 Km/My) is recorded proximal to the Santa Marta-Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26–29 Ma (∼0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30–25 Ma and 25–16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ∼16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ≤1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone

The upper Campanian^Lower Eocene synorogenic sedimentary wedge of the Rancher|¤ a Basin was deposited in an intraplate basin resting on a tilted continental crustal block that was deformed by collision and subsequent subduction of the Caribbean Plate. Upper Cretaceous^Lower Eocene strata rest unconformably upon Jurassic igneous rocks of the Santa Marta Massif, with no major thrust faults separating the Santa Marta Massif from the Rancher|¤ a Basin.The upper Campanian^Lower Eocene succession includes, from base to top: foraminifera-rich calcareous mudstone, mixed carbonate^siliciclastic strata and mudstone, coal and immature £uvial sandstone beds. Diachronous collision and eastward tilting of the plate margin (Santa Marta Massif and Central Cordillera) favoured the generation of accommodation space in a continuous intraplate basin (Rancher|¤ a, Cesar and western Maracaibo) during the Maastrichtian to Late Palaeocene.Terrigenous detritus from the distal colliding margin ¢lled the western segments of the continuous intraplate basin (Rancher|¤ a and Cesar Basins); in the Late Paleocene, continental depositional systems migrated eastwards as far as the western Maracaibo Basin. In Early Eocene time, reactivation of former extensional structures fragmented the intraplate basin into the Rancher|¤ a-Cesar Basins to the west, and the western Maracaibo Basin and Palmar High to the East.This scenario of continent^oceanic arc collision, crustal-scale tilting, intraplate basin generation and fault reactivation may apply for Upper Cretaceous^Palaeogene syntectonic basins in western Colombia and Ecuador, and should be considered in other settings where arc^continent collision is followed by subduction. EAGE