Abstract

The integration of restored basin geometry and internal features of syntectonic units (e.g., stratal architecture, thickness, sandstone composition) with flexural modeling of the lithosphere constrains the evolution of a basin and its flexural history related to orogenic growth (spatial/temporal loading configuration). 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 five tectono-stratigraphic sequences in the foreland basin and five phases of shortening for the Eastern Cordillera. Thermo-chronological and geochronological data support the spatial and temporal evolution of the orogen–foreland basin pair.

Tectono-stratigraphic sequences were identified in two restored cross sections, one located at a salient and the other in a recess on the eastern flank of the Eastern Cordillera. The lower two sequences correspond to late Maastrichtian–Paleocene flexural events and record the eastward migration of both tectonic loading and depositional zero in the Llanos Basin. These sequences consist of amalgamated quartzarenites that abruptly grade upward to organic-rich fine-grained beds and, to the top, light-colored mud-stones interbedded with litharenites in isolated channels. Amalgamated conglomeratic quartzose sandstones of the third sequence record ~15 m.y. of slow subsidence in the Llanos Basin and Llanos foothills during early to middle Eocene time, while shortening was taking place farther west in the Magdalena Valley. The fourth sequence, of late Eocene–middle Miocene age, records a new episode of eastward migration of tectonic loads and depositional zero in the Llanos Basin. This sequence begins with deposition of thick fine-grained strata to the west, whereas to the east, in the Llanos basin, amalgamated quartzarenites unconformably overlie Cretaceous and older rocks (former forebulge). Apatite fission tracks in the axial zone of the Eastern Cordillera, growth strata in the Llanos foothills, and synextensional strata on the forebulge of the Llanos Basin constrain deformation patterns for this time. The post–middle Miocene Andean event initiated with regional flooding of the foreland basin in response the widening of tectonic loading, after which the foredeep was filled with coarse-grained alluvial and fluvial detritus derived from the Eastern Cordillera.

The geometry of tectonic loads, constrained by flexural 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 configuration and had equivalent tectonic loads of 10–11 km.

Integrated analysis is necessary in polyphase orogenic belts to define the spatial and temporal variation of tectonic load and foreland basin configurations and to serve studies that seek to quantify exhumation and three-dimensional analyses of thrust belts. For the Eastern Cordillera, thermochronological sampling must span the width of the Eastern Cordillera rather than be concentrated in a single range.

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