Structural Styles of the Eastern Cordillera of Colombia
By Andreas KAMMER, Alejandro PIRAQUIVE
, Cristhian GÓMEZ, Andrés MORA and Antonio VELÁSQUEZ
Pages 119–137
Manuscript accepted April 15, 2019
Abstract
The Eastern Cordillera of Colombia is bracketed between the moderately west–dipping flank of the Central Cordillera on its western side and the little disturbed to gently bent Guiana Shield on its eastern side. Unlike other Andean foreland–oriented belts, it is completely disconnected from the main Andean trunk system. Transverse shortening of 4 mm/y records a considerable displacement transfer to the upper plate; this is twice the long–term rate of 2.2 mm/y, which is the average for a shortening of 65 km over a period of 30 Ma and suggests an increased recent shortening phase. We differentiate three structural domains. The southern domain records significant shortening by penetrative strain at lower structural levels and folding at higher structural levels, which supports the idea of partitioning into pure–shear deformation within the pre–Cretaceous basement and into buckling in the Upper Cretaceous to Paleogene units. Similar constellations of a relatively weak crustal welt enclosed between domains with backstop characteristics have been examined in analogue and numerical experiments (“vise model”). A northern intermediate domain is characterized by large–scale, basement–cored antiforms, whose formation may be ascribed to the partial reactivation of Late Triassic normal faults. The northernmost domain comprises the Cocuy Syntaxis, which constitutes an antiformal lobe with significant topographic relief. It is affected by secondary folds with a down–slope vergence. These changes in structural style record increased support by the subducting slab, according to the spatial coincidence of the outer slab hinge and the highest topographic relief within this northern Andean flat slab segment. To examine a possible cause for a rheological break between deformable cordilleran crust sandwiched between relatively rigid and strong surrounding basement blocks, we review the Cretaceous back–arc setting. Tectonic subsidence and sedimentation patterns suggest its division into a forebulge and flanking basins that may be ascribed to the framework of an impinging mantle plume. Temporal constraints further suggest that forebulge evolution may have been triggered by an initial foundering of the slab at the onset of a Cretaceous subduction cycle. In such a small–scale convection system, downwelling mantle flow between the back–arc region and the stable shield may maintain a relatively well–defined rheological limit over long periods. This situation complies with the model of an edge–driven convective flow. Rheological contrasts between the back–arc basin and the shield persisted into the Cenozoic and contributed to a protracted evolution of the Andean cordilleran mountain fronts. The eastern mountain front accumulated structural relief of more than 10 000 m during an initial Oligocene to late Miocene shortening phase. Fold growth incited by the buttressing of the strong foreland block of the shield can be tested by the tri–shear model, in which propagation of faulting is halted, and displacement is consumed by fold amplification. During a Pliocene stress reorganization, this Miocene mountain front was breached by a shallowly dipping thrust, which gave rise to a more foreland–oriented deformation front. During this outward–stepped faulting, proximal foreland sequences became involved in a wedge–top position and were exhumed at the thrust tip along emergent ramps. These second–cycle erosion products were widely dispersed into the Llanos Basin and were incorporated into modern fluvial terraces.
Keywords: back–arc basin, fault reactivation, gravitational collapse, Eastern Cordillera of Colombia.