Backarc rifting as a response to a crustal collapse at the western Gondwana margin: The Triassic tectonic setting of the Sierra Nevada de Santa Marta, Northern Andes of Colombia

Since the middle Triassic the long-lived convergent margin of western Gondwana evolved from a relatively steeply inclined into a flat lying slab setting that combined an extensional regime on the backarc side with the telescoping of crustal slices at the continental margin. In the Northern Andes the opening of Late Triassic basins is practically contemporaneous with the outwedging of lower crustal slices, that often alternate with intrusive sheets of S-type granites and mark the limit to a  non-metamorphic roof. A tectonic coupling between backarc collapse and the escape of lower crustal slices can be examined in detail in the northwestern flank of the Sierra Nevada de Santa Marta, a northern-most outlier of the North Andean basement. Remnants of a Late Triassic graben fill attest here to a block tilted toward the hinterland. Its tri-partite sedimentary sequence recycled material sourced from external parts of the continental margin. The basement of a more foreland-oriented block of the Sevilla belt is affected by outward-verging folds, which have formed under greenschist facies conditions in its upper and lower amphibolite conditions in its lower part. The succeeding Inner Santa Marta Metamorphic Belt consists of a stack of high-grade metamorphic basement slices separated by siliciclastic wedges metamorphosed under lower amphibolite conditions. The soles of the basement slices consist of migmatites with remobilized granitic pods and resulting folds oriented in a dip-slip direction. These structures are overprinted by a flattening and a second migmatitic event, which records peak P-T conditions of a lowest crustal level. Accordingly, they contain inclusions of ultramafic rocks. The time-equivalent correspondence between a supracrustal  backarc extension and a foreland-directed stacking of crustal slices suggests some similarity to the model  of a low-viscosity channel of a thickened orogenic crust. An important difference of this flat-slab setting resides, however, in a wholesale mobility of a strongly heated crust that constitutes the backarc and frontal position of this active margin.