Net right-lateral slip of the Eastern Frontal Fault System, North Andes Sliver, northwestern South America

Abstract

The NNE-directed tectonic escape of the North Andean block (NAB) or sliver (NAS) with respect to South America (SA) is generally accepted now. NAS is detached from continental South America (SA) by a 2000-km-long, NE-SW-trending dominant dextral fault system, known as the Eastern Frontal Front System (EFFS; Pennington's name and acronym kept for simplicity) that extends from the gulf of Guayaquil-Tumbes basin (GGTB) in offshore Ecuador to the Caribbean coasts of Venezuela in the Golfo Triste region. From SW to NE, EFFS comprises the following fault systems: In Ecuador, Puná-Pallatanga-Cosangá-Chingual (CCPP); In Colombia, Afiladores, Sibundoy, Algeciras and Guaicáramo; and in Venezuela, Boconó (BF).Several estimates of total dextral slip have been calculated along the EFFS. In Ecuador, the Pallatanga fault (part of the CCPP) displaces several large geologic and morphologic units in about 10 km. The net slip of EFFS at this western front of the Western cordillera can be as much as the double (≈20 km) if strain happens to be distributed between sub-parallel faults. Further south, at the very southern tip of the EFFS, from seismic data acquired from the GGTB, an estimate of 13.5–20 km of lengthening of this basin from the Plio-Quaternary boundary has been proposed.In Colombia, the Algeciras lazy Z-shaped pull-apart basin (Huila Dpt.) provides another estimate. The extent of the Quaternary basin fill measured along the regional fault trend on SGC geologic plate 345, is about 10 km. This is a minimum estimate because the fault shortcuts the basin in more recent times, increasing such slip in at least about 1–2 km. In Venezuela, although more reliable and frequent dextral offsets are of the order of 30 km, a larger 70–80 km offset in Mesozoic rocks was once proposed across BF. A supportive argument to the 30 km net slip along the Mérida Andes axis may be derived from a present mismatch of Bouguer anomaly minima between both adjacent flexural basins of no more than 40 km.From all the estimates of total slip here compiled along EFFS, we can conclude that the NE tectonic escape of NAS is most likely not larger than 30–35 km. This net slip may vary along EFFS strike, being smaller (close to 20 km) in the south. Therefore, despite the large size of NAS, its escape remains modest at plate scale. Remarkably, EFFS net slip is in the same order of magnitude as the ≈20 km for the Sumatra fault, which is as long as the EFFS (≈1900 km long).