Pleistocene to Present North Andean “escape”

This study compiles 20 published field geologic estimates of displacement rates for the northern Andes, such as displaced glacial moraines and offset pyroclastic flow, and compares them to published Global Positioning System (GPS) measurements. Dated displacements compiled in this study were obtained from the Gulf of Guayaquil, Pallatanga, Chingual-la Sofia, and Cayambe-Afiladores-Sibundoy fault systems in Ecuador and southern Colombia and the Boconó fault system in Venezuela. Right-lateral slip estimates on the individual fault segments range from 2 mm/a to 10 mm/a. The mean estimated geologic slip rate for the last 86,000 years is 7.6 mm/a. This estimate is very similar to the GPS measurements of Present day motion at the 2 sigma level. Published GPS results suggest that a large part of the northern Andes is “escaping” to the northeast relative to stable South America at a rate of 6 ± 2 mm/a. The GPS displacement rates of seven sites in Venezuela, Colombia, and Ecuador are statistically identical at the 95% confidence level. Four geologic estimates indicate that slip rates of 4 to 10 mm/a continued back to 1.8 Ma. No geologic slip estimates have been reported for Ecuador prior to that time period. The “escape” of the North Andes is believed to be a result of increased coupling between the obliquely subducting Nazca plate and the overriding South American plate due to the subduction of the Carnegie Ridge in the Ecuador–Colombia trench. If this is correct, the slip estimates for the North Andes suggest that the Carnegie Ridge arrived at the trench prior to 1.8 Ma. In the Eastern Cordillera of Colombia, strike-slip crustal earthquakes reflect slip partitioning on high angle faults located above crustal detachment ramps across a 200 km wide zone. Intermediate depth mantle earthquakes indicate that brittle shearing extends to the base of the lithosphere.