Range-Parallel Extension in the Argentinian Andes: The 2020 Mw 5.7 Humahuaca Earthquake

Normal-faulting earthquakes in mountain ranges are key for studying the dynamics of mountain building. Two styles of high mountain extension have been observed: range-perpendicular and range-parallel. To date, range-parallel extension has only been reported in southern Tibet, limiting our ability to test different models for its dynamic cause. Here, we investigate a new example of range-parallel extension: the 2020 M_w 5.7 Humahuaca earthquake in the Andes of Argentina. We combine InSAR time-series and body-waveform seismology to constrain a source model for the earthquake and show it ruptured a new fault that cross-cuts Neogene fold-thrust belt structures and accommodates pure range-parallel extension. The hypocentre lies ∼70 km west of the Andes range front at 5 km depth. Thrust-faulting earthquakes on the Andes range front adjacent to Humahuaca have slip vectors parallel to topographic gradients and are oblique to Nazca-South America relative motion, consistent with the pattern expected for crustal flow in response to gravitational potential energy contrasts. Interseismic GPS velocities, however, are oblique to the range front and topographic gradients. These velocities may be accommodated by range-parallel shear, with normal faulting at Humahuaca potentially occurring in the step-over of a strike-slip fault or due to clockwise rigid block rotation, although geomorphic evidence is lacking. Notably, we do not see evidence for widespread ‘lateral escape’ in the Andes, as proposed for southern Tibet. In conclusion, range-parallel extension in the Andes may be the result of crustal flow under gravity or back-arc strike-slip faulting. Both models indicate the potential for moderate-magnitude earthquakes within the Eastern Cordillera, which are an overlooked source of seismic hazard .