Kinematics of the active West Andean fold-and-thrust belt (central Chile): Structure and long-term shortening rate

Abstract

West verging thrusts, synthetic with the Nazca-South America subduction interface, have beenrecently discovered at the western front of the Andes. At ~33°300S, the active San Ramón fault stands asthe most frontal of these west verging structures and represents a major earthquake threat for Santiago,capital city of Chile. Here we elaborate a detailed 3-D structural map and a precise cross section of the WestAndean fold-and-thrust belt based onfield observations, satellite imagery, and previous structural data,together with digital topography. We then reconstruct the evolution of this frontal belt using a trishearkinematic approach. Our reconstruction implies westward propagation of deformation with a totalshorteningof9–15 kmaccumulatedoverthelast25 Myr.Anoveralllong-termshorteningrateof0.1–0.5 mm/yris deduced. The maximum value of this shortening rate compares well with the rate that may be inferred fromrecent trench data across the San Ramón fault and the slip associated with the past twoMw>7 earthquakes.This suggests that the San Ramón fault is most probably the only presently active fault of the West Andeanfold-and-thrust-belt and that most—if not all—the deformation is to be released seismically.