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

West verging thrusts, synthetic with the Nazca-South America subduction interface, have been recently discovered at the western front of the Andes. At similar to 33 degrees 30'S, the active San Ramon fault stands as the 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 West Andean fold-and-thrust belt based on field observations, satellite imagery, and previous structural data, together with digital topography. We then reconstruct the evolution of this frontal belt using a trishear kinematic approach. Our reconstruction implies westward propagation of deformation with a total shortening of 9-15 kmaccumulated over the last 25 Myr. Anoverall long-term shortening rate of 0.1-0.5 mm/yr is deduced. The maximum value of this shortening rate compares well with the rate that may be inferred from recent trench data across the San Ramon fault and the slip associated with the past two M-w> 7 earthquakes. This suggests that the San Ramon fault is most probably the only presently active fault of the West Andean fold-and-thrust-belt and that most-if not all-the deformation is to be released seismically.