ACTIVE TECTONICS OF THE ANDES

Nearly 90 mm a-1 of relative plate convergence is absorbed in the Andean plate-boundary zone. The pattern of active tectonics shows remarkable variations in the way in which the plate slip vector is partitioned into displacement and strain and the ways in which compatibility between different segments is solved. Along any traverse across the plate-boundary zone, the sum of relative velocities between points must equal the relative plate motion. We have developed a kinematic synthesis of displacement and strain partitioning in the Andes from 47-degrees-S to 5-degrees-N relevant for the last 5 Ma based upon: (1) relative plate motion deduced from oceanic circuits giving a roughly constant azimuth between 075 and 080; (2) moment tensor solutions for over 120 crustal earthquakes since 1960; (3) structural studies of deformed Plio-Pleistocene rocks; (4) topographic/geomorphic studies; (5) palaeomagnetic data; and (6) geodetic data. We recognize four neotectonic zones, with subzones and boundary transfer zones. that are partitioned in different ways. These zones are not coincident with the 'classic' zones defined by the presence or absence of a volcanic chain or differences in finite displacements and strains and tectonic form; the long-term segmentation and finite evolution of the Andes may not occur in constantly defined segments in space and time. In Segment 1 (47-degrees-39-degrees-S), the slip vector is partitioned into roughly orthogonal Benioff Zone slip with large magnitude/large slip-surface earthquakes and both distributed dextral shear giving clockwise rotations of up to 50-degrees and dextral slip in the curved Liquine-Ofqui Fault System giving 5-degrees-10-degrees of anticlockwise fore-arc rotation. In Segment 2 (39-degrees-20-degrees-S), the slip vector is partitioned into Benioff Zone slip roughly parallel with the slip vector, Andean crustal shortening and a very small component of dextral slip, including that on the Atacama Fault System. Between 39-degrees and 34-degrees-S, a cross-strike dextral transfer, which deflects the Chile trench and the volcanic arc, absorbs the shortening contrast between Segments 1 and 2. In Segment 3 (20-degrees-6-degrees-S), the slip vector is partitioned into roughly orthogonal Benioff Zone slip, crustal shortening, sinistral trench-parallel faulting and northeast-southwest extension. Compatibility between Segments 2 and 3 is maintained by the sinistral east-southeast-trending Cochabamba shear zone and north-trending dextral faults. In Segment 4 (6-degrees-S to 5-degrees-N), the slip vector is partitioned into roughly orthogonal Benioff Zone slip and dextral strike-slip faulting in the fore-arc and volcanic chain.