The first local seismic network established in the remote region bordering the Chile triple junction monitored earthquakes at a rate of three per day with magnitudes M = 0 to M = 4, far smaller than any recorded previously from that region. Focal mechanisms were obtained by the adaptation of the relative amplitude method to local earthquakes. Epicentral locations define a lineament corresponding to the active segment of a subducted transform fault, and events corresponding to the predicted position of a subducted ridge show extensional mechanisms, with the exclusion of thrusting and trench-parallel mechanisms. These data suggest that oceanic spreading and seismic activity continue in the subducted ridge-transform system. Extension in the slab influences the overlying crust, giving rise to dextral transtension. Over the past 10 m.y., the locus of ridge subduction has migrated northward; we postulate that the pattern through time of extension and basin development in the overlying crust has mirrored this.

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