Resumen
The Caleta Coloso Duplex is a brittle strike-slip structure developed along the Atacama Fault System during the Early Cretaceous. A hydrothermal vein system existing within the duplex documents the nature of the link between fluid transport and progressive structural development. The dominantly NW-striking vein system occurs near or at the damage zone of the duplex fault zones. Veins can be classified according to their composition and crosscutting relationships into early chlorite veins, interrnediate epidote-quartz veins and late calcite-limonite veins. Sorne of them exhibit minerals with their long axes oriented orthogonally or obliquely with respect to the vein walls (extension veins and oblique-extension veins), whereas others show mineral fibers that are parallel to striae ofthe hosting faults (fault-veins). The latter show kinematic indicators that are compatible with those ofthe faults, suggesting they were synkinematic with strike-slip duplex development. The vein microstructure suggests that they form from fluid-ti Ued open fractures under subhydrosthatic pressures suggesting that mineral precipitation was triggered by pressure drops at shallow crustallevels ( <4 km) consistent with the suction pump mechanism. 40 Ar/39 Ar and K-Ar ages between 119 and 141 Ma are interpreted as the approximate time window within which the duplex and veins were developed.