Abstract

In the Cordillera de la Costa of the central Andes in northern Chile, Mesozoic arc complexes are cut by a trench-parallel strike-slip fault system: the Atacama fault system. Brittle faulting in the Atacama fault system is superposed on steeply dipping foliations in ductile shear belts. Between 25°S and 27°S, the western part of the fault system was active in Early Cretaceous time as an upper amphibolite facies, down-to-the-east, dip-slip ductile shear zone. In the eastern part of the fault system, ductile deformation is of similar Early Cretaceous age but occurred under lower-grade metamorphic conditions at the greenschist/lower amphibolite facies transition. The mylonites in the eastern part of the fault system were formed by sinistral strike-slip displacement.

The dip-slip and sinistral strike-slip displacements are contemporary with the development of a magmatic arc, and they imply that the tectonic environment in this part of the arc was transtensional. The ductile deformation was partitioned spatially into a dip-slip component associated with the emplacement of magmas and a sinistral strike-slip component.

Brittle fault zones in the El Salado segment of the Atacama fault system define large-scale sidewall ripout structures. Subhorizontai slickenlines, ripout asymmetry, and S-C-type fabrics in fault gouge indicate that brittle deformation involved sinistral strike-slip displacements. The transition from ductile to brittle sinistral strike-slip displacements may have occurred due to cooling, in mid-Cretaceous time, when the magmatic arc was abandoned.

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