Contrasting nature of deformation along an intra-arc shear zone, the Liquine-Ofqui fault zone, southern Chilean Andes

The Liquine-Ofquifault zone (LOFZ) cuts the Patagonian batholith and the mo dern volcanic are of southern Chile for ca 1000 km. The rock fabric along t hree transects of the LOFZ combined with new and published geochronological data suggest marked differences in the nature and timing of deformation al ong strike. In the Liquine transect (39 degrees S), a 1 km wide, northeast- striking subvertical mylonitic zone shows north-plunging stretching lineati ons. This mylonitic zone has been juxtaposed by high-angle reverse faulting against a nearly undeformed Miocene granitoid, Metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions an d sinistral reverse displacement. Deformation pre-dates a 100 +/- 2 Ma unde formed porphyritic dike (hornblende Ar-40-Ar-39 mean age). In the Reloncavi transect (41-42 degrees S), deformation in Cretaceous and Miocene plutons is predominantly brittle. Kinematic analysis of two fault populations yield s compressional and dextral strike-slip stress regimes, interpreted as late Miocene in age. In the Hornopiren transect (42-43 degrees S), a 4 km wide mylonitic zone, developed in plutons and wallrocks, shows subhorizontal str etching lineations and dextral displacement. A single fault population over printing the mylonites supports a dextral strike-slip stress regime. Availa ble U-Pb, K-Ar and Ar-40-Ar-39 dates on deformed Cenozoic plutons and wallr ock range from 9 to 13 Ma on hornblende and from 6 to 3 Ma on biotite. Micr ostructures and mineral assemblages indicate that the youngest ductile fabr ics in the plutons formed at greenschist facies, similar to the biotite Ar closure temperature. Subparallel magmatic and solid-state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons, Dextral-oblique subduction of the N azca plate beneath western South America has driven long-term intra-are def ormation at the southern Andes plate boundary zone; ridge collision, in tur n, has favored dextral displacement at the leading edge of the continent si nce the Pliocene (C) 2000 Elsevier Science B.V. All rights reserved.