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

Abstract

The Liquiñe–Ofquifault zone (LOFZ) cuts the Patagonian batholith and the modern volcanic arc of southern Chile for ca 1000 km. The rock fabric along three transects of the LOFZ combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike. In the Liquiñe transect (39°S), a 1 km wide, northeast-striking subvertical mylonitic zone shows north-plunging stretching lineations. 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 and sinistral reverse displacement. Deformation pre-dates a 100±2 Ma undeformed porphyritic dike (hornblende 40Ar– 39Ar mean age). In the Reloncavı́ transect (41–42°S), deformation in Cretaceous and Miocene plutons is predominantly brittle. Kinematic analysis of two fault populations yields compressional and dextral strike-slip stress regimes, interpreted as late Miocene in age. In the Hornopirén transect (42–43°S), a 4 km wide mylonitic zone, developed in plutons and wallrocks, shows subhorizontal stretching lineations and dextral displacement. A single fault population overprinting the mylonites supports a dextral strike-slip stress regime. Available U–Pb, K–Ar and 40Ar– 39Ar dates on deformed Cenozoic plutons and wallrock range from 9 to 13 Ma on hornblende and from 6 to 3 Ma on biotite. Microstructures and mineral assemblages indicate that the youngest ductile fabrics 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 Nazca plate beneath western South America has driven long-term intra-arc deformation at the southern Andes plate boundary zone; ridge collision, in turn, has favored dextral displacement at the leading edge of the continent since the Pliocene