Tectonics, magmatism and fluid flow in a transtensional strike-slip setting: The northern termination of the Liquiñe-Ofqui fault System

Abstract

One fundamental question in continental margin tectonics is the nature of theinterplay between tectonics and magma/fluid transport in the lithosphere. Deformation-driven faultfracturenetworks are efficient pathways through which magma and/or hydrothermal fluids aretransported, stored and eventually connected to the earth surface. Thus, the state of stress of thelithosphere at the time of magma/fluid transport should control the first and second-order spatialdistribution of dikes swarms, volcanic centers and geothermal reservoirs.We conducted a structural mapping of the geometry, kinematics and relative timing of first andsecond-order fault systems and their spatially associated volcanoes and fault-vein networks atregional and local scales at the northern termination of the LOFS, which is characterized by atranstensional horsetail structure. Volcanoes and hydrothermal vein systems are spatially andtemporally associated with NNE master and ENE subsidiary faults of the LOFS and with NWstrikinglong-lived basement faults. ENE-elongated clusters of minor eruptive centers and flank ventsare common. Compositions range from basalts and basaltic andesites in the ENE-chains to moreevolved magmas at mature stratovolcanoes lying on top of NW-striking structures. We hypothesizethat the kinematics of fault-fracture networks under which magma is transported through the crust isone fundamental factor controlling the wide variety of volcanic forms, volcanic alignment patterns androck compositions observed. Whereas bulk intra-arc compression (vertical σ3) favors longerresidence times in the highly misoriented WNW-striking long-lived structures, transtension associatedwith margin-parallel strike-slip deformation (horizontal σ3) provides subvertical pathways for magmaascent and shorter residence times. This in turn prevents advanced magma differentiation.The overprinted geothermal system is documented by NNE and ENE striking calcite-quartz hybridand extensional vein systems, which appear to be associated with dextral strike-slip displacement onthe LOFS. Fault-vein and vein structure varies from mineral fibers to typical ridge-and-groove striae.Bladed calcite occurs in dilational jogs along the main LOFS master faults; they are interpreted torepresent boiling episodes. Thicker and more pervasive WNW sinistral-reverse fault-vein systemsand breccias bodies suggest that the fault-valve mechanism was active during fluid transport andmineral precipitation. In some sites the WNW-striking system cuts and displaces the active LOFS,suggesting that their active has extended to at least the Pleistocene. Internally consistent structural andkinematic data from fault-fracture systems spatially and temporally associated with volcanoes andhydrothermal systems suggest that the same processes that drive the interplay between volcanism andtectonics may also control the nature, geometry and composition of geothermal reservoirs in thesouthern Andes.