Ushuaia and Santa Rosa plutons. Structural and geophysical features of the Cretaceous magmatism in the Southernmost Andes.

PERONI J. I,; TASSONE A.; MENICHETTI, M.; CERREDO, M. E.; LIPPAI, H,; VILAS, J. F.

Rimini , Italia

Congreso; VII Forum Italiano di Scienze della terra; 2009

Societa Geologica Italiana

Three major strike-slip structures of dominant WNW-ESE trend characterize the southernmost Andes, from N to S: the Magallanes-Fagnano Fault System (which represents the onland boundary between the South America and Scotia plates), the Carbajal Valley Fault System in central Tierra del Fuego and the Beagle Channel Fault System (BCFS) which separates de Tierra del Fuego Island from the southern archipelago. The complex cinematic evolution of the BCFS would have included a Cretaceous transpressional stage, Late Cretaceous contractional activity and dominantly transtensive regime during the Cenozoic. The Ushuaia and Santa Rosa plutons are located respectively on the northern and southern margins of the Beagle Channel, in the southernmost Andes. Both plutons are hosted in the pelite/greywacke sequence of the Lower Cretaceous Yahgán Formation. Presently available radiometric data indicate a Late Cretaceous age (83-93 Ma, hornblende K/Ar age) for the Santa Rosa Pluton (SRP) of Navarino island (Chile), whereas Ushuaia Pluton (UP) of Tierra del Fuego (Argentina) yielded whole rock K/Ar ages within the latest Early Cretaceous (100-110 Ma). The alignment of intrusive bodies along the BCFS has already been reported as a causal relationship for the Palaeocene Yamana Granite Suite in Chilean Tierra del Fuego. With the aim of deciphering the meaning of Cretaceous intrusive bodies located along the BCFS, several field surveys have been conducted in the area of UP and SRP where structural, lithological and geophysical data were acquired. The UP is an epizonal intrusive body with a large compositional variation (cumular ultrabasic facies, monzodiorite and monzonite facies and late stage syenite veins and dykes) of dominant shoshonitic character. The SRP has been regarded as a diapiric intrusive mainly composed of tonalite/granodiorite facies, with common dismembered melanocratic synplutonic dykes.             The area around the Isla Navarino and the Canal de Beagle is characterized by the presence of E-W trending, left lateral strike-slip faults, corresponding at the Canal de Beagle fault systems,  which are associated with several normal faults. These transtensional structures present many fault planes dipping at low angle and are superimposed on the north-verging thrust slices. Magmatic banding and foliations are commonly observed in both plutons. Deformation/crystallization relationships indicate a dynamic scenario for the emplacement and cooling evolution; synmagmatic foliations were recognized both at the meso- and microscales; subsolid high T microstructures parallel the submagmatic ones and both are overprinted by widespread brittle deformation. The down-T set of microstructures reflect a combination of stretching and shearing. Field magnetometric profiles of both intrusive bodies display comparable average gradients (92-97 nT/km) and strong contrast with the sedimentary host. Aeromagnetic anomalies associated with both plutons display similar shapes and sizes, i.e. subcircular pattern with average diameter of around 10 km. Modeling of the aeromagnetic anomaly related to UP yielded a laccolithic body. Preliminary AMS data on UP suggest a probable layered nature which agrees with the laccolithic geometry proposed on magnetic modeling grounds. Ongoing AMS sampling on UP and SRP will provide new insights into the history of pluton assembly. Presently available data support a transtensive scenario for the emplacement of SRP and UP; radiometric studies in progress combined with structural and geophysical surveys (paleomagnetism) will allow to better constrain the cinematic evolution of BCFS at Ushuaia longitude.