Tectonic controls and Cenozoic magmatism at the Torres del Paine, southern Andes (Chile, 51°10'S)

Abstract

Five Miocene intrusive complexes are located along the N-trending, nearly trench-parallel San Lorenzo-Balmaceda Lineament in southern Patagonia. These complexes are characterized by mildly alkaline to calc-alkaline magmatism. The deformation and kinematics in the foreland of the granitoid-dominated Torres del Paine laccolith (12±2 Ma) were studied in order to evaluate the influence of the pre- to syn-intrusive crustal stress field on magma emplacement. Compression and transpression led to large-scale folding, with fold wavelength of up to 10 km, and faulting of the Cretaceous flysch sedimentary rocks as well as alkaline gabbro sills. Geochronological data from folded gabbros point to an Oligocene minimum age for regional folding at 29.4±0.8 Ma. Reverse faults and convergent sinistral strike-slip faults occur as well as conjugate reverse faults. The shortening axes trend ENE- to E. These directions correlate with the oblique convergence between the Nazca- and the South America-plate. The magma ascent was parallel to the NNW-SSE striking Lago Grey-fault zone and probably coincides with the fossil intersection of the oceanic Madre de Dios transform fault. The intrusion correlates with a change from transpressional to transtensional dynamics as documented by a kinematic change from left-lateral convergent strike-slip faulting to left-lateral divergent strike-slip faulting. The changes in dynamics and kinematics correlate in time and space with a reorganisation of the plate tectonic situation during the Miocene. After the collision of the Chile ridge, separating the Nazca-plate from Antarctica, the situation in the south-western part of South America changed from rapid ENE-WSW-directed oblique convergence between the Nazca- and South America-plate, to an east-west-directed slow frontal convergence between the Antarctica and South America plates. The observed geometric divergence of the San Lorenzo-Balmaceda lineament and the apparent control of magma ascent through upper crustal faults may be explained by the interpretation of the SLB as a possible large-scale lower crustal strike slip zone and by shear partitioning from lower crust to upper crust.