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 Me. 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 parailel 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.