We report new high-precision U/Pb ages and geochemical data from the
Chalten Plutonic Complex to better understand the link between magmatism
and tectonics in Southern Patagonia. This small intrusion located in the
back-arc region east of the Patagonian Batholith provides important
insights on the role of arc migration and subduction erosion. The
Chalten Plutonic Complex consists of a suite of calc-alkaline gabbroic
to granitic rocks, which were emplaced over 530 kyr between 16.90 +/-
0.05 Ma and 16.37 +/- 0.02 Ma. A synthesis of age and geochemical data
from other intrusions in Patagonia reveals (a) striking similarities
between the Chalten Plutonic Complex and the Neogene intrusions of the
batholith and differences to other back-arc intrusions such as Torres
del Paine (b) a distinct E-W trend of calc-alkaline magmatic activity
between 20 and 17 Ma. We propose that this trend reflects the eastward
migration of the magmatic arc, and the consistent age pattern between
the subduction segments north and south of the Chile triple junction
suggests a causal relation with a period of fast subduction of the
Farallon-Nazca plate during the Early Miocene. Previously proposed flat
slab models are not consistent with the present location and morphology
of the Southern Patagonian Batholith. We advocate, alternatively, that
migration of the magmatic arc is caused by subduction erosion due to the
increasing subduction velocities during the Early Miocene.