Tectonic evolution of the Sothern Andes in the last 100 My

FOLGUERA, A.; LITVAK, V.; SPAGNUOLO, M.; ORTS, D.; ROJAS VERA E. A.; RAMOS, V.A.

Santiago

Workshop; SFB 574 SUBDUCTION WORKSHOP; 2010

SFB 574

Southern Andean evolution has been governed in the last 100 Ma by changes in slab steepening due to different processes that determined cycles of crustal stacking, arc expansion and within-plate eruptions. These processes comprise i) subduction of non seismic volcanic ridges, ii) subduction of young oceanic crust attached to an approaching spreading ridge, and iii) subduction of highly serpentinized fracture zones producing localized shallow subduction settings. These processes operate at different temporal and spatial scales producing a similar pattern of deformation and magmatism that starts with: a) rapid incorporation of vast sections of the upper crust into the frontal fold an thrust belt; b) rapid expansion of the arc-related activity to the foreland; c) subsequent widespread extension from the foreland to the coastal zone, associated with within-plate volcanism and sedimentation. The Southern Andes have been constructed and destructed repeatedly through this mechanism. It started with the inception of two shallow subduction zones (100-90 Ma) at 37º-39ºS and 40º-48ºS respectively, that lasted up to Eocene times. These shallowings fragmented the foreland uplifting the western north Patagonian massif up to the western Deseado massif. Within-plate activity associated with Eocene-Oligocene extensional depocenters dominated 20 My after the steepening of the subduction zones, creating the Somuncura plateau among others. Latter three Neogene shallow subduction zones established at ~18 Ma in 35º-38ºS, 41º-43ºS, 46º-50ºS, fragmenting the foreland, provoking eastward arc expansions, and triggering after their steepening within-plate extensional settings. Finally an incipient shallow subduction setting could be developing at 36º-38ºS in the last 3 Ma.