Tectonic evolution of Northern Patagonia (36°-48°S) through the inception of shallow subduction regimes from Late Cretaceous to Paleogene times

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Folguera, Andres; Ramos, Victor
Holding Date 03 September 2008

During last years increasing evidence about the inception of shallow subduction regimes has been revealed as decisive factors in constructing Andean relief. These are revealed by i) extreme outward expansion of arc-related sequences followed by ii) periods of crustal thickening, and iii) subsequent location of the arc-front nearer to the trench, coexisting with iv) extensional collapse. Extensional depocenters created at stage iv), controlled the emplacement of both mantle derived materials and crustal melts. Particularly, initial stages in Andean deformation along the western margin of Northern Patagonia (36°-48°S) are associated with previous eastward-arc expansions. Those can be associated with two discrete shallow subduction regimes: 1) 130 to 110 Ma arc-related sequences have expanded over previous Early Cretaceous back-arc depocenters, between 42° and 48°S. This migration represents an eastward jump of more than 300 km between a chain of arc-related plutons (140-120 Ma) located at the Pacific coast and volcanic rocks at the eastern slope of the Andes. Those sequences end up with a series of ignimbritic packages dated at 110 Ma that are separated from lower mesosilicic sequences by an angular unconformity. At the far retroarc area 80 Ma basaltic flows with tholeiitic affinities as well as Paleogene intraplate sequences are unconformably covering Cretaceous deformations. Those are evidenced by a) inversion of Permian-Early Cretaceous rift systems (47°-48°S) more than 600 km from the trench; and b) inversion of Early to Middle Jurassic depocenters m east of the main Andes (43°-46°S), more than 500 km from the trench. This scheme points to a strong Early-Late Cretaceous expansion of arc-related series, followed by distal thick-skinned deformation and then eruption of Latest Cretaceous to Paleogene crustal melts and mantle derived materials presumably during a slab retreat stage. The second proposal for an early-Andean shallow subduction zone in Northern Patagonia comes from 2) eastward expansion of Late Cretaceous to Eocene (75-45 Ma) arc-related sequences (36°-43°30’S). Those describe two separate lobules centered at 36°-39°S and 40°-43°30’S respectively, where arc migration was maximum with values around 400 km. Both correlate with Late Oligocene to Early Miocene intraplate volcanic plateaus at the outer retroarc area. A series of basement-cored structures have uplifted since 100 Ma in correspondence with the area of arc expansion more than 600 km from the trench, previously to an Atlantic transgression at the Cretaceous-Paleocene boundary. This scenario points to a relatively younger shallow episode of subduction between Latest Cretaceous and Eocene, coincidently with the initial Andean phases at these latitudes, and followed by the emplacement of Late Oligocene to Early Miocene intraplate series, previously to reestablishment of normal subduction in the area.