ALONG-STRIKE VARIABILITY OF COUPLED HINTERLAND-FORELAND PROCESSES DURING FORMATION OF THE MAGALLANES FORELAND BASIN, PATAGONIAN ANDES

The Patagonian Andes record the compressional inversion of the Late Jurassic Rocas Verdes back arc basin (RVB) and formation of the Cretaceous-Neogene Magallanes foreland basin. The RVB was floored by basaltic crust with mid-ocean ridge affinities and filled with up to 4 km of mud-dominated marine sediment. Our work on Tierra del Fuego and elsewhere identified differences in the timing and character of foreland basin sedimentation for ~500 km along the strike of the orogen. Throughout Patagonia, the onset of foreland basin sedimentation is marked by a sandstone-mudstone package that records the beginning of turbiditic sand deposition and fan growth. Modal analysis of the turbiditic sandstones, mudstone geochemistry, and U-Pb detrital zircon ages show changes in rock composition and provenance across this transition that record the progressive evolution of the Andean fold-thrust belt. Foreland basin sedimentation coincided with or immediately followed the obduction of the RVB basaltic floor onto continental crust. The age of obduction (~100 Ma) is known in only 2 places but appears to be younger in the south by 2-3 m.y. Other differences between northern and southern sectors include: (1) foreland basin sedimentation on Tierra del Fuego began after 88-89 Ma, several m.y. later than ~700 km north in Ultima Esperanza. (2) The basaltic floor of the RVB and metamorphic basement contributed much less to the foreland basin in the south, probably because they were more deeply buried. (3) In the south, basaltic crust first became emergent at 81-80 Ma, ~10 m.y. later than in the north. These differences may reflect an inherited asymmetry of the RVB, which was wider, experienced greater subsidence, and had a thicker fill in the south. The fold-thrust belt appears to have remained small (~60 km wide) until a period of Paleogene out-of-sequence thrusting. This event partially exhumed metamorphic and igneous basement and marked the beginning of a rapid expansion of the fold-thrust belt into the foreland. These results support existing flexural models of foreland basin evolution and suggest that the beginning of turbidite sedimentation in the Magallanes foreland basin, and the subsequent exhumation of deeply buried rocks in the Andean fold-thrust belt, occurred later in southern Patagonia than in the north by a few million years.