Abstract

The Late Cretaceous closure of the Late Jurassic–Early Cretaceous Rocas Verdes basin resulted in the development of the Patagonian fold-and-thrust belt and Magallanes foreland basin between 50°S and 54.5°S. New geologic maps, structural data, and two retrodeformed, line-balanced cross sections from the Magallanes region of Chile (53°S–54°S) constrain the kinematic evolution and along-strike correlations of deformation that occurred at the base of the fold-and-thrust belt near the brittle-ductile transition. The stratigraphic architecture of the predecessor basin controlled the position of regional décollement levels. During the initial stage of closure (Albian–Campanian), the floor of the Rocas Verdes basin was imbricated and thrust onto the continental margin to form a regional décollement within Jurassic–Lower Cretaceous shale. Continued shortening resulted in the deepening of the décollement to a ductile shear zone that formed <1 km below the basement-cover contact. Ductile polyphase folding accommodated basement shortening and was mechanically decoupled from the overlying fold-and-thrust belt below the lower décollement. Ramps that cut Jurassic volcanic deposits linked the lower and upper décollements and transferred displacement into the nascent foreland basin. A second stage of shortening, characterized by basement-involved reverse faults that cut the early décollements, reflects complete closure of the Rocas Verdes basin by the Maastrichtian–Eocene. Coniacian to Eocene shortening estimates indicate a northwest-southeast increase from 26% to 37% over 100 km along strike and are consistent with regional models of the Patagonian fold-and-thrust belt. The results provide an important example of the kinematic evolution of the base of a retroarc fold-and-thrust belt in an “Andean-style” orogen.

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