Fore-to-retroarc crustal structure of the north Patagonian margin: How is shortening distributed in Andean-type orogens?

Orogenic belts like the Andes experience several changes during long-term evolution of the subduction margin; however, the degree to which their different components (the fore-to-retroarc domains) are jointly or separately deformed is not well understood. To investigate this problem, we provide new field and seismic data of the Chilean forearc at the Chiloé island latitude (~41-44°S) integrated with the retroarc structure, in order to obtain a clear visualization of the crustal architecture in the north Patagonian margin. Field work in the western Andean flank and Coastal Cordillera along with 2D multichannel lines, basement isobaths and borehole data in the forearc basins are used to build a structural framework and stratigraphic correlations between the offshore and onshore areas. Forearc syntectonic strata associated with basement faults show a change in the tectonic regime from late Oligocene-early Miocene extension to middle-late Miocene contraction, coincidently to the previously determined deformational stages in the main cordillera and retroarc regions. This coupled fore-to-retroarc behavior ceased during the Pliocene when glaciations covered the North Patagonian Andes, providing abundant sedimentary supply to the forearc and promoting the growth of the accretionary wedge. During this period the retroarc fold-thrust belt growth stagnated and a strike-slip regime was activated in the western Andean flank. Different mechanisms controlled the forearc basins during these late Cenozoic changing tectonic conditions; whilst the external depocenter at the oceanic platform is part of a west-vergent wedge affected by a major splay-like thrust beneath the Coastal Cordillera, the internal depocenter next to the main orogen is overthrusted by the steep western Andean slope through a major west-directed fault. We refer to this latter structural system as the “Western Patagonian Thrust” and find that positive tectonic inversion is the main deformation mechanism. These west-directed crustal faults are part of the orogenic prowedge that is associated with considerably less crustal shortening (~4.2 km) than the retroarc fold-thrust belt (~18 km), part of the retrowedge. By placing these values in a global context through a comparison with other segments of the Andes and with type-examples of collisional orogens, we find that shortening along the Andes is preferentially accommodated in the retrowedge, while shortening in collisional orogens is mainly absorbed in the prowedge. These results have implications for models of subduction orogeny and the crustal architecture of the Andes.