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

ECHAURREN, ANDRÉS; ENCINAS, ALFONSO; SAGRIPANTI, LUCÍA; GIANNI, GUIDO; ZAMBRANO, PATRICIO; DUHART, PAUL; FOLGUERA, ANDRÉS

GLOBAL AND PLANETARY CHANGE

ELSEVIER SCIENCE BV

Año: 2022 vol. 209

0921-8181

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 separatelydeformed is not well understood. To investigate this problem, we provide new field and seismic data of theChilean forearc at the Chilo´e island latitude (~41-44◦S) integrated with the retroarc structure, in order to obtaina clear visualization of the crustal architecture in the north Patagonian margin. Field work in the western Andeanflank and Coastal Cordillera along with 2D multichannel lines, basement isobaths and borehole data in theforearc basins are used to build a structural framework and stratigraphic correlations between the offshore andonshore areas. Forearc syntectonic strata associated with basement faults show a change in the tectonic regimefrom late Oligocene-early Miocene extension to middle-late Miocene contraction, coincidently to the previouslydetermined deformational stages in the main cordillera and retroarc regions. This coupled fore-to-retroarcbehavior ceased during the Pliocene when glaciations covered the North Patagonian Andes, providing abundantsedimentary supply to the forearc and promoting the growth of the accretionary wedge. During this periodthe 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 splaylikethrust beneath the Coastal Cordillera, the internal depocenter next to the main orogen is overthrusted by thesteep 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. Thesewest-directed crustal faults are part of the orogenic prowedge that is associated with considerably less crustalshortening (~4.2 km) than the retroarc fold-thrust belt (~18 km), part of the retrowedge. By placing these valuesin a global context through a comparison with other segments of the Andes and with type-examples of collisionalorogens, we find that shortening along the Andes is preferentially accommodated in the retrowedge, whileshortening in collisional orogens is mainly absorbed in the prowedge. These results have implications for modelsof subduction orogeny and the crustal architecture of the Andes.