Cenozoic intraplate tectonics in Patagonia: record of main Andean phases in a weak upper plate

ALFONSO ENCINAS; JEREMÍAS LIKERMAN; GUIDO GIANNI; ANDRES FOLGUERA; VICTOR VALENCIA; ANDRES ECHAURREN; CARLOS DAL MOLIN

Simposio; Primer Simposio de Tectónica Sudamericana; 2016

The Patagonian broken foreland is part of the few intraplate belts uplifted behind the Andes. Unraveling its evolution is an opportunity to better understanding tectonic triggers of far-field contraction as well as strain location and distribution in intraplate belts linked to Andean-type settings. To address this issue we studied the Cenozoic record of the central sector of this system (San Bernardo fold and thrust belt) and the Andes (Meseta de Chalia) following a multidisciplinary approach involving structural modeling, growth-strata detection, U-Pb geochronology and analog modeling. Description of eocene growth-strata at different localities(~44-40 Ma) indicates contraction from the Andes to the broken foreland. Recognition of this event constitutes an unprecedented record of the Incaic phase in Central Patagonia. Detection of synextentional deposits suggests that the broken foreland collapsed partially in Oligocene/early Miocene. During middle Miocene the Quechua contractional phase, produced folding of neogene volcanic rocks and Olistostrome deposition at ~17 Ma. Current intraplate activity is evidenced by thrusted quaternary deposits. Unlike other Andean intraplate belts, subduction angle and contractional phases are not correlated. Instead, we attribute a major role to changes in boundary conditions linked to global-scale reorganization driving Incaic and Quechua phases and upper plate strength in facilitating intraplate deformation. The inherited structural-framework linked to rifts basins related to Gondwana break-up was crucial as these localized intraplate contraction in the Patagonian broken foreland. Analog modeling shows that deformation can propagate effectively into a weakened foreland after applying subtle shortening estimated from a regional cross-section in this study. Furthermore, these results illustrate how overall strain transfer and distribution is dictated by foreland strength, explaining end-member scenarios where intraplate belts are often described.