From Back-Arc Drifting to Arc Accretion: the Late Jurassic-Early Cretaceous Evolution of the Guerrero Terrane in Central Mexico (Sierra de Guanajuato)

Three paleogeographic scenarios have been proposed for the Mesozoic volcano-sedimentary successions that compose the Guerrero terrane, western Mexico. In the "type 1" scenario the Guerrero terrane is an exotic Pacific arc accreted to nuclear Mexico by the consumption of a pre-Cretaceous oceanic basin, named Arperos Basin. The "type 2" scenario considers the Guerrero terrane as a fringing multi-arc system, accreted by the closure of relatively small pre-Cretaceous oceanic basins at multiple subduction zones with varying polarities. Alternatively, in the "type 3" scenario the Guerrero terrane is interpreted as a North American west-facing para-autochthonous arc, which drifted into the paleo-Pacific domain by the opening of the Cretaceous back-arc oceanic Arperos Basin, and subsequently accreted back to the Mexican mainland. In order to test these reconstructions and understand the dynamics of the arc accretion, we present here a combined study that includes sandstone provenance, U-Pb geochronology, and structural data from the Arperos Basin in the Sierra de Guanajuato, central Mexico. Our data document that the Arperos Basin developed in a back-arc setting, and evolved from continental to oceanic conditions from the Late Jurassic to the Early Cretaceous. Sandstone provenance analysis shows an asymmetric distribution of the infill sources for the Arperos Basin: continent-recycled sedimentary rocks were deposited along its north-eastern side, whereas magmatic arc-recycled clastic rocks developed at its south-western side. Such an asymmetric distribution closely fits with sedimentological models proposed for present-day continent-influenced back-arc basins. Based on these evidences, we favor a "type 3" scenario for the Guerrero terrane, which is then considered to represent a detached slice of the Mexican leading-edge that drifted in the paleo-Pacific domain during Late Jurassic-lower Early Cretaceous back-arc extension, and subsequently accreted back to the Mexican craton prior to the Aptian. The accretion of the Guerrero terrane produced a ~80 km-wide suture belt that is represented by a complex pile of tectonic nappes. The lowermost nappes are composed of the Arperos Basin successions and are piled up with a top-to-the SW tectonic transport, whereas the uppermost nappes contain the arc succession and show a top-to-the E transport. We interpret this double vergence pile as the result of a two-stage evolution of the arc-mainland collision. Initially, the oceanic substrate of the Arperos Basin was subducted beneath the Mexican craton, producing the accretion and top-to-the SW piling up of the basin infill. Once the oceanic floor was consumed, the arc overthrust to the E the previously accreted basin nappes and locked the subduction process.