Cretaceous Intraplate Contraction in Southern Patagonia: A Far‐Field Response to Changing Subduction Dynamics?

Abstract

The origin, extent, and timing of intraplate contraction in Patagonia are among the least understood geological processes of southern South America. Particularly, the intraplate Deseado fold‐thrust belt (FTB), located in the Patagonian broken foreland (47°–48°30′S), is one of the most enigmatic areas. In this belt, time constraints on tectonic events are limited and synorogenic deposits have not been documented so far. Furthermore, the driving mechanism for intraplate contraction remains unknown. In this study, we carried out a structural and sedimentological analysis. We report the first syntectonic deposits in this area in the Baqueró (Aptian) and Chubut (Cenomanian/Campanian) groups and a newly found unit referred to as the Albian beds (109.9 ± 1.5 Ma). Thus, several contractional stages in late Aptian, Albian, and Cenomanian‐Campanian are then inferred. We suggest that the Deseado FTB constituted the southernmost expression of the early Patagonian broken foreland in Cretaceous times. Additionally, we analyzed the spatiotemporal magmatic arc behavior as a proxy of dynamic changes in the Andean subduction during determined stages of intraplate contraction. We observe a significant arc broadening from ~121 to 82 Myr and magmatic quiescence after ~67 Ma. This is interpreted as a slab shallowing to flattening process. Far‐field tectonic forces would have been produced by increased plate coupling linked to the slab flattening as indirectly indicated by the correlation between Cretaceous arc expansion and intraplate contraction. Finally, the tectonic evolution of the Deseado FTB favors studies supporting inception of Andean shortening since Cretaceous times.

Northward propagation of Andean genesis: Insights from Early Cretaceous synorogenic deposits in the Aysén-Río Mayo basin

“…This methodology is suitable to distinguish distinctive bed fanning in growth strata from drag folds forming local flexures in homoclinal beds near normal or contractional faults in the hanging wall (e.g. Gianni et al, 2018a). To discriminate contractional fanning structures from those developed during extension, such as tectonic rollover growth-strata folds (e.g.…”

Early Cretaceous to Cenozoic Growth of the Patagonian Andes as Revealed by Low‐Temperature Thermochronology

“…The initiation of the Cretaceous cooling phase at ∼120 Ma as shown by models FRO1‐LGC2‐JK1 (Figure 5), could be explained by a proposed shallow subduction phase linked to an eastward arc expansion in Patagonia at 46°–48°S, that would have taken place between ∼121–117 Ma and 84–82 Ma (Gianni et al., 2018). As a result, coupling between the Aluk‐South America plates increased, promoting intraplate shortening reported east of the study area in the Deseado Massif (Figure 1) after the Barremian‐Aptian (Giacosa et al., 2010; Gianni et al., 2018) and in the San Bernardo FTB (Figure 1) during the late Early Cretaceous (Gianni et al., 2015). The onset of shortening in the area would have been accompanied by the onset of rock uplift and cooling.…”

“…These observations were interpreted as the result of Aptian‐Cenomanian contractional deformation and basin inversion (Ronda et al., 2019). Nearby records of the onset of Cretaceous shortening were found in the San Bernardo FTB (Figure 1; Gianni et al., 2015) and the Deseado Massif (Figure 1) and are associated with a broken foreland stage and a proposed Cretaceous flat subduction in Patagonia between 121 and 80 Ma (Gianni et al., 2018). During the Eocene, the Farallon‐Aluk ridge collided with southernmost South America (Cande & Leslie, 1986; Somoza & Ghidella, 2012) and generated widespread back‐arc volcanism such as the Posadas, Meseta Chile Chico and Balmaceda flood basalts constrained between 57–40 Ma, interpreted as a first evidence of an asthenospheric window underneath the area (Espinoza et al., 2005; Ramos & Kay, 1992).…”