Dynamics of deformation and sedimentation in the northern Sierras pampeanas: An integrated study of the neogene Fiambalá basin, NW argentina: Comment and discussion

Abstract

In a recent contribution to the Geological Society of America Bulletin, Carrapa et al. (2008) provided an interesting geochronological and thermoChronological database for the Fiambalá Basin, northern end of the Sierras Pampeanas, which, together with stratigraphic, sedimentological and structural data, led them to interpret a late Miocene–Pliocene foreland reorganization from a simple scenario (e.g., DeCelles and Giles, 1996) to settings dominated by intermontane basement thrusting. They associated this reorganization with the beginning of fl at subduction, which would have been coeval (according to Carrapa et al., 2008) with broken foreland stages in the Bermejo Basin (>400 km south). One of the most crucial issues that Carrapa et al. (2008, p. 1518–1543) address (in their words) is “the structural and sedimentary behavior of broken forelands and their relationships with large-scale plate-tectonic processes such as fl at-slab subduction” that can “contribute to a better understanding of the transition from unbroken foreland (thin-skinned) to broken foreland (thick-skinned) styles of deformation and related sedimentation.” However, for some reason, they did not discuss the entire Andean stratigraphy of the area, superbly exposed much less than 50 km southward and 100 km northward of their study region (in central Famatina and Southern Puna, respectively; see Fig. 1). These two regions are evidently much closer than the Bermejo Basin exposures (see Jordan et al., 2001), which were used as a key correlation. The Bermejo Basin is located at ~31°S, i.e., ~400 km southward of Fiambalá (Fig. 1). Although I do agree that the entire extent of the Sierras Pampeanas shows evidences of basement- thrusting tectonics during the late Miocene to Pliocene and even during the Quater nary (already demonstrated previously; e.g., Jordan and Allmendinger, 1986; Ramos et al., 2002; among others), there are sources that indicate the foreland partitioning would have begun earlier. Particularly, I disagree with: (1) the timing of transformation from “simple fl exural stages” to “broken foreland scenarios” and (2) their interpretation about a synchronous slab shallowing at ca. 6 Ma. This discussion is divided into four lines of reasoning: mapping and stratigraphy of Tertiary sequences in Famatina and Southern Puna; timing of basement thrusting in boundary regions; thermochronology; and evidences of flat subduction between 29°S and 26°S.