Using basin thermal history to evaluate the role of Miocene–Pliocene flat-slab subduction in the southern Central Andes (27° S–30° S)

Title of Publication: 
Using basin thermal history to evaluate the role of Miocene–Pliocene flat-slab subduction in the southern Central Andes (27° S–30° S)
Stevens-Goddard, Andrea L. and Carrapa, Barbara
Publication Info: 
Basin Research (2017) 1–22, doi: 10.1111/bre.12265

Studies in both modern and ancient Cordilleran-type orogenic systems suggest that processes associated with flat-slab subduction control the geological and thermal history of the upper plate; however, these effects prove difficult to deconvolve from processes associated with normal subduction in an active orogenic system. We present new geochronological and thermochronological data from four depositional areas in the western Sierras Pampeanas above the Central Andean flat-slab subduction zone between 27° S and 30° S evaluating the spatial and temporal thermal conditions of the Miocene–Pliocene foreland basin. Our results show that a relatively high late Miocene–early Pliocene geothermal gradient of 25–35 °C km 1 was typical of this region. The absence of along-strike geothermal heterogeneities, as would be expected in the case of migrating flat-slab subduction, suggests that either the response of the upper plate to refrigeration may be delayed by several millions of years or that subduction occurred normally throughout this region through the late Miocene. Exhumation of the foreland basin occurred nearly synchronously along strike from 27 to 30° S between ca. 7 Ma and 4 Ma. We propose that coincident flat-slab subduction facilitated this widespread
exhumation event. Flexural modelling coupled with geohistory analysis show that dynamic subsidence and/or uplift associated with flat-slab subduction is not required to explain the unique deep and narrow geometry of the foreland basin in the region implying that dynamic processes were a minor component in the creation of accommodation space during Miocene–Pliocene deposition.

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Fig. 1. (a) The tectonomorphic setting of the central Andes adapted from Ramos et al. (2002). The depth to slab is indicated relative to these features. (b) A geologic map modified from Fosdick et al. (2017) of the sample locations in this study. Depositional areas in this study are marked with empty circles. Locations of wells are marked with filled circles. In both figures, GN = Guanchin River depositional area, RLT = Rio La Troya depositional area, VH = Valle Hermoso depositional area, VN = Vinchina depositional area, LF = La Flecha depositional area, G = Guandacol, P = Pozuelos,M = Matagusanos.