Constraints on the timing of the rain shadow generation related to the Miocene North Patagonian andean uplift: Multiproxy palaeosol evidences

Abstract

Climatic changes related to orographic barrier uplift are in the geoscience spotlight during last years. Several works focused in the relationships between climate and tectonic processes in order to understand the rain shadow effect generation. The Patagonia constitutes an ideal case to analyze those relationships, since a Miocene climatic change, from wet to dryer conditions, was associated with the Andean uplift. We analyzed a continuous palaeosols succession recorded in a Northpatagonian Miocene foreland basin to understand the moisture regime changes in response to the uplifting of the Patagonian Andes. Based on macromorphological, micromorphological and geochemical analyses, supported with a high-resolution time scheme, the palaeosols were characterized with their corresponding Mean Annual Paleoprecipitation (MAP) and Mean Annual Temperature values (MAT). Alfisol-like palaeosols were identified at the base of the foreland infill (15-14.6 Ma) with MAP of 1229 ± 108 mm/yr. Andisols-like were recognized for the middle section (14.6-12.75 Ma) with MAP of 1056 ± 108 mm/yr, whereas Aridisols-like palaeosols localized at the top of the infill (12.75-11.5 Ma) presents MAP of 677 ± 108 mm/yr. Mean Annual Temperatures has not significant changed from the Miocene (~11 ± 2.1°C) to the present (11°C). Based on the complete tectonic record of the Patagonian Andes the decreasing of MAP was assigned to the rain shadow effect promoted by the uplifting of the Patagonian Andes. The results indicate that although the Patagonian Andes started the uplift around 19 Ma, the rain shadow effects were effectively recorded from ∽14.6 Ma.