--> Structural Evolution of the Ainsa Deep-Water Fold and Thrust Belt in the Central Pyrenees and Syn-Tectonic Evolution of the Related Sedimentary Systems

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Structural Evolution of the Ainsa Deep-Water Fold and Thrust Belt in the Central Pyrenees and Syn-Tectonic Evolution of the Related Sedimentary Systems

Abstract

The Ainsa Basin in the southern Pyrenees comprises up to 4km of deep-marine sedimentary rocks. The Ainsa basin offers excellent outcrops that have provided key observations for the understanding of deep-water depositional systems. However, little is known about the kinematic 3-D evolution of the structures that developed during the sedimentation of the Ainsa Basin infill. The aim of this contribution is to describe the main structural features and evolution of this deep-water fold and thrust belt and their impact on the facies arrangement and internal architecture of the turbidite systems as well as on the sedimentary pathways and sediment dispersal of the slope systems and their shallow-water counterparts. The Ainsa Basin evolves from an initial foredeep stage in front of the south Pyrenean thrust sheets to a final piggyback stage, once the floor of the basin was detached above a Triassic salt layer. At the initial stages the deep-water fold and thrust belt is characterized by an imbricate thrust stack and related folds. Structural style is dominated by the coexistence of thrusts and extensional faults resulting from the gravitational collapse of the highest structural relief areas. Turbidite systems were confined between the tilted carbonate platform in the foreland and the extensional features truncating the fault-related folds. During the piggyback stage the Ainsa Basin was mainly deformed by salt-cored detachment folds and fault-related folds. These folds significantly controlled the facies distribution and the pathways of the turbidite systems. The structures of the Ainsa Basin, at present forming a high angle to the regional structural trend, experienced a clockwise vertical axis rotation of up to 700. Rotation occurred by progressive curvature development triggered by thrust displacement gradient, which in its turn resulted from the salt layer distribution. The study presented here builds on the structural and sedimentological work done in the area by the authors in the last two decades. It integrates surface (structural, paleomagnetic, sedimentary) and subsurface (seismic and wells) data and relies on mesostructural analysis, stratigraphic correlation, section construction, 3D and paleogeographic reconstruction and analogue modelling. Similar tecto-sedimentary relationships have been interpreted from seismic data on deep-water fold and thrust belts both in distal passive margins (Gulf of Mexico, west Africa) and active margins (Makran, Borneo).