Relationships between thrust tectonics and sequence stratigraphy surfaces in foredeeps: model and examples from the Pyrenees (Cretaceous-Eocene, France, Spain)

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Abstract
A new method of chronological and geometrical analysis is used for understanding the sedimentary and tectonic evolution of perisutural basins (foredeep and foreland basins). It may be applied to synorogenic basins whose infilling was controlled by sea-level fluctuations and where, as a result, the tectonic structures are well preserved. This method is based on the mapping of depositional sequence boundaries and maximum flooding surfaces defined by sequence stratigraphy. These planar markers are considered as synchronous and their numerical ages are determined by classical chronostratigraphic methods, by dating and by comparison with the eustatic global cycle chart. The evolution of the studied perisutural basins is controlled by the propagation in space and time of a syn-depositional thrust-and-fold system including a fault-propagation anticline and two synclines (a foreland and a hinterland syncline) over and in front of a blind basal sole thrust. The propagation of this thrust-and-fold system toward the foreland is connected to unconformable depositional sequence sets, which indicate for each system a time span for thrusting of several eustatic cycles. The evolution of a single thrust-and-fold system corresponds to a shortening accommodated by upward imbrication of second-order thrusts inside the anticline. Each of these thrusts is related, at least, to one depositional sequence lying unconformably on the external side of the fold and deformed by the successive development of the branch lines toward the hinterland. This tectono-sedimentary correlation indicates a time span sometimes as short as one eustatic cycle for each second-order thrust. Thrusting and sea-level changes are not exactly synchronous, as the depositional sequence sets (which are tectonic in origin) do not systematically correlate with eustatic supercycles. The apparent time correlation between the two groups of independent phenomena is an artefact of the method which calibrates the tectonic evolution by comparison with eustatic fluctuations. Applied to the South Central Pyrenees, the method has specified the structure of the Late Cretaceous foredeep basin and has revealed the successive stages of forward thrust-and-fold system migration (4 to 5 Ma for each one from 89 Ma) and of hindward second-order thrust migration (1 to 4 Ma for each one). In the North Central Pyrenees as in the South Central Pyrenees, Late Cretaceous foredeeps are characterized by syntectonic turbidite deposits (from 89 Ma onward) and by a significant shortening due to a symmetrical and synchronous structural evolution. The Eocene foreland is a foreland syncline developed in the footwall of the North Pyrenean Thrust. This syncline is due to the occurrence of blind thrusts and it is infilled by fluvial deposits.