Structural inheritance and coastal geomorphology in SW Brittany, France: An onshore/offshore integrated approach

The Variscan crystalline basement exposed along the SW Brittany coast recorded extensive long-term planation processes during Mesozoic times. Detailed onshore-offshore mapping (600 km2) in the Penmarc'h-Concarneau granitic coastal area reveals a km-scale, deeply fractured submarine rocky shelf. High-resolution offshore imagery (bathymetry and seismic reflection dataset), combined to structural field investigations, on these surfaces allow us to identify a preserved network of both ductile and brittle structures. The inherited fault pattern is dominated by the N160°E-trending and long-lived Concarneau-Toulven fault zone (CTFS) that separates two distinct morphostructural blocks, and strongly influences the seaward limit of the Concarneau submarine rocky shelf, as well as the linear coastline of the Concarneau embayment. The structural imprint of the CTFS decreases progressively westwards with respect to a composite network of large-scale N50°E- and N140°E-oriented faults bounding the seaward edge of the Penmarc'h rocky shelf. The latter in turn splits into three large-scale blocks along N50°E- (La Torche Fault - LTF), N140°E- (Saint Guénolé Fault - SGF) and N160°E-trending normal faults. The morphostructural evolutionary model applied here to the Penmarc'h-Concarneau granitic coastal area resulted from the combined effects of structural Variscan inheritance and post-Variscan tectonics. Paleo-stress analysis of striated fault planes indicates three main Cenozoic tectonic events, inferred to have operated from Eocene to post-Oligocene times. The 3D-architecture of the Concarneau embayment, as a rocky shelf partially sealed with quaternary sediments, chiefly resulted from the reactivation of the CTFS during Eocene and Oligocene times. Further west, the surface of the Penmarc'h rocky shelf was tilted southeastward by the brittle reactivation of the LTF, and dissected by a horst-graben network post-Oligocene in age. The present-day morphology of the Penmarc'h and Concarneau domains depends on distinct driving processes: the Concarneau N160°E coastline is clearly controlled by tectonic processes via the CTFS, while the Penmarc'h headland land-sea contact appears to have been shaped by post-Cenozoic eustatism.