Mapping the crustal structure beneath the eastern Pyrenees

Two passive seismic profiles have been acquired in the eastern Pyrenees as part of the Pyrope and Orogen projects to investigate the crustal structure differences between this area and the central and western Pyrenees. Up to 28 broad-band stations were deployed along two orthogonal lines, with an interstation spacing close to 10 km. High frequency receiver functions allowed us to obtain the main lithospheric interfaces along those lines. The NNE-SSW profile shows a well-defined Moho beneath Iberia, slightly deepening northwards. Beneath the Axial zone the Moho appears to be segmented but does not show evidence of crustal imbrication. Further North, the Moho appears again as a continuous interface located around 30 km depth. This image clearly differs from the conspicuous imbrication between the Iberian and Eurasian crusts observed westward. The E-W profile shows a smooth Moho thinning from a 40 km depth to the west of the profile to 23 km close to the coastline, evidencing the crustal thinning related to the Neogene extensional processes. Additional constraints on the geometry of the crust/mantle boundary in the Eastern Pyrenees are obtained from local moderate magnitude earthquakes recorded along the seismic lines during the experiment. In particular, a fan profile built from an event located near the Mediterranean coast suggests that crustal thickness differences between Iberian and Eurasian crusts can still be recognized 30 km westward of the Mediterranean coast, in an area that seems to mark the limit between regions dominated by compressive and extensive processes. We propose a model in which the observed dissimilar Moho structure beneath the Eastern Pyrenees is interpreted as the result of a different Mesozoic pre-shortening margin configuration. Seismic results are consistent with recently proposed tectonic models including an intermediate continental block separating Iberia and Europe by two basins with extremely thin crust or exhumed mantle.