Thin crustal layering in Northern France: observations and modelling of thePMPspectral content

Abstract

SUMMARY variations in spectral content of the P,P seismic phase (the P-wave reflected on the Moho) are investigated using refraction data collected along the North of France ECORS profile for offsets ranging from 60 to 110 km. The low-frequency component of the PMP (10-15 Hz) shows the usual increase in amplitude beyond the critical distance (90 km). An anomalous attenuation of post-critical amplitudes, however, occurs in the high-frequency part of the reflection. This frequency-selective attenuation characterizes the transmission through the reflective deep crust since it does not affect waves reflected from above. Synthetic seismograms are computed to investigate the influence on the spectral content of the Moho reflection of both thin lamellae and anelastic dissipation within the lower crust. They show that a cyclic repetition of thin low- and high-velocity layers efficiently removes high frequencies from the post-critical part of the PMP. A good fit to the observed rapid decrease in amplitude between 15 and 20 Hz requires: (i) sufficient impedance contrasts between highand low-velocity layers (>0.10), (ii) velocities alternating rapidly from high to low values, (iii) low velocities in the range 5.8-6.7 km s-l and high velocities in the range 7.0-7.6 km s-l (iv) thicknesses in the range 80-200 m. The observed frequency-selective attenuation supports the hypothesis of a layered structure of the lower crust. The constraint placed upon impedance contrasts favours a magmatic origin of the lamellae rather than a purely tectonic origin. The frequency-selective attenuation can also be accounted for by anelastic absorption. The required intrinsic Q is found to be 200 for the whole lower crust. Such a low value would imply the presence of pore fluids. However scattering due to thin layering is believed to play a great part in the observed attenuation since it also explains the high reflectivity of the lower crust pointed out by vertical seismics in the southern part of the profile.