A tentative chronology of the Moho undulations in the Celtic sea region

Abstract

A Bouguer anomaly map of the Celtic sea and the Western Approaches of the English Channel has been computed from the free air anomaly which is affected by the variations in sea bed topography. Low pass filters of progressively increasing wavelengths are used to remove the effect of the surfacial features and the broad regional trend associated with the thinning of the continental margin. Application of a low pass filter with a cut off wavelength of 150 km results in a relatively smooth map by suppressing major trends associated with shallow structures; this processing also evidences a few new trends. Another map with a cut off wavelength of 300 km approximates the major gradient associated with changes in the Moho depth over continental margin. These two maps are used to prepare a residual map which exibits three major structural trends, viz., (a) a NE SW trend coinciding with the Mesozoic and Cenozoic Celtic basin is related to the compensation of the near surface mass deficits; (b) a NS trend associated with upwarping of the mantle may represent a subdued echo of the Porcupine Seabight openning; and (c) an EW trend in line with the South Armorican shear zone is distinct from the shelf break of the Western Approaches. A map of the Moho depth over the continental shelf, prepared from the residual anomaly, is explained in terms of basement tectonics. The proposed analysis has been constrained through known seismic refraction and reflection results.