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

A Bouguer anomaly map of the Celtic sea and the Western Approaches of the E nglish Channel has been computed from the free air anomaly which is affecte d by the variations in sea bed topography. Low pass filters of progressivel y increasing wavelengths are used to remove the effect of the surfacial fea tures and the broad regional trend associated with the thinning of the cont inental 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 as sociated with shallow structures; this processing also evidences a few new trends. Another map with a cut off wavelength of 300 km approximates the ma jor gradient associated with changes in the Moho depth over continental mar gin. 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 Mesozo ic and Cenozoic Celtic basin is related to the compensation of the near sur face 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 sheaf zone is distinct from the shelf break of the Western Approaches. A map of the Moho depth over the con tinental shelf, prepared from the residual anomaly, is explained in terms o f basement tectonics. The proposed analysis has been constrained through kn own seismic refraction and reflection results. (C) 1998 Elsevier Science Lt d. All rights reserved.