Teleseismic prospecting of lithospheric contrasts beneath the Pyrenees and Alps

Abstract

The possibility of deriving the three-dimensional structure of the lithosphere by inversion of arrival times from distant earthquakes has previously been assessed using data from permanent seismic arrays¹. Temporary emplacement of arrays in regions of key interest furnishes a practicable tool for investigating their structure^2,3, but the data needed for resolving complex three-dimensional structures are difficult to collect. We have therefore selected here regions where cylindrical symmetry reduces the problems to two dimensions and where the geological structure of interest is likely to produce large, easily detectable, relative arrival time anomalies. In this respect the depth variation of an interface with a large velocity contrast such as the Moho, as it is known to exist on average perpendicularly to the axis of alpine mountain ranges like the Alps and the Pyrenees, is of interest. In the case of the Pyrenees, explosion seismology profiling on parallel east-west lines in the North Pyrenean Zone (NPZ) and the Palaeozoic Axial Zone (PAZ)^4–7 provides evidence of an increase of over 15 km in Moho depth across the range. The variation in this depth beneath the North Pyrenean Fault (NPF)⁸, a feature clearly expressed at the surface in the east as separating the NPZ and PAZ, can be shown to be very sharp, occurring over approximately 5km. These results call into question the hitherto assumed smoothness of deep structure.