A new constraint on the composition of the topmost continental mantle--anomalously different depth increases of P and S velocity

• Published in: Geophysical Journal International
• Main Authors: Gajewski, D., Stangl, R., Fuchs, K., Sandmeier, K. J.
• Format: Text
• Language: English
• Published: Oxford University Press 1990
• Subjects: Articles; Fennoscandia
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/103/2/497; https://doi.org/10.1111/j.1365-246X.1990.tb01787.x

Compared to clear crustal P - and S -waves as well as a strong refracted P n -wave propagating in the topmost mantle (i.e., 10 to 15 km below the Moho), an anomalously weak appearance of the corresponding refracted shear wave S n is observed on refraction seismic profiles in SW Germany, France, coastal Maine (NE United States) and Fennoscandia. It is possible that the observation of P n / P M P ratios close to unity combined with small S n / S M S ratios (i.e., small S n amplitudes) is typical for the continental uppermost mantle since this was observed for quite different types of continental crust. It is shown that the observed phenomenon is not a source effect. Several possible causes to explain the observations are studied, also with the aid of synthetic seismograms. The effects of temperature, attentuation and anisotropy have been investigated. Their influence cannot explain the observations. The most likely candidate is a depth-increasing V P / V S ratio (an increase in the order of 0.027 over a depth range of 10 km), forcing P - and S -waves to propagate on different paths in the topmost mantle. The observed phenomenon provides new constraints on the change of composition with depth for the crust mantle transition zone with basalt depletion as the most likely explanation.