A new constraint on the composition of the topmost continental mantle-anomalously different depth increases ofPandSvelocity

Autor: K.-J. Sandmeier, R. Stangl, Karl Fuchs, Dirk Gajewski
Rok vydání: 1990
Předmět:
Zdroj: Geophysical Journal International. 103:497-507
ISSN: 1365-246X
0956-540X
Popis: SUMMARY Compared to clear crustal P- and S-waves as well as a strong refracted Pn-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 Sn is observed on refraction seismic profiles in SW Germany, France, coastal Maine (NE United States) and Fennoscandia. It is possible that the observation of Pn/PMP ratios close to unity combined with small Sn/SMS ratios (i.e., small Sn 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 VP/VS 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.
Databáze: OpenAIRE
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