Upper-mantle cross-section from California to Greenland
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Vols. 1-18 (1924-1944), ISSN 0044-2801
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Abstract
Pure-path upper-mantle models appropriate for tectonic, shield and old ocean have been recently presented by Grand and Helmberger. This was accomplished by modeling a rather restricted data set of S and SS triplication waveforms as well as the beginning portion of the Love waves. A much larger data set of S, SS and SSS, etc. (multibounce S-wave triplications) with a mixture of tectonic paths is available. In particular, events usually occur at tectonic margins and are recorded on stable continents. We present results of modeling these observations for laterally varying structure, essentially along a profile from California to Greenland. The models are allowed to be locally dipping with the lithosphere thickening with age at the expense of a dwindling low-velocity zone. Lateral variation does not appear to be required for depths greater than 400 km along this particular profile. The best-fitting model has a large increase in lithospheric thickness near the Rocky Mountain Front, roughly an increase of 75 km in thickness over a horizontal distance of 400 km or less. The low-velocity zone, with a velocity of 4.4 km/s, is replaced by a much faster upper 300 km with velocities near 4.7 km/s or a 7% overall increase. The one-way travel time jumps by roughly 4 s across this boundary, which compares reasonably well with the direct S residuals obtained from deep earthquake data although the latter data show large scatter.
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References
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