A Schematic Model Concerning the Formation of the Benioff Zones as Inferred from Examination of Global-Scale Geoid Undulation

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  • ジオイド面起伏の原因とベニオフ・ゾーンの成因について
  • ジオイドメン キフク ノ ゲンイン ト ベニオフ ゾーン ノ セイイン ニ ツ

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Abstract

Taking account of the recent study of Tarakanov et al. based on a new method and attributing the large-scale geoid undulations to the lateral anomalies of density in the lower part of upper mantle, global-scale geoid undulations were examined and then an excellent correlation was found between the undulations given by the GEM-8 model and the P-wave velocity anomalies found for the uppermost part of lower mantle. The negative correlation that the regions appointed to positive geoid anomalies almost coincide with those appointed to negative velocity anomalies and vice versa could be interpreted based on shock-wave experiment data showing that a Mg-rich dunite exhibits higher velocities and lower densities than Fe-rich one. Thus it was inferred that the large global-scale geoid undulations can be attributed mostly to lateral anomalies of density reflecting compositional heterogeneities presumed in the uppermost part of lower mantle.<br>It was noticed that deep-focus seismic zones, called the Benioff zones, seem to extend near along the ocean side of the belts characterized by steepest descent on relief of the geoid surface possibly reflecting large lateral density variations in the uppermost part of lower mantle. This spatial relationship was considered as indicative of a genetic interrelation between the Benioff zones and the large density contrasts, and then a schematic model was proposed concerning the formation of the Benioff zones as a result of differential movements induced in the laterally heterogeneous upper mantle by density currents resulting from the large density differences in the above-mentioned part of lower mantle. In the case of a two-dimensional viscous layer model the velocity of the density currents was tentatively estimated to be of the order of 10-1-10-2km/my, which seems sufficient for the formation of a weak zone corresponding to the Benioff zone during the Meso-Cainozoic tectonic cycle of development of active continental margins.

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