Geological interpretation of the gravity data for a profile across the Pacific Ocean

Abstract

This paper presents a model of geological interpretation of gravity observations along the profile across the Pacific Ocean from the Japan trench across the Hawaiian ridge to the Middle America trench crossing eleven geological regions (the total length of the profile is about 14,000 km). The interpretation was made by comparing the gravitational attractions of the combined layer of the hydrosphere and the crust and upper mantle from sea level to the depth of isostatic compensation by using the fundamental principles of the theory of isostasy. The density of the crust and mantle beneath the M discontinuity was determined from the results of deep seismic sounding (DSS) and the correlation between compressional velocities and densities of rocks. A statistical analysis was made of the data obtained by many scientists to improve the correlation for different rocks at pressures of 0.001–10 kb. Density distribution curves are presented for different rocks and different velocity intervals. The lateral density inhomogeneities in the lower part of the combined layer from a depth of 50 km to the depth of isostatic compensation are supposed to cause regional gravity anomalies of the first order determined from satellite observations, whereas density inhomogeneities of the combined layer of the hydrosphere and the crust and upper mantle to a depth of 50 km account for residual gravity anomalies obtained from free-air anomalies (the normal gravity formula derived by Helmert (1901–1909)) after the regional first-order anomalies are subtracted from the satellite data. For the profile across the Pacific Ocean we obtained curves of regional gravity anomalies of the first order, residual gravity anomalies, ocean-floor topography, variations of the apparent density of the combined layer of the crust and mantle to a depth of 50 km below sea level, variations of the apparent density of the upper-mantle layer to the same depth, variations of heat flow, and a model of a density section to a depth of 50 km below sea level. The density of the crustal rocks varies from 2.7 to 3.0 g/cm3. The upper-mantle layer has its highest density (3.42 g/cm3) directly beneath the M discontinuity under the Hawaiian ridge (from DSS) and under the Japan trench (under the continental slope). Compact rocks lie beneath the northwest Pacific basin and the mid-Pacific mountains and also in fracture zones. The upper-mantle rocks of the lowest density (3.13 g/cm3) lie beneath the east Pacific ridge, the Hawaiian ridge (at a greater depth), and in the form of isolated horizontal lenses beneath the mountains of the Darwin uplift. Anomalously high values of the heat-flow intensity were found. A suggestion is made that the upper-mantle rocks with a density of 3.13 g/cm3 west of the Hawaiian ridge are genetically related to the rocks of about the same density lying at the base of the third crustal layer (density obtained from DSS data). This interesting region requires geophysical investigation and deep drilling to reach the rocks with a 3.13-g/cm3 density at a depth of about 3 km. These studies will throw light on the processes of transformation of rocks in the crust and upper mantle.