The interrelationships of crustal and upper mantle parameter values in the Pacific

The data from some 400-odd seismic refraction measurements of crustal structure in the Pacific Ocean basin are examined from the standpoint of possible systematic interrelationships of crustal and upper mantle parameter values as a function of crustal age, changes in heat flow alone and in combination with crustal age, changes in crustal spreading rate, changes in free air gravity anomaly values, and possible changes in the petrology of newly formed crust on the rise crests. It is found that there are systematic interrelationships between depth of water, thickness of the crystalline rock crust, mean seismic velocity of the crust, and seismic velocity of the mantle and also that these interrelationships change with crustal age. The age periods that appear to have significance in terms of the duration for a given set of interrelationships are 0–25, 25–36, 36–63, 63–90, and 90–≈110 m.y. Although it is likely that this pattern of changes in the interrelationships of crustal and upper mantle parameter values at intervals of between 20 and 30 m.y. continues for ages greater than 110 m.y., the data available for older age areas free of volcanic and tectonic disturbance are too few to permit the establishment of meaningful results. In the first age zone (0–25 m.y.) all parameter values for the crust and upper mantle vary directly with each other, so that on an overall basis the decrease in heat flow encountered in moving away from the rise crest axis is accompanied by an increase in depth of water, a thickening of the crust, and an increase in mean crustal velocity and mantle velocity. The second age zone (25–36 m.y.) marks a zone of transition in which a new pattern is established, the mean velocity of the crust bearing an inverse relationship to the velocity of the mantle and the thickness of the crust. This zone of transition appears to be related to the rise flank zone of low heat flow. The change from a direct to an inverse relationship between mean crustal velocity and the velocity of the mantle appears to be related to an anhydrous change as a consequence of temperatures building up beneath the Moho in association with an underlying phase transformation boundary in the upper mantle. The parameter values of the crust and upper mantle found beyond the zone of low heat flow appear to represent frozen characteristics resulting from the anhydrous transformation's not being able to reverse because the water released escaped. There is a correlation of crustal and upper mantle parameter values with past crustal spreading rate and also free air anomaly values for crust having an age of >36 m.y. The correlation with spreading rate is related to the time available for modification as the crust passed through zones of crustal change such as that associated with the low heat flow zone. The correlation with gravity is related to the original composition of the crust formed beneath the rise crest spreading center and its modified compositional thickness. All the evidence suggests that the original Moho is a chemical discontinuity and that the original composition and structure of the crust and upper mantle is related to density segregation. Once established, this structure is modified by hydrothermal metamorphism, the gabbro-garnet granulite-eclogite transformation, and olivine-serpentine transformation. These transformations appear to go on concurrently where heat flow exceeds 1.7 µcal/cm² s, but in the zone of low heat flow, each appears to become progressively inactive and terminates. Beyond the zone of low heat flow and for ages of >36 m.y. the only active process is densification and subsidence of the crust as a consequence of continued cooling with increasing distance from the rise crest, the subsidence being in part due to the weight of the attendant increase in depth of water. Otherwise, crustal relations are as they were developed in the zone of low heat flow.