INCREASED MANTLE CONVECTION DURING THE MIDCRETACEOUS - A COMPARATIVE-STUDY OF MANTLE POTENTIAL TEMPERATURE

Mantle convection patterns of the past are not well known, yet an unde rstanding of changing mantle convection characteristics is fundamental to understanding the evolution of plate tectonics. There are very few ways to examine mantle characteristics of the past. Changes in spread ing rate and volcanic activity with time have been used to draw conclu sions about historic changes in mantle activity. Mantle temperature ha s been found to be related to crustal thickness. With this relationshi p, crustal thicknesses may now yield new conclusions about historic ch anges in mantle characteristics. We have inferred changes in mantle co nvection patterns throughout the last 180 m.y. by examining variations in assumed crustal thickness within the Pacific basin. Crustal thickn esses were calculated from residual depth anomalies by assuming that r esidual depth anomalies are the result of isostatic compensation of va riations in crustal thickness. Crustal thickness is determined at the time of crustal formation and is dependent upon the temperature of the mantle source material. Intraplate hot spot volcanism effects on crus tal thickness were not ignored. Examination of variations in crustal t hickness of crust of different ages can reveal information about chang ing temperatures of the mantle at the ridge through time. We have lear ned that mantle temperatures at the ridge during the mid-Cretaceous we re more variable than those temperatures at the ridge after the mid-Cr etaceous. Furthermore, we have inferred from the data that mantle temp eratures at hot spots were higher during the mid-Cretaceous than those at hot spots existing after the mid-Cretaceous. We suggest that mantl e convection at the ridge was more rapid during the mid-Cretaceous cau sing a higher variability of temperatures at the ridge. We also note t hat this period of increased mantle convection is concurrent with the increased mantle temperatures at hot spots within the pacific basin.