Hot regions and the origin of marginal basins in the western Pacific

Abstract

Volcanic provinces, showing no genetic relationship to plate boundaries, may be classified into two categories: hot spots which are a few hundred kilometers across and may be nearly fixed in position with respect to deep mantle, and hot regions which are a few thousand kilometers across and can migrate more freely. The mantle upwellings which presumably give rise to them may originate at different depths by different causes. It is observed that the marginal basins, older than 15 Ma, of the western Pacific become progressively younger northward (with the exception of the South Fiji Basin). This leads to the hypothesis that the spreading of marginal basins was caused by the mantle upwelling associated with a migrating hot region, and that a hot region which started from the Australia region in the Late Cretaceous, migrated northward successively creating the Tasman, Coral Sea, Caroline, South China, Parece Vela, Shikoku, and Japan Basins through Paleocene, Eocene, Oligocene, and Early Miocene time. This hot region presently underlies northern China, Mongolia and adjacent areas. Another hot region is assumed to have caused the spreading of the South Fiji Basin and all the currently spreading basins in the southwestern Pacific. Spreading of individual marginal basins occurred during the short period when a hot region was passing the pertinent area. The main merit of this hypothesis is that it can explain the relationship between the location and age as well as the short duration of spreading, of western Pacific marginal basins. Marginal basins are formed commonly (though not always) in the overriding plates near subduction zones, because in such a setting a spreading marginal basin is readily accommodated either by an oceanward movement of the nearby subduction zone or by a continentward movement and subduction of the overriding oceanic plate. There are a few marginal basins that were formed in other tectonic settings and accommodated by some other mechanism of compensative subduction. Presumably the splitting of the supercontinent Pangea was also caused mainly by the effect of the mantle upwelling associated with the same hot region that later migrated northward, creating marginal basins in the western Pacific. The main basis for this presumption is the fact that the splitting of Pangea was completed in the Australia region in Middle Cretaceous time, whereas the spreading of marginal basins in the western Pacific began in the same Australia region in the Late Cretaceous immediately following.