MANTLE FLOW BENEATH THE PACIFIC PLATE: EVIDENCE FROM SEAMOUNT SEGMENTS IN THE HAWAIIAN-EMPEROR CHAIN

摘要

Several lines of evidence suggest that individual en echelon seamount segments along the Hawaiian-Emperor hot-spot chain reflect mantle flow beneath the Pacific plate. A simple kinematic model is proposed in which ascending mantle diapirs, or "plumelets", are sheared by the differential motion of the lithosphere and the underlying upper mantle. In this model, individual seamount segments represent the topographic expression of discrete plumelets that emanate from a single Tower mantle thermal feature. The age and alignment of successive volcanoes within each seamount segment result from the shearing of a single plumelet and indicate the relative motion of the Pacific plate with respect to the underlying upper mantle (12.5 ± 2.0 cm/yr in the direction 33° north of west). The age and alignment of corresponding volcanoes across different segments (for example, the first-formed volcanoes along each segment) describe the relative motion of the Pacific plate in the hot-spot reference frame (8.6 ± 0.2 cm/yr in the direction 22° north of west). The difference of these two vectors describes the motion of the underlying upper mantle in the hot-spot reference frame (4.3 ± 2.0 cm/yr in the direction 40° east of south). This analysis suggests that the upper mantle is flowing opposite and ～30° oblique to the motion of the overriding Pacific plate toward the spreading center along the East Pacific Rise. The model offers an alternative explanation to the apparent shift in the rate (Jackson, Silver, and Dalrymple, 1972) as well as the direction (Cox and Engebretson, 1985) of propagation of the Pacific plate 5 my ago. The volume and age relationships of seamount trends set a minimum estimate for the plumelet radius at 55 km and suggest that on average, one plumelet emerges every 3 my.