Biostratigraphic evidence for dramatic Holocene uplift of RobinsonCrusoe Island, Juan Fernández Ridge, SE Pacific Ocean

[摘要] Hotspot oceanic islands typically experience subsidence due to severalprocesses related to migration of the oceanic plate away from the mantleplume and surface flexural loading. However, many other processes caninterrupt subsidence, some of which may be associated with catastrophicevents. A study of the biostratigraphy and sedimentology of Holocene depositson Robinson Crusoe Island (RCI) on the Juan Fernández Ridge (JFR) indicatedthat dramatic uplift has occurred since 8000 years BP, at a rate of about8.5mm yr^-1. This is evidenced by supratidal flats with tepeestructures and sand layers containing marine gastropods (mostlyNerita sp.) that are now exposed ca. 70 m a.s.l. The activehotspot is located 280 km further west and the last volcanic activity on RCIoccurred at ca. 800 000 years BP. Long-term subsidence is evidenced by deepsubmerged marine abrasion terraces at RCI. As no direct evidence was foundfor the existence of a compensating bulge generated by the present hotspotupon which RCI would be situated, it must be concluded that subsidence in thewake of the mantle plume beneath the migrating plate was interrupted by veryrapid uplift, but on a scale that did not fully compensate for the previoussubsidence. This can be attributed to large-scale landslides followed byisostatic rebound, although this is only vaguely reflected in thelow-resolution bathymetry of the area. To determine if this mechanismproduced the uplift, a detailed bathymetric survey of the area will berequired. If such a survey confirms this hypothesis, it may have implicationsfor the short-term dynamics of vertical variations of oceanic edifices andtheir related effects on ecosystems and human population.

[学科分类] 地球化学与岩石