Abstract
Juan Fernandez Ridge (JFR) is a ca. 800 km long volcanic chain composed by seamounts, guyots and oceanic islands in the SE Pacific. JFR is thought to be related to a deep mantle plume and usually scores high in the hotspots catalogs (e.g., Anderson, 2005; Jackson et al., 2017). High 3He/4He in Robinson Crusoe is probably the most undoubted feature of lower mantle involvement. However, one of the most convincing pieces of evidence for a source rooted deep in the mantle is the age progression along a volcanic chain, which is poorly constrained for JFR. In fact, some scarce K-Ar dates in Alejandro Selkirk and Robinson Crusoe islands, and a total fusion age for O’Higgins Guyot published more than 20 years ago, is the only evidence available for such a hypothesis in previous works. Here we integrate recently published 40Ar/39Ar ages and 17 new results on groundmass step heating experiments from rocks corresponding to the late shield stage (O’Higgins Guyot: 8.4 Ma; Alpha Guyot: 4.6; Robinson Crusoe Island: 3.7 Ma; Alejandro Selkirk Island: 0.83 Ma; Friday Seamount: 0.62 Ma) to document a solid age progression which yields a long-term absolute velocity of ca. 81 mm/yr- for the Nazca Plate. This value is much higher than the velocity prescribed by plate tectonic models that assume fixed hotspots, and still somewhat higher than models that take into account hotspot drift, indicating that the Juan Fernandez hotspot is moving ca. 2 cm/yr towards East Pacific Rise. Present hotspot would be ca. 20 km west of Domingo Seamount. Merging geochronological data with our current understanding of the mantle sources and magmatic evolution, we provide a case for a hotspot possibly rooted in a weak primary plume, and discuss some causes and consequences of that.
Highlights
Seamounts are conspicuous features of the deep seafloor, and together with oceanic islands, form a widespread expression of intraplate volcanism
Juan Fernández Ridge (JFR) is thought to be related to a deep mantle plume and usually scores high in the hotspots catalogs (e.g., Anderson, 2005; Jackson et al, 2017), mostly because of high 3He/4He ratios in Robinson Crusoe Island, but firm geochronology is still lacking
The JFR intersects the continental margin at ∼33.4◦S and its eastern component (O’Higgins seamounts group) is located ∼120 km west of the Chile-Peru Trench (Figure 1), where the Nazca Plate sinks beneath South America with an azimuth of 78.4◦ at a high convergence rate of 74 mm/year according to MATERIALS AND METHODS
Summary
Seamounts are conspicuous features of the deep seafloor, and together with oceanic islands, form a widespread expression of intraplate volcanism. In the last decades more detailed global bathymetry has revealed more seamounts in a wide range of size and geometry, many of them isolated and others forming chains with no clear age progression The latter became evident for the Pacific basin (Clouard and Bonneville, 2001) and new ideas were raised to question the fixed hotspot model, and the mantle plumes theory. The JFR intersects the continental margin at ∼33.4◦S and its eastern component (O’Higgins seamounts group) is located ∼120 km west of the Chile-Peru Trench (Figure 1), where the Nazca Plate sinks beneath South America with an azimuth of 78.4◦ at a high convergence rate of 74 mm/year according to
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