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A Comprehensive Geochronology Study of the Rio Grande Rise: Evidence for Prolonged On and Off-ridge Volcanism
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  • Peter Davidson,
  • Anthony Koppers,
  • Cornelia Class,
  • William Sager
Peter Davidson
Oregon State University

Corresponding Author:[email protected]

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Anthony Koppers
Oregon State University
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Cornelia Class
Lamont -Doherty Earth Observatory
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William Sager
University of Houston
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Expedition NBP1808 on the R/V Nathan B. Palmer completed 32 dredges between October and December, 2018 from locations across the Rio Grande Rise (RGR)—a largely unstudied oceanic plateau on the South American plate—and several seamounts located between RGR and the Mid-Atlantic Ridge (MAR). Eighteen samples from 10 dredge locations on RGR were dated to better understand the geochronological history of this large igneous province and to provide clues to its relationship with the Walvis Ridge and Tristan-Gough hotspot(s) on the conjugate African plate. 40Ar/39Ar results from plagioclase separates (and one biotite) show a prolonged emplacement history throughout RGR ranging from ~84 to 48 Ma. Ages in general decrease towards the MAR in accord with plate motions showing that RGR as a whole was emplaced over at least several Ma and not as a single pulse like some other oceanic plateaus. Using the recently published tectonic reconstruction of Sager et al., most volcanism in the NW and NE sectors on RGR was emplaced off-axis while that in the SE sector was erupted on-axis. This suggests that the plume source for RGR changed from more intraplate to more ridge-centered as the system evolved through time. There is evidence of a possible reversed age progression in the NE RGR which could provide evidence for micro-plate activity that has been suggested in this region, though more ages are needed to confirm this trend. Geochemistry studies are ongoing and will be used in the future to better understand the eruptive processes. Additional age analyses are also ongoing and will focus on the other dredge locations throughout RGR as well as the seamounts to complete the geochronological picture of the emplacement of RGR.