The Evolution of Central Volcanoes in Ultraslow Rift Systems: Constraints From D. João de Castro Seamount, Azores
Creators: name: Romer, R. H. W.
name: Beier, C.
name: Haase, K. M.
name: Eberts, A.
name: Hübscher, C.
Publisher: name: FID GEO
Year: 2021

description: The Dom João de Castro seamount in the Hirondelle Basin (Azores) is a central volcano on the ultraslow diverging Terceira Rift axis. The combination of structural and geochemical data provides insights into the evolution of central volcanoes in oceanic rift systems above the Azores melting anomaly. The orientation of fault scarps and volcanic structures at D. João de Castro and the adjacent Castro fissure zone indicate that the regional SW‐NE extending stress field dominates the morphology of the NW Hirondelle Basin. The regional tectonic stress field controls the crustal melt pathways and leads to dike emplacement along fissure zones and the prevalent eruption of mafic lavas. The occurrence of mafic to felsic lavas at D. João de Castro gives evidence for both a deep and a shallow crustal melt reservoir generating a subordinate local stress field at the seamount. New Sr‐Nd‐Pb isotope data along with incompatible trace element ratios indicate that D. João de Castro and the Castro Ridges originated from similarly heterogeneous mantle source but did not form simultaneously. Our new model implies that central volcanoes along the Terceira Rift form by the growth of volcanic ridges and transitioned into circular edifices after magmatic systems generate local changes in the regional lithospheric stress field. The geometry of D. João de Castro and other magmatic systems along the Terceira Rift combined with the alkaline nature of the erupted lavas, and the large lithosphere thickness indicates that young oceanic rifts are more similar to continental rifts rather than mid‐ocean ridges.
description: Plain Language Summary: Dom João de Castro seamount is a large submarine volcano located in the submarine Hirondelle Basin in the Azores archipelago. The Hirondelle Basin is formed as a result of extensional forces in the oceanic crust along the Azorean Terceira Rift that causes rifting of the Eurasian and Nubian plates. The presence of the D. João de Castro volcano and several elongated volcanic ridges inside the basin shows that the extensive magmatic activity in the Azores contributes to the opening of the basin. By quantifying the orientations of the tectonic and volcanic structures in the basin, it can be shown that the formation is controlled by a dominant SW‐NE directed extensional stress combined with extensive magmatic activity. Based on combined structural and geochemical observations, we conclude that the D. João de Castro seamount formed from the growth of elongated volcanic ridges and transitioned into a circular edifice after a magma system generates a local change in the crustal stress field. The geometry and geochemical composition of volcanic rocks from the D. João de Castro magmatic system, as well as other magmatic systems along the Terceira Rift are more similar to continental rift systems rather than oceanic spreading centers.
description: Key Points: D. João de Castro seamount in the Terceira Rift, Azores is influenced by a SW‐NE regional transtensional and a local radiating stress field. Structural, seismic, and geochemical data imply formation by the growth of volcanic ridges along with local stress field changes. The geometry, chemistry, and rifting rates of the Terceira Rift are more comparable to continental rifts rather than mid‐ocean ridges.
description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659