Abstract

The Lower Cretaceous Fogo Seamounts and the J-Anomaly Ridge formed along the transform margin of the southwestern Grand Banks synchronous with volcanism on the adjacent continental shelf off southeastern Canada. This volcanism took place during the “nonvolcanic” rifting of Iberia from the Grand Banks. Chemical composition and radiogenic isotopes show that basalts from the seamounts range from mildly alkalic to mildly tholeiitic. The voluminous tholeiitic magmas resulted principally from decompression melting, and the ocean-island-basalt signature of the alkalic rocks also suggests upwelling of deeper asthenosphere. The distribution of the seamounts has been determined from magnetic, bathymetric, and seismic data. Some seamounts are flat-topped and resemble guyots in that they are capped by carbonate-platform rocks. The distribution of the seamounts suggests that they developed from edge-controlled convection in the upper mantle due to thermal and density gradients at the transform transition between the spreading ocean and the continental block of the Grand Banks. The thermal effects of the volcanism promoted mid-crustal partial melting and detachment faulting on the Grand Banks and the development of the asymmetric Grand Banks–Iberia rift system. The location of the volcanic centers was strongly influenced by crustal-scale strike-slip faulting. Although the Fogo Seamount chain thus has a different tectonic origin compared with many oceanic seamount chains, the young seamounts from another area, off the transform Californian margin, also show many geochemical similarities and may have a similar origin.

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