The “Azores geosyndrome” and plate tectonics: Research history, synthesis, and unsolved puzzles

Abstract

The Azores volcanic archipelago, the Azores Plateau (AP) and the Azores triple junction (ATJ) between the Eurasia, North America and Nubia plates occupy the summit of a regional feature we refer to as the ‘Azores Geosyndrome’. Included are anomalies in crustal thickness, rock composition, basement depth, plate boundary morphology, seismicity, gravity and geoid, and upper mantle seismic velocity structure, and there are many similarities between the Azores and Iceland geosyndromes. The location of the Azores in the central North Atlantic, technological advances in marine geophysics as well as logistic, geomilitary and geopolitical motivations and advanced research of island geology/volcanology have contributed to make the ATJ the most studied oceanic triple plate junction. However, a unified understanding of the Azores Geosyndrome awaits future deep crustal boreholes (particularly on the AP) and regional sea-floor seismometer arrays to resolve the seismic velocity structure below the AP down to the middle and perhaps lower mantle. Whereas a deep mantle plume appears unlikely to exist below the Azores, it cannot yet be excluded (see O’Neill and Sigloch, Chapter “ Crust and Mantle Structure Beneath the Azores Hotspot—Evidence from Geophysics ”, and Moreira et al., Chapter “ Noble Gas Constraints on the Origin of the Azores Hotspot ”). What is already clear is that the development and evolution of the Azores Geosyndrome has involved dynamic interactions among the North America-Nubia-Eurasia plates and at least the uppermost mantle below those plates—even far from the ATJ area. The plate boundary reorganization that resulted in the triple plate junction jumping from the end of King’s Trough south to create the ATJ was largely complete by Chron 6C (23 Ma) and coincided within dating uncertainties with the jump of the spreading plate boundary from the Norway Basin to the new Kolbeinsey Ridge just north of Iceland. Major geological changes in the Pyrenees and Alpine Tethys region at that time have long been known. In fact, the Palaeogene-Neogene boundary, a time of global change in planktonic biogeography, is placed at 23.0 Ma, in the upper part of C6C. Why the ATJ developed where it did and not elsewhere along the MAR suggests the lithosphere and subjacent mantle had already created a region of plate weakness. The subsequent development of the AP, largely via Mid-Atlantic Ridge spreading, produced a thick crust and more fertile mantle lithosphere-particularly from ca. 12 to 8 Ma. This mantle lithosphere was and continues to be relatively weak and fertile, favouring transtensional fissuring, formation of central volcanoes, as well as oblique hyperslow spreading along the Terceira Rift—particularly in the last 1.5 Ma.