The emergence and evolution of Santa Maria Island (Azores) - the conundrum of uplifting islands revisited
Monday, 15 December 2014
The growth and decay of ocean island volcanoes is intrinsically linked to vertical movements; whilst mechanisms for subsidence are well understood, uplift mechanisms are still very enigmatic. Santa Maria Island in the Azores is an ocean island volcano with a complex evolutionary history with subsidence followed by uplift. The island emerged by surtseyan activity at approximately 6 Ma, entailing in the formation of a subaerial shield volcano that was fully developed by ~5.6 Ma. The edifice then experienced a period of intense erosion and subsidence, during which it was partially or completely truncated to a shallow submarine bank. Around 5 Ma, volcanism resumed on the eastern side of the edifice, with occasional submarine/surtseyan activity synchronous with marine deposition. By ~4.3 Ma, volcanism grew in intensity forming a new edifice centred on the eastern flank of the underlying edifice. This new volcanic edifice started as submarine and then, as it gradually grew upwards and outpaced subsidence, breached sea level and formed a new island. The new edifice kept growing eastwards and northwards until ~3.5 Ma, when volcanic activity waned. At 3.5-3.2 Ma, however, subsidence reversed to an uplift trend that extended throughout the Upper Pleistocene into the present. This uplift trend is responsible for the generation of a staircase of shore platforms on the windward side of the island, which extends up to ~230 m in elevation. The fact that an island located in very young lithosphere experienced such a pronounced uplift trend is remarkable and raises important questions concerning possible uplift mechanisms. The loading of the neighbouring island of São Miguel may account for part of the uplift, but not the majority. Vertical tectonics along the nearby Gloria Fault is not completely implausible but unlikely to be a source of significant uplift due to its dominant strike-slip character. Thus, other mechanisms need to be considered, most notably intrusions at the base of the edifice and crustal thickening, suggesting that these play a significant role even on islands standing on a young lithosphere, such as in the Azores.