Abstract
Mid-ocean ridge interactions with hotspots strongly affect mantle flow processes. This study analyses the anomalies produced as a result of the interaction between a hotspot and an oceanic ridge-ridge-ridge triple junction, in close proximity to one another. The complex three dimensional (3D) nature of the Azores Triple Junction (ATJ), in which two near-collinear faster-spreading ridges are joined orthogonally with a slower-spreading ridge, provides an excellent opportunity to quantify the effect of triple junction geometry on along-axis magmatic accretion and mantle dynamic processes as a result of the interaction with a hotspot. For the ATJ, the faster-spreading ridges are two branches of the Mid-Atlantic Ridge (MAR), and the slower-spreading ridge is the Terceira Rift (TR).
Using shipboard bathymetry and satellite free-air gravity, we obtain mantle Bouguer anomaly (MBA) by eliminating from free-air gravity the attractions of seafloor topography and a reference crust. Along the TR, the Azores hotspot has a maximum MBA axial gravity low of -100 mGal, suggesting localized crustal thickening, elevated mantle temperatures and/or low density mantle. The entire Azores plateau along the TR is associated with a large (~80 mGal) broad low. Dispersion of plume material along the TR, a distance in the range of 550 km, is likely minimized by the rift system's obliqueness, immature nature and hyper-slow spreading rate, as well as the presence of the Gloria Fracture Zone. Further, along-axis profiles along the TR suggest that MBA shows a strong dependence on the tectonic segmentation of the ridge axis.
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