Effects of ridge geometry on mantle dynamics in an oceanic triple junction region: Implications for the Azores Plateau

Plate boundary geometry can affect the nature of magmatism along a mid-ocean ridge. The Azores Plateau is located in a complex geological setting that includes a triple junction (TJ), an oblique and recently-formed ultra-slow-spreading ridge, a zone of diffuse seafloor deformation, a major fracture zone, and a postulated hotspot. The precise character of the hotspot is somewhat debated, as some lines of evidence indicate it may not be a classic deep-seated plume. However, seismic and gravity data suggest plateau crustal thicknesses of 8 km or more, implying some mechanism for excess melting. To assess the role of ridge geometry in creating the Azores Plateau, this study uses a finite element numerical model to isolate the effects of selected aspects of plate boundary configuration on mantle flow and melt production in a TJ kinematically similar to the Azores TJ. The model focuses on the slowest-spreading ridge in the TJ, analogous to the Terceira Rift. The effect of the varying ridge obliquity observed along the Terceira Rift is also assessed using an independent 1-D melting model. In general, relatively little melt production is predicted along the Terceira Rift analogue, except for regions closest to the TJ where the proximity of a faster-spreading ridge increases temperatures within the melting zone. In the 1-D melting model with mantle temperatures of 1350 degrees C, melt thicknesses of 2 km are calculated for the least oblique segments, while more oblique segments produce little to no melt. The presence of a long discontinuity (simulating the Gloria FL) has little effect on mantle dynamics for axial distances <350 km from the TJ, although crustal production is predicted to diminish to zero within similar to 150 km of the discontinuity. When several ridge geometrical effects are combined (i.e., a TJ, time-limited spreading, a ridge discontinuity, and depressed spreading rates within similar to 100 km of the TJ point), similar to 2.5 km of variability in melt thickness can be produced. Overall, while these numerical experiments suggest plate boundary geometry may play a role in modifying Azores crustal accretion, additional factors such as a mantle heterogeneity are likely required to explain the full scale of the observed magmatism. (C) 2010 Elsevier B.V. All rights reserved.