Abstract
AbstractThe Rungwe Volcanic Province (RVP) is a volcanic center in an anomalous region of magma‐assisted rifting positioned within the magma‐poor Western Branch of the East African Rift (EAR). The source of sublithospheric melt for the RVP is enigmatic, particularly since the volcanism is highly localized, unlike the Eastern Branch of the EAR. Some studies suggest the source of sublithospheric melt beneath the RVP arises from thermal perturbations in the upper mantle associated with an offshoot of the African superplume flowing from the SW, while others propose a similar mechanism, but from the Kenyan plume diverted around the Tanzania Craton from the NE. Another possibility is decompression melting from upwelling sublithospheric mantle due to lithospheric modulated convection (LMC) where the lithosphere is thin. The authors test the hypothesis that sublithospheric melt feeding the RVP can be generated from LMC. We develop a 3D thermomechanical model of LMC beneath the RVP and the Malawi Rift and constrain parameters for sublithospheric melt generation due to LMC. We assume a rigid lithosphere and use non‐Newtonian, temperature‐, pressure‐, and porosity‐dependent creep laws of anhydrous peridotite for the sublithospheric mantle. We find a pattern of upwelling from LMC beneath the RVP. The upwelling generates melt only for elevated mantle potential temperatures (Tp), which suggests a heat source possibly from plume material. At elevated Tp, LMC associated decompression melts occurs at a maximum depth of ∼150 km beneath the RVP. We suggest upwelling due to LMC entrains plume materials resulting in melt generation beneath the RVP.
Highlights
Melt intrusions into the lithospheric mantle and crust during extensional tectonics play a key role in weakening the lithosphere during magma-assisting rifting
Some studies suggest the source of sublithospheric melt beneath the Rungwe Volcanic Province (RVP) arises from thermal perturbations in the upper mantle associated with an offshoot of the African superplume flowing from the SW, while others propose a similar mechanism, but from the Kenyan plume diverted around the Tanzania Craton from the NE
We suggest upwelling due to lithospheric modulated convection (LMC) entrains plume materials resulting in melt generation beneath the RVP
Summary
Melt intrusions into the lithospheric mantle and crust during extensional tectonics play a key role in weakening the lithosphere during magma-assisting rifting. Magma-assisted continental rifting involves magmatic intrusions that are sourced from melt generated in the sublithospheric mantle beneath the rift axis, which developed when mantle potential temperatures are higher than average (i.e., McKenzie & Bickle, 1988). The source of melt generation in the sublithospheric mantle beneath rifts has been proposed to originate from thermal perturbations due to plumes (e.g., Burke & Dewey, 1973; Furman et al, 2006; Saunders et al, 1992) or asthenospheric upwelling in response to thinned, extended lithosphere (e.g., Nielsen & Hopper, 2002; van Wijk et al, 2001; White & McKenzie, 1989). Thermomechanical modeling by Koptev et al (2018) suggests that the melt beneath the RVP is sourced from the Kenyan plume that is channeled into three mantle flows by the thick lithospheric keel of the Tanzanian craton and the Bangweulu cratonic block. We hypothesize that the melt beneath the RVP is, at least, partly generated from decompression melting associated with the passive upwelling model
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