Continental rifting is a critical component of the plate tectonic paradigm, and occurs in more than one mode, phase, or stage. While rifting is typically facilitated by abundant magmatism, some rifting is not. We aim to develop a better understanding of the fundamental processes associated with magma-poor (dry) rifting. Here, we provide an overview of the NSF-funded Dry Rifting In the Albertine-Rhino graben (DRIAR) project, Uganda. The project goal is to apply geophysical, geological, geochemical, and geodynamic techniques to investigate the Northern Western Branch of the East African Rift System in Uganda. We test three hypotheses: (1) in magma-rich rifts, strain is accommodated through lithospheric weakening from melt, (2) in magma-poor rifts, melt is present below the surface and weakens the lithosphere such that strain is accommodated during upper crustal extension, and (3) in magma-poor rifts, there is no melt at depth and strain is accommodated along pre-existing structures such as inherited compositional, structural, and rheological lithospheric heterogeneities. Observational methods in this project include: passive seismic to constrain lithospheric structure and asthenospheric flow patterns; gravity to constrain variations in crustal and lithospheric thickness; magnetics to constrain the thermal structure of the upper crust; magnetotellurics to constrain lithospheric thickness and the presence of melt; GNSS to constrain surface motions, extension rates, and help characterize mantle flow; geologic mapping to document the geometry and kinematics of active faults; seismic reflection analyses of intra-rift faults to document temporal strain migration; geochemistry to identify and quantify mantle-derived fluids in hot springs and soil gases; and geodynamic modeling to develop new models of magma-poor rifting processes. Fieldwork will begin in January 2022 and the first DRIAR field school is planned for summer 2022. Geodynamic modeling work and morphometric analyses are already underway.

The leading paradigm for rift initiation suggests “magma-assisted (wet)” rifting is required to weaken strong lithosphere such that only small tectonic stresses are needed for rupture to occur. However, there is no surface expression of magma along the 300 km long Albertine-Rhino Graben (except at its southernmost tip within the Tore Ankole Volcanic Field), which is the northernmost rift in the Western Branch of the East African Rift System. The two prevailing models explaining magma-poor rifting are: 1) Melt is present at depth weakening the lithosphere, but it has not reached the surface or 2) far-field forces driving extension are accommodated along weak pre-existing structures without melt at depth. The goal of this study is to test the hypothesis that melt is generated below the Albertine-Rhino Graben from Lithospheric Modulated Convection (LMC) using the 3D finite element code ASPECT. We develop a regional model of a rigid lithosphere and an underlying convecting sublithospheric mantle that has dimensions 1000 by 1000 by 660 km along latitude, longitude, and depth, respectively. We solve the Stokes equations using the extended Boussinesq approximation for an incompressible fluid which considers the effects of adiabatic heating and frictional heating. We include latent heating such that we can test for melt generation in the sublithospheric mantle from LMC. Using LITHO1.0 as the base of our lithosphere, our preliminary results suggest melt could be generated beneath the Albertine-Rhino graben given a mantle potential temperature of 1800 K. These early results indicate LMC can generate melt beneath the northernmost Western branch of the East African Rift System.