The DRIAR Project: Dry-Rifting In the Albertine-Rhino Graben, Uganda

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 DRIAR Project: Dry-Rifting In the Albertine-Rhino Graben, Uganda The DRIAR project was funded by the NSF Frontier Research in Earth Sciences in late 2020 to investigate continental rifting, which is a process integral to plate tectonic theory. As such, continental rifts have been intensely investigated to understand the physical processes that initiate and sustain continental break-up. 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 (e.g., Buck, 2004;Wright et al., 2006;Muirhead et al., 2016;Jones et al., 2019). However, numerous examples of "magma-poor (dry)" rifting exist worldwide that do not show surface expressions of magmatism and challenge the magma-assisted model. Hypotheses for magma-poor rift segments: 1) Despite the lack of surface volcanism, melt is present at depth and has weakened the lithosphere, allowing for strain localization during the onset of rifting.
2) The lack of surface volcanism is indicative of a lack of magmatism at depth, and pre-existing structures weaken the lithosphere allowing.
We test our hypotheses by studying the magma-poor northern Western Branch of the East African Rift System (EARS; Figure 2), which comprises the Lakes George-Edward Graben and the Albertine-Rhino Graben. This setting provides an unprecedented opportunity to study the hypothetical along-axis transition from wet (magma-assisted) to dry (magma-poor) rifting, and to rift termination. 1. Use magnetotellurics (MT), gravity, and passive-source broadband seismology to determine lithospheric thickness, map the rigidity and density of the lithosphere, mantle-flow direction from deep anisotropy, potentially frozen lithospheric anisotropy, and to assess the presence or absence of melt at depth.
2. Use magnetics, gravity, and seismology to determine the thermal structure and thickness of the crust beneath the rift zone.
3. Constrain surface motions with new GNSS observations. 4. Collect geomorphic samples for Quaternary geochronology of landforms cut and offset by the rift-bounding and intra-basinal faults to determine their fault slip rates.
5. Conduct detailed field observations to examine the geometry and kinematics of rift related structures and pre-rift (Precambrian) structures. Use these observations to evaluate the extent to which the Precambrian structures have been reactivated during rift evolution and possible seismic risks of these structures.
6. Use industry seismic reflection data constrained by well data to investigate fault evolution, as well as, strain migration and evolution through time.
7. Use the geochemistry of hot springs and measurements of magmatic gas fluxes along faults and non-faulted areas to identify mantle magmatic signatures and their variability along strike, and to establish the presence or absence of shallow magma chambers and how fluids/volatiles assist faulting. Simultaneously, quantify the variability of tectonic CO 2 flux to the atmosphere.
8. Use the helium isotope data from recent lavas and xenoliths from the rift axis to determine the magmatic sources beneath the northern Western Branch of the EARS.
9. Jointly use our observations to simultaneously determine temperature, density, and compositional heterogeneities, as well as, use these products as input to develop a new class of geodynamic models for magma-poor rifting.
The DRIAR team plans to initiate fieldwork in January 2022 with a temporary seismic deployment (PASSCAL), magnetotelluric observations, structural geologic fieldwork, campaign GNSS observations, and the installation of 4 new continuous GNSS stations ( Figure 3). In summer 2022,

Open-access data and publications
Educational Videos

Student Mentoring and Training
Underrepresented groups Professional development we also plan to conduct the first of two field schools to train our African collaborators and a few US participants in the techniques that are being used for this project. The field school is part of our comprehensive broader impacts plan that encompasses capacity building, societally relevant scientific outcomes, communicating broadly our results through educational videos and open access data, and by providing professional development and mentoring guidance to students participating in the DRIAR project ( Figure 4).

ACKNOWLEDGEMENTS REFERENCES
The DRIAR project was funded by the NSF Frontier Research in Earth Sciences in late 2020 to investigate continental rifting, which is a process integral to plate tectonic theory. As such, continental rifts have been intensely investigated to understand the physical processes that initiate and sustain continental break-up. 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 (e.g., Buck, 2004;Wright et al., 2006;Muirhead et al., 2016;Jones et al., 2019). However, numerous examples of "magma-poor (dry)" rifting exist worldwide that do not show surface expressions of magmatism and challenge the magma-assisted model. Hypotheses for magma-poor rift segments: 1) Despite the lack of surface volcanism, melt is present at depth and has weakened the lithosphere, allowing for strain localization during the onset of rifting.
2) The lack of surface volcanism is indicative of a lack of magmatism at depth, and pre-existing structures weaken the lithosphere allowing.
We test our hypotheses by studying the magma-poor northern Western Branch of the East African Rift System (EARS; Figure 2), which comprises the Lakes George-Edward Graben and the Albertine-Rhino Graben. This setting provides an unprecedented opportunity to study the hypothetical along-axis transition from wet (magma-assisted) to dry (magma-poor) rifting, and to rift termination. 1. Use magnetotellurics (MT), gravity, and passive-source broadband seismology to determine lithospheric thickness, map the rigidity and density of the lithosphere, mantle-flow direction from deep anisotropy, potentially frozen lithospheric anisotropy, and to assess the presence or absence of melt at depth.
2. Use magnetics, gravity, and seismology to determine the thermal structure and thickness of the crust beneath the rift zone.
3. Constrain surface motions with new GNSS observations. 4. Collect geomorphic samples for Quaternary geochronology of landforms cut and offset by the rift-bounding and intra-basinal faults to determine their fault slip rates.
5. Conduct detailed field observations to examine the geometry and kinematics of rift related structures and pre-rift (Precambrian) structures. Use these observations to evaluate the extent to which the Precambrian structures have been reactivated during rift evolution and possible seismic risks of these structures.
6. Use industry seismic reflection data constrained by well data to investigate fault evolution, as well as, strain migration and evolution through time.
7. Use the geochemistry of hot springs and measurements of magmatic gas fluxes along faults and non-faulted areas to identify mantle magmatic signatures and their variability along strike, and to establish the presence or absence of shallow magma chambers and how fluids/volatiles assist faulting. Simultaneously, quantify the variability of tectonic CO 2 flux to the atmosphere.
8. Use the helium isotope data from recent lavas and xenoliths from the rift axis to determine the magmatic sources beneath the northern Western Branch of the EARS.
9. Jointly use our observations to simultaneously determine temperature, density, and compositional heterogeneities, as well as, use these products as input to develop a new class of geodynamic models for magma-poor rifting.
The DRIAR team plans to initiate fieldwork in January 2022 with a temporary seismic deployment (PASSCAL), magnetotelluric observations, structural geologic fieldwork, campaign GNSS observations, and the installation of 4 new continuous GNSS stations ( Figure 3). In summer 2022,

Communication of Data and Results
Open-access data and publications Educational Videos

Student Mentoring and Training
Underrepresented groups Professional development we also plan to conduct the first of two field schools to train our African collaborators and a few US participants in the techniques that are being used for this project. The field school is part of our comprehensive broader impacts plan that encompasses capacity building, societally relevant scientific outcomes, communicating broadly our results through educational videos and open access data, and by providing professional development and mentoring guidance to students participating in the DRIAR project ( Figure 4). Figure 3. A map of the study area (only in Uganda) showing topography, hot springs, streams, lakes, and major towns. Also shown are proposed transects for the acquisition of geological and geophysical data and seismic profiles. Where possible, the transects will also be used to collect gas samples in addition to samples taken at hot springs.