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Abstract Ethiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co‐located, previously unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub‐lithospheric slow wavespeeds imply that Turkana's present‐day low elevation is explained best by Mesozoic and Cenozoic‐age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.
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This content will become publicly available on September 23, 2025
NEW INSIGHTS INTO THE CRUSTAL STRUCTURE OF THE MAGMA-RICH TURKANA RIFT ZONE IN THE EAST AFRICAN RIFT SYSTEM
Studies on the crustal structure of the Turkana Rift Zone (TRZ) in northern Kenya and southern Ethiopia began in the early 1980s. Initially driven by hydrocarbon exploration, these studies revealed that the rift zone comprises multiple fault-bounded basins ranging in age from the Eocene to the present. They also showed that the area hosts the intersection zone of the N-S trending basins of the Cenozoic East African Rift System (EARS) and the NW-SE-trending Mesozoic-Paleogene Central African Rift System (CARS). However, early seismic reflection and borehole data were mostly concentrated in the southern TRZ, resulting in limited subsurface data for its northern counterpart. This data gap has led to an incomplete understanding of the rift zone's regional crustal structure and how earlier CARS-related rifting influenced the development of the present-day EARS. Here, we leverage newly collected onshore and offshore subsurface industry datasets in the TRZ, spanning a 300 x 150 km region, to characterize the TRZ's crustal structure. We map several key subsurface horizons using a dense grid of 363 2-D seismic reflection profiles, which we tie to surface geology and borehole datasets. Mapping the acoustic basement produced new structure contour maps that provide high-resolution constraints on the TRZ’s crustal structure. Additionally, our isopach maps of key horizons show that strain migrated toward the modern rift axis, located along the center of Lake Turkana, following the widespread eruption of the Gombe Group basalt around 4 million years ago. Together, these results indicate that the area of maximum subsidence is collocated with the area transected by the CARS. Thus, we propose that these earlier episodes of rifting may have influenced the development and evolution of the modern EARS in the northern TRZ. These results provide crucial information for understanding tectonics in the context of hominin evolution and offer new insights into forming a divergent plate boundary.
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- Award ID(s):
- 2426448
- PAR ID:
- 10563057
- Publisher / Repository:
- Geological Society of America
- Date Published:
- ISSN:
- 0000000
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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