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Key Points New seismic data from the Turkana Depression enhances images of East African mantle wavespeed and discontinuity structure Thinnest transition zone in East Africa is below the NW Turkana Depression and is underlain by a mid to lower mantle plume tail Refractory lithosphere in SE Ethiopia governed northern extent of Mesozoic Anza rifting and southeastern limit of flood basaltic magmatism
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Active Deformation Constraints on the Nubia‐Somalia Plate Boundary Through Heterogenous Lithosphere of the Turkana Depression
Key Points New 1‐D velocity model relocates 1716 earthquakes with local magnitude ranging from 1 to 4.5 and b ‐value of 1.22 ± 0.06 in Turkana Depression Brittle strain localizes in narrow areas overlying uppermost mantle low velocity zones and avoids previously thinned crust of Mesozoic rift We redefine the modern Nubia‐Somalia plate boundary across the Turkana Depression; only a part of the 300‐km‐wide region is active
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- Award ID(s):
- 1824417
- PAR ID:
- 10464105
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 24
- Issue:
- 9
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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New field observations and 40 Ar/ 39 Ar geochronology reveal that the Topernawi Formation of the Ekitale Basin, northern Turkana Depression, Turkana County, Kenya was deposited entirely during the Oligocene between 29.7 ± 0.5 Ma and 29.24 ± 0.08 Ma. These bracketing ages are determined via new 40 Ar/ 39 Ar geochronology on a basaltic lava flow at the base of the section and a felsic ignimbrite near the top. A newly discovered basal unit and interbedded lava flow result in a new total sedimentary thickness of 92 m. The Topernawi Formation is the oldest dated syn-rift sedimentary section in the northern Turkana Depression.more » « less
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New field observations and 40 Ar/ 39 Ar geochronology reveal that the Topernawi Formation of the Ekitale Basin, northern Turkana Depression, Turkana County, Kenya was deposited entirely during the Oligocene between 29.7 ± 0.5 Ma and 29.24 ± 0.08 Ma. These bracketing ages are determined via new 40 Ar/ 39 Ar geochronology on a basaltic lava flow at the base of the section and a felsic ignimbrite near the top. A newly discovered basal unit and interbedded lava flow result in a new total sedimentary thickness of 92 m. The Topernawi Formation is the oldest dated syn-rift sedimentary section in the northern Turkana Depression.more » « less
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Abstract The Turkana Depression, located between the Ethiopian and East African plateaus, displays an anomalous rift architecture. It is missing the narrow, magma‐rich morphology observed in the Main Ethiopian Rift that cuts through the Ethiopian Plateau. Instead, diffuse faulting and isolated volcanic centers are widespread over several hundred kilometers. Turkana has also experienced less magmatism over the last 30 Myr than adjacent plateaus, despite having a thin crust and residing above a mantle that is inferred to be hot and partially molten. We hypothesize that lithospheric weakening has been the key control on magma transport across the lithosphere in the Turkana Depression and subsequent rift development. Using poro‐viscoelastic–viscoplastic models of melt transport, we show that magma extraction across a thin, weakened lithosphere is slower than across a thick, elastic lithosphere. Our results suggest that pre‐rift lithospheric strength can explain the magma‐poor character of Turkana for most of its tectonic history.more » « less
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Kinematics of rift linkage between the Eastern and Ethiopian rifts in the Turkana Depression, AfricaAbstract Rift initiation within cold, thick, strong lithosphere and the evolving linkage to form a contiguous plate boundary remains debated in part owing to the lack of time–space constraints on kinematics of basement‐involved faults. Different rift sectors initiate diachronously and may eventually link to produce a jigsaw spatial pattern, as in the East African rift, and along the Atlantic Ocean margins. The space–time distribution of earthquakes illuminates the geometry and kinematics of fault zones within the crystalline crust, as well as areas with pressurized magma bodies. We use seismicity and Global Navigation System Satellites (GNSS) data from the Turkana Rift Array Investigating Lithospheric Structure (TRAILS) project in East Africa and a new digital compilation of faults and eruptive centres to evaluate models for the kinematic linkage of two initially separate rift sectors: the Main Ethiopian Rift (MER) and the Eastern rift (ER). The ca. 300 km wide zone of linkage includes failed basins and linkage zones; seismicity outlines active structures. Models of GNSS data indicate that the ca. 250 km‐wide zone of seismically active en echelon basins north of the Turkana Depression is a zone, or block, of distributed strain with small counterclockwise rotation that serves to connect the Main Ethiopian and Eastern rifts. Its western boundary is poorly defined owing to data gaps in South Sudan. Strain across the northern and southern boundaries of this block, and an ca. 50 km‐wide kink in the southern Turkana rift is accommodated by en echelon normal faults linked by short strike‐slip faults in crystalline basement, and relay ramps at the surface. Short segments of obliquely oriented basement structures facilitate across‐rift linkage of faults, but basement shear zones and Mesozoic rift faults are not actively straining. This configuration has existed for at least 2–5 My without the development of localized shear zones or transform faults, documenting the importance of distributed deformation in continental rift tectonics.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023GC010982
