The Turkana Depression of northern Kenya and southern Ethiopia contains voluminous plume-related basalts that mark the onset of the Paleogene–recent East African Rift System (EARS) at ca. 45 Ma. Thus, the Turkana Depression is crucial to understanding the inception of intracontinental rifting. However, the precise chronology of early rift-basin formation in Turkana is poorly constrained. We present apatite fission-track and (U-Th-Sm)/He thermochronology data from basement rocks from the margins of the north-south–trending Lokichar Basin that constrain the onset of rift-related cooling. Thermal history modeling of these data documents pronounced Eocene to Miocene denudational cooling of the basin-bounding Lokichar fault footwall. These results, along with ∼7 km of Paleogene to middle Miocene syn-rift strata preserved in the Lokichar fault hanging wall, suggest that formation of the Lokichar Basin began as early as ca. 45–40 Ma. Preexisting lithospheric heterogeneities inherited from earlier Mesozoic rifting and Eocene plume magmatism likely facilitated the broadly concurrent nucleation of strain in the Turkana Depression, up to ∼15 m.y. earlier than EARS initiation elsewhere. Late Paleogene extension in the Lokichar Basin and other parts of Turkana significantly predate the Miocene creation of pronounced plume-related topography in East Africa, suggesting that other mechanism(s), such as far-field stresses or mantle basal drag, likely played a critical role during EARS inception.

The Broadly Rifted Zone (BRZ) of southern Ethiopia is a long-lived and structurally complex segment of the East African Rift System. However, due to poor surface exposure of early synrift strata and a dearth of subsurface data, the evolution of the BRZ remains poorly understood. We present new apatite (U-Th-Sm)/He and augmented apatite fission track low-temperature thermochronology data from the Beto and Galana basin boundary fault systems to constrain the tectonothermal evolution of the western and eastern BRZ, respectively. Time-temperature reconstructions suggest that East African Rift System-related extension began concurrently across the BRZ in the early Miocene (20-17 Ma), at least 6 Myr prior to faulting in the Main Ethiopian Rift further north. Increased time-temperature resolution provided by multithermochronometer analyses reveals contrasting along-strike spatiotemporal variations in Beto and Galana margin cooling histories, which appear to mirror the disparate structural geometries of their basin-bounding normal fault arrays. Longitudinal contrasts in basin architecture and rift-related cooling histories across the BRZ may reflect the region's heterogeneous distribution of preexisting basement fabrics, namely, the presence of a previously reported N-NNE trending Neoproterozoic suture zone beneath the eastern BRZ. Its influence may explain both the development of long, curvilinear faults and the gradual basinward migration of strain exhibited by the easternmost BRZ, absent further west. The anomalous evolution of the BRZ compared to the greater Ethiopian Rift, both in its earlier onset and its wider deformation zone, likely results from its inheritance of preattenuated lithosphere, thermomechanically modified by earlier Cretaceous-Paleogene Anza-South Sudan rifting and/or Eocene plume impingement.

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The Turkana Depression of northern Kenya lies at the intersection of the NW‐SE trending late Mesozoic‐early Paleogene South Sudan and Anza rifts and the N‐S trending late Paleogene‐Recent East African Rift System (EARS). A low‐temperature thermochronology study in the Lapur Range reveals a complex tectonothermal evolution related to multiple periods of regional and local tectonism. Zircon (U‐Th)/He data from Precambrian basement record rapid Early Cretaceous denudational cooling. Coeval subsidence in the adjacent Anza and South Sudan rifts (Morley, Bosworth, et al., 1999; Schull, 1988) suggests that the northern Turkana region acted as a basement high separating the grabens, as well as an axial source of sediment. Between ~95 and 90 Ma, a period of reheating commenced with burial of Lapur basement beneath ~500 m of Late Cretaceous‐Eocene Lapur Sandstone and ~1.5–3.5 km of latest Eocene‐early Miocene Turkana volcanics. Apatite fission track (AFT) and apatite (U‐Th‐Sm)/He data record a transition to rapid denudational cooling in the mid‐Miocene (~14 Ma) in response to EARS‐related extension in the northern Turkana Basin. Thermal history models indicate that the Lapur Range experienced ~90–100°C of mid‐Miocene to Plio‐Pleistocene cooling, yielding the first Neogene AFT ages reported from Kenya related to EARS exhumation. We attribute the larger magnitude of cooling in the Lapur Range compared to other regions of the EARS to the attenuated crustal thickness and elevated heat flow of the Turkana Depression, crustal properties inherited from earlier Cretaceous‐Paleogene rifting. The resulting low effective elastic thickness of the Turkana lithosphere allowed for increased isostatic footwall uplift in response to EARS extension.