Influence of Rift Superposition on Lithospheric Response to East African Rift System Extension: Lapur Range, Turkana, Kenya

Abstract

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.

Tectonothermal Evolution of the Broadly Rifted Zone, Ethiopian Rift

“…The presence of a top basement erosional unconformity and large decreases in paleotemperature (~80–90 °C), as indicated by t‐T reconstructions, are consistent with erosional denudation as the mechanism for observed cooling. A similar period of denudational cooling, both in timing and character, has been observed in low‐temperature thermochronology studies performed throughout much of East Africa (e.g., Boone, Seiler, et al, ; Boone, Kohn, et al, ; Foster & Gleadow, , , ; Noble et al, ; Torres Acosta et al, ). This pervasive period of cooling has been interpreted as recording denudation of the Anza rift margin and adjacent hinterlands during the development of the NW‐SE trending, Cretaceous‐early Paleogene graben (e.g., Foster & Gleadow, ).…”

Evolution of the northern Turkana Depression (East African Rift System, Kenya) during the Cenozoic rifting: New insights from the Ekitale Basin (28‐25.5 Ma)

“…The next important event affecting the Ekitale Basin was the activation of the N–S structures (Group 2 faults and dykes) and the opening of the North Lake Basin. The timing of the MRLF activation has been recently estimated between 15 and 9 Ma based on thermochronology (Boone et al, ). This is supported by the age of a N–S‐oriented Group 2 dyke dated ca .…”

Accommodation of East African Rifting Across the Turkana Depression

“…The superposition of at least two phases of rifting in the Mesozoic and Oligo-Miocene to Recent time produced a region of enhanced crustal thinning with crustal thickness of only 20 km beneath Lake Turkana, compared to the 27-35 km beneath the Eastern and MERs (Ebinger et al, 2017;Hendrie et al, 1994;Mechie et al, 1994;Sippel et al, 2017). The NW-SE trending Mesozoic rift structures of the Anza and SE Sudan rifts are oblique to the present-day extension direction, and their reactivation has been invoked as an alternative cause of a lack of uplifted footwalls in parts of the Turkana Depression (Brune et al, 2017;Boone et al, 2018).…”