Birth of the East African Rift System: Nucleation of magmatism and strain in the Turkana Depression

Abstract

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.

Accommodation of East African Rifting Across the Turkana Depression

“…During this period, volcanism was limited to E-W trending dykes along strike-slip faults connecting southern Ethiopia into South Sudan (Ebinger et al, 1993) and eastward to the Anza Graben Morley, Wescott, et al, 1999). In the Paleogene, reactivation of the Anza Rift and extension west of present-day Lake Turkana occurred (Boone et al, 2019;Bosworth, 1992;Bosworth & Morley, 1994;Ebinger & Ibrahim, 1994;Morley et al, 1992Wescott et al, 1999). Magmatism in southern Ethiopia then began in the late Eocene, and by 45-35 Ma was widespread in southern Ethiopia and in the Lotikipi-Lapurr and proto-Kino Sogo regions of Kenya (e.g., Davidson & Rex, 1980;Ebinger et al, 1993;Morley, Wescott, et al, 1999;Vetel & Le Gall, 2006).…”

Plio-Pleistocene sedimentation in West Turkana (Turkana Depression, Kenya, East African Rift System): Paleolake fluctuations, paleolandscapes and controlling factors

“…It corresponds to a 300 km long and 200 km wide lowland that separates the Main Ethiopian Rift to the north from the northern Kenyan rift to the south (Ebinger et al, 2000;Brune et al, 2017). The Turkana Depression consists of several N-S oriented half-grabens and grabens which developed from the middle to upper Eocene up to the present-day (Rosendahl, 1987;Dunkelman et al, 1988Dunkelman et al, , 1989Morley et al, 1999;Tiercelin and Lezzar, 2002;Vétel and Le Gall, 2006;Tiercelin et al, 2012a and2012b;Macgregor, 2015;Boone et al, 2017Boone et al, , 2019Ragon et al, 2019).…”

Linking surface and subsurface volcanic stratigraphy in the Turkana Depression of the East African Rift system

“…1) (Furman et al 2004). Unlike other areas of the EARS, rifting within the Turkana Depression area began when part of the Central Africa Rift system (CARS) formed in the area during the Cretaceous (Morley et al 1999;Furman et al 2006;Boone et al 2019), with further rifting related to both the CARS and the EARS taking place in the Paleogene and Miocene-torecent, leading to a cumulative Beta factor approaching two (calculated from integration of seismic reflection data, refraction data and modelling) (Ebinger and Ibrahim 1994;Hendrie et al 1994). Morley et al (1999) provide a geological history of the EARS with particular respect to Kenya and the Turkana region, based on well results from the early exploration phase in the 1990s and regional seismic reflection lines collected as part of petroleum exploration.…”

“…Within the South Lokichar Basin, the main phase of rifting commenced during the Late Oligocene to Early Miocene, with rifting ceasing into the late Middle Miocene, with the basin then transitioning into a post-rift sag stage which resulted in deposition of a thin Upper Miocene-to-recent fill (see Loperot-1 in Fig. 18) (Morley et al 1999;Vetel et al 2004;Vetel and Le Gall 2006;Boone et al 2019). However, the North Lokichar Basin illustrates a contrast in rift timing where, based on data from Emesek-1 (this paper), the syn-rift package is composed of c. 2 km of Upper Miocene shale and sandstones (see Emesek-1 in Fig.…”