The crust beneath the Turkana Rift has thinned to about 13 kilometers, placing the region in an advanced stage of continental rifting called necking.The transition appears to have accelerated around 4 million years ago, when deformation became concentrated and sediment began accumulating more rapidly.Those tectonic changes may help explain why Turkana preserves one of the world\u2019s richest and most continuous records of early human evolution.<\/p>\n
A 13-kilometer-thick strip of crust beneath northern Kenya may mark one of the clearest places on Earth where a continent is progressing toward breakup.<\/p>\n
The crust beneath the Turkana Rift is thinner than scientists had recognized, according to research published in Nature Communications<\/a>. The work identifies Turkana as the first active continental rift known to be undergoing necking, when stretching becomes concentrated before separation.<\/p>\n The finding also offers a new way to view Turkana\u2019s record of human evolution. The same tectonic process that weakened the continent appears to have created sediment-filled basins capable of preserving fossils over millions of years.<\/p>\n Late Miocene fossil-bearing strata of Lothagam in West Turkana. (CREDIT: Christian Rowan) A continent stretching at its weak point<\/p>\n The Turkana Rift spans northern Kenya and southern Ethiopia within the larger East African Rift System. In this region, the Nubian part of the African plate and the Somali plate are slowly moving apart.<\/p>\n Continental rifting does not always end with a new ocean. Some rifts stop developing, while successful rifts pass through several stages as deformation becomes increasingly concentrated.<\/p>\n Necking is a critical stage in that progression. Much like pulling taffy from both ends, stretching causes the middle to become longer and thinner while material farther away remains relatively unchanged.<\/p>\n \u201cWe found that rifting in this zone is more advanced, and the crust is thinner, than anyone had recognized,\u201d said lead author Christian Rowan, a doctoral researcher at Columbia University\u2019s Lamont-Doherty Earth Observatory<\/a>. \u201cEastern Africa has progressed further in the rifting process than previously thought.\u201d<\/p>\n Using seismic reflection data and borehole information, Rowan and colleagues mapped buried rock layers. They combined those measurements with estimates of the Moho, the boundary between Earth\u2019s crust and mantle.<\/p>\n The crust has narrowed dramatically<\/p>\n The resulting cross section revealed a wedge-shaped structure. Crystalline crust thicker than 35 kilometers near the rift flanks narrows to about 12.7 kilometers, with an uncertainty of 2.8 kilometers, near the rift axis.<\/p>\n Homo erectus crania from the Turkana Rift. Left: WT 15000, \u2018Turkana Boy\u2019 from West Turkana. Right: ER 3733 from East Turkana. (CREDIT: John Rowan) <\/p>\n That geometry resembles necked crust at ancient continental margins where breakup already occurred. Modern earthquake activity is also concentrated in the thinned zone, indicating that deformation continues there.<\/p>\n \u201cWe\u2019ve reached that critical threshold\u201d of crustal breakdown, co-author Anne B\u00e9cel, a Lamont geophysicist, said. \u201cWe think this is why it is more prone to separate.\u201d<\/p>\n The result does not mean eastern Africa is about to split apart on a human timescale. Continental breakup unfolds across millions of years. Necking instead indicates that Turkana<\/a> has entered a more advanced stage than most active continental rifts.<\/p>\n Earlier tectonic history may have helped bring Turkana to this point. The region experienced an older episode of rifting that left weakened and thinned crust for later deformation to exploit.<\/p>\n This inheritance may explain why Turkana matured faster than expected despite relatively slow present-day plate motion. Magmatism also weakened the lithosphere, further concentrating deformation.<\/p>\n A major shift began about 4 million years ago<\/p>\n East African Rift-related faulting began in the Turkana region roughly 45 to 40 million years ago, followed by volcanism around 37 million years ago. For much of that history, deformation was spread across a broad region.<\/p>\n The pattern changed around 4 million years ago. A major phase of basaltic volcanism<\/a> was followed by stronger localization of tectonic strain and faster sediment accumulation.<\/p>\n Christian Rowan at Lothagam fossil site in West Turkana. (CREDIT: Christian Rowan) <\/p>\n The researchers interpret that transition as the beginning of crustal necking. Extension along major faults created more space for sediment to accumulate as parts of the basin subsided.<\/p>\n That timing overlaps with a major change in Turkana\u2019s fossil record. Before about 4 million years ago, fossil-bearing deposits<\/a> were more isolated and discontinuous. Afterward, thick sediments accumulated across a connected basin system.<\/p>\n \u201cIn essence, we now have a front row seat to observe a critical rifting phase that had fundamentally shaped all rifted margins across the world,\u201d said co-author Folarin Kolawole of Lamont.<\/p>\n Tectonics may explain Turkana\u2019s fossil treasure<\/p>\n The Turkana region is one of the most important places for studying human origins. Its Plio-Pleistocene sediments<\/a> preserve thousands of hominin fossils, archaeological remains and other fossils.<\/p>\n Much of that continuous record lies within the Omo Group, including the Shungura, Nachukui and Koobi Fora formations. Together, these deposits preserve a rich record spanning roughly 4 million to 1 million years ago.<\/p>\n Scientists have long considered why Turkana contains such an extraordinary concentration of evidence about early humans. One possibility is that the region itself played an unusually important biological role in hominin evolution.<\/p>\n The new work adds another possibility. Turkana may be unusually important partly because its geology made preservation more likely.<\/p>\n Global active and failed intracontinental rifts and the tectonic setting of the study. (CREDIT: Christian Rowan et al, Nature Communications) <\/p>\n As necking intensified, subsidence produced additional accommodation space, the geological term for room in which sediments can accumulate. Faster sediment buildup could bury bones, artifacts and environmental evidence, helping preserve them.<\/p>\n \u201cThe conditions were right to preserve a continuous fossil record,\u201d Rowan said.<\/p>\n