March 26, 2012

Western Great Rift is 15 Ma. older than believed

This is potentially an important discovery that may affect the chronological frame of the simians (what ultimately include us also).

The Western section of the Great Rift (or East African Rift) is not a lot younger than the Eastern section, as was believed until now, but of the same age (c. 25 million years).

Abstract

The East African Rift System transects the anomalously high-elevation Ethiopian and East African plateaux that together form part of the 6,000-km-long African superswell structure. Rifting putatively developed as a result of mantle plume activity that initiated under eastern Africa. The mantle activity has caused topographic uplift that has been connected to African Cenozoic climate change and faunal evolution. The rift is traditionally interpreted to be composed of two distinct segments: an older, volcanically active eastern branch and a younger, less volcanic western branch. Here, we show that initiation of rifting in the western branch began more than 14 million years earlier than previously thought, contemporaneously with the eastern branch. We use a combination of detrital zircon geochronology, tephro- and magnetostratigraphy, along with analyses of past river flow recorded in sedimentary rocks from the Rukwa Rift Basin, Tanzania, to constrain the timing of rifting, magmatism and drainage development in this part of the western branch. We find that rift-related volcanism and lake development had begun by about 25million years ago. These events were preceded by pediment development and a fluvial drainage reversal that we suggest records the onset of topographic uplift caused by the African superswell. We conclude that uplift of eastern Africa was more widespread and synchronous than previously recognized.

Research area: Rukwa Rift



You can also read the press release at Michigan State University (fragment follows):
This study provides new evidence that the two rift segments developed at about the same time, nearly doubling the initiation age of the western branch and the timing of uplift in this region of East Africa. 

“A key piece of evidence in this study is the discovery of approximately 25 million-year-old lake and river deposits in the Rukwa Rift that preserve abundant volcanic ash and vertebrate fossils,” Roberts said.

These deposits include some of the earliest anthropoid primates yet found in the rift, added Nancy Stevens of Ohio University.

The findings imply that around 25 to 30 million years ago, the broad uplift of East Africa occurred and re-arranged the flow of large rivers such as the Congo and the Nile to create the distinct landscapes and climates that mark Africa today.


Hat tip to Pileta.

8 comments:

  1. "This is a most important discovery that will surely affect the chronological frame of the Homininae"

    Why? according to the press release they found some old simians, which are traditionally thought to have come about around the same age or if not older than this new age for the formation of the Western Rift, which is apparently about the same age as the formation of the eastern rift.

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  2. Maybe not. Maybe you are right. I wrote that before checking the details and actually most hominin findings are in the Eastern branch of the Rift or intermediate features like Olduvai Gorge.

    However it is also in the press release that:

    "These deposits include some of the earliest anthropoid primates yet found in the rift, added Nancy Stevens of Ohio University".

    In truth this is not my field of expertise at all. So I'm listening rather than issuing opinions.

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    1. Also, for clarity, "Ohio University", in Athens, Ohio (in the Appalachian foothills of Southeast Ohio) is not to be confused with its more famous reserach oriented mega-university, "The Ohio State University" in Columbus, Ohio in the central plains of that state.

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  3. I corrected that entrance paragraph to a softer tone. Hopefully better. Thanks.

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  4. I also shortened the title, just in case.

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  5. "However it is also in the press release that:"These deposits include some of the earliest anthropoid primates yet found in the rift, added Nancy Stevens of Ohio University"."

    Yes, and my understanding is that anthropoid primates are simians that are thought to have either branched off to new world monkeys (like squirrel monkeys) 40 million years ago or branched off to old world monkeys (like Gelada baboons of Ethiopia) greater than 25 million Years ago, or stayed into the branch that later became the Hominidae which includes the Homininae plus the organutans. Unfortunately they don't tell us what type of simian they found, I assumed it was the older type, or maybe they have not determined what type it is yet.

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    1. You are right. I read anthropoids as meaning hominin, but it is obviously what you say. It may still be important for the phylogeny of primates overall but surely less directly for our line.

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    2. And again edited to reflect reality as accurately as possible. Thanks for your insights.

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