A 17-MY-OLD WHALE CONSTRAINS ONSET OF UPLIFT AND CLIMATE CHANGE IN EAST AFRICA

Henry Wichura, Louis L. Jacobs, Andrew Lin, Michael J. Polcyn, Fredrick K. Manthi, Dale A. Winkler, Manfred R. Strecker, Matthew Clemens

Abstract
Timing and magnitude of surface uplift are key to understanding the impact of crustal deformation and topographic growth on atmospheric circulation, environmental conditions, and surface processes. Uplift of the East African Plateau is linked to mantle processes, but paleoaltimetry data are too scarce to constrain plateau evolution and subsequent vertical motions associated with rifting. Here, we assess the paleotopographic implications of a beaked whale fossil (Ziphiidae) from the Turkana region of Kenya found 740 km inland from the present-day coastline of the Indian Ocean at an elevation of 620 m. The specimen is ~17 My old and represents the oldest derived beaked whale known, consistent with molecular estimates of the emergence of modern straptoothed whales (Mesoplodon). The whale traveled from the Indian Ocean inland along an eastward-directed drainage system controlled by the Cretaceous Anza Graben and was stranded slightly above sea level. Surface uplift from near sea level coincides with paleoclimatic change from a humid environment to highly variable and much drier conditions, which altered biotic communities and drove evolution in east Africa, including that of primates.

Copyright © 2015 National Academy of Science. All rights reserved.

3D rendered movies of the Turkana ziphiid can be seen at digimorph.

A full online version and .pdf of this article can you find and order here.

selected international media response:
Los Angeles Times
The Science Times
International Business Times
ScienceNews
livescience
Daily Mail

ANOROGENIC PLATEAU FORMATION: THE IMPORTANCE OF DENSITY CHANGES IN THE LITHOSPHERE

Erik Duesterhoeft, Romain Bousquet, Henry Wichura, Roland Oberhänsli

Abstract
Away from active plate boundaries the relationships between spatiotemporal variations in density and geothermal gradient are important for understanding the evolution of topography in continental interiors. In this context the classic concept of the continental lithosphere as comprising three static layers of different densities (upper crust, lower crust, and upper mantle) is not adequate to assess long-term changes in topography and relief in regions associated with pronounced thermal anomalies in the mantle. We have therefore developed a one-dimensional model, which is based on thermodynamic equilibrium assemblage computations and deliberately excludes the effects of melting processes like intrusion or extrusions. Our model calculates the “metamorphic density” of rocks as a function of pressure, temperature, and chemical composition. It not only provides a useful tool for quantifying the influence of petrologic characteristics on density, but also allows the modeled “metamorphic” density to be adjusted to variable geothermal gradients and applied to different geodynamic environments. We have used this model to simulate a scenario in which the lithosphere-asthenosphere boundary is subjected to continuous heating over a long period of time (130 Ma), and demonstrate how an anorogenic plateau with an elevation of 1400 m can be formed solely as a result of heat transfer within the continental lithosphere. Our results show that, beside dynamic topography (of asthenospheric origin), density changes within the lithosphere have an important impact on the evolution of anorogenic plateaus.

Copyright © 2012 American Geophysical Union. All rights reserved.


A full online version and .pdf of this article can you find and order here.

THE FORMATION AND EVOLUTION OF AFRICA: A SYNOPSIS OF 3.8 GA OF EARTH HISTORY

Describtion
The African continent preserves a long geological record that covers almost 75% of Earth’s history. The Pan-African orogeny (c. 600–500 Ma) brought together old continental kernels (West Africa, Congo, Kalahari and Tanzania) to form Gondwana and subsequently the supercontinent Pangaea by the late Palaeozoic. The break-up of Pangaea since the Jurassic and Cretaceous, primarily through opening of the Central Atlantic, Indian, and South Atlantic oceans, in combination with the complicated subduction history to the north, gradually shaped the African continent.
This volume contains 18 contributions that discuss the geology of Africa from the Archaean to the present day. It celebrates African geology in two ways: first, it highlights multidisciplinary Earth science research by viewing the formation and evolution of Africa from 18 different angles; second, it celebrates the work of Kevin Burke and Lewis Ashwal and portrays the wide range of interests and research angles that have characterized these two scientists throughout their careers, working in Africa, and studying African geology.

CHAPTER 14: THE MID-MIOCENE EAST AFRICAN PLATEAU: A PRE-RIFT TOPOGRAPHIC MODEL INFERRED FROM THE EMPLACEMENT OF THE PHONOLITIC YATTA LAVA FLOW, KENYA

Henry Wichura, Romain Bousquet, Roland Oberhänsli, Manfred R. Strecker, Martin H. Trauth

Abstract
High topography in the realm of the rifted East African Plateau is commonly explained by two different mechanisms: (1) rift-flank uplift resulting from mechanical and/or isostatic relaxation and (2) lithospheric uplift due to the impingement of a mantle plume. High topography in East Africa has far-reaching effects on atmospheric circulation systems and the amount and distribution of rainfall in this region. While the climatic and palaeoenvironmental influences of high topography in East Africa are widely accepted, the timing, the magnitude and this spatiotemporal characteristic of changes in topography have remained unclear. This dilemma stems from the lack of datable, geomorphically meaningful reference horizons that could unambiguously record surface uplift. Here, we report on the formation of high topography in East Africa prior to Cenozoic rifting. We infer topographic uplift of the East African Plateau based on the emplacement characteristics of the c. 300 km long and 13.5 Ma Yatta phonolitic lava flow along a former river valley that drained high topography, centred at the present-day rift. The lava flow followed an old riverbed that once routed runoff away from the eastern flank of the plateau. Using a compositional and temperature-dependent viscosity model with subsequent cooling and adjusting for the Yatta lava-flow dimensions and the covered palaeotopography (slope angle), we use the flow as a ‘palaeo-tiltmeter’. Based on these observations and our modelling results, we determine a palaeoslope of the Kenya dome of at least 0.28 prior to rifting and deduce a minimum plateau elevation of 1400 m. We propose that this high topography was caused by thermal expansion of the lithosphere interacting with a heat source generated by a mantle plume. Interestingly, the inferred Mid-Miocene uplift coincides with fundamental palaeoecological changes including the two-step expansion of grasslands in East Africa as well as important radiation and speciation events in tropical Africa.

Copyright © 2011 The Geological Society of London. All rights reserved.


A full online and/or hardcover version of this book can you find and order here.

Bring forth what is true
Write it so it it’s clear
And defend it to your last breath

Ludwig Boltzmann
- -

ACKNOWLEDGEMENTS

My PhD thesis would not have been completed and much more finalized without the help of numerous people - Thanks to all of you!

First of all I would like to thank my long-standing supervisors Prof. Dr. Romain Bousquet and Prof. Dr. Roland Oberhänsli for initiating this research project within the Graduate School GRK1364 and supporting me since my early days as a student in geology.
Romain Bousquet’s inexhaustible abundance of new ideas was the source of several aspects of this study, and his support, open-mindedness, and perpetual readiness to discuss, provided the basic condition for efficient research. His positive attitude to all kind of things and his enthusiasm towards geodynamics were enormously motivating. Even in the hardest times, when problems appeared to be insurmountable, Romain still kept on saying: “Findet man eine Lösung” - a french-accented phrase, which helped always to get through tough periods during my PhD. I will always remember our first year joint fieldwork in a very small car in Kenya.
From Roland Oberhänsli I learned how to look on geology and rocks in detail, even when you run up a mountain. His unpayable field experience, interdisciplinary interest, and knowledge about volcanism always encouraged me to follow my ideas. I profited a lot during fruitful discussions, constructive reviews of the manuscripts and his always-friendly support during the last years.

My study benefited a lot from the contributions by Prof. Manfred Strecker, PhD. His profound knowledge in East African geology and its tectono-volcanic evolution were of great importance for the successful completion of my PhD thesis. His efficient work on numerous manuscripts significantly improved whatever I wrote and always within an incredible short time. This efficiency was extremely helpful during the last month while my thesis was rapidly developing. I will always remember the situation in a seminar when he pointed out the special geological situation of the Yatta Plateau and finally gave the ignition to develop my principal ideas of this thesis.
PD Dr. Martin Trauth brought the results of my thesis into a new light and always saw my work from an interdisciplinary point of view. He helped a lot to find aspects that supported my theory in many ways and shifted this project more into the center of the GRK1364 East African group.

I would like to thank Prof. Cynthia Ebinger, PhD from Rochester University (New York) and Prof. Dr. Sierd Cloetingh from the University of Amsterdam for accepting and reviewing my thesis without any hesitation.

I also would like to thank my colleague and friend Jannes Kinscher who accompanied me during the fieldwork along the Yatta Plateau in Kenya during the second year. He did a perfect job as motivator during hard and frustrating times.
I had the great luck that Ernst Hauber and Joel Ruch performed their PhD thesis at the same time like me. With both I had stimulating discussions on the rheology and emplacement of lava flows and the interpretation of satellite imagery.
Antje Musiol performed XRF-analysis in cooperation with the geochemical laboratories at the GeoForschungsZentrum Potsdam (GFZ). Anne Jähkel was a great help during the sample preparation. As this was really the base for my thesis their help is greatly acknowledged.
Accordingly, I thank my office mates Dr. Paolo Ballato, Heiko Pingel, Dr. Angela Landgraf, Amaury Pourteau, Michael Krause, and Dr. Dirk Scherler for their collegiality.

I am grateful to Dr. Andreas Bergner for coordinating the Graduate School GRK1364 and keeping my head free to concentrate more on scientific and less administrative work. In this respect I shall thank all the PhD students of the GRK1364 for fruitful discussion, unprecedented team spirit, funny evenings in Potsdam’s “Waschbar”, and an unforgettable time during our two excursions to Kenya and the Indian Himalaya.
I thank kindly the German Research Foundation (DFG) for funding this project over the last four and a half years and giving me the opportunity to finish my thesis within the set timeframe.

Finally, I would like to express my deep gratitude for the encouragement and infinite support in so many ways to my family. And last but not least I thank Sarah for her understanding, patience, and love.

EMPLACEMENT OF THE MID-MIOCENE YATTA LAVA FLOW, KENYA: IMPLICATIONS FOR MODELLING LONG CHANELLED LAVA FLOWS

Henry Wichura, Romain Bousquet, Roland Oberhänsli

Abstract
The emplacement of the 13.51 m.y. old Yatta lava flow in Kenya has been investigated using evidence from field observations combined with a novel method of modelling length-dominated lava flows along channels. The Yatta lava erupted as an individual flow from a single vent on the eastern rim of the present-day East African Plateau during the extensive volcanism that occurred in mid-Miocene times. It then followed an old river valley for nearly 300 km, thus forming one of the longest phonolitic lava flows on Earth. For our modelling we combined a composition and temperature dependent viscosity equation with empirical cooling and morphological relationships. By using an average channel width and the known length of the Yatta lava flow but varying the mean thickness and underlying topography, we have improved flow rate calculations for the internal part of the lava, close to the front of the lava flow. Within this zone the lava's motion was treated as steady, uniform, and laminar, following a stepwise cooling from the eruption temperature to the temperature at the cessation of flow. Comparison of eight different compositions ranging from basaltic to rhyolitic has revealed that the length-dominated Yatta lava flow emplacement was rapid (~ 7 days), approximately isothermal (cooling at 0.71 °C/km), and the result of high effusion rates (~ 7900 m3/s). This study shows that morphology of a lava flow, and in particular its length, is not a simple function of rheological properties and effusion rate, but is also affected by many other parameters. Small changes in H2O compounds in the lava chemistry can affect melt viscosity significantly and thus lava flow morphology. Therefore H2O content, together with slope angle and the mean lava flow thickness, ultimately control the length of a lava flow within a channel.

Copyright © 2010 Elsevier B.V. All rights reserved.


A full online version and .pdf of this article can you find and order here.

THE EAST AFRICAN PLATEAU ROSE... TWICE

article & interview by Vanessa Schipani

Beneath the surface of East Africa lies a ballooning expanse of hot magma. For more than 35 million years, the molten rock has driven the land skyward, creating the East African Plateau. Though most geologists agree on how the plateau formed, debate continues on when notable events of uplift occurred. In a recent paper in Geology, researchers provide evidence that supports two separate periods of major uplift, linking the plateau’s growth to important environmental developments in the region, such as the spread of grasslands and the diversification of plant and animal life, potentially including humans.
Researchers previously knew that the East African Plateau underwent significant uplift roughly 6 million years ago, but no one knew if major elevation growth occurred prior to this time. Henry Wichura, a doctoral student at the Institute for Earth and Environmental Sciences at the University of Potsdam in Germany, and his colleagues tried to nail down the plateau’s topographical history using an old lava flow...

© 2008-2010 American Geological Institute. All rights reserved.


The full article is in the October issue of EARTH magazine. You can find and order it here.

EVIDENCE FOR MIDDLE MIOCENE UPLIFT OF THE EAST AFRICAN PLATEAU

Henry Wichura, Romain Bousquet, Roland Oberhänsli, Manfred R. Strecker, Martin H. Trauth

Abstract
Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the ~300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2°, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.

Geological Society of America, P.O. Box 9140, Boulder, CO 80301-9140 USA (http://www.geosociety.org)


A full online version and .pdf of this article can you find here.