The Gregory Rift Valley and Neogene-Recent Volcanoes of Northern Tanzania - Memoir no 33 (Memoir (Geological Society of America)) (No.33)

The structure and volcanic activity of the northern Tanzania sector of the Gregory Rift Valley have hitherto been less well described than those in Ethiopia and Kenya. This book focuses on northern Tanzania where, although the volcanic area is smaller than those to the north, there are major features such as Kilimanjaro, the highest mountain on the African continent; Ngorongoro, one of the largest calderas on Earth; and Oldoinyo Lengai, the world's only active carbonatite volcano. Following an account of the discovery and early exploration of the rift valley, there are descriptions of the individual volcanoes. These are set within the context of the regional geology and geophysics of the rift valley, and in relation to the structural evolution of the rift and its associated sedimentary basins which include Olduvai, an important site in the history of human evolution The volume concludes with a discussion of the volcanism as related to the plume-related African Superswell..Also available:Structure and Emplacement of High-Level Magmatic Systems - ISBN 978-1-86239-256-4The Afar Volcanic Province within the East African Rift System - ISBN 978-1-86239-196-3Alkaline Rocks and Carbonatites of the World: Part 3 - Africa - ISBN 978-1-86239-083-6 The Geological Society of LondonFounded in 1807, the Geological Society of London is the oldest geological society in the world, and one of the largest publishers in the Earth sciences.The Society publishes a wide range of high-quality peer-reviewed titles for academics and professionals working in the geosciences, and enjoys an enviable international reputation for the quality of its work.The many areas in which we publish in include:-Petroleum geology-Tectonics, structural geology and geodynamics-Stratigraphy, sedimentology and paleontology-Volcanology, magmatic studies and geochemistry-Remote sensing-History of geology-Regional geology guides Continental rifting processes continually reshape the Earth's surface, producing sediment-filled rift basins, or rupturing the tectonic plates to form new ocean basins. Rift architecture and tectonics focus volcanic and seismic hazards, as well as geothermal energy resources, while rift systems in Africa have controlled faunal dispersal patterns and influenced human evolution in the past. The response of a plate to extension and heating provides fundamental clues into the plate rheology, and the underlying mantle convection patterns. A number of models have been proposed to explain the success and failure of continental rift zones, but there remains no consensus on how strain localizes to achieve rupture of initially 125-250 km-thick plates, or the interaction between the plates and asthenospheric processes. The seismically and volcanically active East African rift system has long been a classic area for investigating rifting and break-up because its sectors encompass basins in all stages of rift to passive margin development. Its architecture is defined on the basis of both structural and magmatic components. It extends 3000 km from the Afar depression in the north to the Okavango Delta in the south, through Djibouti, Ethiopia, Kenya, Uganda, Burundi, Rwanda, Democratic Republic of Congo, Tanzania, Zambia, Malawi, Zimbabwe and Botswana. The East African rift system overlies one of the most extensive seismic velocity anomalies in the Earth's mantle, extending from the core-mantle boundary beneath the South Atlantic into the upper mantle beneath East Africa (Grand et al. 1997; van der Hilst & Karason 1999; Ritsema & van Heijst 2000). The rift system developed within Archaean-Proterozoic continental lithosphere, providing a unique opportunity to examine the mechanical response of strong, cold lithosphere to extension induced by asthenospheric upwelling and far-field forces. Over 1 000 000 km3 of basalts and more minor rhyolites cover the c. 1000 km-wide Ethiopia-Yemen plateau, which has experienced a maximum basement uplift of c. 1600 m above the mean altitude of the African plate to the west (Pik et al. 2003). The Ethiopian Rift, which transects the plateau, forms the third arm of the Red Sea-Gulf of Aden triple junction within the Eocene-Oligocene so-called Afar volcanic province (also known as the Ethiopia-Yemen flood basalt province) (Fig. 1). Thus, the region records both the evolutio The rifting of continents and eventual formation of ocean basins is a fundamental component of plate tectonics, yet the mechanism for break-up is poorly understood. The East African Rift System (EARS) is an ideal place to study this process as it captures the initiation of a rift in the south through to incipient oceanic spreading in north-eastern Ethiopia. Measurements of seismic anisotropy can be used to test models of rifting. Here we summarize observations of anisotropy beneath the EARS from local and teleseismic body-waves and azimuthal variations in surface-wave velocities. Special attention is given to the Ethiopian part of the rift where the recent EAGLE project has provided a detailed image of anisotropy in the portion of the Ethiopian Rift that spans the transition from continental rifting to incipient oceanic spreading. Analyses of regional surface-waves show sub-lithospheric fast shear-waves coherently oriented in a northeastward direction from southern Kenya to the Red Sea. This parallels the trend of the deeper African superplume, which originates at the core-mantle boundary beneath southern Africa and rises towards the base of the lithosphere beneath Afar. The pattern of shear-wave anisotropy is more variable above depths of 150 km. Analyses of splitting in teleseismic phases (SKS) and local shear-waves within the rift valley consistently show rift-parallel orientations. The magnitude of the splitting correlates with the degree of magmatism and the polarizations of the shear-waves align with magmatic segmentation along the rift valley. Analysis of surface-wave propagation across the rift valley confirms that anisotropy in the uppermost 75 km is primarily due to melt alignment. Away from the rift valley, the anisotropy agrees reasonably well with the pre-existing Pan-African lithospheric fabric. An exception is the region beneath the Ethiopian plateau, where the anisotropy is variable and may correspond to pre-existing fabric and ongoing melt-migration processes. These observations support models of magma-assisted rifting, rather than those of simple mechanical stretching. Upwellings, which most probably originate from the larger super-plume, thermally erode the lithosphere along sites of pre-existing weaknesses or topographic highs. Decompression leads to magmatism and dyke injection that weakens the lithosphere enough for rifting and the strain appears to be localized to Structure And Emplacement Of High-level Magmatic Systems: Introduction / N. Petford -- Large-scale Mechanics Of Fracture-mediated Felsic Magma Intrusion Driven By Hydraulic Inflation And Buoyancy Pumping / G. Albay, J.d. Clemens & N. Petford -- Lithological And Structural Controls On The Emplacement And Morphology Of Sills In Sedimentary Basins / K. Thomson & N. Schofield -- On The Long-distance Transport Of Ferrar Magmas / P.t. Leat -- Hyaloclasticites, Peperites And Soft-sediment Deformation Textures Of A Shallow Subaqueous Miocene Rhylotic Dome-cryptodome Complex, Pálháza, Hungary / K. Németh [and Others] -- Volcanic Craters, Pit Craters And High-level Magma-feeding Systems Of A Mafic Island-arc Volcano: Ambrym, Vanuatu, South Pacific / K. Né́meth & S.j. Cronin -- Constraining Melt Concentration And Strain Distribution Around Basalt Dykes In Partially Molten Olivines / S. Vinciguerra, X. Xiao & B. Evans -- Evolution And Morphology Of Saucer-shaped Sills In Analogue Experiments / A.p. Bunger, R.g. Jeffrey & E. Detournay -- Hydrofracturing-related Sill And Dyke Emplacement At Shallow Crustal Levels: The Eastern Elba Dyke Complex, Italy / F. Mazzarini & G. Musumeci -- Magma Emplacement In A Transfer Zone: The Miocene Mafic Orano Dyke Swarm Of Elba Island, Tuscany, Italy / A. Dini [and Others] -- Emplacement Of The Etive Dyke Swarm, Scotland: Implications Of Dyke Morphology And Ams Data / G.a. Morris, M. Kamada & V. Martinez -- The Structure, Fabrics And Ams Of The Slieve Gullion Ring-complex, Northern Ireland: Testing The Ring-dyke Emplacement Model / C.t.e. Stevenson [and Others] -- Concentric And Radial Joint Systems Within Basic Sills And Their Associated Porosity Enhancement, Neuquén Basin, Argentina / A. Bermúdez & D.h. Delpino -- Textural Analysis Of A Late Palaeozoic Coherent-pyroclastic Rhyolitic Dyke System Near Burkersdorf (erzgebirge, Saxony, Germany) / C. Winter, C. Breitzkreuz & M. Lapp. Edited By K.thomson And N. Petford. Papers From Lasi Ii (laccoliths, Sills And Dykes), Held On The Island Of Skye, Scotland, 1-3 April, 2006. Includes Bibliographical References And Index.

An account of the structural evolution and volcanism of the Gregory Rift Valley in northern Tanzania over the past eight million years. The volume includes a history of the discovery of the African rift valleys, and is illustrated with colour photos of some of the unique features of Tanzanian geology.

vol 147/2 Journal of Geological Magazine

This Memoir effectively summarizes the truly remarkable and visually spectacular volcanic province in northern Tanzania toward the southern end of the Gregory Rift or eastern branch of the East African rift system.

.It is an invaluable addition to the geological literature as it provides an authoritative summary of this region, which contains so many unusual, even unique rock types.

The volume is well illustrated with geological maps, photographs, tables of representative chemical analyses of volcanic rocks, as well as figures showing some of the voluminous geochemical data available. Colour photographs of several of the volcanoes and photomicrographs of some of the rocks enhance the volume. In reviewing the geochemistry of the volcanic rocks, Dawson presents persuasive evidence that the bulk of the magmas, whether basaltic or carbonatitic, originate from the upper mantle, and that any assimilation of continental crustal material is minor or absent.

The author is to be congratulated in giving the geological community a very useful reference guide to this intriguing region and its volcanism.

Apart from their scientific interest, the alkaline rocks and carbonatites are of major, and growing, economic importance. They are significant repositories of certain metals and commodities, indeed the only source of some of them, including Nb, the rare earths, Cu, V, diamond phosphate, vermiculite, bauxite, raw materials for the manufacture of ceramics, and potentially for Th, U, Diamonds, and many more. The economic potential of these rocks is now widely appreciated, particularly since the commencement of the very lucrative mining of the Palabora carbonitite for copper, a host of valuable by-products, and it the exploration efforts of mining companies that have led to the discovery of many important new localities. The present volume is likely to be of considerable interest to mineral exploration companies because there appears to be no published reviews of the economic aspects of the alkaline rocks. Readership: Research scientists in the fields of igneous petrology and volcanology, geologists interested in the regional distribution of igneous rocks and their relationship to tectonics and economic and exploration geologists.

the Seismically And Volcanically Active East African Rift System Is An Ideal Laboratory For Continental Break-up Processes: It Encompasses All Stages Of Rift Development. Its Northernmost Sectors Within The Afar Volcanic Province Include Failed Rifts, Nascent Sea-floor Spreading, And Youthful Passive Continental Margins Associated With One Or More Mantle Plumes. A Number Of Models Have Been Proposed To Explain The Success And Failure Of Continental Rift Zones, But There Remains No Consensus On How Strain Localizes To Achieve Rupture Of Initially 125-250 Km-thick Plates, Or On The Interaction Between The Plates And Asthenospheric Processes. This Collection Of Papers Provides New Structural, Stratigraphic, Geochemical And Geophysical Data And Numerical Models Needed To Resolve Fundamental Questions Concerning Continental Break-up And Mantle Plume Processes. The Focus Is On How Mantle Melt Intrudes And Is Distributed Through The Plate, And How This Magma Intrusion Process Controls Along-axis Segmentation And Facilitates Break-up.

Cover ......Page 1 Front matter ......Page 2 Contents ......Page 8 1. Introduction......Page 10 2. Discovery of the African rift valleys: early work on the Gregory Rift Valley and volcanoes in Northern Tanzania......Page 12 3. Regional geology......Page 17 4. Geophysical evidence for the structure of the crust and upper mantle of the Tanzania Craton and the Gregory Rift Valley......Page 21 5. Tectonic development of the rift structures......Page 29 6. Rift-associated sedimentary basins......Page 40 7. The Neogene–Recent volcanic rocks......Page 46 8. Regional comparisons, petrochemistry and petrogenesis......Page 85 9. Future work......Page 96 Appendix 1......Page 97 Appendix 2......Page 101 Index......Page 103