Geology of Afar (East Africa)

345 Pages·2018·53.89 MB·English

by  Jacques Varet (auth.)

Regional Geology Reviews
Jacques Varet
Geology of Afar
(East Africa)
Regional Geology Reviews
Series editors
Roland Oberhänsli
Maarten J. de Wit
François M. Roure
More information about this series at http://www.springer.com/series/8643
Jacques Varet
Geology of Afar (East Africa)
123
JacquesVaret
Géo2d
Orléans
France
ISSN 2364-6438 ISSN 2364-6446 (electronic)
RegionalGeology Reviews
ISBN978-3-319-60863-1 ISBN978-3-319-60865-5 (eBook)
https://doi.org/10.1007/978-3-319-60865-5
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Foreword
TheAfardepressioninEastAfricaisoneofthemostimportantgeologicalstructuresonEarth.
ItistheareaofthejunctionofthreemajorstructuresoftheEarth’scrust:theRedSea,theGulf
of Aden and the East African Rift. Nearly 50 years have passed since January 1968, when a
French-ItalianteamledbyGiorgioMarinelliandHarounTazieff,ofwhichJacquesVaretandI
were young members, initiated the systematic geological exploration of Afar. I am therefore
very pleased to write this preface to the book by Jacques on the geology of Afar.
Since our first mission, we have been fascinated by the geology of Afar and by the
discovery—along the axis of the depression—of some active volcanic ranges displaying
magmatic and tectonic features suggesting that they were oceanic structures, an indication of
continentallaceration.Itwas,however,onlyaftersomeyears,whensufficientlywidezonesof
Afar had been explored, that general structural interpretation could be attempted. Extensive
radiometric dating of volcanic rocks, coupled with tectonic, petrological and geochemical
studies, enabled us to establish that internal Afar was almost entirely floored by recent vol-
canicrocks(lessthanfourmillionyearsold)andthatolderformationswereoutcroppingonly
on the depression margins. We thus proposed a two-stage evolution model: an initial long
stage of continental rifting which began in Early Miocene (around 25 million years ago) was
followed nearly four million years ago by a stage of continental break-up with the develop-
ment of oceanic structures.
DetailedinvestigationsontheeasternmarginshowedthattheconnectionbetweentheRed
SeaandtheGulfofAdenspreadingaxeshadtobefoundwithinAfar,asaconnectionthrough
Bab-el-Mandeb strait (as until then believed) was geologically unsustainable. The Southern
Red Sea and eastern Afar margin, in fact, have been stable for eight million years and,
therefore, had to be considered as an accretion of the Arabian plate.
Volcanicstructures,analogoustooceanicridgesandfracture zones,wereidentifiedwithin
Afar. Axial ranges—where basalts of a tholeiitic to transitional affinity erupted along open
fissures—displayed geochemical, topographical and geophysical similarities to mid-oceanic
spreading ridges, therefore marking the zone where new oceanic crust was accreting within
Afar.Transversetectoniclineaments,withsubordinatealkalibasalticvolcanism,markedalong
Afar the equivalent of oceanic fracture zones of the same type as those occurring in the
equatorial Atlantic Ocean. By analogy with clay experiments, some sets of en échelon faults
connectingadjacentaxialrangeswereinterpretedasthesurfaceexpressionoftransformfaults.
The identification of these structures was the basis for the reconstruction of the present plate
boundaries within Afar. A complex picture emerged, with a mosaic of microplates located in
thejunctionzoneofthethreemajorArabian,AfricanandSomalianplates,someofthembeing
deeply affected by deformation even far from the margins.
Ontheotherhand,petrologicalandgeochemicalstudiesrevealedacomplexvariationinthe
nature of volcanic products from one type of structure to the other, and even within a single
structure.Thesestudiesshowedthataxialrangesconsistedofbasaltsandsubordinateproducts
of their differentiation, whereas crustal contamination had modified the composition of
products erupted in the volcanic complex located near the Afar margins, where attenuated
continental crust was present.
v
vi Foreword
The occurrence of complete sequences of rocks from basalts to peralkaline rhyolites
enabled the study of the basalt fractionation process. A combined geochemical, petrological
and mineralogical approach enabled us to reconstruct the solid–liquid equilibria during the
fractionation process and to evaluate the controlling physical-chemical parameters.
These studies also showed that few basalts of the Afar axial ranges (Manda Hararo and
Asal) had tholeiitic composition identical to the equivalent rocks of the Red Sea and Gulf of
Aden spreading axes. Elsewhere in the Afar axial ranges, basalts tended to be more alkalic,
althoughremainingin the fieldoftransitional basalts;inthese ranges,fractionation processes
are,ingeneral,moredeveloped.Consideringthestructuralpositionofthevariousaxialranges
inAfar,wewereinclinedtorelatethesedifferencestovariationsinthespreadingrate.Alower
spreading rate would have favoured a lower degree of the mantel partial melting, producing
basalts with a more alkalic affinity. In these low-rate spreading segments, shallow magma
chambersmayhaveformedwhereslowlysuppliedbasaltswerefractionated,formingvarious
differentiates.
The geological studies carried out in Afar in the last few decades confirmed these basic
findings of theFrench-Italian team,including those on the2005–2010 tectono-volcanic crisis
of Manda Hararo, which represents a typical spreading episode of an oceanic axial range in
central Afar.
In 1975 Jacques and I were the first recipients of the Wager Prize, awarded to us by the
International Association of Volcanology and Chemistry of the Earth Interior and the Royal
Society of London for our “fruitful collaborative study of Afar”. I think that the reason was
largely the extraordinary interest of the Afar geology and I am sure that this book will
contribute to the diffusion of the knowledge of this beautiful desert.
Franco Barberi
Università degli Studi Roma Tre Rome Italy
Acknowledgements
The author wishes to express his gratitude to all the people who supported him during these
years of studies of the Afar region:
– His wife, the late Catherine Sornein, who was always so understanding and supportive
during40yearsoflifetogether,withtheirfourchildrenChristohe,Léopold,Sébastienand
Caroline.AbeautifulvolcanointheErtaAlerange(seecoverpage)wasgivenhernameby
H.TazieffandG.Marinelli,andherintelligenceandherkindnesswereequaltoherbeauty.
– ThelateProfessorGiorgioMarinelli,whonotonlywelcomedtheauthorintohislaboratory
inPisa,alwaysprovidingthemostusefuladviceandresearchfacilities,butalsoactedasan
“adoptive father”, hosting the whole family in his house on the Tyrrenian Sea every
summer to facilitate joint work with the team of the University of Pisa.
– The late Haroun Tazieff, who organised the Afar expeditions with his ability to attract
sponsors, thanks to his wide reputation and strong personality, contrasting with the sur-
rounding academic world to which he was contributing.
– Franco Barberi, now emeritus professor at the University of Roma 3, assistant to Giorgio
MarinelliatPisaUniversityatthetimetheworkstartedinAfar,thenProfessoratthesame
university.LaterVice-Ministerfornaturalhazards,heplayedamajorroleintherenewalof
Earth sciences in Italy. Most of the work presented in this book was developed with him.
I also had the honour to share the first Wager prize with him.
– Lul Mengesha Seyoum, “Ras” of Tigré province in Emperor’s Haile Selassie’s time, who
identifiedearly ontheCNR-CNRS teamasapotentialcontribution tothedevelopmentof
this then unknown region of Afar, and constantly supported our work, not hesitating to
comehimselfdrivingabulldozerwhenthewholeteamwasstuckinthixotropicmudafter
unforeseen heavy rains! Despite political changes, he kept his interest and convinced the
new authorities of Afar and Tigré Regional Strates to invite Franco and myself again to
help to develop geothermal in Afar 30 years after having escaped from the country when
the DERG (Coordinating Committee of the Armed Forces, Police and Territorial Army)
took power.
– ThelateProfessorRusselBlack,whoinvitedmetocomefor2yearsasAssistantProfessor
at Addis Ababa University (1970–1972), where I could attract the interest of Ethiopian
students for the fantastic Earth sciences opportunities in their country. This brings about
special thoughts for our students and colleagues, some of whom survived and succeeded,
whereas others were killed by the DERG (such as Johannes and Bill Morton) or while
fighting in the bush.
– The late Prof. Robert Brousse who asked me to become his assistant at the age of 22 and
allowed me to train on volcanic petrology and mineralogy in the French Massif Central.
– Valuable and so friendly colleagues from Italy such as the late Professor Giorgio Ferrara,
Enrico Bonatti and Roberto Santacroce, as well as from France such as the late Françoise
Gasse, Michel Treuil (with whom we jointly presented our “State Thesis”), Hélène
Bizouard, Jacques Demange and the late Jean-Louis Cheminée—I cannot mention all
of them here.
vii
viii Acknowledgements
– ForthemostrecentobservationsinEthiopianAfar,IbenefittedfromthesupportofIsmail
Ali Gardo and Colonel Atsede (Anbesit Nebro), co-founders (with Lul Mengesha and
myself)ofAGAP(thenewAfarGeothermalDevelopmentCompany),fromthesupportof
FouadAye,MinisterofEnergy,WaterandNaturalResources,andfromBGRforrenewed
works in Djibouti in advising the newly-created ODDEG (Office Djiboutien de
Développementdel’EnergieGéothermique).IthankhisdirectorDr.KayadMousaandhis
staff as well as my friend Abdourahman Haga.
Contents
Part I Afar: Global Context
1 Global and Regional Geodynamic Context. . . . . . . . . . . . . . . . . . . . . . . . . 3
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 Relief, Climate, People, Languages, Toponymy and Exploration History. . . 13
2.1 Topography of Surrounding Plateaus. . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Climate and Ecology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Ethnographic Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Afar Topographic Maps and Toponymy. . . . . . . . . . . . . . . . . . . . . . . . 22
2.5 History of Afar Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3 Geophysical Frame: Mantle Plume(s), Triple Points, Rifting Processes . . . . 39
3.1 The Nubian Plateau Uplift and Associated Volcanism. . . . . . . . . . . . . . 39
3.2 From Continental Doming to Rifting: The Continental Rift Stage . . . . . . 43
3.3 Mantle Plume in Afar and East Africa, Geochemical
and Geophysical Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.4 The Afar Triple Junction, a “Textbook Example”? . . . . . . . . . . . . . . . . 48
3.5 Geodynamic Relations Between Doming and Rifting Inferred
from Petrological Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Part II The Early Afar Geological Formations
4 Pre-quaternary Geological Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.1 The Afro-Arabian Plate Before Doming and Rifting . . . . . . . . . . . . . . . 57
4.2 Geology of the Plateau Margins Surrounding Afar . . . . . . . . . . . . . . . . 61
4.2.1 The Nubian Margin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2.2 The Arabian Margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.2.3 The Somalian Margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2.4 The “Danakil Alps” (or Arrata). . . . . . . . . . . . . . . . . . . . . . . 68
4.2.5 The Aisha—Ali-Sabieh Block. . . . . . . . . . . . . . . . . . . . . . . . 71
4.3 The Miocene Peralkaline Granite Intrusions of Afar . . . . . . . . . . . . . . . 74
4.4 The Mio-Pliocene “Polychromatic Formation” . . . . . . . . . . . . . . . . . . . 76
4.5 The Adolei Basalts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.6 The Mabla Rhyolites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.7 The Dalha Basalts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
ix