STRUCTURAL GEOLOGY OF THE EASTERN RIFT TECTONICS ...

Sustainable Development Goals Short Course IV on Exploration and Development of Geothermal Resources

Risper kandie

Geologist-KenGen

Email: [email protected]

STRUCTURAL GEOLOGY OF THE EASTERN RIFT TECTONICS, VOLCANOLOGY AND GEOTHERMAL

ACTIVITY

20th November, 2019

SDG Short Course IV on Exploration and Development of Geothermal ResourcesOrganized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 14-4 Dec, 2019

SUMMARY

GEOTHERMAL ACTIVITY

VOLCANOLOGY

TECTONICS

OVERVIEW

Contents


EAST AFRICA RIFT SYSTEM (EARS) • Evolution

The third arm of the Afar triple junction African plate (Nubian plate) and Somalia

plate separated from Arabian Peninsula plate to the west and east

• Nature Active geological phenomena During Miocene~30-25 ma Recent (Youthful stage of ocean evolution-

Wilson cycle) It’s set up in divergent plate boundary Continental extensional feature Intra-continental ridge & axial rift

• Extent 6400 km long, 48-150km wide Gulf of Eden/Red Sea in SW Asia

propagating south through E. Africa-Mozambique, inhambane

• Branches Eastern & Western

Relief Map of EARS Shuttle Radar Topography Mission (Farr et al. 2007)


EASTERN RIFTReferred as Gregory Rift, comprises of;

Main Ethiopia Rift (MER)

Afar Rift (AR)

Kenyan Rift (KR)

Rifting stages

Advanced rifting (Ethiopian rift)

Oceanic rifting (Afar &Djibouti)

Spreading rates

Red Sea-Gulf of Aden (2 cm/yr)

AR(1 cm/yr)

MER (0.7 cm/yr)

Kenyan Rift (0.5 mm/yr)


WESTERN RIFT• Also referred as Albertine Rift

• Extends northwards from L. Malawi

(Nyasa) in a great arc that include (L.

Rukwa, L. Tanganyika, Kivu and Albert)

• Deepest basins that contain large lakes

and lots of sediment (L. Tanganyika, the

2nd deepest lake in the World and L.

Malawi)

Western and Eastern branch converge n

SW Tanzania forming a triple junction at

Rungwe volcanic complex in Mbeya-

Northern Tanzania divergence


TECTONICS

Model of Rift Formation

Lithospheric extension

Upwelling of

asthenospheric mantle

Decompressing and magma

generation

Fault& rift structure

formation

Generalized E-W section across a typical continental rift showing late stages of its formation

Mantle dynamics, the driving force of plates


TECTONICS CONT’DDriving mechanism

Mantle Superplumes

• Ethiopian dome

• Kenya dome

• Rifting of continental lithosphere is caused by hotspot


GEOLOGICAL-STRUCTURAL TECTONICS SETTING

• Its associated with Proterozoic orogenic processes have influenced rift opening and the subsequent onset of mantle-crust interactions

• Shields that were affected during Proterozoic (but remained stable since) are called Mobile belts. The main ones are:

1. Paleoproterozoic Belts: -Limpopo, Liberian, Dodoman-Nyanzian, Eburnian-Ubendian, Huabian, Buganda-Toro, Usagaran

2. Mesoproterozoic Belts: -Kibaran-Burundian, Irumide, Lurian

3. Neoproterozoic Belts: Pan/East African oregon:- Damaran-Katangan, West Congo, Dahomeyan-Pharusian, Mauritanide, Mozambique, Arabian-Nubian

• Rifting developed in terrane occupied by the Mozambique Belt (MB) close to Tanzania craton

• No volcanism occurs on the cratons

C=Congo, Z=Zambia,T=Tanzania, EAO-East Africa orogeny,AFS=Aswa Fault Zone, GA=Gulf of Aden, RS=Red sea, ANS-Arabian Nubian shieldGeologic outline of EARS Region

Map showing African cratons and zones with presently active volcanism


VOLCANOLOGY

MAIN ETHIOPIA RIFT

Structural setting;

Northernmost part of EARS

• It opens out into the Afar Depression

that opens in a funnel-like fashion as a

northward continuation of the MER

• Bound by 2 plateaus;

Ethiopian Plateau

Somalian Plateau

• The rifts have been the sites of volcanism

since early Miocene with fissure and

central style of eruptions


VOLCANOLOGY CONT.,’D

MER Structural set up

• It is a large graben, about

800km long and up to 60-

80km wide, having a mean

elevation of about 1600m.

• The rift is made up of 2

branches

Northern

Southern


VOLCANOLOGY CONT.’D

MER Volcanics composition and ages

• The dominant part of the rift volcanics

is Plio-Quaternary in age, older

volcanics only limited to the rift

margins. Basaltic lavas are

predominate over intermediate and

acidic products.

• Rhyolitic suites are more in the south

and basaltic in the north.

• In the MER, basic products are

subordinate to evolved lavas and

pyroclastics, which are dominated by

massive eruptions of silicic

ignimbrites from caldera volcanoes.


GENESIS AND EVOLUTION

• Total-Alkali vs. Silica (TAS) plot

for lavas erupted across MER

• MER is represented by a

bimodal distribution of large

amounts of peralkaline rhyolites

(which make up >75% of the

eruptives) and minor basaltic

products with transitional

petrochemical affinity

Alkali versus. Silica classification diagram for volcanic rocks from the northern sector of the main Ethiopian rift. The dashed line is the divide between the subalkaline and the alkaline field


AFAR RIFT

Volcanic formations and distribution of Quaternary magmatic segments.

EA = Erta’Ale, TA = Tat’Ale, AL = Alayta, MH = Manda-Hararo. DMS = the Dabbahu Magmatic

Segment; TGD = Tendaho Gobaad

Discontinuity (thick dashed line) and

• NB. Volcanic activity in Afar has been continuous from E. Oligocene to Present day.

• Dubbi volcano, May 1861, Erta Ale lava lake overspill, 1968-1974 and Ardoukoba fissure eruption, November 1978. In September 2005 a seismotectonic event started on the Dabbahu-Manda Hararo (DMH) rift zone in west-central Afar partly manifested by volcanic eruptions. Between 2005 and 2010, six eruptions occurred in different locations: a small rhyolitic eruption (2005) at Da’Ure locality and three dyke-fed basaltic fissure eruptions (2007, 2009 and 2010) have occurred at Karbahi locality on the DMH magmatic rift segment. Further eruptions took place in 2008 at Alu-Dallafila, part of the Erta Ale volcanic range in northern Afar, and also in 2011 at Nabro volcano in the Nabro Volcanic Range in eastern Afar.


GENESIS AND EVOLUTION

• Total-Alkali vs. Silica (TAS) plot

for lavas erupted across Afar.

• In general there is a strong

bimodal basalt-rhyolite

character to lava compositions.

• However, lava with intermediate

compositions are found at

several volcanoes located along

both axial rift zones (i.e.

Dabbahu) and at transverse

zones (i.e. Assab) with plume-

related OIB. Total-Alkali vs. Silica (TAS) plot for lavas erupted across Afar


GEOTHERMAL PROSPECTS IN ETHIOPIA

At different development

and assessment stages.

Regional

assessment

Detailed

exploration

Pre-feasibility

Feasibility

Pre-feasibility


ERITREA

• Alid Volcanic centre is the better

studied of all geothermal areas in

Eritrea

• It lies within the Danakil Depression

• Characterized by a rhyolitic domal

intrusion which is considered to be the

main heat source for the geothermal

system

• Manifestations occur in the form of

boiling pools, hotsprings &fumaroles

•Detailed geoscientific investigations

revealed a reservoir temp. 250◦C


DJIBOUTI

• Located at the Afar triple junction of 3 active, major coastal spreading centres: EAR zone Gulf of Aden Rift Red sea Rift

• Areas of strong manifestations are located within Asal and Hanle rifts in Afar Depression

• 8 exploratory wells drilled in the Assal geothermal fields (1970-1990)

• High temperature system has been successfully located, problems related to high salinity

• Prospects ready for exploration drilling

• Drilling in 2018-L.Assal Fialecaldera Map of Djibouti showing distribution of geothermal areas


DJIBOUTI CONT.’D

Geothermal prospects 1. Arta 2. Assal rift3. Djibouti 4. Dorra5. Gaggade6. Hanle7. Lac Abbe8. Nord Goubet9. Nord Lac Assal10. Obock11. Rouéli12. Sakalol13. Sud Goubet

Promising prospects1. Assal rift2. Hanle3. Nord Goubet

Photos of Djibouti showing distribution of geothermal areas


KENYA RIFT Structural set up

Linear grabens ~900km, 40-60km wide

Stretching from L. Turkana to northern Tanzania.

Rifting started about Early Miocene in the N around L.

Turkana and migrated southwards being active from

about Middle to L. Miocene in the central segment.

Regionwide rifting started at about 30 Ma (late

Oligocene epoch)

Faulting gave rise to the present day horst and graben

structures that occurred along fissures eruption

Volcanism associated with the rifting began at about 23

Ma (Miocene epoch)

Quaternary (<1.8ma) to Recent volcanism


KENYA RIFT CONT.’D

Geological set up

Characterized by volcanic centres with contrasting

morphologies

Mafic lavas from these volcanoes are transitional-

tholeiitic basalts; the corresponding felsic lavas

are dominantly trachytic in composition.

The rift axis volcanoes are dominantly trachytic to

pantelleritic, comenditic with little to no exposure

of associated basalts. Where present, the basalts

are dominantly transitional tholeiitic, although

the Chyulu Hills lavas are markedly silica-

undersaturated.


KENYA RIFT CONT.’D Quaternary volcanic complexes;

(1) Northern Rift Barrier Namurunu Paka-silali Korosi Arus Bogoria L. Baringo

(2) Central rift Menengai Eburru Olkaria Longonot

(3) Southern Rift Suswa, L. Magadi, Shompole

(4) Off-rift Chyulu Hills Homa Hills


KENYA RIFT CONT.’D

Geothermal development

At different development and assessment

stages.

Regional assessment

Detailed exploration

Pre-feasibility

Feasibility

Pre-feasibility


GEOTHERMAL IN TANZANIA

• Rifting initiated in north andstalled against Tanzania craton

• Magmatism started 8 m.a

• Largest caldera in the world-Ngorongoro

• Active volcano-Oldonyo lengai

• Variety of volcanic suitesprospects at varied explorationstages

Luhoi, Kisaki and KiejoMbaka-Detailed scientificstudies

Mt. Meru-Reconnaissance

Ngozi in Mbeya-prefeasibility-3-slim wells

Songwe-prefeasibility-temperature gradient wells


GEOTHERMAL PROSPECTS

Volcanic hosted prospect• Volcanics of Quaternary and

Tertiary age dominate the area• Numerous central volcanoes• The shield volcanoes are built

largely of basalts, intermediate lavas (trachytes) and rhyolites and the associated pyroclastics.

Style of magmatism • Notably changes along the from

north in Afar where MOR is literally on the surface.

• Through MER and Kenya rift, the Moho is < 25km to northern Tanzania where the crust thickens to ≈40m is exemplified by change from basalt-trachyte-rhyolite to carbonatitic types in northern Tanzania, e.g. Ol Doinyo Lengaivolcano.


SUMMARY

EARS is a continental type rift started ~30 Ma and propagated southwards

Eastern rift branches are MER, Afar, Kenya Rift

The current rift morphology general N-S orientation of the rift structure has been tectonically controlled by the Tertiary to Quaternary rifting

Volcanism and geothermal activity is attributed to domal related mantle superplumes; Ethiopian and Kenya dome.

Presence of Proterozoic mobile belts have facilitated propagation of rift zones with control by existence of cratons

Rifting is more advanced at Afar rift near oceanic floor formation and high volcanic activity

Volcanic materials vary in genesis and evolution at the MER, Afar and Kenya rift

Presence of elevated heat convective mantle plumes ingredients of geothermal resources

Eastern rift is endowed with geothermal resources potentials that are at varying development stages


END

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STRUCTURAL GEOLOGY OF THE EASTERN RIFT TECTONICS ...

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