Proceedings World Geothermal Congress 2020

Reykjavik, Iceland, April 26 – May 2, 2020

Structural Geology, a control of the North and South Kivu Geothermal sites, A

Preliminary Report

1KAMBALE KAVYAVU W and 2KAPAJIKA BADIBANGA.C

1Université de Conservation de la Nature et de Development de Kasugho, Democratic Republic of the Congo, Goma Town

+243999154269, wisdomkambale@gmail.com

2Université de Lubumbashi

Keywords: Eastern DRC, Geothermal, Lithology

ABSTRACT

The study of the North and South Kivu geothermal power plant in Eastern Democratic Republic of Congo merit a detailed

structural survey. Located on the West branch of East African Rift System, the power plants are not dominated by Fumaroles

but hot and cool springs which deduce a structural genetic.

This section of African geothermal belt is reputed to have a high quantity of energy not yet quantified and it’s a result of post

early Eocene extensional deformation. Normal faults and Strike-slip faulting has mostly occurred on reactive normal faults.

Analysis of the faults slip data shows a principal stress, N29°E/80°ESE in some places. For rocks, joints accompanying the

rifting, in the middle σ1 is twice oriented around WNW-ESE, NE-SW; σ2 varies WSW-ENE WNW-SSE, while σ3 is about

N-S, S-N. Oriented approximately N-S and a contemporary, σ3 is in distension. The analysis suggests that structural view has

rotated in clockwise due to some change in orientation of σ3 might reflect counterclockwise rotation of the crust about a

vertical axis. The σ1 and σ2 stress axes apparently switched recently, with σ3 axis remaining changed too.

The majority of the hot and cool springs in Eastern DRC lie along faults due to pan African orogeny. Fractures control the

movement of the geothermal waters. Some springs occur along joints and faults and, in places, hot water flows laterally along

bedding planes. If the fractures also control the movement of water at high depth, other sites the reservoir of the geothermal

waters of North and South Kivu might be located around volcanoes and may be connected to volcanic vibrations too. The

geothermic gradient high in the Kivu, the principal hots springs areas along the region have a paralleled location to the Rift

attested by the Lakes in the region. Location since the majority of the faults dips in that direction. However, if the fault that

seems to have controlled the development of thermal activity, geothermal energy reservoirs in the area of Eastern DRC

around the Kivu Lake may be directly affected by volcanic chamber and water circulation beneath.

I. INTRODUCTION

Similar to western Rwanda and Uganda, Eastern DRC is a region of abundant geothermal activity currently undergoing

significant extension and mixed with high volcanism activity. Thus, the geothermal activity in this area is generally driven by

magmatic heat sources whose lava lake is not deep in the crust. In spite of faults accommodating deep circulation of

hydrothermal fluids of meteoric origin is the primary control on some geothermal springs in this region. Differently from

others, collisional, in the world; African Rift is due to a distension system of East African corner from the Big Africa.

Although most of the geothermal activity is in DRC, no structural control investigations have been conducted on the

individual fields in this part of DRC. The cause of knowledge of such structures may facilitate development of exploration

models and strategies. In this paper we have embarked upon a regional study of the controls on geothermal activity, which

includes detailed analysis of fields in the near south of Albertine GRABEN.

Our findings indicate a variety of structural controls in Eastern DRC but political and financial needs stopped some field

trips. The two hottest geothermal fields in North and South Kivu (Virunga-Kahubzi Biega active volcanic and Rwenzori

mount) are focused near the principal fault of EARS and one of them is at the end of the major fault zone around the graben.

Some small faults are serving as transversal to this one and this fault zone breaks into multiple splays, or horsetails, as it

terminates, thus generating small horsts of higher fracture density and permeability that guide Lake fluid flow. Several other

springs, including Kisumu, Bitaata in Masisi and Walikale zones. Faults might be connected to Kivu Lake and Edouard

through the west of Nyiragongo and

Nyamulagira volcanoes. The hot spring of Rwindi might be due to the same fault. In this area, it’s probably meteoric and

lake’s water infiltrates onto the principal fault network. Some faults are normal or oblique-slip in intersection with major

graben-bounding have springs to confirm geothermal of the region. Nyangezi site is such a case.

I.1 Structural model in geothermal power plant

On the continental areas thermal waters that appear as hot and cool springs (more than 38°C) infiltrate into rocks and straight

columns circulating to depth along Proterozoic to recent block faults. Surficial rivers circulate in valleys where depths are

fractured and fault continental crust. Some of the geomorphologies show the major and local structural lineaments.

KAVYAVU and BADIBANGA

At individual hot and cool springs, pressurized thermal waters are transmitted along fractures formed by the intersection of a

major fault with other faults, fracture zones, anticlinal axes (which may be faulted or fractured), or sedimentary aquifers. This

concretizes the general cases. In other circumstances, fracture zones alone may provide the necessary vertical permeability

conducting cool charges to depth at one side and rising thermal waters at the other side.

During extensive tectonic activity, faults accompanied by grabens and horsts are in the region, cool charges infiltrate and

have a high temperature under high columns causing heat flow in hot rocks or near volcanic reservoirs; due to temperature

gradient, hot charge emigrates to the surface or in valleys by fractures or holes in liquid or vapor states, figure N°1 The rocks

at deep levels are naturally hot and heated water supplies the hot springs at the surface.

Faults are still unique roads serving as open pipes from deep hot reservoirs to the surface, in this open pit the permeability

allows a path where hot fluids easily ascend, conserving its high temperature. Fortunately, these kinds of hydrothermal

systems are commonly appropriate within continents, exposed to be a great source of renewable energy for electricity.

Figure N°1. Rifting and geothermal resources a) Cracking of continents b)Evolution of geothermalism system

Hydrothermal systems involving dormant faults within orogenic belts are rarely targeted for geothermal exploration, partly

because of the complexity of the 3D topography, the unknown permeability of the fault zones, the basement lithology, and

the lack of deep level data (Audrey Taillefer et all, 2018). In all cases, cold charges originate from the infiltration of rainwater

in the highlands. Some studies indicate that high topography controls the circulation of fluids in the crust (until-3700m),

hence the hot springs location at the surface, Figure N°2.

Except springs in Central African grabens, most of the springs in DRC are in low valleys and river flats (M. G. Passau. ? )

In the study area, the sediment deposits covering outcrops on which can be gleaned, details of faults and size are hardly

visible and difficult to evaluate masked outcrops. Geophysical surveys were taken on a field trip for a full understanding of

the geothermal plant.

Figure N°2. Topography influence on springs

I.2 Regional Structural Framework

The purpose of this paper is to discuss the regional and local structural element controls from the occurrence of the warm/hot

springs and their related hydrothermal systems around Lake Kivu, on the DRC part. We summarize the results of surficial

geologic surveys and if possible, deduce shallow faults.

The main goal of this study is to extend ideas about considerations of what fault zones are at this intersection. Whether

outcropping or not these are mostly localized as zones of high secondary permeability and are fortunately areas to focus

hydrothermal discharge and hot spring risings for mining prospects, are traps of hydrothermal deposits.

KAVYAVU and BADIBANGA

Kivu lake watershed is led mostly by crystalline rocks, Precambrian metamorphic rocks, Cretaceous to Tertiary batholiths

and recent deposits in valleys. It contains the greatest number of springs in the country. Therefore, a better understanding and

deep knowledge of the structural conditions will lead to be better understanding of the true permeability of this country

section and its position in Rift zone. That’s the way of analyzing the hot-spring systems.

The Western Branch of the East African Rift System is known for its particular seismic activity with larger magnitude (up to

Ms 7.3) and more frequent destructive Earthquakes than those in the Eastern Branch. Neotectonics activity related to the

Western Rift Branch is in general well expressed and relatively well studied in the Eastern flank of this Rift Branch; in

Uganda, Rwanda, Burundi and Tanzania. In contrast, the western flank of this rift branch, largely exposed in the DRC, has

attracted less attention.

The crystalline rock complex is broken and develops a high permeability of numerous fractures that appear as petroleum

trapped to control occurrences of thermal springs. Reactivated faulting during Cretaceous to early Tertiary time followed by

block faulting further altered the regional fracture mixed to Cenozoic volcanic are truly the key of springs in the area. Due to

high alteration and tropical atmospheric conditions, sedimentary rocks drape the crystalline and preexist sedimentary with

which crustal, fault mirrors and valleys are borrowed.

The role of the inheritance of the leading rift faults from pre-existing basement structures and the result is frequently

occupied by sedimentary basins and Cenozoic rift lakes. (J. KLERKX, K. & all 1998)

Data collected during the colonial times show significant seismotectonic activity in East DRC, not only in the Western flank

of the Western branch, but also extending far westward up to the margin of the Congo basin, the indicated spring around the

rift. In particular, our predecessors paid a special attention to the mapping and description of thermal springs, noticing that

they are often controlled by active faults (Delvaux. D, 2013) The first-and second-order stress field of this region is relatively

well-known stress inversion of earthquake focal mechanisms, but the more detailed stress field related to the interaction of

fault segments has yet to be defined.

The major local structural controls are seen at the intersection of faults, intersection of fault and anticlinal axis, step faults or

other intravalley faults, intersection of fault and sedimentary aquifer, and minor faults and fracture zones in crystalline rock.

The seismotectonic investigation of the western side of the Kivu rift allowed for the identification and characterization of

some seismogenic faults on the basis of tectonic parameters such as the tensor of the orthogonal principal stress (σ1, σ 2 and

σ 3), the tectonic regime index (R and R’) and the azimuths of the maximal (SHmax) and minimal (SHmin) horizontal

principal stresses. Bamulezi.G. G. 2017

Since the Early Permian (Rolet, 1991; Delvaux, 1991), the tectonic evolution of the African continent has encountered rifting

processes that have been classified into six major systems based on the age of the youngest sediments in the rifts: Late

Tertiary to Recent system;

Early Tertiary system; Mid Cretaceous system; Early Middle Jurassic (end of Karoo system pro parte); Late Triassic-Early

Jurassic system; the Permo-Triassic (early Karoo) system.

The Late Permian-Early Triassic (Karoo) event affected all of eastern and southern Africa. It generated many new basins and

reactivated some of the Permian ones. A new rifting paroxysm occurred during the Early to Middle Jurassic. This episode is

related to the opening of the Indian Ocean, due to the initial E-W separation of Gondwana into a Western part, including

Africa, and an Eastern part, including India and Antarctica.

The Mid Cretaceous and Early Tertiary Rift systems affected a large portion of central to Western Africa. They

include the intracontinental Benue Trough, the Congola, East-Niger and Sudan Rifts (Benkhelil, 1989;

Bosworth, 1989; Daly & ai., 1989; Fairhead & Green, 1989).

Many of the geothermal systems in this region lie within or along the margins of major ~E-W trending grabens,

which are bound by major low-angle detachment faults and/or steeply dipping normal fault systems.

I.2.1 LOCAL STRUCTURAL CONTROLS OF HOT SPRINGS

The most important flexure is that which travels to the N. W. end of the studied region, in a N.E.S.W direction. It connects

the highlands bordering Lake Kivu to plateaus lower than 800 meters. The faults describe a vast cross of St. Andrew whose

center is immediately west of Lake Kivu, in the volcanic massifs of Kahusi and Biega.

The eastern branches of the cross correspond to the large western graben in which lakes Albert, Edouard, Kivu and

Tanganyika are housed. They are relatively recent and consist of faults composed of stairs, which delimits a series of

landings. This arrangement is the proof of the fragmentation of the primitive ground of the graben into a series of trenches,

nested within each other; they are all more narrow and deep as they are more recent.

The western branches, whose rejection is very important near the lake, are decreasing towards the west. They are ante-Karroo

because the layers of this system cover them. They have also replayed at recent times.

KAVYAVU and BADIBANGA

We can distinguish a beam of N.E.S.W. and two steering beams N.W.-S.E. The main fault of the N.E.W. is the fault of the

Wall, which probably extends to the North, under Virunga lavas, with the Rutshuru fault; this one borders the mountainous

promontory which separates the plain of Rutshuru from that of Ruindi. The fault disappears in the plain and is relayed by the

fault of Lake Edward along the escarpment of Kabasha and the lake. Further north, the fault line N.E-S.W. is prolonged by

Ruwenzori and Albert Lake fractures (ASSELBERGHS.E, 1939 )

Local modifications of structural framework are clearly noticed around the Kivu Lake. This local structure is superimposed

on the general fractured or block faulted crystalline rock framework. Further, the structure determines the location of most of

the springs. Major local structural controls are especially the result of simple faults, step faults or other intravalley faults,

intersection of sloped fault and lake, and minor faults and fracture zones in crystalline rock.

I.2.1.1 Identification of faults

Faults’ maps were compiled for the region around Kivu Lake. Surface surveys of Faults have been localized by direct

observations in the field, borrowed faults are based on some seismic and borehole data and masse movements. In some places

the data coverage is quite good, and the fault locations well-known, however in other locations the data are rare due to

agricultural activities or in deeper zones under granitic batholith. In addition, since data from different sources were used

from either side of the border, the fault traces did not always match. This discrepancy had to initially be rectified with the

correlation of faults across the border.

I.2.1.1.1 Intersection of faults and hot springs in the watershed

As previously mentioned, the erosional evolution in the tropical areas don’t allow structural surveys and left geologists to

identify all faults. Only some manifestations make earthquakes’ deduction. Springs in this kind of site are carried by

sedimentary rocks and have low temperature due to alluviums effluences. At Sake Springs in Masisi, Paleozoic and

Precambrian dip toward superposed by sedimentary rocks in the valley along the Kihira River valley, springs are hosted by

the East wall of the borrowed fault. This resurgence is near an intersection of two faults N-S, ENE-WSW directions as seen

in figure N° 3. Mesoproterozoic rich of schists and gneiss, are faulted and superposed by volcanic pyroclastics.

The hot spring of Mahyuza is a case of this form too; the site is carried by the East wall of fault in a ENE-WSW and NEN-

SWS direction.

In Kivu Lake watershed, many borrowed and outcropping faults are located. The case of Makelele fault is one, a part

borrowed, mylonitic rocks guided us to identify stable and active faults full of tectonic brechians, figure N°2, these faults are

not geothermally usable.

Figure N°2. Structural framework of Kalungu area

KAVYAVU and BADIBANGA

I.2.1.1.2Minor faults and fracture zones in crystalline rock

Kisuma hot and cool springs in Masisi lies along a block between two borrowed paralleled shear zones ENE-SWS. Today,

some mass movement at Mushake can be interpreted as Normal fault activation.

Nyangezi hot spring is located in the sedimentary basin upwelling a large graben, one Km of NE-SW direction. This segment

does not go out the elongation and the geometrical position of the rift in the region.

The Kivu Lake geological basement is principally characterized by fault networks active in the region with N-S strike, some

of them are in an E-W strike perpendicular to EARS direction (N-S), neither others present NNW-SSE and NNE-SSW strike

(CIRABA et al. 2012). Some faults are locally detected by surficial levels variations. In other faults lie rivers, such is the case

of N’kongwe as seen at Makele, carbonates deposits from Mbumbi hill and Rwamiko for distensive model, cross tectonic

brechians in shear zones perpendicular at Makele and Rwamiko. Some sites offshore are lost by collapse, photo N°4. The

maps indicate that faults could enhance permeability in different lithologies’ reservoirs across the majority of zone around the

lake and the re-activated faults by volcanoes. Good potential is found in the west rift Valley Graben.

Photo N°4. Collapse at activation of faults at Munigi, A, B volcanic faults; C, D at the banks of Lake Kivu Collapse at

Birava on Feb. 3 2008. on the island of Nkombo on 3 Feb. 2008

Others faults in the region are essentially small and unusable in this domain due to no hot springs in the extension, as is the

case of Shasha, with a limestone and schist lithology, deep studies are masked by agriculture and vegetation, the fault zone is

10 m large and essentially cataclastic in the rapid of Shasha river. This fault dip is generally vertical but dips around 80° to

the North East, its N-S strike deviates in some places. The connection with the Rift is large, about 1km hosting sediments.

The second remarkable feature is the half circle fault seen in Kalungu, as indicates the photo, the picture of Mbumbi with

Quartzitic lithology is a mirror of a NE-SW segment large about 20m in the valley ling by Ngonkwe while mirrors are

separated by 800m. Study of tectogliphs and other markers identify senestral and dextral types. The limestone wall of

Makelele is the full deduction of this fault on E-W segment strike. In front of this wall young mylonites and tectonic brechian

vertical and subvertical veins of NE-SW strike illustrate the reactivation of local structural basement in this locality, Photo

N°5.

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Photo N°5. Fault mirror in crystalline rocks, a) satellite view, b) Mbumbi Quarzitic pic c) Makelele limestone escalle

Extension may cause strike slip movement along faults with dilation at fault jogs and dilation of properly oriented fractures

within a preexisting fault-fracture. In the South of the past fault, at Nyabibwe, is another active fault. Its activity is deduced

from its mass movements and topographical differential levels here and there on road walls and housing, Photo N°6.

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Photo N°6. Nyabibwe local structural framework, a) Google view, b) Fault dimension, c) Mass movements around

Nyabibwe city, d) Normal fault near road

KAVYAVU and BADIBANGA

I.3 GEOLOGICAL SETTING

In DRC, the Western Rift (i.e. the Western Branch of the East African Rift) goes from N to S, by the Ruwenzori horst, the

Lake Edward, the Virunga volcanic massif, the Lake Kivu, the Ruzizi valley, Lake Tanganyika, Lake Moero (also spelled

"Mweru") and the Upemba graben; all running along the Eastern limit of the country.

The crust of all the province is considered to be a basement complex of Precambrian metamorphic rocks, in some

appointments, intruded by Precambrian gneiss, metamorphosed batholiths. The basement is sometimes covered by

metasedimentary rocks linked to the Belt and Paleozoic to Mesozoic sedimentary and volcanic rocks of Tertian age.

The West branch Rift is filled with alluvial, eluvial and colluvial Pliocene, Pleistocene and Holocene deposits, dated on the

basis of paleontological studies, including investigation of prehistoric sites. In the Ruzizi valley, area between the Lakes Kivu

and Tanganyika, these deposits may display a thickness ranging between 1500 and 2000m (Lepersonne, 1974) cited by

Ilunga.

Diatomite beds are linked to the Pliocene and Pleistocene deposits. The Western branch of the East-African rift lies mainly in

mobile belts of Lower Proterozoic origin, that evolve around the Archean cratons of Tanzania and Bangweulu. Lower

Proterozoic times, extensive granitization occur in the Bangweulu block. The Upper Proterozoic period is characterized by

the Pan-African (sensu-Iato) orogenesis which evolved in brittle-ductile

conditions. Tectonical manifestations have dislocated the Precambrian sole causing graben depressions of which flats are

scaled victims of erosional events (DENAEYER-E.M and Heinrich HART, 1952).

The Karoo series occur (1) in N-S trending grabens along the East-African coast, (2) in long elongated basins of general

ENE-WSW trend in Southern Tanzania and adjacent countries, (3) in small grabens aligned along the NW-SE trending

Tanqanyika-Rukwa- Malawi (TRM) lineament and also (4) in the Eastern margin of the Zaire basin (plate 1). The recent

Lake Malawi rift cuts across the NE-SW trend of long elongate Karoo basins, while the recent rifts of the TRM zone

developed relatively parallel to the discontinuous occurrences of Karoo along the NW-SE TRM trend. The sedimentary and

structural evolution in Kivu Lake coastal surveys indicates an extension activity in the basin. Tertiary volcanism started in the

Miocene, 10-12 Ma ago, in the Kivu volcanic province at the Northern end of Lake Tanganyika (Pasteels & a1.1989;

Ebinger, 1989b).

I.4 STRUCTURAL CONTROLS AND KIVU HOT SPRINGS

The Western branch of the East-African rift system is composed of a series of deep troughs forming a typical rift valley. The

image of the figure N°3 is a key of rifting geothermal zones. This alignment has a sigmoidal shape, bordering the Tanzania

craton to the West. Several transverse NE-SW shallow depressions are also related to the Western branch of the rift system

(Upemba flats, Mweru-Mweru Wantipa lakes, Usangu flats and Kilombero-Makata valleys).

Tertiary volcanism occurs in four isolated provinces along the Western rifts (Toro-Ankole, Virunga, Kivu and Rungwe

provinces). They coincide with accommodation zones, suggesting an intimate relationship between volcanism and faulting

during the initial stages of continental rift development. The Ruwenzori massif (5119m of elevation), displaying Precambrian

formations consists of a horst uplifted between the Lakes Albert (620 m; previously known as "Lake Mobutu Sese Seko" and

Edward (91 2 m; previously known as "Lake Idi-Amin"). Small-sized horsts are locally observed in the rift depression.

KAVYAVU and BADIBANGA

Figure N°3 Tectonic setting around Lake Kuvu , Congo and Rwanda block, resulting from the distension of African

and futreSomalian plate, has induced late Eocene to recent.

I.4.1 Katana hot springs

Baghalwa et al (2015) found a decrease in temperature of thermal sites between 2006 and 2008 followed by a slight increase

in temperature between 2008 and 2010 after a strong earthquake on 03 February 2008 leading to the increase of temperature

in the different springs sites even the drying up of some griffons in Kankule and Cimenki. These kinds of behavior are caused

by shears of the compartments or blocking faults. It must be admitted, however, that the opening of the walls causes a loss

while the closure increases the success rate of a total or instantaneous drying.

The geochemical analysis of springs of this region confirms diversified concentrations which reflect many different origins.

The high and grouped mineralization reflects their circulation in reservoirs whose lithological bedrock is varied. The richness

in chloride, sulphate and calcium, testifies a deep circulation through a crystalline base whose nature would be calcareous

and/or dolomitic or all both. Since the permeability is not recognized in this lithology, it is affected by major tectonic

activities from which crusting flush to un-flush thermal waters carried Ca and Cl precipitating in roll front model generating

carbonated deposits.

I.4.2 Nyangezi springs

Lithological surveys of Nyangezi valley attest a sedimentation basin, pebbles and sands are the most elements in this area.

The geomorphology structure deduces a small graben linking to Ruzizian graben. As previously mentioned, the surficial

hydrological network overlies the deep fracture’s images.

I.4.3 Sake springs

Located in east of Hango granite, high topographic characters parallel with the oriental synclinorium axis suggest an overlap

in Kahuzi-Nyamukubi (LAVREAU.J 1977)

2. CONCLUSION

At the end of the day, we found that the Kivu region is too fractured, this tectonic location all faults are not of prospecting

interest to confirm the thermal richness of the crust. The numerous thermal springs of Province Orientale are related to the

radial fracture bundles that make up the Great Central African Graben and their western extensions from it to Lualaba

following the two main directions combined NE-SW and NW-SE.

The sources are particularly abundant in the region of intersection of the bundle of contiguous faults loosely delimited by the

graben arc and the westward tangents to Lake Albert, and the northern portion of Lake Tanganyika. We must admit here that

KAVYAVU and BADIBANGA

fracking is young compared to the deposit of meta-sediments in grabens and prior to recent sediments. These are able to

influence the physicochemical characteristics of the surface.

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