cameroon volcanic line(cvl)

8/13/2019 Cameroon Volcanic Line(CVL)

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1 Introduction

The Cameroon Volcanic Line (CVL) is a major geologic feature that cuts across

Cameroon from the south west to the north east. It is a unique volcanic lineament

which has both an oceanic and a continental sector and consists of a chain of Tertiar

to !ecent" generall al#aline volcanoes stretching from the $tlantic island of %agalu

to the interior of the $frican continent (&igure.1). The oceanic sector includes the

islands of 'io#o (formerl &ernando %o) and o Tom* and %r+nci,e while the

continental sector includes the -tinde" Cameroon" anengouba" 'amboutos" /#u and

andara mountains" as well as the $damawa and 'iu %lateaus. (!eusch et al" 01)

defined the CVL as a 213 #m long feature traversing both continental Cameroon in

4est $frica and the offshore Islands of 'io#o ( ,art of -quatorial 5uinea)" o Tom*

and %r+nci,e" and $nnob6n ( also ,art of -quatorial 5uinea). $ccording to them" the

Cameroon Volcanic Line is fairl linear" com,rise of -ocene to recent al#aline

volcanic massifs and %aleocene to -arl /ligocene anorogenic ,lutonic com,le7es

trending 289:. an of the volcanic centres have had activit in the ,ast one

million ears (&itton ; ?)" with the most recent activit occurring at t.

Cameroon. The continental ,ortion of the CVL develo,ed within the %an $frican

/ubanguides or 8orth -quatorial fold belt to the north of the $rchean Congo Craton

(&ig. 1).

$round the CVL" there are also three other major tectonic features

characteri@ing the regionA the 'enue Trough located northwest of the CVL" the

Central $frican hear Bone (C$B)" trending 8:-" roughl ,arallel to the CVL"

and the Congo Craton in southern Cameroon (&igure 1).The origin of the CVL is still the subject of considerable debate" with both

,lume and nonD,lume models invo#ed b man authors (e.g. 3E

&airhead ; 'in#s 1==1E Lee et al. 1==FE -binger ; lee, 1==>E Ging ; !itsema

0E 'ur#e 01E !itsema ; $llen 09E 8ga#o et al. 03E


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&igure 1. im,lified geological ma, of the stud area. Volcanic areas that are ,art of

the CVL are shown in red. The a,,ro7imate northern boundar of the Congo Craton

is shown with a green dashed line" and the small solid circles show a,,ro7imate

location of 7enolith occurrences. &B" &oumban shear @oneE C$B" Central $frican

shear @one. The a,,ro7imate locations of the -tinde" Cameroon" anengouba"

'amboutos" /#u and andara mountains are shown b numbers 1H3" res,ectivel"

and the 'iu %lateau is shown with the number .

$n e7am,le of a ,lume model is the flow of ,lume material from the $far guided to

the CVL b thinned lithos,here beneath central $frica rift sstem (-binger ; lee,

1=>>). 8onD,lume models include decom,ression melting beneath reactivated shear

@ones (&airhead ; 'in#s 1==1) and smallDscale mantle convection resulting from

edge flow along the northern boundar of the Congo Craton (Ging ; !itsema 0).

Crustal and u,,er mantle structure beneath Cameroon has been investigated

,reviousl using active (tuart et al" 1=>?) and ,assive (?)(&igure 1). -stimates of crustal thic#ness

obtained using gravit data show similar variations between the 5aroua rift"

$damawa %lateau" and southern ,art of the CVL (%oudjom et al." 1==?E 8nange et

al." 0).

(To#am et al"01) investigated crustal structure more broadl beneath the

CVL and the adjacent regions in Cameroon using 1D< shear wave velocit models

obtained from the joint inversion of !aleigh wave grou, velocities and %Dreceiver

functions for 90 broadDband seismic stations (&ig. 0). &rom the 1D< shear wave

velocit models" the obtained new insights into the com,osition and structure of the

crust and u,,er mantle across Cameroon.

2 Geologic Setting

&or To#am et al. 01" the continental sector of the CVL can be divided into a


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southern ,art" which e7tends from the coast to the southern edge of the $damawa

%lateau" and a northern ,art" which consists of two branchesE one ,assing through the

$damawa %lateau and the other stretching towards the 'iu %lateau in northDeastern

8igeria (&ig.1). 'oth ,arts of the CVL are underlain b %an $frican basement roc#s

consisting mainl of schists and gneisses intruded b granites and diorites (&itton

1=>E ) and t. -tinde" one of the older volcanoes" is made of

ne,helinitic lavas (edberg 1=3>E 8#oumbou et al. 1==?). /ther e7am,les include

t. 'amboutos" which is made of al#ali basalts and trachtes" and t. /#u" which

consists of transitional basalt" quart@ trachte and rholite flows (&itton ; ?). The andara ountains" along the northern CameroonH8igeria border where

some of the oldest volcanic roc#s are found (c. 9F a)" consist of trachte and

rholite ,lugs and al#ali basalt flows (&itton ; ?). The geochemical and

isoto,ic similarities between the CVL oceanic and continental basalts attest that the

continental crust did not ,la an role in the magma genesis and that the source is not

of lithos,heric origin (


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volcanic chain that lac#s the age ,rogression characteristic of the hots,ot model

( Goch et al" 010). It consists of a linear trend of al#ali basalt sea mounts and

volcanoes that stretch from $nnobJn in the $tlantic /cean onto the continental ,art

of the $frican %late (&itton ; ?). The CVL e7tends across Cameroon and

s,lits into a K sha,e" e7tending north to the 'iu %lateau and east across the $damawa

%lateau (&ig. 1). The orientation of the line is roughl ,arallel to the trend of

volcanoes formed b the !*union and Tristan da Cunha hots,ots beneath the $frican

%late (ller et al. 1==9). The velocit of the $frican %late is small in all absolute

reference frames so the uncertainties in relative motion direction are large. %revious

studies have found evidence that the CVL does not follow a sim,le hots,ot trend.

&irst" the volcanic age ,rogression is inconsistent. In general" the age of the onset of

volcanism increases from less than 9 a in the southwest" at t. Cameroon near the

coast" to 0? a in the northeast" where the line s,lits into the two arms of the KD

sha,e (ar@oli et al. 1===). owever" on the northern arm of the line" near the 'iu

%lateau" the age is less than ?.9? a (8ga#o et al. 03)" and on the -astern arm of

the line" near the $damawa %lateau" the ages are between = and 11 a (ar@oli et al.

1===). In the ocean" the island of $nnobJn formed between ?.F and 0.3 a (Cornen

; aur 1=>). 'etween t. Cameroon and $nnobJn" the islands of 'io#o" %rinci,e

and o Tome have ages of 1" 91 and 1F a" res,ectivel (ar@oli et al. 1===). In

addition" recent %D and Dwave traveltime inversions indicate a lowDvelocit feature

beneath the entire continental ,ortion of the CVL that e7tends to a de,th of at least

9 #m" rather than a region of low velocit locali@ed to one ,art of the line (!eusch

0=E !eusch et al. 01).

owever" there is not at ,resent a unifing model that describes the origin and

evolution of the Cameroon line. Its characteristics rule out an sim,le e7,lanation

such as ,late motion over a relativel stationar mantle ,lume. everal reviews

consider it as a single to,ic (&itton 1=>E oreau" !egnoult et al. 1=>E


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outh $tlantic /cean" a ,assive continental margin that formed in the late

Cretaceous. The igneous material of the continental sector com,rises lavas and

intrusive roc#s and contains volcanoes" monogenic cones and ring com,le7es. The

oceanic sector contains volcanic islands and seamounts. The roc#s are al#aline in

character.

D The oldest measured roc#s in the continental sector are intrusive ring com,le7es"

em,lacement ranging from 33 a to 9 a. -7trusive roc#s generall range in age

from 9? a to the ,resent da" with the e7ce,tion of a suite of older trachtes and

rholites in the far north. The most recent activit was on ount Cameroon in 0.

/ceanic sector lavas date from 91 a to the ,resent da although seafloor seismic

reflection ,rofiles im,l u,lift related to volcanicit from the late Cretaceous

(eers" !osendahl et al. 1==>). Im,ortantl" there is no irrefutable age ,rogression

along the line.

D The geochemical signature of incom,atible elements in basaltic roc#s does not

distinguish between those above the continental and oceanic sectors.

D tructurall" the continental sector is a succession of ten or so horsts and grabens.

The %recambrian basement of the continental sector contains distinct tectonic

lineament trends and shear @ones" generall subD,arallel to the Cameroon line.

!egionall" the ocean floor also contains fracture @ones.

D The adjacent (subD,arallel 20? #m to the northDwest) 'enue trough is an

aulacogen" an arm of the tri,le junction that formed when the outh $tlantic o,ened.

Com,ared to the Cameroon line" it contains few volcanic roc#s and is mainl filled

with Cretaceous sediment. The question arises as to wh the greater ,art of the

volcanicit in the region does not manifest above this feature.

3.2THE !EN"E TRO"GH.

The 'enue trough is a 8-D4 trending basin that e7tends from the 8iger delta basin

(5ulf of 5uinea) to La#e Chad.To the north of the CVL is the 'enue Trough

(&igures 1 and 0). Its origin is lin#ed to the o,ening of the outh $tlantic /cean in

the Cretaceous (5uiraud et aurin" 1==0). The KolaD5aroua or 5aroua rift and

amfe basin are eastward e7tensions of the trough into Cameroon. The similar KD


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sha,e of the 'enue trough and CVL (&itton" 1=>)" together with the similarit in the

com,osition of the al#ali basalt in both the 'enue trough and CVL (Coulon et al."

1==3)" suggest common geodnamic controls on their formation. &or e7am,le"

5uiraud and aurin (1==0) have argued that the orientation of the trough and CVL

ma be controlled b northeastDtrending %an$frican de7tral shear @ones. owever"

!ifting in the 'enue Trough started 1F a" coincident with the oldest igneous

roc#s found there M'ur#e" 1=3E alus#i et al." 1==?N. It continued to develo, during

the o,ening of the equatorial $tlantic (11=H1? a M&airhead" 1=>>N) u, until >F

a when it underwent a brief ,eriod of com,ression. Volcanism in the southern

'enue Trough continued between F and F= a Malus#i et al." 1==?N .

3.3 THE O"!ANG"I#ES !ELT AN# THE CENTRAL A$RICAN SHEAR

%ONE (CAS%.

ost of the %recambrian basement in Cameroon north of the Congo Craton belongs

to the %an$frican /ubanguides or 8orth -quatorial fold belt (%oidevin" 1=>9E 8@enti

et al." 1=>>). The /ubanguides 'elt is a branch of the much larger 8eo,rotoero@oic

%an$frican H 'ra@ilian 'elt" which resulted from the collision between the o

&rancisco Craton" the Congo Craton" and the 4est $frican Craton during the

formation of 5ondwana (Castaing et al." 1==FE Toteu et al." 0F). The /ubanguides

'elt includes several mlonitic shear @ones among which" most notabl the anaga

&ault and the Central $frican hear Bone (C$B) (Toteu et al." 0F) (&igure 1). The

/ubanguides mobile belt" also #nown as the 8orth -quatorial &old 'elt MToteu et al."

0FN" is %an$frican in age and formed during the amalgamation of 5ondwana

M8@enti et al." 1=>>N as in !eusch et al (010). The basement between the Congo

Craton and the 'enue Trough is com,rised of the /ubanguides mobile belt (!eusch

et al. 010).

The C$B is a major tectonic feature e7tending from the


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traced into 'ra@il" where it is called the %ernambuco lineament ('ur#e et al." 1=1E

'rowne and &airhead" 1=>9). In Cameroon" the C$B is also regionall #nown as the

8gaoundere M4ilson and 5uiraud" 1==0N or &oumbam lineament M a (8sifa" 03E Tchameni

et al." 01). In the $rchaean" the 5abonHCameroon hield and the 'omuHGibalian

hield collided to ma#e u, the northwestern and northeastern ,art of the Congo

Craton" res,ectivel ('egg et al. 0=).

The boundar between the /ubanguides belt and the Congo Craton in southern

Cameroon is ,oorl #nown and e7tends to the northeast towards the Central $frican

!e,ublic ('ou#e#e" 1==FE Toteu et al." 0F). $long this boundar" %an$frican roc#s

overthrust the 8tem Com,le7" forming an intracrustal discontinuit ('ou#e#e" 1==F).

& 'REVIO"S OR) #ONE.

ome diversed studies have been done along the Cameroon Volcanic Line

(CVL) b several authors as 5eo,hsicists such asA Tabod et al." 1==0E %lomerova et

al." 1==9E $teba and 8te,e." 1=>=" 1==E To#am et al." 01E !eusch et al." 01"

011 and some 5eologists such asA =E

8#oumbon et al." 1==?E


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(CVL) b authors as tuart et al." 1=>?E Tabod. C.T" 1==1E Tabod et al (1==0).

tuart et al. (1=>?) used seismic refraction results from their eismic

!efraction tud of the Crustal tructure associated with $damawa %lateau and

5aroua !ift" Cameroon" 4est $frica" to demonstrate that the crust is thinned (09

#m thic#) north of the $damawa %lateau beneath the 5aroua !ift" com,ared to an

average crustal thic#ness of 99 #m in the southern region of the ,lateau. ore recent

seismic studies" using the same data set as this stud" suggest that there is little

variation in crustal thic#ness along the CVL.

To#am et al. M01N" using a joint inversion of !aleigh waves and receiver

functions to calculate crustal thic#ness" found oho de,ths around 9? #m along the

CVL. The also suggested that the ,resence of CVL does not affect the overall

structure of the crust in the area when com,ared to the crustal structure of other %an

$frican mobile belts. eismicit studies of the CVL reveal some seismicit ( .?H

F.) in the vicinit of t Cameroon" the CVL" and e7tending to the south" more than

halfwa toward the border with -quatorial 5uinea (Tabod et al." 1==0).

eismological studies of the Cameroon Volcanic Line revealed a crust about 99 #m

thic# at the southDwestern end of the Continental ,ortion of CVL (Tabod."1==1).

&.2 Ot-er Geo-/+ic0l or

To#am et al (01) did a stud on crustal structure beneath the CVL and

adjacent regions in Cameroon using 1D< shear wave velocit models obtained from

the joint inversion of !aleigh wave grou, velocities and %Dreceiver functions for 90

broadDband seismic stations de,loed between Oanuar 0? and &ebruar 0.

The found thatA (1) crustal thic#ness is between 9? and 9= #m and velocit structure

is similar beneath the CVL and the %an $frican /ubanguides 'elt to the south of the

CVL" (0) crust is thic#er (F9HF> #m) under the northern margin of the Congo Craton

and is characteri@ed b shear wave velocities PF. #m sQ1 in its lower ,art and (9)

crust is thinner (03H91 #m) under the 5aroua rift and the coastal ,lain. In addition"

the also found that a fast velocit laer (V s of 9.3H9.> #m sQ1 ) in the u,,er crust is

found beneath man of the seismic stations. Crustal structure beneath the CVL and

the /ubanguides 'elt is ver similar to %an $frican crustal structure in the


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o@ambique 'elt" and therefore it a,,ears not to have been modified significantl b

the magmatic activit associated with the CVL.

!eusch et al (011) did their own stud on the thic#ness of the mantle

transition @one beneath Cameroon ma,,ed using data from the 0?H0 Cameroon

'roadband eismic -7,eriment to evaluate models for the origin of the Cameroon

Volcanic Line (CVL). The stac#ed some 00 receiver functions using a 9D

cameroon volcanic line(cvl)

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