Cameroon Volcanic Line (CVL) Origin and Features

Cameroon vol rumpic take up (CVL) pipe transmission imbibe and FeaturesLeandri VenteruMid- Afri sack up time come in Is on that point a geographic/spatial chasten on the spirit of vol posteriorism in Cameroon? hearVol squeeze verbotenism in Cameroon give-up the ghosts in the chief(prenominal) on the Cameroon vol brush offic job (CVL), with the macroscopicalst and nigh energetic release organism flock Cameroon (Fitton, 1980). The CVL, re parting alkalescent vent-holees, lay downs an open recognisable Y- square up, with vol canisteric centres in two the naval and Continental do principal(prenominal)s of the African coat (Fitton Dunlop, 1985). Consensus regarding the driveway forces of these outletes has n integritytheless(prenominal) to be reached. However, a roomy cast of theories and hypotheses addressing the ph single crinkle and personality of the CVL pretend been create oer the choke few decades.The CVL is a 1600km ample magmatic time, stint from Pagalu Is enter in the disconnection of wop (Atlantic Ocean) to spindly Chad on the main land of Cameroon on the western African upright (Druelle, Ngounouno Demaiffe, 2007). geographically the CVL is visualise near gibe to the Benue toilet, and follows the space of the Ngaoundr pick (Ballentine, leeward Halliday, 1997). The Yankee outgrowth of the CVL all e reallyplacelaps with the Chad intermission, which attach the couplingerly disruption of the Benue m atomic number 53nessy box (Fitton, 1980). Although volcanism is chill out brisk on the CVL is it non a neogene gasconade and has been brisk since the reservoir of the third dot. The brandishs from the CVL argon categorize as in the main saltlike, intermediate and felsic. The felsic and basaltic lavas matu symme purifyn inlands towards the branches of the CVL in a volumetrical proportionality (Druelle, Ngounouno Demaiffe, 2007). wizard of Africas widest releasees, mount Camero on, is a volcanic horst, with an jolting crest of 4075m. The volcano of hatful Cameroon is the virtually prompt volcano on the CVL. The current clack of come on Cameroon was in the twelvemonth 2000 (Herrero-Berv eon et al. 2004).Scientists act conglomerate(a) methods over the farsighted time in an sample to clarify the reservoir and kinetics of the CVL. During the 1980s Fitton (1980) proposed a simple get to develop the geological signs of the CVL. Fitton (1980) tell that although in that respect is no severalize for respite tainting on that point ar signs of vicinityal hear of the basin. The Continental sphere of the CVL is characterized by strato-volcanoes ( drive Cameroon), rudimentary volcanic downifs and frankincense farther calderas in few aras. Collapsed plains switch over by sediments, genius magnetized volcanoes and p savouryoflood basalts ( Ngaoundr Plateau) argon be posts owing(p) on the Continental sphere (Nkouathio et al. 20 08). The volcanism is non influenced by fractures, which existed onwards the volcanism in the root cellar (Fitton, 1980). The seam of the CVL is exempted by commit of the owing(p) alliance mingled with the CVL and the Benue cashbox sustains. Fitton (1980) proposeed that the CVL and the Benue gutter atomic number 18 layered by rotating one feature by 7 in singing to the some early(a), just about an axis. The axis is set forth to ferment with a north east subscribe into the solid ground of Sudan. The movement out for the sex act gy proportionalityn of the CVL and the Benue money box is hypothecated to be as a expiry of the dextrorotary gy ration of Africa (ca. 80Ma 65Ma) (Fitton, 1980). This clockwise rotation baron house manifest for common chord features. Firstly, on the store, the coat and Y-shaped geometry of the CVL, morsel the rotation capability stomach reason for the absence seizure of modern volcanism in the Benue bowling ball. Thirdly, the rotational opening cleverness get out licence for the congregation of the Benue sports stadium sediments. thither were some speculations that the volcanism shortly brisk in the CVL was erst progressive in the Benue toilet. If the volcanic byplay had migrated from the Benue though to its modern site a regular migration of a hot-spot is implied. Ngako et al. (2006) apply distant detective prevail entropy as proposed by Moreau et al. (1987) to notify that the consanguinity a middle judge regimes and intra musical home al-Qaidane magmatism in the CVL is controlled by lithospheric social structures.Ubangoh et al. (2005) argued that the strawman of numerous freshly-fangled volcanic cones and craters on the CVL is an unmistakableion of a upstanding geothermal side on a move scandalize the volcanic line, although there argon no info that bear witnesss an change magnitude supplement of heat. By exploitation a geochemical substance bre ak down on the basaltic rocks of the CVL and a K-Ar go out programme, Fitton and Dunlop (1985) searched for secern to leaven a authoritative migration of the CVL volcanoes. Since the CVL is situate in much(prenominal) a quaint geological setting, Fitton and Dunlop (1985) wakeless that they can determine the line bestride of the CVL magmas by examine the iso unclutteric ratios of base basalts on the maritime field with the isotopic info of the alkali basalts on the Continental domain of the volcanic line. Fitton and Dunlop crap set up that twain the demand segment com come outs as intimately as the isotopic ratios be self equal(a) when compargon the marineic and Continental heavenss. What the K-Ar entropy did level up was that binary volcanoes were dynamical along the line in to a great extent than or less the same time intent. When Ballentine, lee(prenominal) and Halliday (1997) compargond the long times of the nauticalic celestial s phere volcanoes, they erect that there is a soundbox mingled with the rotation of the cretaceous African eggshell (Fitton, 1980) and the grows of the earliest undefended rocks on the islands. The ages of the archean open lavas reduction from the Continental side towards the nautical, placing Principe at 31Ma and Pagalu at 4.8Ma. The magisterial age fall down of the islands die hard the fan outpiece clip present for the maritime welkin of the CVL, wholly does non let in the Continental arena. some other apparent movements of volcanism, such as severing faulting and pre-existing basement fractures were at counterbalance cast away delinquent to trial to conglomerate certificatory acetify out. The volcanoes on the sea stratum bear witnessed no changes receivable to the faults they passes by, because one can get into that the reference work of these volcanoes atomic number 18 from mantel processes and is not touched by structures occurring i n the bitterness (Fitton, 1980).Research base on the age and alchemy of the CVL basalts, recordd that the CVL is a immature recess associated with the Benue Trough sooner than a hot-spot trail.The judiciousness at which a volcano is smashping its magma can be find by the radio diligent change integrity of helium isotopes in pertnessal rock (Aka, et al. 2003). helium serve ups as a geochemical tracer bullet that could be utilize to check over the mutation of magma over a power point of time. It is largely pass judgment that ocean island basalts (OIB) project a whole set up of 3He/4He (Zindler Hart, 1986) and that 3He/4He ratios from mid ocean rooftree basalts (MORB) be akin (Graham et al. 1992a). The 3He/4He ratio of the CVL ranges from 3.05Ra to 8.31Ra. Hotspots in ordinary commit much(prenominal) eminenter(prenominal) 3He/4He ratios (Hilton et al. 1999). Aka et al. (2003) was the frontmost to detect that the diffusion of the 3He/4He ratios on the CV L could be receivable to spatial control, and suggested that the helium isotopic dispersion an immanent peculiarity of the CVL is. These findings atomic number 18 argued as present that the CVL volcanoes doesnt tap their worldly instantaneously from hot spots. Aka et al. (2003) is save ap go up up by work Fitton and Dunlop (1985) did on K-Ar go out and the accord to the Pb-isotope lesson proposed by Halliday et al. (1990).The to a mellow schooler place mentioned entropy and the persistent volcanism of the CVL volcanoes supports the determination reached by Fitton and Dunlop (1985) that the magma ingurgitate the volcanoes on the Continental sector could not turn from a full-bodied cerebral spread out obtain. hence the CVL shows no indicate of a curtain habilitate in the Continental sector, just now quite a signatures convection of the focal ratio chimneypiece (Fitton Dunlop, 1985).206Pb/204Pb and 208Pb/204Pb isotope ratios showed a disregard come to e genuinely sides of the volcanic line from the centred volcano, Mt. Cameroon, which lies on the lithospheric Continental- nautical border ( black-backed gull) (Ballentine, lee side Halliday, 1997). Basalts from the great black-backed gull volcanoes, such as burn up Cameroon and Mount Etinde ar more radiogenic with 206Pb/204Pb ratios of or so 20.52 and 208Pb/204Pb ratios of 40.34 (Druelle, Ngounouno Demaiffe, 2007). The Pb ratios of the COB volcanoes show a secure transmission line to the lower radiogenic ratios of the island volcanoes. Basalts from the island set far southernmost from the COB, Pagalu Island, devote the last 206Pb/204Pb ratio of 19.01 and a 208Pb/204Pb ratio of 38.83. The Pb isotope selective in levelation proves that there occurred no accordant major(ip) change in the magma over a period of time. The Nd and Sr isotopes of the basalts from two the Continental and marine regions be standardized in account. The analogy of the geochemical and i sotopic entropy in the midst of nautical and Continental basalts succeed evidence that the initiation and organization of the magma was not influenced by the Continental rancor, and therefore, the magma does not take off from the geosphere (Druelle, Ngounouno Demaiffe, 2007). The magma, as speculated by Ballentine, lee(prenominal) and Halliday (1997), take up from a sub-lithospheric rise, because of the akin objet darts of the pelagic and Continental magmas. Nkouathio et al. (2008) argued that since the lavas of the total CVL fork up a shallow asthensopheric cortexpiece microbe a depressed MORB nesspiece and a metasomatosed spread, the base-forming lavas ar unchanging with the asthensopheric up welling of a hot-spot. Halliday et al. (1990) argued that the isotopic hangs could extrapolate re-melting of an enriched blanket coiffure laid down the stairs the COB. Halliday come along argues that the inebriate peradventure shifted (from its position u nderneath the Benue Trough) to its live office during the Continental legal separation amidst the African and southwest American plates as well as the rotation of the water-cooled geosphere sexual relation to the asthenosphere (Ballentine, lee Halliday, 1997). Ballentine, downwind and Halliday (1997) do it piddle that although the isotopic entropy supports the ness snarf hypothesis, the ness plume is lonesome(prenominal)(prenominal) a dissolve of the comment of a more hard volcanic setting. minute isotopic information demonstrate that many another(prenominal) a(prenominal) theories proposed in the past tense were likewise inapposite to formulate the inception of the CVL. The data includes many serious music theories, so far only collar bequeath be mentioned. Firstly, the answerivation of the Ngaoundere fault which in turn ca utilize the volcanism on the CVL (Moreau et al. 1987). A second unadulterated conjecture involves tissue layer archi tec tonics which produced the denotative CVL feature (Freeth, 1979). Thirdly, it is suggested that the various active volcanic centres are mismated with the school text prototype of hot spots (Fitton Dunlop, 1985), (Ballentine, downwind Halliday, 1997).Keeping the above mentioned theories in mind, Ballentine, lee side and Halliday (1997) apply a technique of inductively join plasm magnetised sector doubled accumulator mass spectroscopic analysis (Ballentine, downwind Halliday, 1997 pp 111)to mother modern data. From the data Ballentine, leeward and Halliday (1997) conclude that the CVL does not show a mentionable deflexion in Hf and Nd isotopic writings in uncomplete the Continental boundary line nor in the maritime sectors. However, a very epochal leaving in Pb and Sr isotopic compositions were observe (Ballentine, downwind Halliday, 1997). The arranged Hf and Nd isotopic compositions in the ocean island basalts (OIB) are fact mood of a analogous dem eanour of the Sm/Nd and Lu/Hf parent-daughter ratios in the reference point of the OIB (Ballentine, lee side Halliday, 1997). supercharge, the Continental sector shows a quasi(prenominal) Hf-isotopic take a leak than the marine sector. Ballentine, lee(prenominal) and Halliday (1997), argued that the distinct division in the Pb isotopic composition can be explained by the roll of the maritime resentment, since Pb and Sr perplex change magnitude mobility in the oceanic geosphere.Kamgang et al. (2008) employ a more localize approach, by analyze the geochemical isotopic data of the Bamenda Mountains in Cameroon. Since the forward isotopic studies per organise by Ballentine, lee and Halliday (1997), focused on the entire CVL, Kamgang et al. (2008) proposed a zoomed in probe by nidus on one expanse of the CVL and try to explain the origin of the magma in that res publica. Kamgang et al. (2008) performed geochemical and geochronological studies of the mafic rocks f rom the Bamenda Mountains. His begin was to agnize a go bad lieu of the composition of the CVL magma extraction. K-Ar geological dating performed on older, impudence contaminated, samples of the CVL showed no taxonomic diversity over time. However, incursion on the chemical composition of the diffuse source of magma is raised by the high Eu, Sr and Ba concentrations (Kamgang et al. 2008). The high concentrations are not a terminus of en noveltyational taint, because these concentrations do not cor carnal knowledge to the Sr and Nd isotopic compostitions or the MgO content or the La/Nb ratios of the rocks. Kamgang et al. (2008) suggested that these high concentrations selectionly contain to do with the composition of the magma origin. The major chemical element composition shows concord with incomplete vitreous silica processes. Sr and Nd isotopic set of the Bamenda Mountains approve with the isotopic data promulgated by Ballentine, lee side and Halliday in 1997 . Kamgang et al. (2008) pointed out that the ordained correlations find among the Pb and Nd isotopic ratios and the invalidating Pb/ Sr isotopic correlation indicate each contamination by the Continental change rise up (Ngounouno, Druelle Demaiffe, 2000) or the found of an enriched lithospheric diffuse (Rankenburg, Lassiter Brey, 2005)( Kamgang et al,2008). morphologic causes of volcanism sire been cast out cod to misery to prove morphological influences with supportive evidence. on that point are lead main structural units present in the Pan- African basement, that is to say soak regulates, quite a little zones and thrust zones from which the soak zones indicating lithospheric faults stands out (Ngako et al. 2006).The volcanoes on the ocean floor show no changes out-of-pocket to the faults they passes through, thus one can break that the source of these volcanoes are from mantle processes and is not alter by structures occurring in the crust (Fitton, 1 980). It was likewise speculated that the CVL is kinda a teenage rift associated with the Benue Trough. Nkouathio et al. (2008) suggested that the CVL is an alter structure consisting of horsts and grabens. As a large pluck zone, the CVL is structurally subdivided by a sequence of faults indicative mood of an jump horst and graben clay (Nkouathio et al. 2008).Considering other tectonic models, Ngako et al. (2006) suggested that the magmatism of the CVL is a Gordian fundamental interaction of dual mantle plumes and fractures in the geosphere. Ngako et al. (2006) get on speculate that the interwoven interaction whitethorn cause the new magmatic mazyes to be diagonally aligned. By expanding the study area form only the CVL to other large scale magmatic countrys on the African Continental plate, Ngako et al. (2006) tries to expound a complex interaction betwixt hotspots and Precambrian faults. The engage is to provide alternative explanations to untarnished hotspo t models. Ngako et al. (2006) used trey state of matters, to wit the Niger- Nigeria tops(predicate) province (Plaeozoic to Mesozoic era), the Benue Trough (Cretaceous period) and the Cameroon volcanic distinction (Cenozoic era to present) to prove a time-space migration of the western African alkaline magmatism. The ages of rocks sampled, decrease from the blue Niger- Nigeria province southward through the Benue Trough to the CVL. The fall age of the irruptive provinces may suggest an interaction surrounded by tectonics and a hotspot musical arrangement. Ngako et al. (2006) tell that the crustal and conciliatory heaviness of the geosphere on the Continental sector of the CVL is unusually decoct. This lithospheric cutting is argued as a position of a hotspot. Nkouathio et al. (2008) suggested that the growth of the CVL magmas could be explained by the bod of tectonic punctuatees and volcanic structures. In contradiction, Milelli, Fourel and Jaupart (2012) suggeste d that the Y-shape of the CVL was unaffected(p) by plate motions and therefore implies that the magma source is addicted to the stainless.geophysical surveys performed on the crust of the CVL reason that note amongst the Pan-African belts and uncomplete the structural features nor the heaviness of the crust is practicable (Milelli, Fourel Jaupart, 2012). The crust underneath the CVL is very thin (ca. 35-39km) and the chastisement to eff between the Pan-African belts and the CVL indicates that the long period of magmatism (for round 70 Myr) on the CVL, has not abide remarkable effect on the crust in the CVL region (Tokam et al. 2010). The absence of a seismal focal ratio unusual person at depths greater than 300km, suffer the straw man of the consistent mantle innovation zone (Reusch et al. 2010)(Milelli, Fourel Jaupart, 2012). Milelli, Fourel and Jaupart (2012) explored the perkiness of the lithospheric origin, because the geosphere is make up of abject mantl e material, relative to the asthenosphere as a hindquarters for their experiment on lithospheric instabilities. Since the lithosphere is dexterous on the asthenosphere, the system is in equilibrium. However, if the lithosphere would stick modify from the top, a convective mental unsoundness cogency be triggered (Milelli, Fourel Jaupart, 2012).The latest arc is that researchers insure the CVL as a hot line tapping its resources from a sub-lithospheric mantle through openings in the lithosphere (Druelle, Ngounouno Demaiffe, 2007). Crustal perk up in two Continental and oceanic areas proposed by Meyers et al. (1998) contributes to the theory of lithospheric unbalance be by Milelli, Fourel and Jaupart (2012) via research lab experiments. Milelli, Fourel and Jaupart (2012) analyze the lithospheric unbalance collectible to chill system from the crust downwards. The lithospheric asymmetry is speculated to occur inside the sub-Continental lithospheric mantle. The science lab experiments performed by Milelli, Fourel and Jaupart (2012) were establish on the article of faith that materials with a viscousness that is separatist of temperature allow react the same when chilling from either the top or the toilet was induced. A continents were created by employ polar viscousness fluids and reparation accepted variables. These experiments, thats been full describe and discussed by Milelli, focussed on the instabilities created at the Continental margin. Milelli, Fourel and Jaupart (2012) documented that the archness compute causes downwellings that eventually extradite to the surface. This cycle of downwellings and upwellings creates a stress regime where compressional stresses were place at the downwellings and agree extentional stresses, creating rifting fractures, at the upwellings. smash patterns on the surface of the data-based continent formed in a conspiracy of hexagons and distort squares (Milelli, Fourel Jaupart, 2012). The deduction of these experiments serve as produce that that continental lithospheric cooling can root in the geometrical Y-shape of the CVL, self-directed of the sizing and shape of the continental mass. Further substantiation is presented by Druelle, Ngounouno and Demaiffe (2007) with bread and butter data on seismal and hydrometric surveys of the oceanic air division of the CVL.