Cameroon Volcanic Line

Botala Boloso, JorgeResearch Paper Spring 09, Geol 399 Independent Study

Evolution and Crustal Structure of the Cameroon Volcanic Line, West Africa

Outline

00. Abstract

I. Introduction

II. Age and Rock Types of the Cameroon Line

III. Crustal and lithospheric structure of the Cameroon Line

IV. Origin of the Line

V. Effect on oil and gas fields

VI. Conclusions

Questions to answer

a. What is the age of the Cameroon Line and how do we know?

b. What rock types form the Cameroon Line?

c. What is the crustal and lithospheric structure of the Cameroon Line? Does the crust thicken and lithosphere thin like over a hot spot?

d. What caused the Cameroon Line and when?

i. Hot spot, hot line?

ii.Other ideas

a. Is the Line still volcanically active?

b. How does the line affect oil and gas fields?

i. Sediments from volcanic islands

ii.Thermal maturation (higher thermal gradient)

Miscellaneous questions:

00. Abstract

The Cameroon Line volcano-swells formed 30 Ma. The establishment of shallow-mantle

convection, developed at 30 Ma underneath the African plate, and localized beneath the zone of extension

that joined the 711 plume to the right-angled bend in the continental margin, triggered the formation of

the Cameron Line. In this paper I discuss the different theories that have been presented by many authors

concerning to the development of the different facets of the Cameroun volcanic line, its lithologic

structure and compositions, as well as its bulk lithospheric structure. In addition, West African, where the

Cameroon line is located, has benefitted with heavy inversions and the flow of exploration data from its

shallow and deep marine basins. This data have let to the discovery of the major oil fields in West Africa.

Some of these major oil fields and the source of their origin are also listed in this paper.

I. Introduction

The break-up of Pangea 200 Ma ago originated major deformation in the Gulf of Guinea. This

complex tectonics event was followed by other processes ranging from gravity sliding and salt

movement, shale diaprism, compressional inversion and uplift; and strike-slip faulting. the

Cameroon volcanic line is a NE-SW trending line of volcanic centers including Mt. Cameroon

volcano, the offshore volcanic island of Bioko, Principe, Sao Tome, and Annobón, and a number

of sea mounts (St. Helene) (Lawrence et al., 2002). The spacing of volcanic centers along the

oceanic sector of the line has a crude regularity at least from Bioko to Pagalú (Annobón),

(Meyers and Rosendahl, 1991). The Cameroon volcanic line is a series of islands and seamounts

separated by saddles, situated on oceanic crust and sediments (Meyers and Rosendahl, 1991,

Hedberg, 1969). Volcanism had begun about 65 Ma along the continental sector and 35 Ma

along the oceanic sector [Hedberg, 1969].

Fig. 2. Major cratons and major tectonics elements of the African Continent. (from the Geological Society of South Africa). See the position of the Sao Francisco Craton, where the offshore sector of the Cameroun volcanic line is located. Also see the position of the West African Craton, Madagascar, and India compared to their current locations. And Notice the rotation of the African Plate during the past 600 Ma. (Depicted from the Geological Society of South Africa, ppt. presentation)

Fig. 1. Early stages of the breakup of Pangea. Observe the Tristan plume in between African and South America. This plume may have been the driven force that causes this rupture between Africa and South America. this rupture (depicted from the Geological Society of South Africa, ppt. presentation)

II. Age and Rock Type of the Cameroon Line

The Cameroon Line is constituted by 10 volcano-capped-swells of 1000-km-long. The

swells are small, 100 km across; arranged in a straight line. Half of these swells lie onshore and

half offshore. Irregularly shaped basins are intermingled among the swells. The landward sector

of the Cameroon volcanic line, from west to east, has a Y-shape form and comprises the Jos, Biu

and, Ngaoundere swells. The Biu Plateau, Nigeria, at the northern end of the CVL, is constituted

by Cenozoic Volcanism (Rankenburg et al, 2004). Lavas from the Biu plateau range in

composition from hy-normative basalt to basanite, with K/Ar ages from 5.3 to 0.84 Ma (Grant et

al., 1972). Hence, the timing of the Jos plateau is thought to be much related to that of the other

CVL volcanic centers. The magacryst suites of the Biu and Jos Plateaux comprise chemically

homogeneous crystals of clinopyroxene (cpx), garnet (gnt), plagioclase (plag) and ilmenite (ilm)

(Rankenburg et al, 2004). Data from these authors reveal magmatic textures, temperatures and

lack of zoning or exsolution lamellae, suggesting that the cpx and gnt megacrysts do not derive

from an older period of volcanism unrelated to the recent CVL. Megacrysts must have been

derived from magmas related to the recent (_5 Ma to present) magmatism on Biu and Jos Plateau

(Rankenburg et al, 2004), as suggested by these authors. In addition, crystallization of the

Cameroon Cpx preceded that of plagioclase in the basic rocks, while it was subcontemporaneous

to alkali feldspar and scarce oligoclase crystallization in the silicic rocks (Salvioulo et al., 2000).

Since Late Cretaceous, the volcanic history of the CVL has been dominated by alkali

magmatism (Fitton, J.G, 1987), corresponding to magmatic activity ranging from 66 to ~ 30Ma.

Many authors argue that these plutonic complexes were found to be constituted of gabbro,

syenite, and alkali granite, with composition ranging from alkali Basalt to trachyte and alkali

rhyolite.

On the other hand, there are no isotopic age determinations for the older basalts

(Rankenburg et al, 2004) from the Jos Plateau; their age estimate was around Paleocene. In

addition, younger volcanics of both the Biu and Jos Plateaux are characterized by abundant

inclusions of mantle xenoliths and megacrysts, as stated by these authors.

Furthermore, lavas along the CVL do not reflect simple fractionation or equilibrium

crystallization products, but instead reflect mixing of primitive and evolved batches of magma

shortly before eruption (Rankenburg et al, 2004). Virtually, crustal contamination has affected, to

some extent, some continental CVL lavas, particularly those lavas with 206Pb/204Pb <19_8

(Rankenburg et al, 2005).

Table. 1 (from Ubangoh et al, 1998) summarizes the ages of rocks from the Cameroon

volcanic line. Notice that rocks from the Cameron line are the youngest, compared to rocks from

other volcano-capped swells across Africa.

Figure 3. Basins and swells of the African plate. The Cameroon Line of 10 volcano-capped swells straddles the continental margin close to long 10_E. The line forms a distinctive part of the African plate’s active swell population:(1) the swells are small, 100 km across; (2) they are arranged in a straight line; and (3) half the swells lie onshore and half offshore. The ca. 75 swells of the African plate are indicated as ellipses. Irregularly shaped basins are interspersed among the swells. Volcano-capped swells are shown with black marks indicating the locations of volcanic rocks. The three swells at the landward end of the Cameroon Line are, from west to east, the Jos, Biu, and Ngaoundere swells. Offshore swells are shown only where there is evidence of dynamic support such as

Fig. 4. Eruption ages of the major volcanic centers of the Cameroon Volcanic Line and the Gulf of Guinea (adapted from Fitton and Dunlop et al 1985, Rankenburg et al, 2005).

Fig. 5. Rock types of the three major islands of the Cameroon Volcanic Line (Hedberg, 1969, Jay Pulliam, Institute for Geophysics, University of Texas, Austin, ppt. presentation)

Fig. 6. Primitive mantle normalized trace element patterns for representative primitive samples of the younger and older Biu Plateau from St. Helena (Thirlwall, 1997, ) and the HREE-depleted pattern of the mean of five melt inclusion found in ilmenite megacrysts.

Fig. 8. Reported ages refer to the stratoid volcanism of the continental sector (Gouhier et al., 1974; Fitton & Dunlop, 1985; Njilah, 1991; Lee et al., 1994a; Plomerova et al., 1993; Poudjom Djomani et al., 1995)

Fig. 7. Sr-Nd isotope compositions of Biu and Jos Plateau lavas and megacrysts along with literature data for oceanic rocks and continental CVL rocks MgO>5 wt % [from Halliday et al. (1988, 1990), Lee et al (1994), Marzoli et al. (1999, 200) and Rankenburg et al. (2004)]

An analysis of the study area in Fig. 7., where most lavas from the oceanic island of St. Helena, Tubuai and Mangaia plot in the field HIMU, confirmed that rocks from the Biu and Jos plateuax span much of the range of CVL lavas as a whole. Where the presence of isotopic systematics may be explained by mixing between a DMM (depleted MORB mantle) component and a CC (continental crust) and/or EM (enriched mantle) component, as stated by these authors. Similar to Fig.7., Fig. 8. (left) include the West Africa Craton listed in Fig. 7, the Kalahari Craton and Mount Banboutu caldera that was missing from Fig. 5. Tables of the isotopics analysis of Jos and Biu plateau lavas and their respective major (w %) and trace (ppm) are provided by (Rankenburg et al, 2004)

Hedberg (1969) studied the geology of the Sao Tomé (see fig. 4) and the southern

Cameroon. He found that the oldest rocks in Sao Tomé are quartzose sandstones that outcrop on

the Ubabudo Plantation in the east central part of the island. He also stated that the remainder of

the island is volcanic with exception of recent bioclastic limestones found near sea level at

several points along the cost. The map of Sao Tomé given in Fig. 4. summarizes Hedberg’s

observations regarding to the type of rocks that constitute Sao Tomé Island and the rest of islands

and capped-swells along the Cameroon volcanic lines.

Moreover, isotopic analysis data reveals that the older plateau building rocks of the Biu

Plateau range from basanites to trachybasalts. Whereas, the younger cinder cones have more

variable chemistry ranging from basanite to phonolite (see Table1. and Table 2. from

Rankenburg et al, 2004). Data from these authors also demonstrated that only the younger suites

of the Biu and Jos Plateaux contain xenoliths (peridotites, pyroxenites and crustal rocks) and

megacrysts. However, the absence of cooling features, such as recrystallization, diffusional

gradients or exsolution lamellae, commonly found in pyroxenite xenoliths rules out the

interpretation that the megacrysts were precipitated by an earlier magmatic event and stored in

cooler lithosphere for a significant amount of time (Rankenburg et al, 2004). Hence, these

authors propose that the megacrysts derive from magmas related to the recent (_5Ma to present)

magmatism on the Biu and Jos Plateaux.

III. Crustal and lithospheric structure of the Cameroon Line

The volcanic centers and seamounts along the Cameroon Volcanic Line are comprised of

uplift oceanic basement; thick pre-Miocene sedimentary overburden and Neogene Volcanic

cover (Meyers et al., 1998). In addition, CVL is comprised by Aptian to Late Cretaceous oceanic

basement of uplifted, thick pre-CVL volcanism sedimentary overburden, Oligocene and younger

volcanic rooks intermixed with sediments, and overlapping deep-water sediments, as stated by

these authors. Evidence of basaltic rocks in the oceanic and continental sectors of the CVL were

found to be geochemically and isotopically (87Sr/86Sr), suggesting a line or zone of hot

upwelling asthenospheric mantle underneath the region, partial melting of which has generated

parental magmas without any substantial involvement of the overlying lithosphere (Fitton &

Dunlop, 1985, Rankenburg et al, 2005).

Furthermore, recent data suggest that coeval Miocene crustal uplift extends from Sao

Tome into the onshore portion of the CVL. Some authors rule out the possibility that this portion

of the CVL represents a hotspot trace. On the other hand, many authors agree that volcanic

Fig. 9. Uplifts, Rifts, Basins and volcanic rocks of the past 30 Ma on the African Plate. See Figure 1 and table 1 for information on the ages of the rocks and the name of the cratons. Notice the Y-shape structure of the Cameroon line, traced from Jos and Biu Plateaux (onshore) to St. Helene Island (offshore). (Depicted from the Geological Society of South Africa, ppt. presentation)

centers and inner-island ridges along the CVL are not entirely made up of thick piles of intraplate

volcanics, but are instead combination of uplift lithosphere with significantly less volcanic

material. On this regard, Meyers and others (1998) found that at Principe Island, the underlying

basement has been uplifted by as much as 3 km to form a crustal arch less than 200 km wide

perpendicular to the CVL trend. The reflection geometry from this crystalline basement has been

found to be typical of oceanic crust. These authors also raised the point that the vertical faults

along the crustal arch present offsets and that dikes are common along this arch. The Reflection

Moho was found (by these authors) to be shallow and parallel to the uplifted crust along margins

of the arch, but was not observed directly below the arch axis where volcanism and faulting are

pervasive, therefore, concluding that the episode of crustal uplift was marked by a prominent

reflection unconformity. On the other hand, data from gravity modeling on Principe Island,

adjacent to Sao Tomé on the Cameroon volcanic line, indicates that an elongate wedge of

relatively less dense lithospheric mantle material underlies this island to a depth of 40 km. This

evidence is also supported by the authors cited on this section.

Fig. 10. Interpretation of unmigrated PROBE reflection profile showing uplift of flank of Príncipe Island. See figure 4 for location. T20 = to T60 = sequence boundary identifications; T40

= uplift sequence boundary; UHZ = uplift hinge zone; IVS = intercalated volcanic and sedimentary rocks; L2 = top of layer 2; and RM = reflection Moho

IV. Origin of the Line

The origin of the Cameroon volcanic line remains a controversial issue. Burke (2001) invoked

reactivation of a long-standing plume (the “711 plume”), at lat 7_N,long 11.5_E, responding to a

change in plate tectonic regime about 20 million years ago to create a shallow, short-wavelength

mantle convection pattern, and the stress conditions created at the right-angle bend in the

continental margin. These author also defined three remarkable distinct phenomena that

originated the Cameroon volcanic line: (1) a position over the 711 plume for the past ca. 140

m.yr.; (2) a location adjacent to a right-angled bend of the continental margin for ca. 125 m.yr;

(3) the establishment of a new platewide pattern of shallow-mantle convection at ca. 30 Ma when

the African plate came to rest.

Fig. 11. Deep-imaging reflection crossing seamount between Bioko and Príncipe Islands. Note how sedimentary reflections continue through seamount indicating crustal uplift. RM = reflection Moho; L2 = top of oceanic basement layer 2; IVS = intercalated volcanics and sediments; UU = uplift unconformity.

In addition, the Cameroon Volcanic Line is defined as a hot line essentially resulting

from passive rifting (Nkouation et al, 2008), and interpreted as a mantle upwelling inside zones

of weakness in the lithosphere after intra-continental extension. These volcanic chains form

parallel hotlines that are products of mantle upwelling between Reyleigh-Bernard-type

cylindrical convective rolls formed in the upper mantle (Meyers et al, 1998).

V. Effect on Oil and Gas Fields

Fig. 13. Hotspot track of the African Plate. Notice that the Cameroon line hot spot track is traced all the way from Mount Cameroon (onshore) to St. Helene (offshore). (Depicted from the Geological Society of South Africa, ppt. presentation)

Fig, 12. Nupe rift, Benue rift, and the northernmostrift of the south Atlantic rift system are suggested to have formed at ca. 140 Ma on the crest of a dome, shown as an ellipse, that was elevated over 711 plume. The dashed line with an arrowhead between Afu and Dutse indicates how the greater exposure of volcanic rocks in the north and plutonic rocks in the south is interpreted to have resulted from tilting of the surface of the Earth during doming (Depicted from Burke, 2001).

VI. Effect on oil and gas fields

There are speculations that the Cameroon line may be playing a crucial role in oil

regeneration in the Golf of Guinea, West Africa. However, more studies need to be conducted

to confirm this fact. Instead, many of the papers been published today about the driving factors

of the presence of oil in West Africa’s golden oil triangle refers to faults systems, salt domes,

and sediment deposition in deep marine basings generated during the break up of Pangea ~200

Ma. In addition, the major petrolifeurous basin systems of the eastern Gulf of Guinea extend

from the Niger Delta to North Gabon. Also, Rio Muni-Cameroon margin includes the Rio Muni

and Douala/Kribi-Campo basins (Lawrence et al, 2002), which mainly contains Rift faulting

associated with thick Aptian sedimentary section. These added that the break-up history, which

is shared with the conjugate margin of South America, and subsequent “African” events have

produced a complex and diverse structural history involving rifting, gravity-sliding and salt

movement, shale diaprism, compressional inversion and uplift; and strike-slip faulting.

Fig. 14. Regional stratrigraphy of the Gulf of Guinea showing sediment progradation, uplift, salt movement and the basin floor (Zane Jobe & Jim Hewett, Amerada HESS, ppt. presentation)

Furthermore, the structural sequence described above have been important in generating

hydrocarbon traps for either straightforward structures (Alba, Kribi, Sanaga Sud fields) or as

some kind of combination trap. (Zafiro, Ceiba fields). The Zafiro Field, offshore Bioko,

Equatorial Guinea, is formed by deepwater clastics drapping a toe-thrust complex. Whereas,

the Ceiba Field, offshore Rio Muni, is formed by sands associated with a saltcored structure

(Dailly, 2000). (Lawrence et al., 2002)

ShShelf edge

elf edge

Slopecanyons

(200 m incision)

Pock marks(piracy)

Fig. 15. Neogene evolution of a slope canyon system with

emphasis on Canyon fill architecture, offshore equatorial guinea (Zane Jobe & Jim Hewett, Amerada HESS, ppt. presentation)

Fig. 16. Locations of Crude Oil samples along the South Atlantic Basin. (Depicted from Geomark Research, Inc, 1994.) Notice that most of the oil in the Golf of is within the rifting zone of Pangea.

VII. Conclusions

Ovarall, understanding the complexity of the Biu and Jos Plateaux is essential to draw

any inference related to the age and rocks compositions comprising the Cameroon volcanic line;

whereas, studying the lithospheric structure of any of the volcano-swells can reveal deep inside

of the lithospheric structure for the overall Cameroon line. Rock samples from the Jos and Biu

Plateaux are constituted by basalts, and their compositions range from nepheline to hypersthene-

normative. Reflection profile from the Príncipe Island shows that the underlying basement of the

Cameroon volcanic line has been uplifted.

Fig. 17. Location of significant oil fields along the rifting fault zone of the Golf of Guinea,West Africa

(Geomark Research, Inc, 1994)

References:

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Due 15 page report Thursday April 30, 2009Date since Project Assignment March 17, 2009

Date Submitted May 0, 2009