The Cameroon Volcanic Line (CVL) is an 1800-km-long line of Cenozoic volcanoes that does not show a chronological progression consistent with hotspot-related volcanism. We investigate seismic anisotropy to determine the upper-mantle lattice preferred orientation and constrain the mantle flow pattern using a temporary array of 32 broad-band seismographs deployed throughout Cameroon between 2005 and 2007 along with two additional permanent seismographs in adjacent countries. We determine the fast direction and lag time beneath each station by stacking SKS and SKKS splitting measurements from multiple events. The results indicate four regions with different splitting parameters. The Congo Craton in southern Cameroon and the Garoua rift region in northeast Cameroon have northeastsouthwest-oriented fast directions and split times of about 1 s. Between the Congo Craton and the CVL, in central Cameroon, the fast directions are variable and have small splitting times of 0.3 s or less. Along the CVL, where previous studies show a strong slow velocity anomaly in the mantle, the fast direction is oriented approximately northsouth, with splitting times of about 0.7 s. We interpret measurements from southern Cameroon and northeast Cameroon as indications of lattice-preferred orientation frozen into the Congo Craton and subcontinental lithosphere related to relict plate motion and deformation. The distinct pattern of splitting along the CVL suggests the existence of small-scale convection in the asthenosphere related to the formation of the CVL, perhaps driven by the adjacent cold edge of the Congo Craton.

The origin of the Cameroon Volcanic Line (CVL), a 1600 km long linear volcanic chain without age progression that crosses the ocean‐continent boundary in west‐central Africa, is investigated using body wave tomography. Relative arrival times from teleseismic P and S waves recorded on 32 temporary seismic stations over a 2‐year period were obtained using a multichannel cross‐correlation technique and then inverted for mantle velocity perturbations. The P and S wave models show a tabular low‐velocity anomaly directly beneath the CVL extending to at least 300 km depth, with perturbations of −1.0 to −2.0% for P and −2.0 to −3.0% for S. The S wave velocity variation can be attributed to a 280 K or possibly higher thermal perturbation, if composition and other effects on seismic velocity are negligible. The near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the CVL that invoke plumes or decompression melting under reactivated shear zones, but are possibly consistent with a model invoking edge‐flow convection along the northern boundary of the Congo Craton lithosphere. If edge‐flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the CVL, then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness.

Mount Cameroon is an active volcano located in the Gulf of Guinea, west of Central Africa. After the March-April 1999 eruption on the SW flank, another eruption of the volcano occurred in 2000. It took place from three sites on the southwest flank and near the summit. The first eruptive site was located 500 m to the southwest of the summit, at 3900 m altitude. Activity on this site was mainly explosive with no lava flow. The second site was located between 3220 and 3470 m altitude. Lava was emitted along NNE-SSE fissures from this site and flew towards Buea, the main city of the area, stopping similar to 4 km from the first houses. The last site was located in the south western flank at 2750 m altitude. The lava ejected from an old cone near the first 1999 eruptive site was divided into two branches, for a total length of around 1 km. The location of active volcanic cones in 1999 and 2000 seems to be linked to the local tectonics. The pre-eruptive period was characterized by a seismic swarm which may be a precursor recorded in March 2000 by an analogue seismic station. The main shock was a magnitude 3.2 event, and was felt by the population in Ekona town located on the eastern flank. It had a Modified Mercalli intensity of III-IV. When the eruption started, a temporary network of short period 3-component seismic stations was set up around the volcano to improve the monitoring of seismic activity. The co-eruptive period from late May to September was characterized by sequences of earthquake swarms, volcanic tremor and a family of earthquakes having similar waveform and appearing regularly in August and early September. Some of the earthquakes were felt by the population in Buea and its environments. The largest seismic event recorded had a magnitude of 4. During the post-eruptive period from mid-September to December, seismicity returned to its background level of 1-3 earthquakes per 3 days. Hypocenter locations reveal a linear narrow structure under the summit zone which could represent the magmatic conduit of the volcano. The frequency/magnitude relationship revealed a b-value of 1.43 higher than those previously determined, but more representative of volcanic media. Seismic energy release was gradual after the 2000 eruption started. (C) 2008 Elsevier B.V. All rights reserved.

Volcanic rock samples collected from 154 sites on the Cameroon Line (CL) have been analysed to study their petromagnetic characteristics. Thermomagnetic experiments and electron microscope observations on the samples indicate that the principal magnetic mineral and carrier of Natural Remanent Magnetisation (NRM) in these rocks is titanomagnetite rich in titanium. These also indicate that the level and stability of magnetisation are dependent on the quantity and composition of the magnetic minerals (which are controlled by the composition of the original liquid magma), their oxidation states and grain sizes. The contribution of high- and low-temperature oxidation observed in some of the magnetic minerals was that of increasing their magnetic resistance. Rocks with abundant ilmenite lamellae in their titanomagnetite showed increased magnetic resistance during demagnetisation experiments. In the analysis, the samples exhibited a considerable magnetisation spectrum indicating that most of them are strongly magnetic. The general tendency in experimental magnetism of an increase in magnetic resistance with decrease in grain sizes of discrete magnetic minerals is not respected in this natural system because of the effects of the presence of many ilmenite lamellae and mini-fractures in some of the titanomagnetite grains. In this way, the conditions of low crystallisation temperature, high fO2 and high PH2O, which were the conditions for the crystallisation of the most stable rocks (the hawaiites) on the line, were the conditions favourable for the acquisition of a stable magnetisation in the region. The low Curie temperatures (74–250 °C) found for a majority of the rocks indicate that the lower crust in the region and the upper mantle could be nonmagnetic. The high regional negative magnetic anomaly over the Cameroon Line that is not consistent with the small depth previewed by the Curie system and by the paramagnetic effect of the acid volcanic rocks in the region is probably derived principally from the differentiated basalts.

Mt. Cameroon erupted simultaneously in March-April 1999 in two distinct places on its southwestern flank. A hawaiite (with An 83-76 plagioclase phenocrysts) and an alkali basalt (with Fo 84-80 olivine phenocrysts) have been simultaneously emitted respectively at 1 400 and 2 650 m of altitude a.s.l. The eruption of these co-magmatic lavas attests to the occurrence of a unique magmatic reservoir inside which the magma differentiation took place by crystal set-ring of denser mineral phases. (C) 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

Recent seismological studies of the Cameroon Volcanic Line show that Mt. Cameroon is the most active centre, so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986. The network was reinforced with temporary stations up till 1987, and the local seismicity was studied. Here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations. Four swarms lasting 9 to 14 months are identified at intervals of 2–3 years. Most earthquakes are felt (intensity and magnitude, respectively, less than VI MM and 5) during the first three swarms and a few during repose periods. The main focal regions are the northwest and southeast flanks, the Bimbia and Bioko regions in the South of the volcano. Hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities. The subcrustal events are observed only in the southeast flank, they are the most regular earthquakes with a monthly frequency of 9 to 15 events. They are characteristic earthquakes with magnitude 2.8 ± 0.1. Between 1984 and 1992, their yearly mean time interval between successive events range from 50 to 86 hours. For that period their occurrence can be modelled as a stationary renewal process with a 3-day period. But the analysis of variance shows possible significant differences among yearly means. A Weibull's distribution confirms that the time intervals between successive deep events are not independent, and in 1993 a swarm of deep earthquakes is recorded, hence a non-loglinear magnitude/frequency relation. The deep seismicity is thought to be associated with a zone of weakness (perhaps a magmatic conduit) and may have some close relationship with the magmatic activity.

Historical and recent instrumental studies of the central region of the Cameroon Volcanic Line clearly indicate the occurrence of earthquake swarms of volcanic origin. Analyses of more than 3000 micro-earthquakes recorded between 1985 and 1992 show a well-defined seismic pattern characterised by single and swarm events with duration magnitudes between 2 and 3 at depths down to 20 km. On average, the earthquakes here occur at the rate of about 2 events every 3 days with occasional earthquake swarms, which greatly increase this number. The seismic swarms comprising felt earthquakes are shown to be sometimes preceded by, simultaneous with, or followed by swarms from Bimbia and Equatorial Guinea. Mapped epicentres of some of these swarms correspond to regions of volcanic gas emissions and are parallel to the fissures on Mt Cameroon and to the inferred direction of the underlying shear zone.The quiescent periods between swarms are seen to double each year since 1986. This observation was used to predict a major seismic swarm which occurred in 1993. The data coupled with historical data, are used to infer the involvement of a magma chamber in the generation of the earthquake swarms in the region. Since the installation of the network, no eruption has been observed on Mt Cameroon. This warrants more observation in order to study the seismicity that may precede, accompany or follow an eruption of the mountain. (C) 1997 Elsevier Science Limited.

Results of seismic monitoring of the continental segment of the Cameroon Volcanic Line for the period 1982 to 1990 are reported. Mount Cameroon continues to be seismically the most active area with a well defined seismicity pattern characterized by single and swarm events with duration magnitudes between 2 and 3 occurring at depths down to 55 km. On average, the earthquakes here occur at the rate of about 2 events every 3 days. Further north, the seismicity is low except for isolated felt events located along the Foumban Shear Zone which probably controls the location of the earthquakes as well as the volcanic centres making up the volcanic line.During this eight-year period two fatal carbon dioxide gas emissions from crater lakes occurred in 1984 (L. Monoun) and 1986 (L. Nyos) causing 37 and 1700 deaths respectively. Seismic monitoring indicates that no seismic activity was associated with these lakes thus favouring an aseismic cause to the disasters.These gas emissions, the increased number of felt earthquakes and a recent natural explosion on Mount Cameroon suggest either a more active phase of the Cameroon Volcanic Line and/or a greater awareness by the local population of the phenomena associated with this volcanic province.

Three moderate earthquakes with a maximum intensity IV were felt in the Kribi region in September 1987. Though there were no major damages reported, the phenomenon appeared quite strange to the local authorities and population. Two years later, in July 1989 another light earthquake was felt in the area. This prompted the analysis of the causes of that recent activity which is thought to be associated to the mobile zone/Congo craton margin of Central Africa.