Planar structures (foliations and fractures) around the Tombel graben (southwestern end of the Central African Shear zone system) have been investigated and analyzed with the aim of unraveling the tectonic evolution of the basement.The foliations show two major trends, an older N-S-trending gneissose layering of uncertain age reworked by a later Pan-African (600 ± 50 Ma) NE-SW ductile trend that is contemporaneous with sinistral shearing and mylonitization.The brittle phase characterized by NW-SE-trending open and partially filled fractures is younger than the mylonitization event and although it has not been dated, it is suggested that the origin of these fractures is linked to the onset of volcanism along the Cameroon volcanic line ~ 31 m.y.ago.The mylonitic foliation is recognized for the first time and supports a tectonic evolution model for the Tombel graben in which ductile non-coaxial deformation was succeeded by brittle failure.
Mount Cameroon, a 4095 m high stratovolcano is arguably the most active volcano in Equatorial West Africa, with seven eruptions during the last century. It is the only active member of the 1600 km long volcanic belt – “Cameroon Volcanic Line (CVL)” - with a history going back ~ 80 Ma at the initial opening of the Central Atlantic Ocean during the Cretaceous. In order to decipher recent petrogenetic and geotectonic evolution of the CVL at this volcano, Lava samples of unknown ages but older than all 20th century lavas are investigated for their mineral chemistry and whole rock geochemistry and compared with the 20th century lavas that have been the only focus of petrological investigations for the past three decades. The data show all lavas as within plate and lava types (basalt, basanite/trachy-basalt, hawaiite and basaltic trachy-andesite) indicating these historical eruptions involved even more evolved magmas than those of the 20th century. The lavas are nepheline normative and show an alkaline affinity associated with rift-graben structures, fed from fissures and central vents. Most samples are olivine, clinopyroxene and plagioclase feldspar phyric with additional Fe-Ti oxide phenocrysts. Mineral chemistry data show the compositional range of olivine for older lava samples to be very wide (Fo58%-84%) and a broader plagioclase composition extending to intermediate andesine, a composition which has not been reported for any 20th century lavas. Despite these differences, primitive mantle normalized trace element spidergrams show similar pattern for these older lavas as the 20th century lavas, similar to OIB, confirming they all have a common mantle source of HIMU type. Thus, whilst the precise origin of the CVL is still disputed, it is clear that its mantle source area is not a transient plume. This has major implications for understanding mantle processes marginal to evolving passive Atlantic margins.
Abstract The morphology and structure of the 1999 lava flows at Mount Cameroon volcano are documented and discussed in relation to local and source dynamics. Structures are analysed qualitatively and more detailed arguments are developed on the processes of levee formation and systematic links between flow dynamics and levee–channel interface geometry. The flows have clear channels bordered by four main types of levees: initial, accretionary, rubble and overflow levees. Thermally immature pahoehoe lava units with overflow drapes define the proximal zone, whereas rubble and accretionary levees are common in the distal region bordering thermally mature aa clinker or blocky aa flow channels. Pressure ridges, squeeze-ups and pahoehoe ropes are the prevalent compressive structures. Standlines displayed on clinkery breccias are interpreted to represent levee–channel interactions in response to changing flow levels. These data complement previous knowledge on lava flow morphology, thus far dominated by Etnean and Hawaiian examples.
Abstract Mount Cameroon is characterized by an elongated summit plateau, steep flanks, and topographic terraces around its base. Although some of these features can be accounted for by intrusion‐induced deformation, we here focus on the contribution of edifice‐scale gravitational spreading in the structure of Mount Cameroon. We review the existing geological and geophysical data and morphostructural features of Mount Cameroon and surrounding sedimentary basins. Volcanic ridge gravitational spreading is then simulated by scaled analogue models on which fault formation is recorded using digital image correlation. Three sets of models are presented (i) models recorded in cross section (Type I), (ii) models recorded from above with a uniform (Type IIa), and (iii) nonuniform ductile layer (Type IIb). Type I models illustrate the formation of faults accommodating summit subsidence and lower flank spreading. Type IIa models favor displacement perpendicular to the long axis, with formation of a summit graben and basal folds, but fail to reproduce the steep flanks. Type IIb models investigate the effect of spatial variations in sediment thickness and/or properties consistent with geological evidence. Directional spreading of the volcano's central part perpendicular to the long axis is accounted for by a sediment layer with restricted lateral extent and increasing thickness away from the volcano axis. The later model closely reproduces key features observed at Mount Cameroon: steep upper flanks are accounted for by enhanced lateral spreading of the lower flanks relative to the summit. The relevance of these findings for understanding flank instabilities at large oceanic volcanoes is finally highlighted.
'%3e%3cg id='b'%3e%3cpath id='a' d='M0-8v8h4z' transform='rotate(18 0 -8)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(72)'/%3e%3cuse xlink:href='%23b' transform='rotate(144)'/%3e%3cuse xlink:href='%23b' transform='rotate(216)'/%3e%3cuse xlink:href='%23b' transform='rotate(288)'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
