Field evidence for flank instability, basal spreading and volcano-tectonic interactions at Mt Cameroon, West Africa

The Mt Cameroon volcano is the highest and most active volcano of the Cameroon Volcanic Line. Little geological information is available for improving the understanding of the structure of this large volcanic system and its relationship to regional tectonics. After reviewing the tectonic evolution of the region, the analysis of a Digital Elevation Model and results from a field campaign dedicated to mapping geological structures in the summit area and at the SE base of Mt Cameroon are presented. Mt Cameroon is a lava-dominated volcano with long steep (over 30A degrees) flanks. It is elongate parallel to its well defined rift zone. The summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults. Geological profiles were measured along rivers cutting through a topographic step at the SE base of Mt Cameroon. This step is associated with deformed Miocene sediments from the Douala basin that are overlain by volcanic products. Weak sediments of this area are deformed by 050A degrees-060A degrees and 020A degrees-030A degrees trending asymmetrical folds verging toward the SE, and thrusts faults related to the spreading of the volcano over its mechanically weak substratum. Combined remote sensing and field observations suggest that spreading is accommodated by summit subsidence and flanks sliding. Both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice, repeated dyke intrusions, the weak sedimentary substratum and tectonic structures.