Spatial Distribution and Structural Analysis of Vents in the Anivorano Volcanic Field, Northern Madagascar
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The purpose of this research thesis is to investigate the relationship between the spatial distribution of Neogene-Quaternary volcanoes and the underlying geological structures in the Anivorano Volcanic Field (AVF), northern Madagascar using remote sensing techniques. Understanding the interplay between continental monogenetic volcanic fields and tectonic features in an extensional regime is very important in unraveling the genesis and development of continental rifts. This study focuses on the analysis of the geomorphology, morphometry and spatial distribution of mafic volcanoes in the AVF; and the relative contribution of tectonic processes and pre-existing crustal structures to the formation of the AVF. One hundred and forty-six volcanic cones were identified in the AVF and are distributed over an area of 2500 km2. On the basis of this investigation, the AVF comprises 146 recognisable cones of an inferred volcanic nature, and these have been subdivided into six morphological types. This study found that topography has a negligible effect on cone morphology. However, it plays a significant role on the location of individual volcanic cones, as the majority of the cones are found in areas of lower elevation (<400 m) and on slopes of <10˚, especially in the southwest part of the study area. The study also found that volcanism in the AVF is intimately tied to fault systems in the region. Commonly, volcanic cones are located with mapped geological structures. The general trends of vent alignments within the volcanic field, and the orientations of the elongated vents lie along with known major tectonic features across Madagascar. The identified N000 o -N015 o and N030 o lineaments can be attributed to the pre-existing structures developed during the Pan-African orogenic events. Whereas the N315 o -N330 o trends are associated with fractures related to the ongoing opening of the Somalia basin. In addition, the dextral displacement deduced from the Ankarana Plateau in the southern part of the AVF is consistent with the dextral strike-slip faults observed within the western Somalia basin, which run through the Somalia-Kenya border to the middle of the Mozambique Channel and then to the north of Madagascar. These results suggest that pre-existing structures in the region were reactivated and play an important role in the location of individual monogenetic volcanoes in the AVF. In addition, the high concentration of volcanic cones in the AVF coincides with the presence of low velocity zones beneath the region, strongly suggesting the presence of abundant melts in this region. Based on the data obtained from this study, the fundamental factors controlling the location of volcanic cones within the AVF include; 1) pre-existing structures, 2) abundant mantle-derived melts beneath the area, and 3) topographic variations.